BRIEF HISTORICAL REVIEW: THE DISCOVERY OF X-RAYS
Wilhelm Conrad Roentgen, a German physicist, discovered X-rays on November 8, 1895. The year 1995 became the Centenary year for X-rays. Roentgen was investigating the behavior of electrons in high-energy cathode ray tubes with air evacuated from the tube and the tube enclosed in black cardboard. A short platinum electrode was fitted into each end and on passing a high-voltage discharge through tube, he noticed a faint light glowing on a work bench about 3 ft away. He discovered that the source of the light was the fluorescence of a small piece of paper coated with barium platinocyanide. He concluded that some unknown type of ray was produced when the tube was energized. We can imagine his excitement as he investigated the mysterious new ray. He held his hand between the tube and the screen and, to his surprise, the outline of his skeleton appeared on the screen. By 28th December 1895, he investigated properties of the rays and was awarded the first Nobel Prize for Physics in 1901.
Tool/Procedure | Discovered by | |
|---|---|---|
1. | X-rays | WC Roentgen |
2. | Color Doppler | Christian Doppler |
3. | CT scan | Godfrey Hounsefield |
4. | Nobel Prize for MRI (2003) | Peter Mansfield and Paul Lauterbur |
5. | Theory of NMR elucidated by | Felix Block and Edward Purcell |
6. | Arterial Cannulation and Angiography | Seldinger |
7. | PTCA | Grundzig |
8. | Cardiac catheterization | Werner Forssmann |
Types of Radiations and their Uses
Atomic number | Number of protons in nucleus (Z) |
Neutron number | Number of neutrons in nucleus (N) |
Atomic weight or Mass number | Number of mass particles in nucleus (A) (A = N + Z) |
Neutron excess | Excess of neutrons over protons (N − Z) |
Types of ionizing radiation are: | |
|---|---|
Alpha radiation | • Particulate radiation • Consists of nucleus of helium atoms • Positively charged with “+2” • Least penetrating ability stopped by thin sheet of paper or skin • High LET radiation • They have highest ionizing potential • Predominant alpha emitter: Uranium, Plutonium. |
Beta radiation | • Particulate radiation • Consists of electrons • Negatively charged with “−1” • More penetrating than alpha particles • Can pass through 1–2 cm of water or tissue or a few mm of aluminum • Predominant β-emitter: Phosphorus-32, Strontium-89, Yttrium-92. |
Gamma radiation | • Are electromagnetic radiation (nonparticulate) • Highly penetrating • Can pass through the human body • Cannot be absorbed completely • Least ionization potential • Are emitted from nucleus in excited state (radioactive isotopes) • Predominant γ-emitter: Co-60, Radium-126. |
X-rays | • Are electromagnetic radiation (nonparticulate) • Emitted when fast moving charged particles (like electrons) are stopped (like by anode) • Have penetrating power less than gamma rays but more than alpha and beta rays • Photoelectric effect is important for production of X-ray of diagnostic range • Compton effect is important for X-ray in CT and radiotherapy • Can pass through human body • Cannot be absorbed completely. |
Neutrons | • Are uncharged particulate radiation • Present in nuclear reactors and at high altitudes • Have higher penetrating power • Water and paraffin wax are effective in absorbing it • Predominant neutrons emitter: Californium. |
Points of difference | X-rays | γ-rays |
|---|---|---|
Origin | Extranuclear | Intranuclear |
– X-ray tube | Radioactive isotopes like Co-60, | |
– Linear accelerator | 99mTc | |
Penetrating power | Intermediate | Highest |
Ionization potential | Intermediate | Least |
Diagnostic use | – Radiography | Nuclear scan/ |
– Mammography | Scintigraphy/ | |
– Contrast radiography (IVU, Ba studies, etc.) | Gamma-imaging | |
– Xeroradiography (Outdated) | ||
– Fluoroscopy | ||
– CT scan | ||
– DEXA | ||
Therapeutic use | Teletherapy (Ortho < Super < Megavoltage X-ray therapy) | Tele- as well as brachytherapy |
VARIOUS MODALITIES FOR IMAGING
- Conventional radiography
- Computed/digital radiography
- Contrast radiography
- Thermography (Outdated)
- Xeroradiography (Outdated)
- Mammography
- Ultrasonography (USG)
- Computed tomography (CT)
- Magnetic resonance imaging (MRI)
- Radionuclide imaging/scintigraphy/nuclear scan
- Emission tomography (SPECT and PET).
Diagnostic imaging modality and resolution | |||
|---|---|---|---|
Modality | Resolution | Typical imaging time | Radiation dose |
Plane X-ray | <mm | <1 sec | Low |
Mammography | <mm | <1 sec | High |
Compound tomography (CT) | mm | Few seconds | High |
SPECT and PET | 0.5 – 1 cm | 10 – 30 minutes | High |
Digital radiography | mm | <1 sec | Low |
MRI and MRS | mm | 10 – 40 minutes | Nil |
CONVENTIONAL RADIOGRAPHY
X-ray Production
X-rays are produced mechanically, by making electrons strike a target, which causes the electrons to give up their kinetic energy as X-rays (X-rays are produced extranuclearly).
While gamma rays are produced by nuclear disintegration of radioactive isotopes (Gamma rays are produced intranuclearly).
Properties of X-rays
- Affect photographic plate
- Bombard scattered/secondary radiations
- Chemical and biological changes produced
- Dual nature (emitted as well as absorbed)
- Electrically neutral
- Electromagnetic rays
- Fluorescence producing rays
- Gases ionized (indirectly)
- Highly penetrating
- Heterogeneous (polyenergetic)
- Heat energy produced (in small amount) on passing through matter
- High frequency
- Invisible rays
- Short wavelength (extremely short)
- Straight line traveling rays
- Speed same as that of light (3×108 m/s).
X-ray Tube
X-rays are produced whenever a stream of fast-moving electrons undergo rapid deceleration and these conditions prevail during operation of special thermionic vacuum tube called hot filament or Coolidge X-ray tube.
A typical X-ray tube is a thermionic diode consisting of a tungsten filament cathode, a tungsten target anode, an evacuated glass tube enclosure (Pyrex glass) and 2 circuits to heat the filament and to drive the space charge electrons to anode.
- A hot metal filament (cathode) gives off electrons by the process of thermionic emission.
- If no kilovoltage is applied, the emitted electrons remain near filament as an electron cloud or space charge.
- If kilovoltage is applied between the filament and target so as to place a negative charge on filament (cathode) and a positive charge on target (anode), space charge electrons are driven over to anode at high speed by the large potential difference. The electron stream crossing the gap between cathode and anode constitute the tube current, measured in milliamperes (mA).
- If supplied kilovoltage and resulting electron spin are high enough, the electron strikes and enters the target, their kinetic energy being converted to heat (99.4%) and X-ray (0.6%) and thus X-rays are produced.
- The wavelength of the characteristic radiations produced by the target of X-ray tube is not changed by the potential difference (kVp) applied.
- Intensity is proportional to (kVp)2.
- The quantity of X-rays produced depends on atomic number of target material, kVp and mA, while the quality depends on kVp only.
- Heat dissipation in stationary anode tube occurs by absorption and conductivity, provided by massive copper anode.
- In rotating anode tube, absorption of heat by the anode assembly is undesirable because heat absorbed by the bearings of the anode assembly would cause them to expand, bind and get damaged. Because of this problem the stem, which connects the tungsten target to the remainder of anode assembly is made up of molybdenum, which has high melting point and is a poor conductor of heat.
- Thus in rotating anode X-ray tube the heat generated in rotating tungsten anode disc is dissipated by radiating through the vacuum to the wall of the tube, and then into the surrounding oil and tube housing.
- When an object is placed in the X-ray beam, it will cast a “shadow” on the film that will show some degree of enlargement.
- If X-rays were emitted from a point source, the magnification could be determined by ratio of target-film distance to the target-object distance, which is called geometric magnification.
- But in reality, X-rays are emitted from an area, the focal spot, hence the magnification that results with X-rays from focal spot, is called the true magnification.
Under usual radiographic situations, magnification of the image should be kept to a minimum. Two rules apply for this purpose:
- Keep the object as close to the film as possible.
- Keep the focus-film (X-ray machine to photographic plate) distance as large as possible.
X-ray filters are sheets of metal (aluminum filters are commonly used) placed in the path of X-ray beam near the X-ray tube housing to absorb low energy radiation before it reaches the patient. they are simple and inexpensive. their main function is to protect the patient from useless (low energy) radiation. they reduce skin exposures by as much as 80 percent. NCRP recommends an equivalent of 2.5 mm of aluminum permanent filtration for diagnostic X-ray beams of energy more than 70 kVp.
Heavy metal or k-edge filters are used to remove higher energy photons from the X-ray beam by taking advantage of the increased mass attenuation coefficient at the k-edge of certain elements. Compared to aluminum, k-edge filters enhance contrast, reduce patient dose, and increase tube loading.
An X-ray beam restrictor is a device that is attached to the opening in the X-ray tube housing to regulate the size and shape of an X-ray beam.
They are of three types:
- Aperture diaphragm.
- Cones and cylinders.
- Collimators.
Closely collimated beams have two advantages over larger beams:
- Less scattered radiation and thus improved film quality.
- Smaller area of patient exposed and hence decreased patient exposure.
A major disadvantage of aperture diaphragm, cones and cylinders is the severe limitation they place on the number of available field sizes and hence have no role in modern radiology.
Collimator
Collimators are best general-purpose restrictors/ they offer following advantages:
- Light beam shows the center and exact configuration of the X-ray field.
- Accurate localization of the patient due to X-ray field illumination is permitted.
- It provides an infinite variety of rectangular X-ray fields.
Grid
Grid, a device invented by Dr Gustave Bucky in 1913, is the most effective way of removing scatter radiation from large radiographic fields.
Radiographic grid consists of lead foil strips separated by X-ray transparent spacers.
They are used to absorb scatter radiation (and not primary radiation) and to improve radiographic image contrast.
There are two types of grids—stationary and moving grids.
Chief advantage of moving grids is elimination of image of the lead strips from the film, but they require a little greater exposure factors.10
Air gap technique is an alternative method of eliminating scatter radiation with large radiographic fields (obsolete).
The Five Basic Ways that an X-ray Photon can Interact with Atom/Matter
- Photoelectric effect.
