Simultaneous transmission/emission imaging for SPECT using energy spectrum fitting

Energy information is used to generate emission (^99mTc, Eγ=140 keV) and transmission (⁵⁷Co, Eγ=122 keV) images from a combined projection data set. These components are separated from each other and from Compton scatter by fitting the spectra at each pixel with a simple model. The method is tested using a cylindrical phantom containing inserts with differing attention. Emission and transmission position and energy (xyE) data were acquired in list mode and later summed to simulate simultaneous acquisition. Therefore, the true emission and transmission images are known. The quality and quantitative accuracy of the technique are evaluated by comparing images extracted from the combined data set to the “true” scatter-corrected transmission and emission images, respectively. Compared to using two energy windows, the spectrum fitting technique substantially reduced crosstalk. Although the resulting bias in the emission images was typically less than a few percent, the bias in the transmission images varied from -11% to +16% as the emission activity was increased with constant transmission source activity. Also, the variance added to the emission images obtained by the fitting procedure was found to be less than the variance caused by the stochastic nature of the emission and detection processes