Perfusion Only Scans with and without SPECT/CT in the Era of COVID-19

1390 Introduction: Due to the risk of COVID infection, the European Association of Nuclear Medicine and the Society of Nuclear Medicine and Molecular Imaging have both issued warnings against performing lung ventilation studies with aerosolized agents. Therefore, the use of perfusion-only studies, with- and without SPECT/CT have supplanted more conventional lung ventilation and perfusion studies for various indications, including the diagnosis of pulmonary embolism. There is a paucity of data supporting lung perfusion only studies with- and without SPECT/CT. The aim of our study is to systematically evaluate the diagnostic accuracy of lung perfusion-only scans, and perfusion combined with adjunctive SPECT/CT studies. Methods: Perfusion-only scans were performed in consecutive patients from March 1 - August 31, 2020, using 2-4 mCi (for body mass index below or above 35 respectively) of 99mTc-labeled macroaggregated albumin particles administered intravenously over 3-5 slow deep breaths. Conventional anterior/posterior, and bilateral anterior and posterior oblique planar images were acquired using a gamma camera with a parallel hole collimator. In 30 cases (32.3%) SPECT-CT was performed adjunctively using a NM/CT 870 CZT or Discovery670 Pro scanners (GE Healthcare). Patient age, sex, BMI, D-dimer levels, pretest probability (Wells score), and any results of adjunct imaging tests such as CTA were recorded from the electronic medical record. Diagnostic parameters including sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy were calculated for perfusion only studies, and perfusion-only studies with SPECT-CT using the final clinical diagnosis on which the patient was treated (pulmonary embolism / other diagnosis to account for symptoms) as the gold standard. Results: 93 patients (40 M, 53 F), mean age 60.4 years, and mean Wells score 3.8 (moderate risk) were enrolled retrospectively. The positivity rate was 7.5% (13.3% for SPECT/CT). 4.3% of the patients (and 10.0% of patients with SPECT-CT) had a non-diagnostic or equivocal exam. 88.2% of the patients had a normal study. Perfusion-only studies (n=93) and perfusion-only studies performed with SPECT/CT (n=30) demonstrated sensitivities of 66.7% and 100%, specificities of 95.3% and 88.5%, positive predictive value of 50% and 57.1%, negative predictive value of 97.7% and 100%, and diagnostic accuracy of 93.7% and 90.9%, respectively. Conclusion: Perfusion-only studies performed with the addition of SPECT/CT increases the overall sensitivity for detecting pulmonary embolism, with excellent negative predictive value. Given the tradeoff of detecting more false negatives for more false positives, and the cost of performing SPECT/CT, the utilization of SPECT/CT may be selectively limited to higher-risk patients with high-pretest probability or equivocal planar imaging. These findings suggest a nuanced role for SPECT-CT during the COVID-19 era where the ability to perform concurrent ventilation scans is limited. Figure 1. Planar perfusion images in the anterior, posterior (A-B), left anterior oblique, right posterior oblique (C-D), right anterior oblique, and left posterior oblique (E-F) orientations demonstrate a segmental perfusion defect of the lateral segments of the right middle lobe. Figure 2. SPECT-CT imaging more clearly demonstrates the right middle lobe segmental defect (E-G).