- Coherent scattering.
- Compton scattering.
- Pair production.
- Photodisintegration.
Element | Atomic number | K edge (keV) |
|---|---|---|
Hydrogen | 1 | 0.013 |
Carbon | 6 | 0.28 |
Copper | 29 | 9.0 |
Lead | 82 | 88.0 |
Photoelectric Interaction
- The photoelectric effect is the predominant interaction with low energy radiation and with high atomic number absorbers.
- It generates no significant scatter radiation and produces high contrast in the image, but exposes the patient to great deal of radiation.
- The photoelectric effect is inversely proportional to cube of energy of incident photon and directly proportional to cube of atomic number of interaction material.
- It predominates in diagnostic radiology.
- The atom consists of a central nucleus and orbital electrons. The positively charged nucleus holds the negatively charged electrons in specific orbits, or shell. The innermost shell is called K-shell, and the more peripheral shells are named consecutively L, M, N, and so forth. These shells have limited electron capacity and specific binding energy.
- The K-shell can hold only two electrons.
When an incident photon, with little more energy than the binding energy of K-shell electron, encounters one of these electrons, it ejects it from orbit and the photon disappears, giving up all its energy to electron. This electron now flies into space and is absorbed. Thus the atom is now left with an 11electron void on the K shell, which is filled up soon by as an electron from adjacent shell drops into K shell, giving up energy in the form of X-ray photon. This is photoelectric effect.
Thus, it is the predominant interaction of low energy radiation with high atomic number absorbers, generating no significant scatter radiation, producing high contrast X-ray images, but exposing the patient to great deal of radiation.
X-rays are ionizing electromagnetic radiations, essentially produced when a stream of K shell electrons of an atom accelerated by a high voltage applied between the filament (cathode) and the target (anode), strikes the target and the electrons give up their energy producing characteristic radiations, i.e. the X-rays.
Primary radiation: It goes from cathode to anode of X-ray tube. It comes direct from of X-ray tube. Except for the useful beam, the bulk of this radiation is absorbed in the tube housing.
Secondary radiation: It is radiation other than primary and is emitted by any matter irradiated with X-ray, which are often loose called scattered radiation.
Scattered radiation: Radiation, which during passage through a substance has been deviated in direction. It may also have been modified by an increase in wavelength (Compton effect). It is one form of secondary radiation.
Stray radiation: Secondary radiation and any radiation other than the useful beam coming from within X-ray tube housing (such as item radiation). This is the radiation against which special protection is needed. Useful beam is that part of primary radiation which passes through aperture, cone or other device for collimating X-ray beam.
Compton Scattering
- When incident photon has enough energy to dislodge a loosely bound electron, the emerging photon, undergoes a change in direction—and it is called scattered photon.
- Thus, frequency and energy of the scattered photon is less than that of incident photon.
- These are responsible for scatter radiation, which constitute film fog, impairing image contrast.
In addition, therapeutic radiation acts by Compton effect [Compton effect predominates in CT scan and RT].
X-ray Film
There are following layers in an X-ray film:
- Supportive base made up of polyester plastic.
- Adhesive or Subbing layer for proper binding of the emulsion to the base.
- Emulsion containing silver halide (most commonly used is silver bromide). Crystals suspended in gelatin (photosensitive layer) – the key component of a X-ray film.
- Protective anti-abrasive supercoat of pure gelatin.
- Non-curl backing (only in single coated film) to prevent curling of the film.
- Antihalation layer of dye added to non-curl backing or to the base to prevent reflection and unsharpness.
The behavior of X-ray film (silver bromide emulsion) with respect to light spectrum (VIBGYOR) is known as its spectral response.
An X-ray film is far more sensitive to blue, violet and ultraviolet light than to the rest of the spectrum (monochromatic).
An X-ray film is least sensitive to red light and if sensitizers are added to X-ray film, the spectral response can be extended into green (up to 570 nm), known as orthochromatic emulsion, or even as far as the red (up to 700 nm) in polychromatic emulsion.
Films Used in Medical Imaging
- Double emulsion/coated (duplitized) films: Have emulsion, applied to both sides of the plastic base in order to increase sensitivity, e.g.
- Direct-exposure (nonscreen type) films
- Intra-oral dental films
- Radiation monitoring films
- Screen type films (most commonly used type in routine X-ray filming and used with two intensifying screens)
- Single emulsion/coated films: Have emulsion applied only to one side of its base. Their main advantage is high quality images. To identify the emulsion side of single-coated film a small notch is provided into one edge of each film. The emulsion side is facing if the film is held with the notch in the top right-hand corner, e.g.
- Screen type film (used with single intensifying screen)
- Photofluorographic film
- Cathode-ray tube (CRT) photography
- Subtraction film
- Laser imaging film
- Mammography film
- Computed tomography (CT) film
- Radionuclide imaging film
- Diagnostic ultrasound film
- Computed radiography (CR system) film.
CHEMICAL PROCESSING OF An X-RAY FILM
- Developer – major constituent – mixture of phenidone and hydroquinone
- Fixer – major constituent – sodium thiosulfate/ammonium thiosulfate.
Complex chemical processing of exposed film causes deposition of metallic silver on the film resulting in blackening of the film.
Intensifying Screens
Screens are composed of thin cardboard base coated with crystals of phosphorus (luminescent material).
These phosphorus have unique ability to emit light, when exposed to radiation. Hence screens are stuck to the inside of the cassette or film holder.
The aerial image is temporarily formed on the screen, resulting in various degree of brightness. These, in turn, ultimately produce corresponding difference in darkening of the radiograph.
Ninety-eight percent of the blackening of film exposed with screens may be photographic in origins, i.e. due to light. Rest 2 percent of the density is due to exposure to X-rays directly.
Hence, use of screens causes considerable reduction in the radiation exposure to the patient.
Phosphor composition/following chemicals can be used
DIGITAL OR COMPUTED OR FILMLESS RADIOGRAPHY OR PHOSPHOR PLATE TECHNOLOGY
It is one of the most modern imaging methods in which selective window settings of some image enhance visualization of lung fields, mediastinum or bones as desired.
Optic drum scanners/laser scanners can digitize conventional film radiographs.
Types
- Phosphor plate CR (e.g. europium activated barium fluoride).
- Selenium detection CR → Excellent quantum efficiency considerable dose reduction possible.
- Large area, thin film transistor detector CR → Rapid image, excellent resolution.
Although it is most mature radiographic technology and uses conventional radiographic equipment but employs reusable photostimulable phosphor or selenium plate (europium-doped barium fluorohalide) instead of convocational film cassette.
Major Advantages of CRS
- Linear photoluminescence dose response, which is much greater than that of conventional film
- Hide attitude
- Post processing of images possible
- Advantage of image archiving and transmission
- Excellent resolution.
PICTURE ARCHIVING AND COMMUNICATION SYSTEM
- A picture archiving and communication system (PACS) aims to replace conventional analogue film and paper, clinical request forms and reports with a completely computerized electronic network whereby digital images are viewed on monitors in conjunction with the clinical details of the patient and associated radiological report displayed in electronic format.
Types of PACS
- Central PACS
- Distributed PACS
- Thus, PACS must replace the function of traditional X-ray film, i.e. image acquisition, storage, transportation and display.
- True efficiency benefits can only be realized once a PACS is hospital-wide, as any more limited installation means running two systems in parallel, i.e. it entails continuing to produce conventional film and moving it around the hospital, as well as the cost of installing and maintaining a PACS.
- The hospital information technology (IT) network is likely to need upgrading to enable large amounts of image data to be transported and it is advisable to install multiple PACS ‘drops’ (work-station outlets) so that more PACS work-stations can easily be added at a later date.
Advantages of PACS
- Once correctly acquired into PACS, no image can ever be lost or misfiled and is always available when needed.
- It facilitates easy comparison of a patient's current and past examinations.
- All images remain accessible from the PACS archives day and night.
- Simultaneous multilocation viewing of same image is possible on any connected work-station.
- Image retrieval is infinitely quicker from PACS that is using conventional film.
- All images correctly and permanently reside under appropriate imaging study, remain in their correct orientation and are automatically chronologically ordered.
- Viewing of images on monitors allows many post processing soft copy manipulations, a range of different window width and level setting can be applied to CT images.
- Cost saving (major reduction in film budget, film packet cost and chemical processing).
- Substantial time saving incurred as never have to search for or retrieve films.
- It sets stage for introduction of teleradiology over a wide area of network.
- After hospital wide PACS, old films can be progressively removed from store.
Disadvantages of PACS
- Expensive technology
- Technological complexity
- Dedicated maintenance program required
- Hospital is no longer equipped to run a film-based service
- Changing from a hard copy to soft copy necessitates change of work pattern.
CONTRAST RADIOGRAPHY
Definition
Contrast radiography means radiography (taking X-rays films) with use of contrast (for all practical purpose, broadly only two types of contrast agents are used, barium sulfate and waters soluble iodinated contrast agent). Contrast is injected in some of other space or lumen of the body and films are taken.
BARIUM STUDIES
Barium studies form one of the still most commonly used radiological procedures.
Barium sulfate is inert and best for examination of GI tract, except in few settings like TO fistula and bemls perforation.
However, compared to barium sulfate, the major advantage of aqueous contrast agents is their rapid absorption from the interstitial spaces and peritoneal cavity.
This property makes them uniquely useful for examining patients with suspected TO fistula and perforation of a hollow viscus. No permanent deleterious effects from the presence of these aqueous contrast materials in the mediastinum, pleural cavity, or abdomen have been shown.
NONBARIUM STUDIES
Apart from Barium procedures (done for gastrointestinal imaging), other several procedures include:
- Sialography
- Intravenous urography (IVU)
- Urethrography
- Myelography
- Hysterosalpingography
- Fistulography
- Arteriography (Conventional and DSA)
- Venography.
SIALOGRAPHY
It is used to diagnose stones (sialolithiasis), chronic or recurrent inflammation, and tumors in parotid and submandibular glands. It is contraindicated in acute sialadenitis (parotitis) for fear of exacerbating the condition.
MYELOGRAPHY
It is the radiographic investigation of the spinal canal for the diagnosis of space occupying and obstructive lesions and requires the contrast agent to be injected into the subarachnoid space (which lies between pia mater and the arachnoid mater) usually following a lumbar puncture.
