Accurate spill-down and scatter correction for Tc-99m/In-111 SPECT

Simultaneous Tc-99m/In-111 dual isotope single photon emission computed tomography (SPECT) provides coregistered data sets. The Tc-99m images are often used to provide anatomic information, while the In-111 labeled antibody images are used to reveal occult tumor sites. In addition to the Tc-99m and In-111 images being spatially coregistered, it would be desirable to minimize the spill-down contamination of the Tc-99m images by the In-111. For quantitative analysis, scatter reduction and attenuation correction should also be applied. Two methods of spill-down and scatter correction were compared using an anthropomorphic tub phantom containing a small Tc-99m source and a small In-111 source: 1) the convolution method, and 2) the triple energy window (TEW) method. Both methods markedly reduced the intensity of the In-111 source in the Tc-99m images. For this phantom, the convolution method accurately corrected for the spill-down, while the TEW method over-corrected slightly.