Only by understanding normal findings can you develop the skills to identify abnormal results and correctly diagnose the condition causing them. In radiology, various imaging modalities are used; each has its own way of producing an image. Knowing the basic concepts underlying the different types of radiological examination will enable you to interpret the images produced and understand the pathological processes occurring, even if the actual diagnosis is unknown.
1.1 Plain radiography
The plain radiograph remains an important and useful diagnostic tool. This is especially true in musculoskeletal radiology, as radiographs are quick, widely available and inexpensive. They are well tolerated by most, if not all, patients. Fractures and focal bony abnormalities are easily detected.
However, radiography exposes the patient to ionising radiation in the form of X-rays. Although the radiation burden of radiography and other radiological examinations is small (Table 1.1), the risk of developmental problems and lifetime cancer risk is increased. Therefore any examination must be clinically justified.
How it works
X-rays are passed through a part of the body and the resultant image is captured on an imaging plate (traditionally a film but nowadays a digital detector). The X-rays are either absorbed or scattered by the different layers of tissue. The degree of absorption or scattering depends on the density of the tissue. Thus differences in tissue density are visualised as differences in contrast in the overall image.2
Radiographic densities
The four main classes of radiographic density are gas, fat, soft tissue and bone. Metal may also be seen on radiographs (Figure 1.1).
Figure 1.1: Radiograph of the right shoulder, showing five different radiographic densities in an acromial fixation: in increasing order of density, gas or air Ⓐ, fat Ⓑ, soft tissue Ⓒ, bone Ⓓ and metal Ⓔ.
Gas
Gas (not always air) has the lowest density and therefore absorbs very few X-ray photons. Most of the energy passes through areas of gas, which therefore appear black on the final image.
Fat
Fat has low density but absorbs X-ray energy slightly more than gas does. Therefore areas of fat appear a shade lighter than black, i.e. a dark grey, on the image. Dark-grey areas of fat are seen between layers of soft tissue and help delineate these layers.
Soft tissue
Soft tissue partially absorbs and scatters X-rays, resulting in a grey shadow on the image. Adjacent soft tissues of the same density are indistinguishable if there is no intervening fat, gas or metal.
Bone
Bone contains calcium, which makes it very dense. Therefore bone appears light grey to white on radiographs. The exact shade of grey depends on which part of the bone is being viewed. For example, the light grey medullary cavity is clearly distinguishable from the white cortex in a long bone.
Metal
Metal has the highest density. Its presence in the body may be intentional (e.g. when a screw fixation is used) or unintentional (e.g. in cases of a retained suture needle).
Principles of assessment of radiographs
The general principle is to use a systematic approach to assess the entire image.
- Alignment: check that all the bones and joints are in anatomically correct alignment. Loss of alignment can result from fractures or dislocations.
- Cartilage: cartilage is not visible on radiographs, but check that the joint spaces are present and congruent throughout the joint. Joint space narrowing or widening may indicate underlying pathology.
- Soft tissue: check for the presence of soft tissue changes which can indicate underlying pathology even when the bones and joints appear normal.
1.2 Ultrasound
Ultrasound is a particularly useful tool in musculoskeletal imaging, because it is good at visualising superficial structures due to the high-resolution images it generates. Also, ultrasound images of some structures, such as tendons, are more detailed than those of magnetic resonance imaging (MRI).
However, ultrasound is operator-dependent; the quality of ultrasound images and the accuracy of diagnosis is entirely dependent on the expertise of the operator, and ultrasound skills take a long time to acquire. Also, ultrasound has limited ability to visualise deeper structures or those masked by dense structures such as bone.
How it works
A pulsed wave of ultrasound (2–15 MHz) is transmitted. It loses energy as it passes through the body. The amount of energy lost depends on the amount of energy absorbed by the material. The rate of absorption depends on the type of material through which the pulse passes and the frequency of the ultrasound.
The absorption rate of a material is specified by its attenuation coefficient. The lower the coefficient, the more easily the ultrasound pulse penetrates the material (Figure 1.2). Therefore materials with a lower attenuation coefficient are more anechoic and look darker on ultrasound. Conversely, materials with a higher attenuation coefficient are more echogenic and look brighter on ultrasound.5
Figure 1.2: Ultrasound of the arm, showing various tissue densities: fluid Ⓐ in the tendon sheath of the long head of the biceps tendon Ⓑ, lying on the cortical bony surface Ⓒ, with overlying deltoid muscle Ⓓ and superficial subcutaneous fat Ⓔ.
