🔬 CT Pulmonary Angiogram

CT Pulmonary Angiogram (CTPA) is the primary imaging modality for the diagnosis of acute pulmonary embolism. It uses IV contrast and rapid CT acquisition to visualise filling defects in the pulmonary vasculature. It is highly sensitive and specific but requires careful consideration of renal function and radiation exposure.

CTPA is indicated for the definitive diagnosis of acute pulmonary embolism (PE) in patients with a 'likely' Wells score (>4) or a positive D-dimer test. It is also used when PE is suspected in patients where V/Q scanning is unsuitable, such as those with pre-existing lung disease or abnormal chest X-rays. Clinical features triggering this include pleuritic chest pain, sudden-onset dyspnoea, tachypnoea, and haemoptysis.

CTPA is a rapid, non-invasive helical CT scan performed during the peak opacification of the pulmonary arteries. The patient is injected with iodinated contrast via a large-bore cannula (usually in the antecubital fossa) using a pressure injector. The scan timing is often 'bolus-tracked' to ensure the pulmonary trunk is optimally visualised during the first pass of contrast.

A normal result shows complete and uniform opacification of the main, lobar, segmental, and subsegmental pulmonary arteries without any filling defects. The pulmonary trunk diameter should be within normal limits (<29mm), the heart size should be normal, and the lung parenchyma should be clear with no signs of infarction or alternative pathology.

Interpretation focuses on the presence of thrombus, seen as a dark void against the bright contrast-filled vessel. Central emboli (saddle) carry the highest risk. Radiologists also assess for 'secondary signs' such as parenchymal lung changes or pleural fluid. It is vital to correlate radiological findings with the clinical Wells score and the patient's hemodynamic stability (PESI score criteria).

Key abnormal findings include intraluminal filling defects within the pulmonary arterial tree, which may be occlusive or non-occlusive. Right ventricular (RV) strain can be inferred from a dilated RV (RV:LV ratio >1.0) and bowing of the interventricular septum. Chronic pulmonary hypertension may present with enlarged central pulmonary arteries and peripheral pruning. Other findings might include pulmonary infarction (Hampton's hump) or pleural effusions.

CTPA is the 'gold standard' for diagnosing PE in the UK. Prompt diagnosis is critical as PE carries a high mortality rate if untreated. The imaging results directly dictate the need for anticoagulation or, in massive PE cases, thrombolysis. It also provides important prognostic information regarding right heart strain, which guides the level of clinical monitoring required.

False positives can occur due to 'flow artefacts' or breathing motion, which can mimic thrombus. Small subsegmental clots may be missed if the scan quality is poor. Conversely, a negative CTPA does not entirely exclude PE if the clinical suspicion is extremely high, although it is considered sufficient to stop anticoagulation in most UK protocols.

The primary limitation is the requirement for intravenous iodinated contrast, which is contraindicated in patients with severe renal impairment (e.g., eGFR <30) or a history of anaphylaxis to contrast. It also involves a significant dose of ionising radiation, which is a particular concern in younger female patients due to breast tissue exposure. Breathing artefacts can reduce diagnostic quality in tachypnoeic patients.

Students must remember the 'YEARS' algorithm and Wells score to avoid over-requesting CTPAs. Be aware of the 'triple rule out' CT scans for chest pain (PE, dissection, ACS) though these are less common. Understand the role of V/Q scans as an alternative in pregnancy or renal failure.