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A ventilation/perfusion (V/Q) scan is a nuclear medicine procedure that uses radioactive material to assess the flow of air and blood in the lungs. It is used by physicians to detect PE, which is a blood clot in the lungs.
Compared with CTPA, the breast radiation dose from V/Q is approximately 40 times less in women and 90 times less in pregnant women.2 That’s why the American Thoracic Society recommends V/Q over CTPA in pregnant patients with a normal chest X-ray.3 The radiation from CTPA is particularly a concern for young women because the breast tissue is particularly radiosensitive due to its higher metabolic activity,4 which is why the Society for Nuclear Medicine and Molecular Imaging recommends avoiding CTPA in young women with a low-pretest probability of PE or a negative D-dimer.5
*Estimated CTPA radiation exposure to gravid breast ranges from 1000–2000 mrad, compared to 22–28 mrad from V/Q scan.2
The Society for Academic Emergency Medicine has advocated for evidence-based clinical decision rules to optimize CT utilization, stating that, “Research is needed to refine the indications for CT and to identify the scenarios in which alternatives to traditional CT are most appropriate … and when CT can be avoided without adversely affecting patient-centered care.”6
*Validated at one of the busiest emergency departments in the US.
†Unstable patients undergo CTPA.
‡For PE alone.
An algorithm that incorporates V/Q scans to evaluate patients for PE in the emergency department has been shown to reduce overall patient radiation exposure by 20%.1 This approach uses chest radiography as an initial step to triage all suspected PE cases. Patients with a normal X-ray result underwent a V/Q scan, and patients with abnormal results received a CTPA. In the same study involving more than 2,000 patients at one hospital that employed this algorithm showed that the false-negative rates were 0.5% and 0.6% for V/Q and CTPA respectively, demonstrating that there was no statistical difference inoutcomes for both procedures.1,2 The use of other preliminary diagnostic criteria/ triage variables, such as validated prediction rules for risk stratification and screening with D-dimer assay, can be effectively utilized as well to diagnose PE while minimizing risk of unnecessary radiation burden.7
Although CTPA has been demonstrated to be more sensitive than V/Q, with a false-negative rate of 0.4% vs 1%, respectively, during 3 months of follow-up, its use inadvertently resulted in a significantly greater number of venous thromboembolism diagnoses than did V/Q (19.2% vs 14.2%; P=0.01).8 There is evidence that these minor, subsegmental PEs may not be clinically relevant as a diagnosis or for therapy. Three epidemiological studies found that despite detecting twice as many PEs as V/Q, CTPA did not reduce patient mortality.9-11 By detecting clinically insignificant PEs, CTPA may cause patients to undergo unnecessary tests and treatments—such as anticoagulation therapy—increasing the risk for complications and the total cost of care. Available data, in fact, indicate that patients diagnosed with subsegmental PEs via CTPA are more likely to experience complications from anticoagulation than adverse outcomes from recurrent thromboembolism.12,13
Recognizing the risk associated with overdiagnosis and overtreatment, the American College of Chest Physicians recommends clinical surveillance over anticoagulation in patients with a subsegmental PE (no involvement of more proximal pulmonary arteries) and no proximal deep vein thrombosis in the legs who have a low risk for recurrent venous thrombolism.14
Several decades ago, ventilation-perfusion (V/Q) scan interpretation of pulmonary embolism was performed using probability-based assessments: normal, very low probability and low-probability. Although this approach served well the technology at the time,it was not well-understood by many clinicians. With the development of more advanced imaging equipment and new technology, the Nuclear medicine community is evolving in the diagnostic approach of PE.
Recent advancements allow physicians to obtain the same degree of interpretive clarity with a V/Q scan as they can with CTPA. Some hospitals have implemented a simplified interpretative scheme for V/Q that mimics that used with CTPA—PE-absent, PE-present, and nondiagnostic. Adoption of this trinary V/Q reporting system has been shown to produce a comparable false-negative rate compared with the traditional, probability-based interpretive scheme.15 More importantly, the trinary V/Q reporting system also facilitated communication of V/Q results with referring physicians, and radiology and nuclear medicine residents have expressed much greater comfort interpreting V/Q studies.15 With minimal training, an experienced radiologist using this interpretation scheme can read a V/Q study in just a few minutes.
Findings consistent with PE: If this finding is discordant with objective clinical assessment, further investigation is warranted
No evidence of PE: If this finding is discordant with objective clinical assessment, further investigation is warranted
Nondiagnostic study: Further study with V/Q (CTPA) may be helpful