Dual Energy Imaging for Optimisation of P/V Ratios in VP SPECT

PurposeVentilation-perfusion single photon emission computed tomography (VP SPECT) plays an important role in pulmonary embolism diagnosis. Rapid results may be obtained using same-day ventilation followed by perfusion imaging, but generally requires careful attention to achieving an optimal count rate ratio (P/V ratio) of ~3:1. This study investigated whether the ratio of counts simultaneously acquired in adjacent primary and Compton-scatter energy windows (E_ratio) on V SPECT was predictive of final normalised perfusion count rate (PCRnorm) on P SPECT using [99mTc]Tc-macroaggregated albumin (MAA), allowing optimisation of P/V ratio. MethodsSame-day VP SPECT studies acquired using standard protocols in adult patients during a 2-year period were assessed. Studies were included provided they were acquired with correct imaging parameters, and injection site imaging and laboratory records were available for quality control and normalised count rate corrections. Extraction of DICOM information, and linear regression were performed using custom Python and R scripts. A predictive tool was developed in Microsoft Excel.ResultsOf 643 studies performed, the scans of 343 participants (median age 30.4 years, 318 female) met inclusion criteria. A single outlier with high influence was excluded after an obvious cause was identified. Final analysis of the remaining 342 scans yielded a significant regression equation (F(1,340) = 1057.3, p <.000), with an adjusted R2 of .756 and MSE of 0.001089. A prediction tool designed for routine clinic use was developed for predicting final P/V ratio.ConclusionThe ratio of simultaneously acquired counts in adjacent energy windows on V SPECT is linearly related to perfusion count rate after administration of a known activity of [99mTc]Tc- MAA. A predictive tool based on this work may assist in optimising the dose and timing of [99mTc]Tc-MAA administration in same-day studies to the benefit of patients and workflows.