Pulmonary embolism (PE) is a common form of venous thromboembolism in the middle-aged and older adult population . PE can be rapidly fatal especially in the frail and elderly . Those who survive are at risk of serious long-term complications including recurrent thromboembolism and chronic thromboembolic pulmonary hypertension. Treatment often includes chronic anticoagulation, which in itself is not without significant risk. Fast and reliable diagnosis is, therefore, essential. The clinical presentation of acute PE, however, is often nonspecific. Even after the introduction of D-dimer testing to screen out low risk patients, the majority of patients suspected of PE undergo imaging to establish the diagnosis .
Pulmonary artery computed tomography angiography (PA-CTA) is now the well-established gold standard imaging test for the diagnosis of PE . An optimal quality PA-CTA study will provide peak enhancement of the pulmonary arteries while reducing artifact related to cardiac and respiratory motion. Modern scanners provide submillimeter resolution with near-isotropic images that lend themselves to excellent two- and three-dimensional reconstructions and give us the ability to detect thrombi below sixth-order pulmonary artery branches . Improvements in image quality have, however, come at the cost of higher radiation dose, a significant concern given the widespread use of PA-CTA, and also make images more susceptible to motion artifact.
Strategies to reduce radiation exposure and minimize artifact include improved reconstruction algorithms, high-pitch scanning, peak kilovoltage (keV) reduction, and using magnetic resonance pulmonary artery angiography, which may be performed without intravenous contrast and does not expose the patient to ionizing radiation . These techniques will be briefly reviewed here.
The patient population most commonly in need of evaluation for PE (the elderly in respiratory distress) is also the most likely to have impaired respiratory function and is often unable to cooperate with breath-holding instructions. This will often produce artifactual filling defects in the pulmonary arteries that can be mistaken for emboli either directly due to motion or due to poor mixing of blood and contrast from transient interruption of the contrast column . To a certain degree, this can be addressed by coaching the patient prior to the scan with careful breath-hold instructions. Reducing scan time, however, will significantly improve image quality in patients who cannot cooperate with breath-hold .
High-pitch scanning has been in use mainly in cardiac imaging and in pediatric patients due to faster scan times and a significant reduction of radiation exposure . With older, single-detector systems, the maximum pitch is limited to 1.5 to ensure gapless coverage along the scan axis. Helical pitch of higher than 3 has become possible using dual-detector systems. In recent studies, high-pitch scanning has shown promise in routine chest CT with a marked reduction of artifact from cardiac and diaphragmatic motion while maintaining diagnostic image quality even in patients who are free breathing . Both dose reduction and reduction in scan times are maximized when dual-detector systems are used.
Another concern with PA-CTA is the need for intravenous contrast, which in a significant subset of patients cannot be safely administered due to impaired renal function. Some patients with renal impairment may be scanned with a reduced contrast dose; this, however, often results in suboptimal scan quality due to decreased opacification of the pulmonary arteries. Contrast dose also may be significantly reduced with a reduced keV technique, which increases intravascular contrast resolution because iodine resorption is inversely proportional to tube potential. Consequently, administering a lower dose of contrast will not reduce diagnostic quality for PE. Radiation dose savings in the order of 30%–50% also result when keV is reduced, at the cost of modest increases in image noise .
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