Either a negative contrast agent like air or oxygen is used or more usually a positive nonionic water soluble low osmolar, organic iodine compound (Iohexol).
Oily preparations like iophendylate (Myodil, Pantopaque) is abandoned as it is known to be toxic and causes chronic adhesive arachnoiditis. Amipaque (metrizamide) has replaced myodil (iophendylate) due to its advantages like freely miscible with CSF, flows along subarachnoid spaces around the nerve roots and is absorbed from subarachnoid space within 48 hours. It is isotonic with CSF in concentration usually used for lumbar myelography made up to a volume of 10 cc. It also has low viscosity and thus a narrow bone needle can be used reactions like headache, and nausea seldom last for more than 24 hours.
Iohexol (Omnipaque) is nonionic iodinated contrast agent commonly used today. Iohexol has more evenly distributed OH groups; hence, has less subarachnoid toxicity. Although arachnoiditis remains the most dangerous, but very rare complication if myelography.
Now, myelography has almost completely supplanted by MRI.
EXCRETORY UROGRAPHY (INTRAVENOUS UROGRAPHY, IVU, IVP)
Since 1929, when the first intravenous contrast agent was developed by Swick, excretory urography has been the primary modality for imaging the urinary tract.
The renal margins and parenchyma (nephrogram), as well as the entire collecting system (pyelogram), including the ureters and bladder, can be visualized diagnostically.
After intravenous injection, whether by bolus or by infusion, these preparations leave the vascular system in two ways. First, rapid permeation of the capillary wall and equilibration with the extracellular fluid occurs. At the same time, contrast in the bloodstream undergoes glomerular filtration and subsequent excretion in the urine. As plasma contrast concentration falls as a result of ongoing renal excretion, there is a continual redistribution of the extracellular contrast into the vascular system.
The quality of the urogram depends on good pelvicalyceal concentration of contrast media as well as sufficient distention of the collecting system.
Indications
Infections, acute genitourinary pain, hematuria (microscopic or gross), renal transplantation, neurogenic bladder, congenital anomalies, and investigation of complications following a surgical procedure.
There are several indications for immediate urography. These include massive gross hematuria of unknown cause, and suspected ureteral calculus.
In the past, indications for urography have included the evaluation of hypertension. The hypertensive urogram involves obtaining coned-down films of the kidneys in a rapid sequence following bolus injection of contrast. The minimum series includes films at 1, 2, and 3 minutes post injection. The radiographic criteria for renovascular hypertension include delayed visualization of contrast in the collecting system on the affected side, decreased renal size, and delayed wash-out of contrast on the later films in the urogram. A secondary finding is that of notching of the proximal ureter on the involved side due to development of collateral flow via the periureteral plexus reconstituting the renal artery.
Recently, digital subtraction angiography has been claimed to provide good visualization of the renal arteries and may eventually become a satisfactory screening examination for renovascular hypertension.
The present day urogram is reliable test for functional assessment of the kidneys. But IVU has been replaced by DTPA/MAG3 scan as best test for residual function of kidneys.
Useful information can be gained with regard to the presence or absence of obstruction in many patients with renal failure, assuming tomography is used for better visualization of the poorly opacified collecting systems. A better modality for evaluating such patients, however, is renal ultrasound, which is an excellent screening examination for suspected urinary tract obstruction. Its usefulness is based on its ability to detect hydronephrosis. However, it must be realized that there exist a significant number of conditions that can mimic or produce dilatation of the collecting system in the absence of obstruction. Renal sonography suggestive of hydronephrosis should be followed by additional diagnostic studies to confirm or exclude obstruction, which may mainly include CT scan, especially with respect to ureteric stones.20
Contraindications
- Combined renal and liver failure
- Multiple myeloma
- Pregnancy
- Previous reactions to contrast media
- History of allergy
- Infancy
- Thyroid disease
- Renal failure
- Diabetes mellitus.
Patient Preparation
- Bowel preparation
- Informed consent
- Nil by mouth.
Both the plain film and the urogram should be exposed at 66 kV(p)-70 kV(p), although patients built is vital consideration.
Contrast Administration
Doses in the range of 17 g to 20 g of iodine for the average size adult will result in diagnostic studies if careful monitoring is performed.
In children, weight of the patient is a more prominent factor, and recommended doses are based on this parameter.
Film Sequencing
- Plain film of abdomen
- 1-minute film (nephrogram)—seen due to contrast in collecting tubules
- 5-minute full view (pyelogram)—seen due to contrast in pelvicalyceal system
- Full view post voiding.
Tailoring
Delayed films are essential when an obstructive nephrogram is seen on the routine early views. A recommended sequence for these delayed views is 30 minutes, 1 hour, 2 hours, 4 hours, 8 hours, and 24 hours. When obtaining delayed views, the patient should be instructed to void prior to exposure of the film so that a calculus at the level of the ureterovesical junction will not be obscured by the full bladder. Unless the degree of obstruction is extremely severe, at some point during this sequence of delayed films contrast can usually be seen columning in the ureter to the point of obstruction. If the degree of obstruction is such that there is no columning by 24 hours, further films are not indicated. In such patients further imaging modalities will be necessary to determine the anatomy of the obstructed system. These may include antegrade or retrograde pyelography.
Modifications in IVU Include
Rapid sequence IVU: Done in patients with suspected renovascular hypertension. Films taken at 1, 2 and 4 minutes after injection of contrast medium in addition to the routine filming sequence.
Infusion urography: Done in patients with compromised renal function 40–50 gram of iodine (as against 16 gram in usual procedure) is injected into 200–500 cc of glucose and given as infusion.
Diuretic urography: It is done in patients with PUJ obstruction. Patient is not dehydrated prior to procedure, IV furosemide injected immediately following contrast which causes copious contrast secretion, thus dilating the renal pelvis to greater extent and demonstrating the pathology nicely.
Abnormal Nephrogram
- Rim nephrogram (Rim of cortex receiving collateral blood flow)
- Acute complete occlusion of main renal artery
- Renal vein thrombosis
- Acute tubular necrosis
- Severe chronic urinary obstruction
- Shell/rim nephrogramSever hydronephrosis
- “Swiss cheese” nephrogramPolycystic kidney disease (ADPKD)
- “Sunburst” nephrogram (Striated nephrogram with persistent radiating radioopaque streaks on delayed images)Infantile polycystic kidney disease
- Striated nephrogram (Streaky linear bands of alternating hyper and hypodensity parallel to axis of tubules and collecting ducts)
- Acute ureteral obstruction
- Acute pyelonephritis
- Renal contusion
- Renal vein thrombosis
- Intratubular obstruction
- Systemic hypotension
- Autosomal recessive PCKD
- Medullary sponge kidney
- Medullary cystic disease
- Dense persistent nephrogram
- Systemic hypotension
- Intratubular obstruction/tubular damage
- Renal artery stenosis
- Renal vein thrombosis
- Urinary tract obstruction
- Focal parenchymal disease
- Increasingly dense nephrogram
- Systemic arterial hypotension
- Severe renal artery stenosis
- Acute tubular necrosis
- Acute renal vein thrombosis
- Acute glomerular disease
- Intratubular obstruction
- Acute ureteral obstruction
- Black nephrogram/negative pyelogram: Hydronephrosis
- Soap bubble nephrogram: Hydronephrosis.
URETHROGRAPHY
Retrograde Urethrography/Ascending Urethrography
Direct retrograde examination is appropriate, particularly if the anterior urethra is of paramount interest. Although an uncommon procedure in the female, examination of the male urethra by this technique is frequently done to evaluate urethral trauma or obstruction secondary to inflammatory disease or neoplasm.
Opacification of the male urethra in retrograde fashion allows visualization of the anterior urethra but is usually accompanied by relatively poor filling of the posterior urethra because of the resistance encountered at the external sphincter. The anterior urethra will be well-defined and distended and the level of the external sphincter clearly identified. The posterior (membranous and prostatic) urethra, on the other hand, will usually not be well distended and anatomical landmarks will be more difficult to identify.
The appropriate contrast material should be water soluble, minimally irritating, and handled in a sterile manner. Methylglucamine diatrizoate or iothalamate is readily available in any radiology department and serves the purpose well.
Voiding Cystourethrography/Micturating Cystourethrography/Descending Urethrography
The primary indications for cystourethrography are to evaluate the presence of vesicoureteral reflux and to investigate abnormalities of the bladder neck and the posterior urethra. Functional assessment of bladder contractility and micturition is also possible using this technique. When radiographic complete assessment of the urethra is of primary concern, cystourethrography should be used in conjunction with a retrograde urethral study. Current infection of the lower urinary tract is a contraindication to the procedure.
Indications
To demonstrate the various abnormalities in the neck of bladder and urethra:
- Recurrent UTI especially in children
- For complete assessment of cases of bladder diverticuli
- For demonstration of VUR
- For demonstration of bladder contractions and the control of micturition.
- The MCU is indicated in all boys under 1 year of age with UTI.
- Any child who requires imaging of kidneys and or urinary tract for whatever reason, with the exception of trauma cases, should undergo a USG examination as the first investigation.
- The MCU is however the definitive method of assessing prostatic urethra and bladder.
- It is necessary also in all boys to assess by MCU when there is any suspicion of urethral pathology.
- Congenital abnormalities affect both anterior and posterior urethra, the most common being hypospadias, which has little radiological importance.
- Posterior urethral valves is one of the most common GUT congenital abnormalities in males causing obstructive uropathy.
- USG usually strongly suggests the diagnosis and may detect complication like urinoma. This may, however, be followed by an MCU especially for confirmation and follow-up.
HYSTEROSALPINGOGRAPHY
Definition
Visualization of the uterine cavity and Fallopian tubes by using negative contrast media (normal saline in hydrosalpingosonography) or positive contrast media [Echovist in sonosalpingography and the iodinated nonionic low-osmolar contrast agent in hysterosalpingography (HSG)].
Ideal Time to Perform HSG
Between 7th and 10th day of menstrual cycle (postmenstrual, but preovulatory period) for following reasons:
- No risk of early pregnancy
- Isthmus is most easily distensible
- Tubal filling occurs readily.
