X-rays are electromagnetic radiations having wavelength of 0.1–1 Å and speed is same as that of light (3 × 108 m/sec). They behave as waves as well as particles and are considered as a form of modified electrons.
Modern X-ray tubes are based on hot cathode tube principle invented by Coolidge in 1913 which enables excellent control of kVp (kilovolt peak) and mAs (milliampere second). kVp is responsible for penetration of X-ray beam, low kVp gives high contrast. mAs is responsible for the film blackening. The radiation intensity on the cathode side of the X-ray tube is higher than on the anode side and this principle is called as the Heel effect. The heat generated in the tube is dissipated in three ways: conduction, convection and radiation. Diagnostic X-ray machine uses voltage upto 150 kV whereas machines used for radiotherapy use high voltage > 200 kV.
In diagnostic radiology about 85% of X-rays arise from Bremsstrahlung radiation and 15% from characteristic radiation.
X-ray tube is a diode consisting of tungsten filament cathode and a rotating anode target of tungsten held in an evacuated glass. Tungsten anode is inclined at an angle so that it works on line-focus principle.
Tungsten (atomic number 74) is used as target material for X-ray production. Molybdenum (atomic number 42) is used as the target in mammography. X-rays are produced when the electron beam strikes the anode made of tungsten or molybdenum.
X-ray filter made of aluminum absorbs low energy radiation and decreases unnecessary patient exposure and thus improves film contrast. Grid made of parallel lead lines with intervening radiolucent material absorbs scattered 3radiation. Cones and collimators restrict field size and decrease scatter.
Distance from X-ray tube to the radiograph is called focus film distance (FFD). It is 100 cm for usual radiographs of extremities, abdomen and skull. However, for standing radiograph of chest it is 180 cm (6 ft) so as to reduce the magnification.
Ultrasonography (USG) works on piezoelectric effect of crystals made from lead zirconate titanate. Here the sound waves having frequency more than 20,000 hertz are generated by piezoelectric crystals and are used in forming images by using pulse echo principle. Quartz is the naturally occurring piezoelectric crystal. Gray scale (B) mode is used in general imaging. Motion (M) mode is used for echocardiography.
Color Doppler ultrasound is used for imaging of vessels. Christian Doppler put forth the principle of doppler.
Computed tomography (CT scan) was developed by Godfrey Hounsfield and he was awarded Nobel prize in 1979. It works on the principle of linear attenuation of X-rays. The density of a structure on CT scan is measured in terms of Hounsfield unit (HU). One of the most important uses of multislice CT is in cardiac applications, i.e. coronary angiography.
Magnetic resonance imaging (MRI) scan was introduced by Lauterbur and Mansfield and they were awarded Nobel prize in 2003. The theory of nuclear magnetic resonance was put forth by Felix Block and Edward Purcell. Patient is not exposed to any ionizing radiation in MRI Scan. Hydrogen protons in the body are used for imaging in MRI. The images are obtained using gyromagnetic properties of hydrogen proton under high magnetic field of field strength ranging 4from 0.2–3.0 Tesla. To prevent MRI room from external electromagnetic radiations, a cage made of copper is used which is called as Faraday's cage.
MRI is better than CT scan for imaging brain, spine and soft tissues. CT scan is better than MRI for assessing bones, small calcifications and fresh bleed.
Radioisotope imaging is done using Tc99m (Technetium) and Gamma camera. Single photon emission computerized tomography (SPECT), images the distribution of radiopharmaceutical which emits photon on decay in the body. Whereas positron emission tomography (PET scan), shows the distribution of positron emitting nuclides in patients.
Dual energy X-ray absorptiometry (DXA scan) is used for measuring bone mineral density and it is the gold standard for assessing osteoporosis.
Radiographic film has 2 parts: The base (polyester/cellulose triacetate) and the emulsion (silver halide crystals-photosensitive layer). The cassette is the rigid holder that contains the intensifying screen and film. Intensifying screen made of phosphor converts and amplifies the energy of X-ray beam into light photons so that good radiographic contrast can be obtained at lower dose of X-ray. Latent image on the exposed film is converted into a visible image by processing the film in developer solution made of metol and hydroquinone and fixing it in a fixer solution made of sodium or ammonium thiosulphate.
Radiography has now become ‘film less’ due to the availability of digital radiography (DR), which uses photostimulable phosphor plate. This is also called as computerized radiography (CR).
Picture archiving and communication system (PACS) has become possible only due to the advent of digital radiography.5
Newer units are used now for measuring radiation. Curie, which was the unit of measuring radiation activity earlier, has now been replaced by Becquerel (Bq).
One Bq means one radiation disintegration per second.
Roentgen which was the unit of radiation exposure earlier is now replaced by Coulomb/kg.
Gray (Gy) replaced Rad as the unit of absorbed radiation dose.
One Gray is equal to 100 rads.
Sievert has replaced Rem as the unit of dose equivalent. One Sievert is equal to 100 rem.
Radiation exposure is measured in units of sieverts. On one X-ray chest the patient receives around 0.2 milli Sieverts (mSv) of radiation. One CT scan of chest gives 8 mSv radiation to patient.
Thermoluminiscent dosimeter (TLD) is used to measure exposure of radiation workers. TLD contains calcium sulphate as the thermoluminescent material, which is activated with disprosium.
In radiology department, lead apron is used for protection against radiations. Recommended thickness of lead in the apron is 0.5 mm.
Methods to decrease scattered radiations include reduction in beam size by using collimator, cone, grid, intensifying screen and film cassette.
ALARA principle emphasizes that the radiation should be kept as low as reasonably achievable (ALARA). Children are 10 times more sensitive for hazards of radiations than adults. Hence high kV and low mAs technique is recommended in children.
Inverse square law states that intensity of radiation at a given distance from the source of radiation is inversely proportional to the square of distance.6
Acute radiation at doses in excess of 100 Gy to the total body, usually result in death within 24 to 48 hrs from neurological and cardiovascular failure. This is known as the cerebrovascular syndrome.
Chronic radiation causes radiation pneumonitis and even permanent scarring that results in respiratory compromise. Preconception maternal irradiation in therapeutic doses gives rise to defects in 1 out of 10 exposed children. Non-urgent radiological testing should not be done between 8–17 weeks of gestation, which is the most sensitive period for organogenesis.7
Maximum permissible radiation dose for a pregnant patient is 0.5 rem. The ten day rule for radiological investigation in a female patient states that any investigation using X-ray should be done only during the first 10 days of the menstrual cycle.