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Background
Ventilation and perfusion (V/Q) lung scans are used to evaluate for pulmonary embolism (PE), and can be performed as perfusion only exams using metastable technetium macroaggregated albumin (99mTc-MAA) to evaluate pulmonary perfusion. Due to the recent international shortage of intravenous iodinated contrast, there was decreased availability of pulmonary angiograms and an increased need for lung perfusion imaging to evaluate for PE.

Educational Goals / Teaching Points
This presentation highlights pearls and pitfalls in the interpretation of perfusion-only lung imaging. Viewers of the presentation will become familiar with PISAPED interpretation criteria. Viewers will see the utility of single-photon emission computed tomography (SPECT) for detection of pulmonary emboli and evaluating for other causes of perfusion abnormalities.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Planar images are obtained using a 2-dimensional gamma camera over 30 - 45 minutes. 3-dimensional images can also be obtained and subsequently fused with low dose CT images to form SPECT/CT images. SPECT/CT increases the sensitivity (from 76 - 80% to 96 - 99%) and specificity (from 78 - 85% to 91 - 98%) for detection of PE compared to planar imaging alone. It also increases the dose from approximately 2 mSv to 3 - 7 mSv. At our institution, standard planar images are obtained for initial interpretation and if there is an apparent abnormality, then a SPECT/CT can be obtained for further characterization. The perfusion-only PISAPED criteria is essentially a binary interpretation of PE present of PE absent and had no nondiagnostic studies using the PIOPED II trial data. An examination is considered PE present if there is at least one wedge-shaped perfusion defect. An exam is considered PE absent if there are non-wedge-shaped perfusion defects or perfusion defects caused by an enlarged heart, mediastinum, the diaphragm, or underlying pulmonary abnormality. If the planar images demonstrate no peripheral wedge-shaped perfusion defects, then SPECT/CT is unlikely to be of benefit. If the planar images have non-wedge-shaped perfusion defects, SPECT/CT can help determine if there is an underlying pulmonary abnormality that causes the perfusion abnormality (pleural effusion, cardiomegaly, pneumonia, etc.). SPECT/CT increases sensitivity and specificity for detection of segmental and subsegmental PE. A peripheral wedge-shaped perfusion defect with an underlying pulmonary abnormality may represent an acute PE with associated pulmonary infarct. Planar imaging alone may be nondiagnostic, but with SPECT/CT the examination can typically be interpreted as positive or negative.

Conclusion
Perfusion-only lung imaging performed with SPECT/CT images can improve the sensitivity and specificity for the detection of PE and other pulmonary abnormalities. Understanding key pearls and pitfalls in the interpretation of these studies is critical for the practicing radiologist and nuclear medicine physician.