Echogenicity
Fluid
Fluids are anechoic and thus appear dark on ultrasound. Different fluids, for example blood and water, have different reflective properties. Water appears totally anechoic or black on the screen, whereas blood pooled within a vein appears almost black on the screen, with a slight turbidity due to the cellular components within.
Fat
Fat appears as a bright (hyperechoic) area.
Soft tissue
Soft tissue appearances vary according to the exact type of tissue.
- Muscle is hypoechoic. In the short axis (transverse plane) it looks dark with small speckled dots (due to perimysial connective tissue within it). In the long axis (longitudinal plane) it is dark with hypoechoic cylindrical structures (fascicles), resembling parallel lines of spaghetti.
- Tendons have a fibrolinear pattern, seen on US as parallel lines in the longitudinal axis. In the transverse axis, tendons are round or ovoid. Tendons may be surrounded by either a synovium-lined sheath or a dense connective tissue known as the paratenon (Figure 1.3).
- Ligaments look similar to tendons. However, ligaments have a more compact fibrolinear architecture and hence more hyperechoic pattern.Figure 1.3: (a) Longitudinal ultrasound of the knee, showing fibrolinear parallel lines (arrow) in the patellar tendon, arising from the lower pole of the patella (arrowhead). (b) Transverse ultrasound showing the ovoid tendon (long arrow) with a thin paratenon (short arrow).
- Nerves have fascicular structures that are slightly less echogenic than tendons and ligaments.
Bone
The cortical layer of bone appears as a thin, well-defined, hyperechoic line casting an acoustic shadow deep to its surface.7
Figure 1.5: Transverse ultrasound of the fingers, showing the common flexor tendons. (a) Anisotropic artefact in the ring finger (arrowhead). A digital artery (*) lies between the tendons. (b) Anisotropy resolves (arrow) when the probe position is adjusted.
At joint surfaces, the articular cartilage appears as a thin hypoechoic rim paralleling the echogenic articular cortex.
Principles of ultrasound assessment
It is essential to use the correct ultrasound in order to produce optimal diagnostic images. Choice of probe (low or high frequency) depends on the depth of the tissue that is being reviewed. In principle, use the highest frequency probe possible for the area examined, understanding that what is gained in higher resolution is lost in reduced depth. Target the examination to a specific area, and assess all relevant structures in that area systematically and thoroughly. If an abnormality is found, use basic principles to understand which tissue is involved, and look for other changes such as vascularity and compressibility to assist in unifying the underlying diagnosis. Doppler ultrasound allows detection of vascular flow within the vessels and tissues.
1.3 Computerised tomography
Computerised tomography (CT) produces detailed cross-sectional images of the body. CT is faster to perform than MRI and has a high spatial resolution. It is used in musculoskeletal imaging primarily to assess bones and bony lesions. CT is especially useful when planning surgery for complex fractures.8
Computerised tomography is well tolerated by most patients. However, it carries an even higher radiation burden than that of radiographs (Table 1.1). Therefore CT should be reserved for instances in which other imaging modalities cannot provide the information needed.
How it works
Computerised tomography produces images by using a series of narrow beams of X-rays, in contrast to radiography, which uses one narrow beam. A computer programme uses the obtained X-ray absorption data to generate images called tomograms. Each tomogram represents a cross-sectional slice of a three-dimensional structure. Modern CT uses voxels (3D pixels) to allow multi-planar reconstruction (MPR) review. Contrast material may be injected to enhance the appearance of the tissues.
Computerised tomography provides good cross-sectional images, which can be reconstructed in multiple planes. The intensity scale used in CT is related to the density of the material and is known as the Hounsfield unit (HU) scale.
Computerised tomography densities
As with radiographs, the key to interpreting CT scans is an understanding of the normal appearance of tissues, each demonstrating its own attenuation value. The attenuation scale ranges from −1000 HU for air or gas, through 0 HU for water and to 3000 HU for dense bone (Figure 1.6).
Gas
Gases, such as those in air, do not absorb X-rays emitted by the CT scanner and therefore appear black on the image.
Fat
Fat on average measures −50 HU, so on CT it appears darker than water but lighter than gas.
