Transmission attenuation map with measured downscatter correction

As the attenuation correction becomes clinical reality in SPECT, accurate estimation of emission downscatter to transmission during simultaneous emission-transmission acquisition becomes critical. In this study, we proposed a technique that directly measures the downscatter contamination from 140-keV emission photons to the 100-keV transmission window. The technique combines energy window discrimination and spatial window discrimination to separate emission (EM), transmission (TR) and downscattered (DS) photons. A spatial mask (S) aligned with the opposing scanning line source and a gap (G) at each end of S are first defined. Photons detected inside the S belong to TR if their energy fall inside the 100 keV window; those detected outside the S and G belong to either EM if their energy inside the 140 keV window, or DS if they are inside the 100 keV window. The TR is corrected from the downscatter contamination via subtraction a fraction of the DS. The effectiveness of the technique is evaluated based on the uniformity of the ROI in the reconstructed attenuation map from a series of phantom experiments. The technique is compared with another technique using an adjacent energy window measurement. For typical cardiac studies, the uniformity of the transmission map from both techniques are comparable in the heart area, while for the cardiac study with high concentration present in the gall bladder, the uniformity is significantly improved from the proposed technique.