Indications for HSG
- Infertility
- Recurrent miscarriage
- Congenital abnormalities
- Post-uterine and/or tubal surgery
- Abnormal uterine bleeding
- Evaluation after major pelvic trauma and/or surgery
- Prior to artificial insemination and in vitro fertilization, for tubal patency (other tubal patency tests are laparoscopic chromopertubation, CO2 insufflation and hydrosonosalpinography test).
Contraindications for HSG
- Pregnancy
- Bleeding
- Immediate pre- and postmenstrual phases
- Recent untreated pelvic infection
- Tubal or uterine surgery within last 6 weeks
- Contrast medium sensitivity
- Recent D and C procedure
- Severe renal or cardiac disease
- Migrated IUCD.
Complications of HSG
- Pain
- Bleeding/hemorrhage
- Intravasation (venous)
- Vasovagal episode
- Pelvic infection
- Pregnancy irradiation
- Allergic reaction
- Transient nausea, vomiting and headache.
Genital Tuberculosis and HSG
Initially, fallopes are involved.
In 50 percent of such cases, uterus is secondarily infected.
Tuberculous Salpingitis
HSG
- Bilateral tubal occlusion (isthmic or ampullary part)
- Tubal contours—smooth gross thickening of longitudinal mucosal folds
- Irregular or ragged outline
- Multiple strictures giving ‘beaded’ appearance of fallopes
- Cavities or sinus tracts
- Rigid straight ‘pipe-stem’ appearance
- Tubointestinal and tubovesical fistulae
Uterine tuberculosis: Polypoidal lesions, hyperplastic endometrium, ragged saw-toothed uterine contour, shriveled and deformed uterus, and fibrosis. Venous or lymphatic intravasation.
XERORADIOGRAPHY
- In xeroradiography instead of an X-ray film, a thin layer of semiconductor is used to produce a latent image which is transferred onto paper.
- Characteristically the images have marked edge enhancement and good resolution.
- Thus in breast, blood vessels, ducts, and skin the small calcification and tumor edges stand out clearly.
- Hence, was commonly used in breast cancer detection.
- Further, it also gives a more uniform exposure.
- It also produces good images with low kV tungsten tube.
- The radiation dose is, however, 60 to 100 percent greater than with low dose film techniques.
- It is not in use nowadays and has been replaced totally by mammography.
MAMMOGRAPHY
It's a special radiographic technique for imaging of breasts, basically used for screening purpose.
It has replaced xeroradiography because apart the effect of greater depth dose due to the use of higher X-ray tube kilovoltage, the average glandular dose in xeromammography is about three to six times greater than in screen-film mammography.
The required image should have high contrast, high spatial resolution, and low noise.
It differs from routine radiographic technique in that instead of tungsten filament; molybdenum is used as target material. Rhodium and rhenium can also be used. This is because, molybdenum produces low energy X-ray beam after bombardment with electrons, which are must in mammography as it is used as a screening tool for breast lesions and its repeated use is likely and one of the predisposing factors for carcinoma breast is radiation itself.27
It requires high soft tissue resolution (aim is to image breast tissues), low radiation dose (aim is to avoid radiation hazard to breast), special compression views (mediolateral view and craniocaudal are the most important one) and equal radiation exposure to all parts of breasts.
Overall detection rate of carcinoma breast by mammography is 58 to 69 percent and 8 percent only, if the lesion <1 cm in size. Hence, mammography is a screening modality and not the best diagnostic measure.
- The “triple assessment” for carcinoma breast includes–
- Clinical examination
- Mammography
- FNAC/Biopsy
- In young females, mammography is not a good screening tool as young breasts are more dense, i.e. glandular tissue is more fatty in young females, hence in young females with family history/BRCA-1; BRCA-2 gene caries MRI used.
1 mGy = 100 mrad = 0.1 rad
Indications for Mammography
- Before breast surgery, as it may avert an unnecessary biopsy demonstrating that the palpable mass has a characteristically benign appearance.
- Follow-up of breast cancer patients.
- Work-up a patient with metastases from an unknown primary.
- Mammographic Screening is best screening method for carcinoma breast.
ORTHOPANTOMOGRAPHY OR PANORAMIC RADIOGRAPHY
Pantomography is a special radiographic technique that produces a panoramic radiograph of a curved surface.28
Orthopantomography (OPG) is currently the extraoral technique of choice in dental radiology as it depicts both the upper and lower jaws in their entirety together with the floor of maxillary sinuses and temporomandibular joints on a single film. The rounded configuration of the mandible and teeth makes them especially suitable for OPG. Pantomograms of jaw thus show the Tm joints on either side of film with the teeth laid out between them.
It is a modified form of tomography where jaws are positioned in a predetermined image layer, which is thinner in the anterior region. The patient is in chair and remains stationary throughout the examination. The X-ray film holder and the tube both rotate during the exposure. The film holder has a protective lead front, considerably lower than the film. The film is exposed through a narrow slit in its holder. The film moves across this slit and the X-ray tube rotates and the radiographic image is laid out as the film passes by the tube in much the same way that paint is applied to a wall with a roller.
The resulting image is flattened out image of a curved surface, but it is sensitive to errors of positioning, in particular of incisor teeth, leading to increased distortion and blurring.
ULTRASONOGRAPHY/ULTRASOUND STUDY
Jean-Daniel Colladon a Swiss physicist/engineer discovered sonography with an underwater bell in 1826. He accurately determined the speed of sound through water.
In 1881, Pierre Curie found a connection between electrical voltage and pressure on crystalline material. This was the breakthrough that was needed to create the modern ultrasound transducer.
Ian Donald, invented and improved on many devices used in pregnancy and fetal development. During World War II, he became interested in radar and sonar. He became known in the 1950s when a woman with a diagnosis of inoperable stomach cancer came to his attention. He studied the case with his new equipment and found that she had an ovarian cyst, which was safely removed. He became the father of obstetric ultrasound. He also invented the B-mode scanner. He was able to detect a twin pregnancy.
Dr. John Wild and John Reid modified standard medical imaging equipment and produced a hand held B-mode instrument that could swing side to side and get cross views from various angles to detect breast tumors. This type of unit was the forerunner of the modern machines used today as they produced the first breast ultrasound. They also invented an A-mode scanner for the detection of ovarian cancer.
- Ultrasound is a sound with a frequency greater than 20, 000 cycles/sec (Hertz, Hz).
- Thus, sounds with a frequency above 20 kiloHertz (20 kHz) are called ultrasonic (beyond the range of human hearing).
- The sounds used for sonar are well into the ultrasonic range, with frequencies of 2 to 20 megaHertz (MHz).
- Medical sonography employs frequencies between 1 MHz and 20 MHz.
- It does not involve use of ionizing radiation (the greatest advantage).
- The piezoelectric crystals in the ultrasound probe are nothing but innumerable dipoles arranged in geometric pattern and electric field causes sudden change in their physical dimensions (shape) by realigning them, thereby starting a series of vibrations that produce sound waves.
- Thus, by the virtue of piezoelectric effect in the ultrasound probe one form of energy (electric energy) is converted in to another form (sound energy) the body/organs parts are imaged.
- It works on pulse echo principle and B-mode is used for transmission during all routine including abdominal ultrasonography.
- Real-time B-scans allow body structures which are moving to be investigated. The simplest type of scanner is just a speeded up version of the 2-D B-scan, allowing a rapid series of still pictures to be built up into a video of the movement.
- The probe or the transducer is any device that converts one form of energy to another. In case of ultrasound, the transducer converts electric energy to mechanical energy and vice versa.
- The ultrasound transducer uses the principle or property of piezoelectricity which occur naturally in some materials whereby an applied electric field produces a change in linear dimensions.
- Quartz, natural ceramic is a naturally occurring piezoelectric material having the unique ability to respond to the action of an electric field by changing shape and to the change in polarity of voltage applied by generating small potentials and thus producing an ultrasound image.
- Currently Lead zirconate titanate (PZT), synthetic ceramic is the most widely used material in the ultrasound sound transducers/probes replacing the firstly discovered Barium titanate. Although some naturally occurring materials possess piezoelectric properties (e.g. quartz) but most crystals used in medical ultrasound are man made with artificial ones, known as Ferroelectrics.
- These ceramic crystals are made up of innumerable tiny dipoles but, to possess piezoelectric characteristics, the dipoles must be arranged in specific geometric configuration which requires to be heated to a high temperature to be strong electric field. The curie temperatures for several crystals to possess the piezoelectric crystals property is as follows: Quartz 573°C, Barium titanate 100°C, PZT-328°C, PZT-365°C.
In most diagnostic applications, frequencies in the range 2 to 20 MHz are commonly used.
The frequencies of ultrasound required for medical imaging are in the range 2 to 20 MHz. These frequencies can be obtained by using piezoelectric materials. When an electric field is placed across a slice of one of these materials, the material contracts or expands. If the electric field is reversed, the effect on the material is also reversed. If the electric field keeps reversing, the crystal alternately contracts and expands. So a rapidly alternating electric field causes the crystal to vibrate.
The piezoelectric effect occurs in a number of natural crystals including quartz, but the most commonly used substance is a synthetic ceramic, lead zirconate titanate. The crystal is cut into a slice with a thickness equal to half a wavelength of the desired ultrasound frequency, as this thickness ensures most of the energy is emitted at the fundamental frequency.
- Percentage of the beam reflected at the tissue interface depends on the tissue's acoustic impedance and the angle of incidence of the beam.
- Two factors determine the acoustic impedance of the tissue, density of the tissue and the velocity of ultrasound. But as the velocity of ultrasound in tissues is almost constant (1540 m/s), reflection of the ultrasound beam (change in impedance) is mainly dependant on density.
- More the dense a tissue is, more will be the acoustic impedance, reflection of the ultrasound beam will be more, producing more acoustic shadow.
- The acoustic impedance of bone is maximum in our body (7.8 Rayls); hence it produces dense/maximum acoustic shadow.
- Calculus also produces acoustic shadow.
- Medium acoustic impedance (in standard unit):1. Air0.0004292. Water1.503. Blood1.594. Fat1.385. Muscle1.706. Bone6.50
Clear liquids allow ultrasound to pass directly through without much alteration, so that echoes that come from tissue behind liquid are usually enhanced (brighter). This is known as “acoustic enhancement”.31
Dense materials such as bones or calculi cast shadows on the structures behind them, as the ultrasound waves do not go through them. This is known as “acoustic shadowing” (Postacoustic shadow). Postacoustic shadow is of immense importance in detecting gallbladder and renal calculi. To quote a classical example, WES triad (Wall Echo Shadow) is diagnostic sign of gallstones on ultrasonography, where wall of gallbladder, echo of calculus, and postacoustic shadow of the calculus seen together is diagnostic of cholelithiasis.
