Does a computed tomography (CT) scan of the chest for pulmonary embolism require a CT chest angiogram or a chest scan with contrast?

From the Guidelines

To diagnose pulmonary embolism (PE), a CT pulmonary angiogram (CTPA) with contrast is specifically required, not just a routine chest CT with contrast. This is because a CTPA uses specialized timing and technique to visualize the pulmonary arteries when they are optimally filled with contrast, allowing for detection of filling defects caused by clots 1. The contrast injection protocol differs from standard chest CT with contrast, as it requires precise bolus timing and often higher injection rates to adequately opacify the pulmonary arterial system. Regular chest CT with contrast is not optimized for pulmonary artery visualization and may miss pulmonary emboli, particularly smaller ones in subsegmental arteries.

When ordering imaging to evaluate for PE, clinicians should specifically request a "CT pulmonary angiogram" or "CTPA" rather than simply "chest CT with contrast" to ensure the appropriate protocol is used. This distinction is important because the technical parameters of the scan (including contrast timing, injection rate, and image acquisition) are specifically tailored to evaluate the pulmonary arterial system in a CTPA. According to the NCCN guidelines, CT angiography (CTA) with contrast is the recommended imaging modality for evaluating patients suspected of having acute PE 1.

Some key points to consider when evaluating for PE include:

• Clinical suspicion of pulmonary embolism (PE) based on symptoms such as unexplained shortness of breath, chest pain, tachycardia, apprehension, or tachypnea 1
• Comprehensive medical history and physical examination, including laboratory tests such as CBC with platelet count, PT, aPTT, liver and kidney function tests, and N-terminal prohormone B-type natriuretic peptide (NT-proBNP)/troponin 1
• Imaging modalities such as CT angiography (CTA) with contrast, X-ray pulmonary angiography with contrast, magnetic resonance angiography (MRA) with contrast, or ventilation/perfusion (VQ) scan if CTA is contraindicated 1

It's worth noting that while regular chest CT with contrast may not be optimized for pulmonary artery visualization, incidental PE can be accurately diagnosed on routine chest CT with contrast 1. However, this should not replace a dedicated CTPA when PE is suspected.

In terms of radiation exposure and safety, the relative radiation level (RRL) of CTA chest with IV contrast is higher than other imaging modalities, but it is still considered a necessary test when PE is suspected 1. Ultimately, the choice of imaging modality should be based on the individual patient's clinical presentation and medical history, as well as the availability of imaging modalities at the institution.

From the Research

CT Scan Requirements for Pulmonary Embolism

• A computed tomography (CT) scan of the chest for pulmonary embolism typically requires a CT chest angiogram, also known as CT pulmonary angiography (CTPA) 2, 3, 4.
• CTPA is regarded as the preferred imaging method in diagnosing pulmonary embolism (PE) and involves the use of contrast material to visualize the pulmonary vasculature 2, 3.
• The use of low tube voltage and low-iodine-concentration contrast agent in CTPA protocols has been shown to achieve equally subjective image quality and a positive rate for diagnosing PE, while reducing radiation dose and iodine content of contrast agent 2.
• CT angiography of pulmonary embolism has diagnostic criteria that include partial and complete intraluminal filling defects, which should have a sharp interface with intravascular contrast material 4.
• Factors that cause misdiagnosis of pulmonary embolism may be patient related, technical, anatomic, or pathologic, and the radiologist needs to determine the quality of a CT pulmonary angiographic study and whether pulmonary embolism is present 4.

Alternative Diagnostic Approaches

• Ultrasound assessment has been studied as a potential alternative to CTPA for diagnosing pulmonary embolism, with a bedside ultrasound protocol showing promise in predicting the need for CTPA 5.
• The accuracy of CT in the diagnosis of pulmonary embolism has been systematically reviewed, with combined sensitivities ranging from 66% to 93% and combined specificities ranging from 89% to 97% 6.
• However, the use of CTPA remains the gold standard for diagnosing pulmonary embolism, and alternative diagnostic approaches should be used in conjunction with, rather than in place of, CTPA 3, 4, 6.