A physical phantom evaluation of absolute quantitation of simultaneous Tc-99m/In-111 SPECT

In dual-radionuclide ^99mTc/¹¹¹In SPECT imaging, the contamination of ¹¹¹In photons into ^99mTc energy window can destroy both the quality and accuracy of the ^99mTc image. In this study, we proposed a model-based reconstruction strategy for practical and quantitatively correct ^99mTc/¹¹¹In down-scatter compensation. In our approach, instead of time-consuming modeling of down-scatter, we re-scaled the second-order scatter distribution, analytically (using Klein-Nishina formula) calculated for the ¹¹¹In photopeak. Specifically, our algorithm includes the following steps: (i) Quantitative reconstruction of the ¹¹¹In image (with corrections for attenuation, resolution loss and with analytically modeled self-scatter) from the energy window positioned around 245keV photopeak; (ii) Calculation of the spill-down component (SDC) using extrapolation of the calculated in the previous step noiseless distributions of second-order self-scatter; (iii) Quantitative reconstruction of ^99mTc with a SDC included in the forward step of the OSEM algorithm. A series of experiments with a clinical hybrid SPECT/CT system (Infinia Hawkeye, GE Healthcare) were designed to examine how accurately our algorithm can reconstruct the ^99mTc activity from the projection data containing various levels of ¹¹¹In contamination. We used four 30ml plastic containers with different ratios of ^99mTc and ¹¹¹In activities placed inside the Jaszczak phantom. The absolute activities for these containers were reconstructed with errors 0.8-7.0% (¹¹¹In) and 1.6-13.1% (^99mTc). Especially, in one of our experiments, the developed SDC compensation technique decreased the absolute activity errors from 44.8% to 8.2% (container #1 filled with ^99mTc activity only); from 61.5% to 1.6% (container #2 with the ratio of 2:1 between ^99mTc and ¹¹¹In concentrations), and from 132.8% to 13.1% (container #3 with the ratio of 1:2 between ^99mTc and ¹¹¹In concentrations).