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 referrers (1).
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).
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.
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 indications (2)(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?
References
  1. InsideRadiology (RANZCR): patient and referrer imaging information
  2. RANZCR: Referral guidance for new MBS MRI items
  3. RACGP: Clinical guidance for MRI referral

Modality choice should follow the specific clinical question and local radiology advice.

Last updated from Sen’s notes on 9 October 2026.