Reduction of injected activity for MBF with a CZT Cardiac SPECT Camera

Introduction: Myocardial blood flow (MBF) measurement can add to the diagnostic and prognostic value of relative perfusion imaging. MBF can be determined using cadmium-zinc-telluride(CZT)-based cardiac SPECT cameras which have increased sensitivity compared to conventional systems. Increased sensitivity can be used to reduce radiation exposure as long as it does not result in reduced image quality or test performance. This study uses a retrospective analysis of multi-center data to determine a lower bound on injected activity for SPECT MBF analysis.

Methods: List mode data from a multi-center trial were subsampled to create dynamic SPECT image sets with 50% reduced counts. Data were acquired on a pinhole CZT cardiac camera. The half-count images were reprocessed to calculate SPECT MBF and compared to MBF measured with the full-count images. Using an F-test, the variance of the changes in MBF were compared as a function of the injected activity which was normalized by the patient weight.

Results: 379 rest and stress tetrofosmin scans from 6 different sites were analyzed. 22 studies (12%) were acquired with a 2-day protocol using similar activity at both rest and stress. The remaining studies used a 1-day protocol with lower activity for the first scan. Analysis of the fractional difference in MBF between full- and half-count scans showed decreasing variation with increasing amounts of normalized injected activity. Dividing the activity into ranges of 0 – 5 MBq/kg (n=113), 5-10 MBq/kg (n=140), 15 – 20 MBq/kg (n=55), and >20 MBq/kg (n=51), the standard deviation in the relative difference was 24.4% (p=0.0016), 21.4% (p=0.025), 16.2% (p=0.98), and 16.2% respectively – p-values are for comparison to the > 20 MBq/kg category.

Conclusions: The variability of MBF difference between full- and half-count acquisitions increases significantly for injected activities below 10 MBq/kg suggesting a lower limit of 5 MBq/kg for SPECT MBF using this protocol.

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