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A bstractWe have performed a quantitative SPECT study in a porcine model of myocardial perfusion with the correlated X-ray CT traiismission and SPECT emission (ETCT) system developed at UCSF. Pixel values in the reconstructed SPECT images were calibrated in units of activity concentration and compared with the activity concentration determined from corresponding excised myocardial tissue samples. An object specific attenuation map was obtained by linear scaling from the X-ray CT. Attenuation maps with lower spatial resolution and higher signal to noise ratio were also derived from the X-ray CT image. The pixel Values and intensity distribution in reconstructed myocardial SPECT image was not sensitive to spatial resolution or noise in the transmission map for the given examples. An MLEM algorithm with regularization was used to determine mean and maximum pixel values for ROIs in the myocardial SPECT images. The use of the maximum pixel value did not sufficiently compensate for contrast degradation due to partial volume errors. A separate measurement of the myocardial wall thickness was used to adequately correct for partial volume errors.

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We have developed an emission-transmission CT (ETCT) system capable of both single-photon emission computed tomography (SPECT) imaging and x-ray transmission CT imaging using a common photon counting detector. In principle, SPECT and x-ray CT projection data can be acquired simultaneously with the ETCT system; however, doing so results in contamination of the SPECT projection data due to pulse pile-up caused by the relatively high x-ray fluence rate. In this study, we characterize the effects of pulse pile-up for simultaneous ETCT imaging through computer simulation and experimental studies. We demonstrate that pulse pile-up in the SPECT energy window can be well approximated by a simple quadratic relationship between the pile-up rate and the x-ray fluence rate for sufficiently small x-ray fluence rates. Using this quadratic relationship, we developed a simple pile-up correction scheme that subtracts the pile-up counts from the emission data and also truncates the exterior regions of the emission projection data. Analysis of difference images and profiles indicate that this method permits us to reconstruct SPECT images with no apparent noise or resolution degradation in comparison to those obtained via sequential emission and transmission scans.