Abstract
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 (Eratio) on V SPECT was predictive of final normalised perfusion count rate (PCRnorm) on P SPECT using [99mTc]Tc-macroaggregated albumin (MAA), thus allowing for optimisation of P/V ratios.MethodsSame-day VP SPECT studies acquired using standard protocols in adult patients during a 2-year period (training dataset) 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. This tool was then validated using a second (validation) dataset of same-day studies acquired over a subsequent 7-month period. Accuracy of the prediction tool was assessed by calculating the mean absolute percentage error (MAPE).ResultsOf 643 studies performed, the scans of 342 participants (median age 30.4 years, 318 female) were included in the training dataset, the analysis of which yielded a significant regression equation (F(1,340) = 1057.3, p < 0.0001), with an adjusted R2 of 0.756 and MSE of 0.001089. A prediction tool designed for routine clinical use was developed for predicting final P/V ratio. Of an additional 285 studies, 198 were included in the second (validation) dataset (median age 29.7 years, 188 female). The Excel-based tool was shown to be 91% accurate (MAPE: 9%) in predicting P/V ratio.ConclusionThe relationship between the ratio of simultaneously acquired counts in adjacent energy windows on V SPECT and perfusion count rate after administration of a known activity of [99mTc]Tc-MAA can be linearly approximated. 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.
Highlights
The perfusion component of a ventilation–perfusion single-photon emission computed tomography (VP SPECT) study is typically undertaken on the same day as, and frequently immediately following the ventilation component
While software can perform correction for residual ventilation counts in the final perfusion image, slight differences in radiopharmaceutical distributions, and imperfect co-registration can lead to significant artefacts, and it is recommended that the perfusion count rate (PCR—corrected for residual ventilation counts) exceeds the ventilation count rate (VCR) by a factor of at least 3 [2,3,4]
This study aimed to investigate whether a simple ratio of counts acquired in two adjacent energy windows on the ventilation SPECT (Eratio) is predictive of the final perfusion count rate normalised for [99mTc]Tc-macroaggregated albumin (MAA) activity (PCRnorm) on the perfusion SPECT; and if so, whether such a relationship can be used to develop a simple tool to adjust either the activity of [99mTc]Tc-MAA administered or the timing of the perfusion SPECT, such that an optimal P/V ratio may be achieved
Summary
The perfusion component of a ventilation–perfusion single-photon emission computed tomography (VP SPECT) study is typically undertaken on the same day as, and frequently immediately following the ventilation component. Higher ratios may be considered undesirable since these would require high doses of the perfusion radiopharmaceutical [99mTc]Tc—macroaggregated albumin ([99mTc]Tc-MAA), which would result in higher radiation dose, and increased risks posed by the administration of a higher number of [99mTc]Tc-MAA particles. The former is especially relevant in VP SPECT studies performed to diagnose pulmonary embolism (PE) in pregnant or lactating women in whom radiation exposure poses an additional risk; while the latter is relevant especially when administering [99mTc] Tc-MAA in patients with known or occult disorders of the pulmonary vasculature.
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