From the Guidelines
For patients at high risk for contrast dye reactions who need evaluation for pulmonary embolism (PE), ventilation-perfusion (V/Q) scanning is the recommended alternative diagnostic approach. This nuclear medicine test evaluates both airflow and blood flow in the lungs without using iodinated contrast. For patients with normal chest X-rays, V/Q scanning has good sensitivity and specificity for PE diagnosis, as noted in the 2019 ESC guidelines for the diagnosis and management of acute pulmonary embolism 1.
Key Considerations
- Lower extremity venous ultrasound can also be helpful as a complementary test, as finding a deep vein thrombosis (DVT) in a patient with symptoms suggestive of PE can justify anticoagulation treatment without directly confirming the PE.
- In some centers, non-contrast MRI pulmonary angiography may be available, though it's less commonly used and has lower sensitivity than contrast-enhanced CT.
- D-dimer testing can be useful in low to moderate risk patients to rule out PE if negative, but has limited utility in high-risk patients due to its low specificity, as discussed in the evaluation of patients with suspected acute pulmonary embolism by the American College of Physicians 1.
- Clinical prediction rules like the Wells score or Geneva score should be incorporated into the diagnostic approach to guide testing decisions, as recommended by the European Heart Journal 1.
Diagnostic Approach
The ultimate goal is to balance the risk of missing a potentially fatal PE against the risk of contrast-induced complications in vulnerable patients. According to the NCCN Clinical Practice Guidelines in Oncology for Cancer-Associated Venous Thromboembolic Disease, Version 2.2024, the preferred imaging technique for the initial diagnosis of PE is CTA, but alternative imaging modalities such as V/Q scan should be considered in patients with contraindications to CTA 1.
Recommendations
- Use V/Q scanning as the alternative diagnostic approach for patients at high risk for contrast dye reactions who need evaluation for PE.
- Incorporate clinical prediction rules and D-dimer testing into the diagnostic approach to guide testing decisions.
- Consider lower extremity venous ultrasound as a complementary test to justify anticoagulation treatment without directly confirming the PE.
From the Research
Diagnostic Approaches for Pulmonary Embolism
- Computed Tomographic Pulmonary Angiography (CTPA) is considered the most sensitive test for diagnosing pulmonary embolism, but it involves significant radiation exposure 2.
- Ventilation-Perfusion (V/Q) scanning is an alternative diagnostic approach that offers a lower risk of radiation exposure, particularly suitable for certain patient populations 2, 3.
- Single Photon Emission Computed Tomography (SPECT) V/Q scanning has emerged as a new diagnostic modality with improved diagnostic performances compared to planar V/Q scanning 3.
Considerations for High-Risk Patients
- For patients at high risk for dye and requiring exclusion of pulmonary embolism, diagnostic algorithms using V/Q scanning or SPECT V/Q scanning may be preferred due to lower radiation exposure and risk of contrast dye 2, 3.
- The choice of diagnostic approach should be guided by clinical scoring systems and D-dimer testing to enhance the yield of diagnostic testing 2.
- In cases where CTPA is not feasible due to high risk of dye, V/Q scanning or SPECT V/Q scanning can be used as alternative diagnostic modalities 4, 5.
Diagnostic Accuracy and Safety
- Negative multidetector CTPA study results can essentially rule out the diagnosis of pulmonary embolism without the need for additional tests 2.
- Diagnostic algorithms using V/Q scanning or SPECT V/Q scanning have been shown to be safe and effective in excluding pulmonary embolism 2, 3.
- The accuracy of spiral CT in diagnosing pulmonary embolism has been demonstrated to be higher than V/P scanning, with high sensitivity and specificity 6.