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.

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.

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 presentations (4).

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 present (1)(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.
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