CT pulmonary angiogram

Overview
CT pulmonary angiogram (CTPA) is a medical diagnostic test that employs computed tomography to obtain an image of the pulmonary arteries. Its main use is to diagnose pulmonary embolism (PE).

Diagnostic use
CTPA was introduced in the 1990s as an alternative to ventilation/perfusion scanning, which relies on radionuclide imaging of the blood vessels of the lung. It is regarded as a highly sensitive and specific test for pulmonary embolism.

CTPA is typically only requested if pulmonary embolism is suspected clinically. If the probability of PE is considered low, a blood test called D-dimer may be requested. If this is negative, risk of a PE is considered negligible and CTPA or other scans are generally not performed. Most patients will have undergone a chest X-ray before CTPA is requested.

After initial concern that CTPA would miss smaller emboli, a 2007 study comparing CTPA directly with ventilation/perfusion scanning found that CTPA identified more emboli without decreasing the risk of long-term complications compared to V/Q scanning.

Contraindications
CTPA is generally avoided in pregnancy due to the amount of ionizing radiation required, which may damage the fetus.

CTPA is contraindicated in known or suspected allergy to contrast media or in renal failure (where contrast agents could worsen the renal function).

Acquisition
The best results are obtained using multidetector computed tomography (MDCT) scanners.

An intravenous cannula is required for the administration of the 50-150 ml of radiocontrast. This is injected, usually automatically, by a syringe driver, at a rate of 4 ml/second. Many hospitals use bolus tracking, where the scan commences when the contrast is detected at the level of the proximal pulmonary arteries. If this is done manually, scanning commences about 10-12 seconds after the injection has started. Slices of 1-3 mm are performed are 1-3 mm intervals, depending on the nature of the scanner (single- versus multidetector).

State of the art CT machines can complete a scan in approximately five seconds and it is possible to complete the entire procedure (set-up, injection and scanning) in the space of five minutes.

Interpretation
On CTPA, the pulmonary vessels are filled with contrast, and appear white. Any mass filling defects (embolus or other matter such as fat or amniotic fluid) appears darker. Generally, the scan should be complete before the contrast reaches the left side of the heart and the aorta, which could result in artifacts.