Air (Gas) can present a variety of sonographic patterns; beams can be scattered, reflected, refracted and absorbed and may hence also produce acoustic shadowing. But, it produces dirty postacoustic shadow, hence, air can be said to the enemy of ultrasound and whenever air/gas comes ultrasound fails to image the underlying organ. To quote common examples regarding this we can recall that for detecting of pancreatic lesions and ureteric calculi, CT is far superior to ultrasonography because bowel gas shadow degrades imaging by ultrasound [CECT for pancreatic lesions and NCCT for ureteric stones].
Fat is echogenic but does not produce acoustic shadows.
How things look on USG | ||
|---|---|---|
Hyperechoic | Hypoechoic | Anechoic |
Air | Bile | Cyst |
Bone | Chronic hemorrhage | – |
Acute | hemorrhage Abscess | – |
Fat | Tumors (Most) | – |
Stones | – | – |
Liver hemangioma | – | – |
- Hydrocephalus in infants
- Gallstones
- Acute cholecystitis (although theoretically HIDA scan is best)
- Screening for rotator cuff injuries (Best initial investigation, although MRI is best)
- Renal colic in pregnancy
- Minimal ascites
- Obstetrics indications
- Adenomyomatosis of gallbladder (Comet-tail sign)
- CHPS
- Intussusception
- Screening tool of choice for blunt trauma abdomen (in FAST protocol)
- Hydronephrosis
- Initial test of choice for obstructive jaundice
- Initial test of choice for acute abdominal pain
- Verical calculus
- DDH
- Popliteal cyst
- Retinal detachment and vitreous hemorrhage
- Choroidal melanoma.
Theoretic Safety Risks from Ultrasound
Ultrasound continues to be the major technique for imaging in pregnancy, especially the second and third trimester. Theoretic safety risks from ultrasound energy include thermal damage (due to a rise in temperature) and cavitation (production and collapse of gas-filled bubbles) with subsequent tissue injury.
However, no confirmed deleterious biologic effects on patients or instrument operators caused by exposure at intensities typical of present diagnostic US instruments have been reported. It has no radiation (ionizing) hazard.
Transrectal Ultrasonography
Prostatic Lesions
- Transrectal ultrasonography (TRUS) is an excellent adjuvant to physical examination, it does not serve as a screening investigation. However, the combination of digital rectal examination (DRE) and serum PSA level is more sensitive then TRUS.
- But once suspected, prostatic carcinoma is most effectively confirmed by TRUS-guided needle biopsy.
- The staging accuracy of TRUS does not match the accuracies attainable by MRI, especially endorectal coil MR (ERMR).
- TRUS-guided biopsy is the best utility of TRUS for prostatic lesions.
- CT is not recommended for routine tumor staging as it is insensitive and nonspecific. CT is useful in advanced cancer for evaluation of adenopathy and metastases.
- Major role of MRI is in local staging of disease.
Ca Rectum
- In rectal carcinoma the depth of penetration can be best achieved with TRUS, while the involvement of perirectal nodes can be better assessed by MRI in addition to depth of penetration.
Seminal Vesicle Lesions
COLOR DOPPLER IMAGING/STUDY
When ultrasound is reflected from a moving surface, the frequency of the sound is altered slightly in a manner that depends on the speed of movement of the surface. This is due to the Doppler effect.
The Doppler effect is now commonly used in ultrasound imaging to examine the movement of liquids, such as blood flow in arteries and veins, allowing the location of blockages to be determine precisely. Another common medical application is in fetal heart monitoring and cardiac ECHO.
In Color Doppler Imaging areas of blood flow are represented as color within image.
When the ultrasound waves strike upon the moving RBCs, they are not reflected; they are scattered in all directions and hence the blood vessels are seen as echo-free structures.
It has become common practice to represent flow towards transducer as red and flow away as blue.
Velocity information in special form and color flow helps in diagnosis.
Many indices of waveform analysis have been devised including resistance index (RI) and pulsatilty index (PI), the commonly used ones.
The main advantages of Color Doppler imaging include:
- Confirmation that a structure is a vessel or that a known vessel is patent. It shows vessels that are too small to be seen by 2D image.
- Direction of flow can be easily confirmed, as important in the portal vein.
- It also permits the assessment of number and distribution of vessels within a tissue volume which is important to second blood flow signals from vessels like renal accurate and uterine arteries and in assessment of vascularity in and around focal lesion.
- Doppler frequency shift data also needed to measure blood flow velocity directly.
ECHOCARDIOGRAPHY
Echocardiography is a safe, painless, portable and relatively inexpensive method of acquiring high quality tomographic images of the heart in a variety of patients. It is “first-test” of choice in the noninvasive examination of cardiac anatomy and function.
Posterior structures (pulmonary veins, the atria and their appendages and atrioventricular valves) are especially well imaged by transesophageal echocardiography (TEE) and hence the thrombi, which are common in atria and their appendages (especially the left atrial thrombus) can be easily detected.
TRANSESOPHAGEAL ECHOCARDIOGRAPHY
Even in the hands of experienced echocardiographers, some portion of the adult population, due to obesity, chronic obstructive lung disease, or abnormalities of thoracic musculoskeletal anatomy, may not be amenable to transthoracic (i.e. precordial, apical, suprasternal, or subcostal) echocardiographic imaging. In these patients, the development of transesophageal methods has permitted superior visualization of certain portions of cardiac anatomy. In particular, assessment of structures near the esophagus (left atrium, right atrium, interatrial septum, atrioventricular valves, pulmonary veins, and aorta) may be visualized from transesophageal windows.35
Common Indications
- Detailed assessment of LA for thrombus
- Diagnosis of dissection of thoracic arch (preferred in hemodynamically unstable patients)
- Assessment of LV function perioperatively
- Assessment of cardiac surgical repairs perioperatively
- Detailed assessment of native or prosthetic mitral valve
- Assessment interatrial septum
- Assessment of heart valves in known or suspected endocarditis
- Assessment of aortic valve
- Assessment of some forms of CHD
- Assessment of some right sided cardiac lesions
- General echo indications in ‘hard to image’ patients
- It is also a cardiovascular monitoring technique
- It is the most sensitive and practical technique for detection of myocardial ischemia in the perioperative period.
COMPUTED TOMOGRAPHY
(Synonym: CAT scan-computed axial tomography)
Godfrey N. Hounsfield (1973), while working with central research laboratories of EMI (Electromusical instruments) England, first described an elaborate technique in which X-ray transmission readings were taken through the head at a multitude of angles. He got a Nobel Prize of Medicine in 1979 for describing this first effective scanning system, which was called EMI scanner, the Generation I Computed tomography (CT) machine.
Hounsfield originally developed first generation scanners. In this machine the X-ray beam is collimated to dimensions of roughly 2 × 13 mm. Here 13 mm is the slice thickness, X-ray tube and detectors system moves continuously across the patient, making 16 multiple measurements during translation. At the end 36of each translation, X-ray tube and detector system are rotated 1 degree and the translation is repeated. Major disadvantage is long scanning time. It requires 5 minutes to gather 28,800 ray sums.
Such instruments have been designed along 3 lines, since their introduction by Godfrey Hounsfield.
- Generation II CT machine: Scanners in which the X-ray tube and detectors are made to move in translate rotate type of mechanical motion.
- Generation III CT machine: Scanners that employ a rotating motion in which the detectors and X-ray beam rotate around the object.
- Generation IV CT machine: Scanners in which detectors are stationary and the X-ray source is moved around the object.
The first modification was simply to convert the X-ray beam to a fan shape with diverting angle of between 3 and 10 degrees. Also more detectors are used, so number of angular rotations could be decreased and an adequate number of views obtained in much shorter intervals such second generation scanners were able to obtain a scan in periods as short as 18 seconds.
Next development involved widening the X-ray divergent angle so that it could entirely compass the object without performing any translatory motion. Thus rotate only scanner was developed referred to as the third generation scanner. It can produce scan in 1 to 10 seconds. This advent of spiral or helical CT in 1989 was a dramatic development, which helped CT to mature into a true volume imaging modality. It combines continuous gantry rotation with continuous table feed. It thus eliminates interscan delay. Spiral CT can be single or multiple slice machine.
The pursuit of faster scanning and higher resolution in volume scanning led to concept of multislice CT. The technological innovation of the new millennium is the world of spiral CT. A multislice CT system in contrast uses multiple detector rows and can therefore acquire multiple slices per rotation ranging from 4, 8, 16, 32 to 64 slices per sec. Speed of gantry rotation is increased, resulting in overall increase in scan speed. This reduction in scan time allows larger volumes to be scanned in the same time and same volume in much reduced time or in the same time at a narrower collimation leading to higher axial resolution.
Removing detectors from rotating gantry and mounting them in stationary positions around the patient allowed a further decrease in scanning time. These machines are named fourth generation scanners. In this scanner the X-ray tube is on continuously during a scan and each detector receives X-ray beam for a significant portion of the scan cycle.37
CT Number/HU Value
Basic principle of CT is linear attenuation of X-ray.
Incident X-rays are linearly attenuated by their interaction with orbital electrons of tissues. Measurement of attenuation of emerging/detected beam → gives density of intervening tissues and this density forms basis of signal intensity variation obtained in X-ray tomograms.
For unenhanced CT, there is an essentially linear relationship between voxel signal intensity (image brightness) and the X-ray linear attenuation coefficient, which is scaled relative to air and water and converted to an integer.
This is expressed in Hounsfield units (HU), which range from –1000 to +4000.
Actually the linear attenuation coefficient of each pixel converted to a new number is called a “CT number” which allows computer to present the information gathered as picture/image with large gray scale.
CECT makes use following phases of iodinated contrast medium, vascular enhancement, tissue opacification, and opacification of urinary tract/bowel lumen/cisternal spaces.