Fluid
Attenuation of water is 0 HU, but most fluid in the body measures approximately 15–25 HU. Fluids such as water are lighter than fat on CT.9
Figure 1.6: Computerised tomography of the pelvis, showing various degrees of tissue attenuation. Ⓐ Fluid in the bladder, Ⓑ bones of the pelvis and femur, Ⓒ muscles, Ⓓ subcutaneous fat. Small pockets of intraluminal gas (arrowhead) are present in the rectum.
Soft tissue
Soft tissue has a wide range of attenuation values, ranging from 30 HU for muscle to 90 HU for tendon.
Bone
Different types of bone have different attenuation values, ranging from 700 HU for cancellous bone to > 1000 HU for dense bone. Bones appear white on the normal soft tissue window setting (since all structures hyperdense to 75 HU appear white) and are best visualised on the bone window setting (centred at 300 HU, with width of 1500 HU).
Principles of CT assessment
Use a systematic approach to assess every structure separately and how each structure affects surrounding tissues. To help clinicians, describe bony fragments and their relation to each other, and provide an overall grading of the injury or disease.10
1.4 Magnetic resonance imaging
Magnetic resonance imaging provides excellent contrast resolution of tissues. Therefore it is a very sensitive modality for detecting subtle or early pathology, particularly oedema, a sensitive and early suggestion of underlying pathology. MRI is now the mainstay of complex musculoskeletal imaging. MRI is also good for the local staging of bony and soft tissue tumours, because of its superb ability to differentiate tissue types.
However, there are contraindications for MRI. Magnetically activated implant devices (especially pacemakers) and ferromagnetic metals (especially in the brain or eye) are contraindications for MRI. Also, patients who are prone to claustrophobia may be unable to tolerate MRI.
How it works
In MRI, a very strong magnet is used. The magnetic field aligns hydrogen protons, whilst radiofrequency (RF) pulses disrupt their alignment. The protons then realign, giving off signals, to form images. Various pulse sequences are used. The two commonest sequences produce T1-weighted and T2-weighted images. T1-weighted images (Figure 1.7a) are generally best for showing anatomical structures. T2-weighted images (Figure 1.7b) are typically used to show pathological conditions.
Gadolinium contrast helps to distinguish different pathologies based on the degree of enhancement. It is hyperintense on T1-weighted images. T1-weighted fat-saturated images are obtained before and after gadolinium injection: in these, the fat signal is ‘disrupted’ by a selective radiofrequency pulse, and appears dark.
Short T1 inversion recovery (STIR) is a pulse sequence similar to that used in T2 weighting. However, in STIR sequences, an inversion recovery pulse is used to nullify the signal from the fat, so it appears hypointense or dark (Figure 1.7c).11
Figure 1.7: (a) T1-weighted, (b) T2-weighted and (c) short T1 inversion recovery (STIR) magnetic resonance imaging of the pelvis. Fluid in the bladder Ⓐ is dark on the T1-weighted image but bright on the T2-weighted and STIR images. Medullary and subcutaneous fat Ⓑ is bright on T1- and T2-weighted images but dark on the STIR image. Musculature Ⓒ gives an intermediate signal on the T1-weighted image, appearing slightly brighter than on the T2-weighted image; it is dark on the STIR image. Cortical bone Ⓓ and fibrous ligaments (not shown) are dark on all sequences. Ⓔ Air or gas.
Typically, the remaining hyperintense signal is from fluid only, and this fluid signal often shows the pathological tissue. All other signal intensities remain the same. STIR is often used in musculoskeletal MRI.12
Signal intensity
Because of the nature of MRI, different materials have different signals depending on the sequence used. By looking at several sequences, it is possible to identify which tissues are present (Table 1.2).
Gas
Gas has a low signal on all sequences because of the absence of any hydrogen atoms.
Fat
Fat is the only tissue that returns an increased signal on both T1-weighted and T2-weighted images, therefore it should always be distinguishable. STIR or fat-saturated sequences are designed to eliminate this signal, resulting in low signal from fat.
| |||||||||||||||||||||||||
Fluid
Fluid is classically hypointense on T1-weighted images and hyperintense on T2-weighted images. To help determine whether a sequence is T1 weighted or T2 weighted, always look for physiological areas of fluid, such as the bladder, the brain and spinal cord (containing cerebrospinal fluid), and the joints.
Soft tissue
The signal intensity of soft tissue on MRI depends on the amount of water it contains. Structures lacking water, such as tendons and ligaments, show no or low signal on all sequences.
Bone
Cortical bone lacks free water and so gives no signal on all sequences. However, the medullary cavity may give a fatty signal (with yellow marrow) or a more fluid signal (with red marrow).
