A general-spectral method for improved quantitative SPECT imaging of indium- 111

Improved Indium-111 SPECT imaging should lead to better dose specification in radiotherapy treatment planning; however, photon scatter and down-scatter from the 245- to the 171-keV photopeak represent significant challenges for quantitative SPECT. We have extended to ¹¹¹In a general-spectral (GS) scatter-compensation method. List-mode acquisitions of a 1.6-cm-diam. sphere containing ¹¹¹In were obtained with the sphere in three different locations on the major axis of a water-filled elliptical cylinder. The sphere was then removed, the phantom was injected with ¹¹¹In, repositioned, and scanned repeatedly. High-count, list-mode data from separate sphere and background scans were rebinned into 10-keV windows from 100-280 keV. Attenuated projections, free of scatter and noise, were simulated for use in a least-squares procedure to optimize GS coefficients for a sphere activity-estimation task. Scattered photons in both photopeaks were estimated separately from synthetic noisy projections under different conditions of activity contrast and sphere location; projections were reconstructed with our joint, multi-energy OSEM program. The bias of sphere activity values estimated from projections was less than 7% for both photopeaks and for all contrast values ranging from 2:1 to 8:1; the bias (precision) of sphere activity estimated from OSEM reconstructions was less than 8.5% (3%) for all sphere positions. The GS compensation method is a promising approach for quantitative ¹¹¹In SPECT.