Computed tomography (CT) consists of directing X-rays at an object from multiple orientations and measuring the decrease in intensity along a series of linear paths. This decrease is characterized by Beer's Law, which describes intensity reduction as a function of X-ray energy, path length, and material linear attenuation coefficient. A specialized algorithm is then used to reconstruct the distribution of X-ray attenuation in the volume being imaged.
Generation of CT | CT machine |
|---|---|
First generation CT (Outdated) | EMI scanner |
Second generation CT (Going out of use) | Conventional CT |
Third generation CT (Most common type in India at present, | Spiral helical CT |
but rapidly being superceded by MDCT) | (It can be single or multislice CT) |
Fourth generation CT (No beam-hardening artefacts) | MDCT (Multidetector CT) |
Electron beam CT/Ultrafast CT/Real-time CT/mSec CT | EBCT |
SPECIAL APPLICATION OF MULTISLICE CT
- CTA
- Triple phase CT of liver and pancreas
- Cardiac CT
- Coronary calcium scoring (“Noninvasive coronary angiography”)
- CT perfusion
- CT enteroclysis
- Virtual colonography
- CT urography.
HU value scale | |||||
|---|---|---|---|---|---|
Hypodense | Isodense | Hyperdense | |||
–1000 | –50–100 | 0–10 | 20–40 | 50–60 | 1000 |
Air | Fat | Water | Soft tissue | Blood | Bone |
- Acute SAH (As the age of blood advances, its density decreases as it gets degraded into its degradation products, i.e. oxyhemoglobin → deoxyhemoglobin → methemoglobin → hemosiderin, hence for chronic bleeds CT is not good).
- Head injury
- Intracranial calcification
- Fractures of pelvis/vertebral fractures/fractures of facial bones
- Minimal pneumoperitoneum
- Ureteric calculi (most sensitive investigation for acute renal colic)
- Detecting calcification (e.g. as in retinoblastomas, ovarian dermoids, hydatid cysts, corpus callosal lipomas, neuroblastoma, etc.)
- CT PNS for sinonasal polyps and recurrent chronic sinusitis and pre-FESS noncontrast CT for “Road-map”.
Contrast-enhanced/ CECT is investigation of choice for
- Calcified tumors:
- Oligodendroglioma
- Craniopharyngioma
- Central neurocytoma
- Retinoblastoma.
- Lung carcinoma (Except pancoast tumor)
- Mediastinal tumors (Except posterior mediastinal tumors, which are neurogenic)
- Pancreatic lesions (Except neuroendocrine tumors of pancreas)
- Staging of renal cell carcinoma (Except for RCC with renal/IVC thrombosis)
- Abdominal adenopathy
- Small bowel tumors
- Blunt trauma abdomen
- Advanced Ca prostate (staging)
- Penetrating (stab injury) abdominal trauma with stable patient
- Sensitive and specific screening tool for renal artery stenosis
- Juvenile angiofibroma
- Ca maxilla
- Focal hepatic lesions–triple phase CECT (except for FNH)
- Small bowel tumors (CT better than enterolysis)
- Diverticulitis and abscess
- Complicated appendicitis
- Screening tool of choice for acute mesenteric ischemia.
High Resolution Computed Tomography
Basic Principles
Three factors which significantly improve the spatial resolution of CT such that it can be described as high resolution computed tomography (HRCT) and be used for studying the greater details of lung parenchyma/petrous temporal bone are:
- High spatial reconstruction or edge enhancing or sharp or “bone algorithm” (for image reconstruction, as it reduces image smoothing and makes structure visibly sharper).
- Narrow beam collimation (reduces volume averaging within the section and so increases spatial resolution causing marked effect on appearance of lungs, notably the vessels and bronchi).
- Small field of view.
This makes HRCT an ideal modality for correct identification of subtle diffuse abnormalities of lung parenchyma even in very early stage and thus evaluating interstitial lung disease in the best manner. It is also helpful in imaging of temporal bone.
* Slice thickness of 1 mm is ideal for HRCT.
Clinical Applications
- Lung diseases:
- Bronchiectasis
- Interstitial and diffuse lung diseases
- Emphysema.
- Temporal bone imaging:
- Ossicular chain disruption
- Cholesteatoma
- Mastoiditis and petrositis
- Fracture of facial canal.
- HRCT is of proven value in the diagnosis of diffuse lung disease (like interstitial lung diseases), particularly in the early stages when the chest radiograph is normal and for follow-up.
- HRCT clearly depicts distribution and higher definition of appearances of pulmonary parenchymal disease.
- Nowadays HRCT is used for detection of bronchiectasis, and surgery is undertaken without preoperative bronchography. Severity and extent of bronchiectasis is demonstrated.
- HRCT can identify regions most suitable for biopsy at a time when the chest radiograph is normal.
- Mediastinal or chest wall involvement by lung pathway may also be demonstrated.
Safety Aspect of Computed Tomography
Examination | Effective total dose (mSv) |
|---|---|
Chest X-ray | 0.06 |
Skull X-ray | 0.2 |
Pelvis X-ray | 0.65 |
Lumbar spine X-ray | 1.3 |
Upper GI series | 2.45 |
Abdomen X-ray | 0.55 |
Barium enema | 4–9 |
IVP/IVU | 1.6 |
Extremities | 0.01 |
Enteroclysis | 1.5 |
CT chest | 8 |
CT abdomen | 10 |
CT head | 3.5 |
Bone scan | 4.2 |
MCU | 1 |
Mechanism for dose reduction at CT |
|---|
• X-ray beam filtration • X-ray beam collimation • X-ray tube current modulation and adaptation for patient body habitus (automatic exposure control) • Peak kilovoltage optimization • Improved detection system efficiency • Noise reduction algorithms |
Advantages
- Less time of acquisition
- High spatial and temporal resolution
- Best to detect calcification
- Best to detect cortical bony lesions and fine osseous details
- Extremely sensitive to detect free air in abdomen/pleural space/cranial.
Disadvantages
- Modality that gives highest radiation dose (Ionizing radiation hazard)
- No multiplanar capability, only axial images possible (except for MDCT)
- Contrast related toxicity
- Due to ‘beam hardening’ artifact not good for posterior fossa tumors
- Poor soft tissue resolution compared to MRI.
MAGNETIC RESONANCE IMAGING
(Synonym: NMR–nuclear magnetic resonance)
- It is a noninvasive method of mapping the internal structure of body by producing images by the virtue of gyromagnetic property of protons, with the greatest advantage of not using ionizing radiation for imaging.
- It employs radiofrequency (rf) or radiowaves waves/radiation in the presence of carefully controlled magnetic fields in order to produce high quality cross sectional images of body in any plane.
- It portrays the distribution of hydrogen nuclei and parameters relating to their motion in water and lipids.
- Prucell, pioneer of MRI, took the advantage of the fact that 60 percent our body is made up of water, i.e. protons. Nuclei of certain atoms (protons or neutrons) when placed in a magnetic field, absorb and emit energy of a specific frequency.
- Almost all images produced to date have been by the virtue of gyromagnetic property of protons and the nuclear magnetism of hydrogen nucleus (or proton), which is a particularly favorable nucleus from magnetic resonance imaging (MRI) standpoint, and is present in virtually all biological material. Because nuclei are spinning, they respond to magnetic couple like gyroscope to their axes, are tilted so that they come to rotate at exactly same frequency about the magnetic field; direction of movement is known as ‘precession’.
Where f | = resonant frequency |
γ | = gyromagnetic ratio |
B | = applied field. |
- Michael Faraday (1791-1867) created a complete cage of metal or metallic meshwork. If a region in space is completely surrounded by a Faraday Cage (MRI rooms), ambient electromagnetic waves are effectively screened from enclosed region. Copper or Aluminum foils can transform any room into Faraday Cage. A continuous sheet or wire-mesh of copper or aluminum is used to shield the MRI rooms so as to protect the imager from external electromagnetic radiations, which is known as Faraday Cage. All NMR examinations including MRI imaging and MRI spectroscopy have to be performed in a Faraday Cage to prevent radiofrequency waves from ambient sources from interfering with reception of radio waves emitted from the sample (patient) during examination.
- The induction coils (transmitter/receiver coils) used in MRI are Maxwell coils.
Principle of MR Imaging
Pulse of oscillating radiofrequency (rf pulse) field is applied on a group of protons (a body part) placed in a strong magnetic field ⇒ this produces a resonant effect on protons, called magnetic resonance (providing that the frequency of oscillation of protons is equal to their precession frequency) ⇒ this resonant effect on protons causes change in magnetization due to absorption of rf energy from transmitter coil causing motion of elementary magnets to be disturbed so that direction of total nuclear magnetization is altered ⇒ and as the altered net magnetization returns to equilibrium in exponential manner it induces a small voltage in receiver coil which is placed next to the patient → this small electrical voltage signal induced in receiver coil following an rf pulse is known as free induction decay or FID, the magnitude and length of which is determined by nuclear relaxation times which reflect molecular motion ⇒ and then the FID, i.e. electrical signal produced in receiver coil is digitized and analyzed in a computer using a complex mathematical technique called Fourier analysis, to produce amazing images.
When Protons flip towards RF pulse it results in:
- T1 Spin lattice relaxation
- T2 Spin spin relaxation.
The first of these relaxation times, T1 or the longitudinal relaxation time, represents the time taken by the system of nuclei to return to thermal equilibrium after the rf pulse (For T1, TE and TR are short).
The second T2 or transverse relaxation time represents the characteristic decay time of FID and is due to irresponsible dephasing of initially coherent precession of nuclei, which follows the rf pulse (For T2, TE and TR are long).
Unlike CT images in which contrast is determined by differences in one parameter (the linear X-ray attenuation coefficient mu), multiple parameters influence MRI Signal including nuclear or proton density, T1 and T2.
In human tissue T1 is usually 10 times longer than T2.
A local change in magnetic field homogeneity, e.g. due to local iron or deoxyhemoglobin content, causes a reduction in T2 which is called T2*.
The induction coil in MRI unit is made up of niobium-titanium.
The principle pulse sequences are:
- Partial saturation (PS) or gradient or field echo
- Spin echo
- Inversion recovery.
Technical advances in the form of high field strength MRI machines [(3T) and ultrafast MRI pulse sequences (Echo planar imaging)] have included improvements in spatial resolution, contrast and speed imaging and major advances have related to development of MRI for following special uses:
- Diffusion weighted MRI (DWRI): Excellent for acute and hyperacute.
