From the Guidelines
To rule out pulmonary embolism (PE), a CT pulmonary angiogram (CTPA) is the recommended imaging study in patients with high pretest probability of PE, as stated by the American College of Physicians 1. This specialized CT scan uses intravenous contrast to visualize the pulmonary arteries and detect filling defects caused by blood clots. The patient should have an IV placed prior to the scan, and renal function should be checked before administering contrast. Patients should be instructed to hold their breath briefly during the scan to minimize motion artifact.
Key Considerations
- CTPA is preferred over other imaging modalities because it provides high sensitivity (85-90%) and specificity (>95%) for detecting PE, while also allowing visualization of alternative diagnoses that may explain the patient's symptoms.
- For patients with contrast allergies or renal insufficiency, a ventilation-perfusion (V/Q) scan may be considered as an alternative, though it is less specific than CTPA.
- D-dimer testing may be used prior to imaging to help determine if CTPA is necessary, particularly in patients with low clinical probability of PE, as recommended by the American College of Physicians 1.
Diagnostic Approach
- In patients with low pretest probability of PE, the Pulmonary Embolism Rule-Out Criteria (PERC) should be applied, and if all criteria are met, no further testing is needed 1.
- In patients with intermediate pretest probability of PE, a high-sensitivity d-dimer measurement should be obtained as the initial diagnostic test, and if positive, imaging studies should be performed 1.
- In patients with high pretest probability of PE, imaging with CTPA should be obtained, and V/Q scans should be reserved for patients with contraindications to CTPA or if CTPA is not available 1.
From the Research
CT Scan for Ruling Out Pulmonary Embolism (PE)
- To rule out PE, Computed Tomography Pulmonary Angiography (CTPA) is the imaging test of first choice 2.
- CTPA has been shown to have a high sensitivity and specificity for the diagnosis of pulmonary embolism 2.
- The presence or absence of pulmonary embolism can be determined with sufficient certainty without the need for additional imaging tests after a negative CTPA 2, 3.
- A normal CTPA result alone can safely exclude PE in all patients in whom CTPA is required to rule out this disease 3.
- There is no need for additional ultrasonography to rule out VTE in patients with a normal CTPA result 3.
Alternative Imaging Modalities
- Ventilation-perfusion (VQ) scans can be used as an alternative imaging technique for diagnosing PE in those where CTPA is contraindicated 4.
- Compression ultrasonography is reserved for patients with concomitant symptomatic deep vein thrombosis 2.
- Magnetic resonance pulmonary angiography is not a suitable alternative for CTPA 2.
CTPA Protocol
- A CTPA protocol with low tube voltage (80 kVp) and low-iodine-concentration contrast agent (270 mg I/ml) can achieve equally subjective image quality and a positive rate for diagnosing PE, with a 63.6% decrease in radiation dose and a 22.9% reduction in iodine content of contrast agent 5.
- The use of iso-osmolar contrast agent may improve vascular opacification in CT venography after CT pulmonary angiography 6.