ATTENUATION CORRECTION STRATEGIES FOR MULTIENERGY PHOTON EMITTERS USING SPECT

The aim of this study nas to investigate whether the photopeak window projections from different energy photons can be combined into a singl e window for reconstruction or if it is better to not combine the proj ections due to differences in the attenuation maps required for each p hoton energy. The mathematical cardiac torso (MCAT) phantom was modifi ed to simulate the uptake of Ga-67 in the human body. Four spherical h ot tumors were placed in locations which challenged attenuation correc tion. An analytical 3D projector with attenuation and detector respons e included was used to generate projection sets. Data were reconstruct ed using filtered backprojection (FBP) reconstruction with Butterworth filtering in conjunction with one iteration of Chang attenuation corr ection, and with 5 and 10 iterations of ordered-subset maximum-likelih ood expectation-maximization (ML-OS) reconstruction. To serve as a sta ndard for comparison, the projection sets obtained from the two energi es were first reconstructed separately using their own attenuation map s. The emission data obtained from both energies were added and recons tructed using the following attenuation strategies: 1) the 93 keV atte nuation map for attenuation correction, 2) the 185 keV attenuation map for attenuation correction, 3) using a weighted mean obtained from co mbining the 93 keV and 185 keV maps, and 4) an ordered subset approach which combines both energies. The central count ratio (CCR) and total count ratio (TCR) were used to compare the performance of the differe nt strategies. Compared to the standard method, results indicate an ov erestimation with strategy 1, an under-estimation with strategy 2 and comparable results with strategies 3 and 4. In all strategies, the CCR 's of sphere 4 (in proximity to the liver, spleen and backbone) were u nder-estimated, although TCR's were comparable to that of the other lo cations. The weighted mean and ordered subset strategies for attenuati on correction were of comparable accuracy to reconstruction of the win dows separately. They are recommended for multi-energy photon SPECT im aging quantitation when there is a need to combine the acquisitions of multiple windows. This work can also be extended to include other mul ti-energy photon emitters such as In-111.