- MR Volumetry: Excellent for mesial temporal sclerosis (Temporal lobe epilepsy)
- Functional MRI (Bold-blood oxygen level dependent MRI): For cognitive function of brain and physiological studies
- MR tractography (Diffusion tensor imaging): For WM tracts
- MRCP: Excellent screening tool for biliary anomalies, sclerosing cholangitis, etc.
- MR angiography: Excellent for imaging almost all vessels
- Flair sequence: Excellent for chronic SAH.
- Water (CSF) looks white (Hyperintense) on T2 (*Hint to remember → World War-2)
- Thus, CSF looks hyperintense on T2 weighted image and hypointense on T1 weighted image.
Substance | T1 weighted | T2 weighted |
|---|---|---|
Water/Vitreous/CSF | black | Light gray or white |
Fat | White Light | gray |
Muscle | Gray | Gray |
Air | Black | Black |
Fatty bone marrow | White light | Gray |
Brain: White matter | Light gray | Gray |
Brain: Gray matter | Gray | Very light gray |
Advantages
- No use of ionizing radiations (radiowaves are used in MRI).
- Greater inherent soft tissue contrast (e.g. tumors of brain including cerebral metastases are usually better demonstrated on MRI than on CT due to greater inherent soft tissue contrast).
- Provide direct multiplanar imaging, which helps to define the relationship of the tumor to adjacent structures, and thus helps in planning of surgery (e.g. facilitates the distinction between intra-axial and extra-axial tumors).
- MRI is vastly superior to CT in evaluating posterior fossa tumors as CT is frequently hampered by ‘beam-hardening’ artifact from the base of skull.
- Vascular imaging possible without use of intravenous contrast.
Disadvantages
- Longer time of acquisition
- Claustrophobia
- Metal is absolutely contraindicated
- Costly
- Not the best for detecting calcification or air.
Applications of MRI are many, but amongst the commonly imaged parts are brain, spine and musculoskeletal tissues.
MRI is investigation of choice for
- Empty sella syndrome
- Hyperacute and acute infarct (DWMRI)
- Chronic SAH (MRI with FLAIR sequence)
- To detect cerebral metastases (CEMRI and DWMRI)
- Demyelinating and dysmyelinating diseases of brain (e.g. Multiple sclerosis, SSPE, motor neuron disease, central pontine myelinolysis, etc.)
- Congenital brain anomalies (e.g. Arnold Chiari malformation, etc.)
- To differentiate between arachnoid cyst and epidermoid cyst (DWMRI)
- All brain tumors, especially the posterior fossa tumors
- All spinal cord pathologies (e.g. Prolapsed intervertebral disc, metastases, spinal cord tumors, traumatic paraplegia, transverse myelitis, etc.)
- Imaging of breast with silicon implants
- Juvenile angiofibroma (Invasive one)
- Ca nasopharynx (CT better for delineating skull base/bony erosion)
- All bone tumors (except osteoid osteoma, for which CT is better)
- Avascular necrosis
- Bone marrow edema
- Cartilage, meniscal, or ligament injuries
- Rotator cuff injury
- Preoperative evaluation of carcinoma endometrium and cervix (CEMRI)
- Early detection of Ca prostate
- Assessment of depth of penetration and perirectal nodes in rectal cancer
- Dissection of aorta/ aortic aneurysm
- RCC with tumor thrombus in renal vein or IVC (CEMRI)
- Vascular rings and anomalies.
Contraindications to MRI
- Cochlear implants
- Intraocular metallic foreign body
- Aneurysmal clips
- Cardiac pacemakers (absolute contraindication)
- Prosthetic heart valves
- Bone implants
- Claustrophobia (Phobia of closed spaces).
RADIONUCLIDE IMAGING
This encompasses γ-scans (isotope scans/scintigraphy), SPECT and FDG-PET scan. Isotope scanning is discussed in section B.
EMISSION COMPUTED TOMOGRAPHY
Conventional planer imaging provides only a 2-dimentional projection of a 3-dimentional presentation of activity. Emission computed tomography (ECT) provides an in vivo 3-dimensional distribution of radiopharmaceutical within the body. It also provides improved image contrast and quantification.
It includes
- Single photon emission computed tomography (SPECT)
- Positron emission tomography (PET).
Single Photon Emission Computed Tomography
It involves detection of gamma (γ) rays emitted singly from radionuclide like 99mTc and thallium 201. It is 3-dimensional examination. Most of its use is in brain and heart. Perfusion studies and functional imaging of brain is possible with it.
Positron Emission Tomography/18-FDG-PET Scan
Positron is a positively charged electron.
48Basic principle: Coincidence detection of paired high energy (511 keV) annihilation gamma photons from positron emitting radionuclides like carbon-11, nitrogen-13, oxygen-15 and fluorine-18.
Agent used: Fluorine-18 (F-18) is most commonly preferred. To make the agent to go specifically to the site of interest, F-18 is coupled with deoxyglucose which gets more concentrated in malignant cells as they have much more metabolic demand compared to normal cells.
Mechanism: PET compounds are radiolabeled with positron emitting radionuclides and injected into patient. The compound concentrates at the target and after some time positron is emitted. Positron after being emitted by the given agent combines with an electron in the tissue resulting in formation of high energy annihilation photons, which are emitted in diagonally/exactly opposite directions; in turn detected by coincidence circuit through simultaneously arriving detectors on opposite sides of patient. An electronic collimation occurs through coincidence circuit (thus, no need of lead collimators) hence has better resolution and sensitivity than SPECT. Thus, technically the patient is injected with the 18F-FDG iv and after about 45 minutes imaged with PET camera.
Imaging time → 1-10 minutes.
Positron emission tomography (PET) camera: Routine gamma camera is not used for PET imaging. PET detectors are made up of special material, called BGO (Bismuth germinate) crystals, which sensitively and specifically detects the high energy (511 keV) gamma photons produced after electron and emitted positron combine with each other.
Advantages
- The kind of radiopharmaceutical that can be used most physiological molecules in body are made of carbon, nitrogen and oxygen, enabling them to be labeled with “11C, 13N and 15O and 18F” which are position emitters.
- It is a ‘unique tool’ to study and quantify physiological and pathological function of human tissues and organs.
- Imaging modality that permits noninvasive in vivo examination of metabolism (biochemical imaging), blood flow, electrical activity and neurochemistry.
- Only radiological modality with which metabolic imaging is possible.
- Most accurate noninvasive method of detecting and evaluating most cancers.
Clinical Applications
- Oncology (Glucose and oxygen utilization of tumors measured)
- Breast cancer
- Colon cancer recurrence
- Lung cancer and pancreatic cancer
- To differentiate benign from malignant solitary pulmonary nodule (SPN)
- Differentiation of incidentalomas from metastasis in adrenals
- Best to detect lymph node metastasis anywhere in the body (especially from head and neck cancer)
- Best to detect metastases anywhere is body except cerebral metastases, for which MRI is best.
- Cardiology: Quantification of myocardial blood flow and best to differentiate between a salvageable and nonsalvageable myocardium.
- Neurology: (Metabolic and functional assessment, cerebral blood flow quantification).
- Stroke
- Encephalopathies
- Brain tumor
- Residual/recurrent tumor versus neurosis
- Response to chemoradiation
- Prediction of patient's average survival in it
- Lymphoma staging
BONE DENSITOMETRY
- It is currently the most accurate bone density measurement technique or a method for predicting patients at risk of osteoporotic fracture (Gold Standard).
- Bone density measurement techniques are:
- – Radiogrammetry: Cortical thickness measured from a PA hand radiograph and subjected to a variety of mathematical formulae for estimating skeletal status and cortical bone volume. Its drawback is that it doesn't provide an absolute measurement of bone density.
- – Photodensitometry: In single absorptiometry (SPA), a single energy source of 125-I used to produce a highly collimated beam of low energy photons and the absorbed radiation is detected by Na T scintillation counters. An area measurement of bone mineral density as well as bone mineral content can be obtained by SPA.
- – Quantitative computed tomography (QCT): A measurement of pure trabecular bone mineral density (BMD) in lumbar spine is obtained using a conventional computed tomography (CT) scan. Its advantages over other BMD measuring techniques include: a pure measurement of trabecular bone (sensitive) and measurement of bone that does not include any extraneous calcification that may artefactually increase BMD. A disadvantage is higher radiation dose than DXA.
- – Dual photon absorptiometry (DPA): With DPA it became possible to measure BMD and body composition regionally as well as in the whole body. A radionuclide source (153 Gd) is used to produce two discrete energies by using which the different attenuation values of bone and soft 51tissue could be calculated. It was a simple, accurate precise technique and required only a low dose of radiation. But it had a number of drawbacks, like problem with longitudinal measurements, long scanning time and poor image quality.
- Dual energy X-ray absorptiometry (DXA)
- DXA has superseded DPA and is now the most commonly used technique for measuring BMD throughout the world.
- It provides a very useful noninvasive clinical tool to evaluate and monitor bone mineral density and body composition in both adults and children.
- The result is accurate and precise, with added advantage of very low radiation dose to the patients.
- The first generation DXA scanners used a pencil beam of X-rays coupled to a single detector and although a low radiation dose is produced it has an increased scan time as compared to latest generation of DXA scanners.
- The development of Fan beam DXA scanners (2nd generation DXA) has provided faster scan times and improved image resolution and quality and ability to measure vertebral deformity.
- With dedicated software, measurements of forearm, small animals, periprosthetic bone and vertebral height can be obtained or spinal morphometric X-ray absorptiometry (MXA) for vertebral deformities can be performed.
- The principle of DXA is very similar to DPA, but it uses a beam or X-ray emitted from an X-ray tube rather than an isotope source to provide dual energies.
- With energy switching system there is simultaneous calibration of emergent X-ray beam by a rotating wheel containing internal reference standards for bone, soft tissue and air.
- The K-edge filter systems require photon counting discriminating detectors but, with energy switching technique, current integratory detectors are used. In K-edge filters (cerium or samarium) the output of contrast potential X-ray tube is filtered to produce energies of 40 and 70 keV, while in rapid switching technique, switching between 70 and 140 kVp is done to produce energies of 45 and 100 keV.
EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY
- The development of extracorporeal shock wave lithotripsy (ESWL) by Eisenberger and Chaussy in Munich in 1980 revolutionized the treatment of upper tract renal calculi.
- The first generation lithotripter was an electrohydraulic machine in which a shock wave induced by the underwater discharge of 20 kV across a spark gap.
- Second generation ultrasonic/electromagnetic lithotripter machines have proved to be the simplest and most successful.
- Complications:
- Some degree of ureteric obstruction, as multiple stone fragments build up in distal ureter.
- Failure of treatment
- Rapid recurrence of calculus, may be due to residual fragments acting as nidus
- Renal contusion, perinephric hemorrhage, fracture transverse process (due to misdirected shock waves) are rare
- Increased incidence of long-term hypertension.
Endoscopic Retrograde Cholangiopancreaticography
It has become primary method of direct cholangiography and has developed considerable therapeutic potential also.
A side viewing endoscope is required and the pancreatic tree should be visualized before the biliary tree. The number of side branches of the pancreatic duct opacifying at ERCP decreases with age.
Advantages
PTC include less patient discomfort and fever complications, and a success rate, which is independent of the biliary tract. It also offers ability to examine the upper gastrointestinal tract, the papilla of vater, and the pancreatic duct.
Disadvantage
The procedure is more prone to technical failure than percutaneous transhepatic cholangiography. PEP (Post ERCP Pancreatitis) is one of the most common complications.
Pancreatic pseudocysts and esophageal varices are contraindications for ERCP.
Recent Advances
- Digital mammography, sonomammography, mammography scintigraphy.
- Tissue harmonic imaging (USG of focal lesions in liver)
- IVUS (Intravascular ultrasonography) – needs > 20 MHz probe
- EBCT (Electron Beam CT/Generation V CT/ultrafast CT/mSec CT)
- Virtual endoscopy (Multislice CT with navigator software fecal tagging and colonic distension with in air makes it possible)
- MR and CT enteroclysis for small bowel imaging
- Fetal MRI (especial for complex cranial anomalies)
- Intraoperative MRI
- PET–CT fusion (Revolution in oncoimaging especially due to function and anatomical fusion scanning.
Key indications for various radiological investigations | |
|---|---|
Radiological tool | Prime indications |
IVP/IVU | – Renal tuberculosis – Renal anomalies |
Esophagography | – TO fistula (Water-soluble contrast study) – Motility disorders (initial evaluation) – Dysphagia |
HSG | – InfertilityQ |
(Postmenstrual-preovulatory period) | – Congenital uterine anomalies – Tubal block |
Urethrography | – PU valves (RGU) – VUR (RGU) – Trauma – Stricture |
Ultrasound | – Hydrocephalus in infantsQ – Thyroid nodule – Initial evaluation of rotator cuff injury/subacromial bursitis/bicipital tendinitisQ – Synovial cysts – Pleural/Pericardial effusionQ – First investigation done for acute abdomen and obstructive jaundiceQ – Congenital hypertrophic pyloric stenosis and intussusception – GallstonesQ – Adenomyomatosis of Gallbladder (“Comet tail” sign) – Initial evaluation of focal liver lesions – Initial evaluation of blunt trauma abdomen (FAST protocol)Q – Minimal ascitesQ – Cystic hygroma – Prostatic pathologies (TRUS) – Seminal vesicle pathologies (TRUS) – Staging of early rectal/pancreatic head malignancies (EUS) – Neuroectodermal pancreatic tumors—insulinoma and gastrinoma (EUS) – Scrotal pathologies – Developmental dysplasia of HipQ |
CT Scan | HRCT (Bone algorithm) Lungs: – ILD – Bronchiectasis – Emphysema Temporal bone: – Petrositis – Mastoiditis – Ossicular chain disruption – Cogenital anomalies [e.g. Mondani's malformation] – EAC osteoma – Cholesteatoma – Fracture facial canal Noncontrast CT (NCCT) – Initial test of choice for acute stroke – Fractures:
– Acute hemorrhage
– Minimum air:
– Calcification anywhere in body – Ureteric calculi CTA (CT angiography) – Screening of intracranial internal carotid (ICA) lesions – Screening of circle of Willi's lesions – Chronic abdominal angina – Pulmonary embolism |
– Noninvasive coronary angiography – Coronary anomalies – Pulmonary sequestration – Rapid screening tool for aortic lesions – Sensitive and specific screening tool for renal artery stenosis CT brain – Oligodendroglioma – Screening for cortical venous thrombosis (Empty delta sign in lateral sinus thrombosis) – Evaluation of an acute change in mental status – Evaluation of an focal neurologic findings – Conductive hearing loss – Craniopharyngioma – Craniosynostosis – Krabbe's disease (Globoid cell dystrophy) CECT neck – Staging of Ca larynx – Staging of Ca thyroid – Cold abscess – Nodal characterization CT PNS – Chronic recurrent sinusitis (NCCT) – Noninvasive fungal sinusitis (NCCT) – Sinonasal polyps (NCCT) – Pre-FESS CT (NCCT) – Neoplasms of maxilla (CECT) – Juvenile angiofibroma (CECT) – Screening for Glomus jugulare (Phlep sign) – CSF rhinorrhea (CT cisternography) CT orbit – Metallic foreign body (NCCT) – Optic drusen – Retinoblastoma (CECT) CECT chest – Screening detection and staging of lung cancer – Lung hamartoma – Pleural lesions – Diaphragmatic hernia | |
– Anterior and middle mediastinal lesions – Congenital labor emphysema – Congenital adenomatoid lung malformation – Morgagni's hernia – Mediastinal lymph node characterization – Tubercular pericarditis – Calcified cardiac tumors CECT abdomen – Blunt trauma abdomen (FAST USG for screening only) – Abdominal lump – Abdominal wall hematoma – Advanced rectal/esophagus/stomach/bladder cancer – Detection and staging of Ca gallbladder and small bowel tumors – Focal hepatic lesions – Adrenal imaging (Nonfunctional lesions) – Renal carcinoma detection and staging – Oncocytoma (Central stellate scar) – Renal and perirenal infections – Complicated ADPKD – Mesenteric cyst – Enteric duplication cyst – Abdominal lymph node and peritoneal TB – Diverticulitis and diverticular abcess – Subdiaphragmatic abscess – Bowel obstruction evaluation – Complicated appendicitis – Appendicitis epiploicae CECT pelvis – Adnexal mass – Ovarian dermoid – Ca urinary bladder – Advanced prostatic cancer staging – Sacrococcygeal teratoma | |
MRI | MRI advantages |
• Superior soft tissue contrast resolution—excellent pathological discrimination • No ionizing radiation • Direct multiplanar imaging (transverse, coronal, sagittal, any oblique) • Noninvasive—vascular studies can be performed without contrast | |
MRI disadvantages • Expensive • Long scan times • Audible noise (65–115 dB) • Isolation of patient (cloustrophobia monitoring of ill patients) • Exclusion of patients with pacemakers and certain implants Cranial MRI – Chronic hemorrhage (GRE < FLAIR) – Sensitive most for acute and hyperacute ischemic stroke (DWMRI) – Demyelinating disorders (e.g. MS, ALS, SSPE, SACD, SMON, CPM, ADEM, PML, PVL, PRES, etc.) – Infectious processes (encephalitis, meningitis) – Abscesses – Brain neoplasms (supra as well as infratentorial) – Neurofibromatosis – DAI – Vascular disorders (AVM's, aneurysms, vasculitis, Moya-moya disease) – Metastasis – Internal auditory canal pathology – Pituitary pathology – Hydrocephalus especially in adults – Cranial nerve pathology (e.g. vestibular Schwannoma) – Congenital anomalies (for anatomical review) – Epilepsy (seizures in general) – Parameningeal tumors – Low CSF volume headache Spine MRI – Cauda equina syndrome – Tethered cord – Arachnoiditis – Marrow-replacing processes – Degeneration disk disease – Discitis – Congenital anomalies – Radiculopathy – Spinal cord tumors – Trauma/contusion – Syringomyelia – Metastasis – Vascular disorders | |
– Cord edema – MS plaques – Traumatic paraplegia – Retroperitoneal tumor with spine extension – Pott's spine – Myelomalacia Musculoskeletal MRI – Meniscal pathology – Ligament/tendon injury – PVNS – Muscle/nerve impingement – Rotator cuff tear – Avascular necrosis – Labral tears (shoulder, hip) – Chondromalacia – Inflammation (acute osteomyelitis) – Primary bone tumors – Spinal metastases – Soft tissue tumors – Perthe's disease – Cartilage injury Breast MRI – Screening in young females (as glandular tissue is more than fat in young breasts)Q – Breast with silicone implants (Augmented breast) – Sensitive most investigation to detect DCIS Chest MRI – Imaging of Pancoast's tumor (superior sulcus tumor)Q – Imaging of posterior mediastinal masses – Demonstration of vascular slingQ – Investigation of choice in aortic dissectionQ – Best diagnosis for dissecting aorta (aortic dissection)Q Cardiac MRI – Vascular rings (e.g. Double aortic arch; Aberrant right subclavian; Pulmonary sling) – Post-up complex cyanotic congenital heart diseasesQ – ARVD – Coarctation of aorta – Cardiomyopathies (in general) | |
– Malignant cardiac tumors – Pericardial malignancies Abdominal MRI – Investigation of choice for a pregnant lady with upper abdominal massQ – The most sensitive and specific investigation for renal artery stenosis – Preoperative staging of endometrial and cervical cancerQ – Anorectal sepsis, anorectal malformation and anorectal tumors – RCC with suspected RV or IVC invasion – Fetal cranial abnormalities – Liver pathologies | |
MRA (Gd-enhanced MRA > PC or ToF MRA) | |
• Best for circle of Willis lesions | • Renal artery stenosis |
• AVMS | • Acute mesenteric ischemia (Laparotomy test) |
• Aneurysms | • Cortical venous thrombosis (MR venography) |
• Aortic lesions
| • Vertebral artery dissection/thrombosis |
• Vascular rings | • Corticocavernous fistula (Pulsatile exophthalmos) |
• Carotid lesions | • Coronary artery aneurysms |
• Carotid body tumors | |
• Paragangliomas (South Peeper appearance) | |
Note: ‘Q’ as a superscript indicates ‘vital indications’ | |