What is Computed Tomography Pulmonary Angiography (CTPA) in a patient with suspected pulmonary embolism?

What is CTPA (Computed Tomography Pulmonary Angiography)?

CTPA is a specialized CT scan using intravenous contrast to directly visualize blood clots in the pulmonary arteries, and it is now the recommended first-line imaging test for diagnosing pulmonary embolism in hemodynamically stable patients. 1, 2

Technical Definition and Methodology

• CTPA uses multidetector CT scanning with thin-slice imaging (2-3 mm slice thickness) to visualize pulmonary vessels from the main pulmonary arteries down to the subsegmental level 2
• The test requires intravenous iodinated contrast material timed precisely to opacify the pulmonary arterial tree, with images viewed at workstations using multiplanar reformations and 3D renderings 1, 2
• Meticulous attention to contrast timing is essential to achieve diagnostic quality images comparable to published series 1

Primary Diagnostic Role

• CTPA is the recommended initial lung imaging modality for non-massive pulmonary embolism when PE cannot be ruled out by clinical decision tools (Wells score, Geneva score) and D-dimer testing 1, 2
• The test directly demonstrates intravascular thrombus as filling defects within contrast-enhanced pulmonary arteries 1
• CTPA can also show secondary effects of PE including wedge-shaped opacities from pulmonary infarction and characteristic right ventricular changes indicating RV strain 1

Diagnostic Performance

• CTPA demonstrates excellent accuracy with sensitivity of 83-99% and specificity of 96-100% for detecting pulmonary embolism 2, 3
• The test reliably identifies proximal clot in 94-96% of cases, though subsegmental clot detection remains less reliable 1
• Patients with a good quality negative CTPA do not require further investigation or treatment for PE, with only 1.1% recurrence rate at 3 months—comparable to the 0.9% recurrence after negative conventional angiography 1, 2
• The negative predictive value is 96% in patients with low or intermediate clinical probability 2

Key Clinical Advantages

• CTPA is readily available 24/7 in most medical centers with short acquisition times, allowing rapid diagnosis in emergency settings 2, 3
• When PE is excluded, CTPA identifies alternative diagnoses in a substantial proportion of patients, including pneumonia, pulmonary edema, aortic dissection, pericarditis, and malignancy 1, 2, 3
• The test provides prognostic information through assessment of RV/LV diameter ratio, pulmonary artery diameter, and septal deviation that correlates with clinical severity and short-term outcomes 2, 4, 5
• CTPA is superior in specificity to ventilation-perfusion (V/Q) scanning and allows quantitative assessment of clot burden 1, 4

Important Limitations and Contraindications

• CTPA involves radiation exposure of 3-10 mSv with significant breast tissue exposure, particularly concerning in young women 2
• The test requires iodinated contrast, creating contraindications in patients with iodine allergy, hyperthyroidism, or severe renal failure (eGFR <30 mL/min) 2
• Pregnancy and breastfeeding require careful risk-benefit consideration, with V/Q scanning often preferred in these populations 1, 2
• A small proportion of examinations (3-5%) are technically unsatisfactory due to poor contrast timing, motion artifact, or body habitus 1, 2

Clinical Application Algorithm

• For patients with high pretest probability (Wells score >4 or revised Geneva score indicating high risk), proceed directly to CTPA without D-dimer testing, as a negative D-dimer will not obviate the need for imaging 6
• For patients with low or intermediate pretest probability, obtain high-sensitivity D-dimer first; if elevated, proceed to CTPA 6, 7
• In hemodynamically unstable patients with suspected massive PE, perform bedside echocardiography if CTPA is not immediately available or the patient is too unstable for transport 6
• For patients with contraindications to CTPA (renal failure, contrast allergy, pregnancy), consider V/Q scanning as an alternative, though it is diagnostic in only 30-50% of cases 6, 7

Critical Pitfalls to Avoid

• Do not use D-dimer as a screening test in patients with intermediate-to-high probability, as it has low negative predictive value in this population and delays definitive diagnosis 6
• Do not dismiss subsegmental PE findings without clinical correlation, though their clinical significance remains controversial 2
• Recognize that interobserver agreement is good even with relatively inexperienced assessors, but familiarity with interpretation pitfalls (such as lymph nodes, motion artifact, and flow-related artifacts) is essential 1
• In high probability patients with negative CTPA where clinical suspicion persists, consider additional testing such as lower extremity venous ultrasound before definitively ruling out PE, as false-negative rates can range from 5.3% to 40% in this population 6