Principles of MRI assessment
The key to assessing MRI results is to use all the various sequences and planes covering the relevant structures, and to understand the normal signal appearances of each tissue. Pathological changes can be detected by identifying the abnormal signal, which can be further distinguished in some pathologies by using gadolinium contrast.
1.5 Nuclear medicine
Nuclear medicine (radionuclide imaging) is another method of assessing certain musculoskeletal diseases. Isotope bone scaning (bone scintigraphy) is used specifically for detecting osteoblastic bony activity, including fractures, infection and bony tumours. More specialised tests, such as a leucocyte-labelled study, can be even more specific for infections, particularly those in a joint prosthesis.
Nuclear medicine is relatively expensive but widely available and very sensitive. Its high sensitivity makes it an excellent tool to exclude bony metastasis. However, it has a low spatial 14resolution and has low diagnostic specificity. Also, like radiography and CT, it carries a radiation burden.
How it works
The principle behind nuclear medicine is the use of a marker specific for the intended organ or system, attaching this marker to a radioactive tracer, typically a radioactive isotope. The labelled marker is injected intravenously, and travels to and is taken up by the intended organ. The isotope emits radiation when it decays: a gamma camera detects areas in which the tracer has localised. These so-called hot spots show the presence of pathological changes.
In an isotope bone scan, methylene diphosphonate is used as the marker because it is taken up by bone. This marker is attached to a tracer, the metastable technetium-99m isotope, which emits gamma rays when it decays to its stable technetium-99 form.
Tissue visualisation
Bone
Methylene diphosphonate–technetium-99m is widely used for isotope bone scans. It is taken up throughout the skeleton, with intense uptake in the physis of the long bones due to osteoblastic activity. Marrow-containing flat facial bones in children are also hot spots.
Accumulation of the technetium-99m tracer decreases with age, but some areas shows persistent increased uptake symmetrically: the acromial and coracoid process, medial ends of the clavicle, sternomanubrial joint, sacral ala and sites of tendinous insertion (e.g. the anterior and posterior iliac spine). Areas of dental treatment also may show focal increased uptake.
The bones at the major joints, such as the shoulders and hips, show mild increased uptake symmetrically. The pattern 15of increased uptake at the sternoclavicular joints and manubrium sterni is variable. Further increased uptake can be present when there is arthropathy. Common degenerative (and possibly asymptomatic) arthropathic sites include the shoulders, hips, knees and smaller carpal and tarsal joints. Facet joint arthropathy may cause unilateral uptake in the spine.
A triple-phase bone scan is done for suspected infection. Normal uniform uptake is visible in all three phases if no pathological changes are present (see Chapter 2 for details). Equivocal results may indicate specialised leucocyte scanning, in which white cells harvested from the patient are labelled with a suitable isotope (usually indium-111) and reinjected into the patient. Accumulation of the isotope indicates local infection.
Soft tissue
Nuclear medicine is not primarily used for visualising soft tissue pathology. However, in an isotope bone scan there is physiological soft tissue uptake, and it is important not to mistake this for a pathological change. The isotope is excreted through the urinary system, so the kidneys, ureters and bladder all show increased uptake. Tracer uptake is often seen at sites of intravenous injection too. Sometimes, some unbound (free) technetium will also accumulate in the thyroid.
Principles of bone scan assessment
A good understanding of what constitutes normal uptake is needed. Look carefully for areas of increased uptake, particularly asymmetrical uptake (Figure 1.8). Distinguishing physiological from pathological uptake is important. It is equally important to be aware of areas of photopaenic defect (so-called cold spots). These areas often indicate loss or destruction of bone, and the pathology may lie in the cold spot. If in doubt, radiographs of the affected area can help increase the specificity of the diagnosis. Further anatomical correlation of lesions can sometimes be obtained with MRI.16
Figure 1.8: (a) Anterior and (b) posterior bone scan of the whole body, showing normal skeletal uptake, including areas of increased uptake at the sacral ala Ⓐ, coracoid Ⓑ and sternum Ⓒ. The anterior and posterior iliac spine Ⓓ has tendinous insertions. Focal uptake in the cervical spine Ⓔ, lumbar spine Ⓕ and tarsal bones Ⓖ is consistent with joint de0generation. Dental uptake is present Ⓗ. Soft tissue uptake includes that at an intravenous site Ⓘ, the thyroid Ⓙ, the renal system Ⓚ and the bladder Ⓛ.