Imaging foundations

Choosing and requesting imaging

Overview and approach

Imaging findings must be interpreted with pre-test probability and the clinical question.

  • Pre-test probability determines whether imaging is needed: use a validated clinical pathway first (see below for PE)2.
  • PE: CTPA is used to rule PE in or out; it can also show alternative thoracic diagnoses.
  • Ask a specific question: plain chest and abdominal films should be ordered to answer a defined clinical question, not as routine; non-specific abdominal pain on its own is not a reason for an abdominal X-ray6,7.
  • What an abdominal X-ray is good for: bowel obstruction, free intraperitoneal gas and radio-opaque stones. About 90% of renal stones are visible, but only about 10% of gallstones, so ultrasound is the test for gallstones; up to half of aortic aneurysms have enough wall calcium to be seen7.
Assessment and clinical features

Specify symptoms, onset, examination, relevant blood results and the question to be answered.

  • CTPA both diagnoses and excludes PE and shows other chest pathology; follow the scanner staff's breath-hold instructions.
  • Before you interpret any film: is it the right patient and date? Is it one of a series, and are older films available for comparison? What projection (PA, AP, lateral) and position (erect, supine, decubitus) was used? Was contrast given, and for what (6)?
Differentials and investigations

CTPA assesses pulmonary arteries for embolism but can also reveal alternative thoracic pathology. CT head assesses acute haemorrhage and mass effect; MRI is more sensitive for many neurological conditions.

  • Stroke: immediate non-contrast CT (or MRI) is required; CT is the rapid first test to exclude haemorrhage and identify infarction, early ischaemic change and hyperdense thrombus; CTA is added for potential thrombectomy, CT perfusion or MRI for delayed presentations4.
  • CTPA: multidetector CT shows the pulmonary arteries out to small peripheral branches in one short breath-hold. It also shows right ventricular enlargement (a marker of higher short-term mortality) and alternative diagnoses such as pneumonia, emphysema, fibrosis, a mass or aortic disease, and occasionally an incidental early lung cancer8.
  • V/Q scan: a radiolabelled particle injected into a vein lodges in the lung capillaries and maps perfusion; an inhaled radiolabelled gas maps ventilation. A perfusion defect where ventilation is normal suggests PE, while matched defects suggest airway or lung disease such as COPD or asthma. Two or more segmental perfusion defects with normal ventilation is a high-probability scan. It avoids iodinated contrast, so it is used when contrast cannot be given8; see also pregnancy below.
  • Chest X-ray in PE: often normal or nearly normal. Classic but uncommon signs are Westermark (focal oligaemia), Hampton's hump (a peripheral wedge-shaped opacity above the diaphragm) and Palla's sign (an enlarged right descending pulmonary artery)8.
  • Leg vein ultrasound: the main sign of DVT is a vein that does not collapse with gentle probe pressure. A confirmed DVT can stand in for PE because both are treated with anticoagulation, but a normal leg scan does not exclude PE: about half of patients with PE have no visible leg DVT8.
Management and follow-up

Use validated clinical pathways before ordering CTPA. Follow scanner instructions, including breath-hold when requested. Discuss equivocal or discordant results with radiology.

  • Suspected PE in pregnancy: Wells and PERC are not validated in pregnancy and D-dimer is generally unreliable, especially from the second trimester. Obtain a chest X-ray; choose V/Q if it is normal and the patient is stable, and CTPA if unstable, the X-ray is abnormal or another lung diagnosis is suspected. Do not routinely do bilateral leg ultrasound first; use it if DVT symptoms are present1,2,3.
Red flags and clinical pearls

A normal scan does not exclude every disease. Reassess persistent or deteriorating symptoms rather than offering blanket reassurance.

  • Reassess persistent or deteriorating symptoms rather than offering blanket reassurance after a normal scan.
  • A normal chest X-ray does not exclude PE, and a normal leg ultrasound does not either8.
  • Viva: how do you decide between CTPA and V/Q? What are the limits of a CT head in suspected stroke?
  • CT brain: hydrocephalus is classified as communicating (non-obstructive) or non-communicating (obstructive). Image:
References
  1. SOMANZ: Guide to diagnosis and management of pulmonary embolism in pregnancy, 2021
  2. Rural Specialist (RHCE): Suspected pulmonary embolism, medical imaging module
  3. NSW ACI: Pulmonary embolism, evaluation in the pregnant patient
  4. Australian and New Zealand Living Clinical Guidelines for Stroke Management, Chapter 2: Early assessment and diagnosis
  5. InsideRadiology (RANZCR)
  6. 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.
  7. Talley NJ, O'Connor S. Clinical Examination: A Systematic Guide to Physical Diagnosis. 7th ed. Sydney: Elsevier Australia; 2014. Chapter 16, A summary of the gastrointestinal examination.
  8. Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson JL, Loscalzo J, editors. Harrison's Principles of Internal Medicine. 18th ed. New York: McGraw-Hill; 2012. Chapter 262, Deep venous thrombosis and pulmonary thromboembolism.

Last updated from Sen’s notes on 22 October 2026.