Imaging foundations
Introduction to radiology and modality selection
Overview and approach
Choose imaging to answer a specific clinical question, not simply to obtain a scan.
- Match modality to the question: plain X-ray for bones and chest; CT for rapid assessment of acute trauma, lung and bone; MRI for soft tissue, spinal cord and brain; ultrasound for dynamic, non-ionising assessment (summary below).
- The Royal Australian and New Zealand College of Radiologists (RANZCR) patient information site (InsideRadiology) describes each modality for patients and referrers1.
- How X-ray and CT make contrast: tissues absorb (attenuate) X-rays by density. Gas and fat attenuate little and look black; bone attenuates most and looks white; soft tissue and fluid are grey. Intravenous iodinated contrast raises the attenuation of vessels and enhancing tissue4,5.
- How CT works: the X-ray source spins around the patient as the table moves (helical scanning), and a computer turns the data into axial slices that can be rebuilt in any plane or in 3D. Thinner slices from multi-row scanners show smaller lesions4,5.
- How MRI works: a strong magnet lines up hydrogen protons; radiofrequency pulses knock them out of line, and the signal they give off as they relax builds the image. It gives better contrast between soft tissues than CT, can image in any plane and uses no ionising radiation; gadolinium can be given to enhance it4.
- MRI weighting: on T1, fat (and fatty marrow) is bright and fluid dark, so T1 suits anatomy; on T2, fluid, oedema and blood are bright, so T2 shows disease. Cortical bone, tendons and ligaments are dark on both4.
- Nuclear medicine: a radiopharmaceutical chosen to follow a particular pathway is injected and a gamma camera maps where it goes. It shows function (areas of high or low uptake) rather than fine anatomy, e.g. V/Q scan, bone scan, thyroid and renal scans4.
- PET: a positron-emitting tracer such as FDG (a glucose analogue) collects in metabolically active tissue. It is used to find cancer, stage it and check whether treatment is working4.
Assessment and clinical features
Confirm indication, relevant examination, prior imaging, pregnancy possibility, renal function when relevant, and contrast or MRI risks.
- Patient positioning changes how findings look (e.g. erect versus supine films); see the chest X-ray page for examples.
- Ask about implants, metal, contrast reactions and kidney function before CT with contrast or MRI (see Contrast, radiation and MRI safety).
- CT radiation: CT doses are high compared with plain films (a chest CT is many times a chest X-ray), so avoid unnecessary CT, particularly in children, young adults and women of childbearing age4,5.
- Iodinated contrast risks: acute kidney injury and allergic-type reactions are the main concerns5.
- MRI safety: metal clips, some stents, pacemakers, cochlear implants, some prosthetic valves and metal fragments may make MRI unsafe; check each device's MRI status4.
- Artefacts: metal clips, stents, prostheses and residual barium cause streaks on CT that can hide nearby pathology4.
Differentials and investigations
X-ray is useful for bones and chest; CT is rapid and strong for acute trauma, lung and bone; MRI provides superior soft-tissue, cord and brain detail. Ultrasound is dynamic without ionising radiation.
Modality comparison:
| Modality | Strengths | Weakness | Appearance | Examples |
|---|---|---|---|---|
| Simple XR | Bones and chest | |||
| CT | Rapid; acute trauma, lung and bone | CT KUB non-contrast: stranding = inflammation | ||
| Contrast CT | Arterial phase: bleeding, aorta opacification. Venous phase: more common and homogeneous, opacification of portal vein. Bone excellent, soft tissue poor | SMA obstruction; aortic aneurysm on abdominal CT | ||
| MRI | Soft tissue, cord and brain detail | Excellent soft-tissue detail, poor bone detail | ||
| PET | ||||
| Ultrasound | Dynamic, no ionising radiation | Operator dependent |
- Caveats: "bone excellent, soft tissue poor" for contrast CT is relative (CT soft-tissue contrast is lower than MRI but contrast CT is widely used for solid organs and vessels); and "MRI poor bone detail" is misleading because MRI is sensitive for marrow pathology, though cortical bone detail is better on CT.
CT contrast phases and what each is for5.
| Phase | Timing after IV contrast | Best for |
|---|---|---|
| Non-contrast | No contrast | Calcification and stones (CT KUB), baseline for fresh blood |
| Arterial (CTA) | About 20 seconds | Aneurysm, active arterial bleeding, organ or tumour perfusion |
| Portal venous | About 70 seconds | Routine abdomen: solid organs, portal vein, most pathology |
| Delayed | Several minutes | Contrast leaks, urinary tract (collecting system, ureter, bladder) |
- Reading a CT abdomen: look at the scout image first (it changes or makes the diagnosis in up to about 10%); scroll the whole scan for big abnormalities (dilated bowel, ascites, aneurysm, mass); then go organ by organ for size, margins and enhancement; check the dependent spaces for free fluid and free gas; finish on lung and bone windows5.
- Fat planes: the edge between a normal organ and the surrounding fat is sharp. Blurring or stranding of that fat suggests inflammation, haemorrhage or tumour infiltration5.
Management and follow-up
Discuss the most appropriate modality and protocol with radiology. Give a focused request, check the report and act on critical results.
- For MBS-funded MRI, RANZCR/RACGP referral guidance sets out appropriate indications2,3.
Red flags and clinical pearls
MRI can assess marrow and other bone pathology; describing it as poor for all bone disease is misleading. Radiation and contrast risks must be balanced against benefit.
- Viva: which modality would you choose for suspected stroke, renal colic, appendicitis in a child, and a cord compression? Why?
- Viva: explain why fat is black on CT but bright on T1 MRI, and which CT phase you would ask for in a suspected active bleed4,5.
References
- InsideRadiology (RANZCR): patient and referrer imaging information
- RANZCR: Referral guidance for new MBS MRI items
- RACGP: Clinical guidance for MRI referral
- Burnand KG, Black J, Corbett SA, Thomas WEG, editors. Browse's Introduction to the Investigation and Management of Surgical Disease. Boca Raton: CRC Press; 2010. Chapter 1, Principles and methods of investigation; Chapter 7, Fractures, joint injuries and diseases of bones.
- Talley NJ, O'Connor S. Clinical Examination: A Systematic Guide to Physical Diagnosis. 7th ed. Sydney: Elsevier Australia; 2014. Chapter 12, A summary of the respiratory examination; Chapter 16, A summary of the gastrointestinal examination.
Modality choice should follow the specific clinical question and local radiology advice.
Last updated from Sen’s notes on 22 October 2026.