Plain radiographs
A radiograph is a two-dimensional shadow of X-rays that have passed through the body, and its contrast comes from differences in how much each tissue absorbs. Dense tissues stop more X-rays and appear white (radiopaque); less dense tissues let more through and appear dark (radiolucent). Five basic densities are recognised, from darkest to brightest:
- Air (black)
- Fat (dark grey)
- Soft tissue and fluid (mid grey)
- Bone and calcium (white)
- Metal and contrast media (brightest white)
An edge is visible only where two different densities meet. This is the basis of the silhouette sign: on a chest radiograph, the right heart border is formed by the right atrium and disappears when the adjacent right middle lobe fills with fluid. The left heart border is formed mainly by the left ventricle, with the left atrial appendage, pulmonary trunk and aortic knuckle above it.
Because a radiograph flattens depth, two views at right angles are needed to locate a structure or fracture. A chest radiograph is ideally taken posteroanterior (PA), with the X-rays entering from behind, which keeps the heart close to the detector and reduces magnification. An anteroposterior (AP) film, used for patients who cannot stand, makes the heart look larger.
Computed tomography (CT)
CT uses a rotating X-ray source and detectors to build cross-sectional images, removing the overlap of plain films. Each voxel is given a density value in Hounsfield units (HU), set so that water is 0 and air about -1000. Fat is negative, soft tissues are positive, and bone is strongly positive.
The eye cannot separate thousands of grey levels, so images are displayed with a window: a chosen centre and width of HU values. A lung window shows airways and vessels in the lungs; a soft tissue window shows organs, muscle and fat; a bone window shows cortical and trabecular detail. Intravenous iodinated contrast brightens vessels and enhancing organs, and scans can be timed to arterial or portal venous phases.
CT is acquired as axial slices and viewed from below, as if standing at the patient's feet, so the patient's right is on the viewer's left and anterior is at the top. Coronal and sagittal images are reconstructed from the same data. CT is fast and widely available, but it gives a higher radiation dose than plain radiographs.
Magnetic resonance imaging (MRI)
MRI uses a strong magnetic field and radiofrequency pulses to generate a signal from hydrogen nuclei (protons), mainly in water and fat. It uses no ionising radiation and gives the best soft-tissue contrast of the three methods. Signal intensity is described as hyperintense (bright), isointense or hypointense (dark).
| Tissue | T1-weighted | T2-weighted |
|---|---|---|
| Fat | Bright | Bright (unless fat-suppressed) |
| Water, CSF, oedema | Dark | Bright |
| Cortical bone, air | Dark | Dark |
| Flowing blood | Usually dark (flow void) | Usually dark (flow void) |
T1 images are good for anatomy; T2 images show fluid and so highlight oedema, inflammation and many lesions. Fat-suppressed sequences, such as STIR, make fluid stand out further. Gadolinium-based contrast shortens T1 and brightens enhancing tissue. MRI takes longer than CT, and some implanted devices, ferromagnetic foreign bodies and claustrophobia limit its use.
Ultrasound
Ultrasound forms images from reflected high-frequency sound and is used for superficial structures, solid organs, vessels and pregnancy. Echogenic (hyperechoic) tissues appear bright, fluid is anechoic (black), and air and bone block the beam and cast acoustic shadows. It is safe, portable and dynamic, but it depends on the skill of the operator.
Clinical relevance
The choice of imaging follows the question being asked. Radiographs are first-line for fractures and the chest. CT is used for trauma, acute abdominal problems, staging of cancer and detailed bony anatomy. MRI is preferred for the brain, spinal cord, joints, soft tissues and bone marrow. Knowing the normal anatomy, orientation conventions and appearance of each tissue is what lets a clinician recognise when something is abnormal, and radiological images form part of anatomy assessment at every level of surgical training.