Myocardial perfusion imaging with a combined x-ray CT and SPECT system.

UNLABELLED: We evaluated a novel combined x-ray CT and SPECT medical imaging system for quantitative in vivo measurements of 99mTc-sestamibi uptake in an animal model of myocardial perfusion. METHODS: Correlated emission-transmission myocardial images were obtained from 7- to 10-kg pigs. The x-ray CT image was used to generate an object-specific attenuation map that was incorporated into an iterative ML-EM algorithm for reconstruction and attenuation correction of the coregistered SPECT images. The pixel intensities in the SPECT images were calibrated in units of radionuclide concentrations (MBq/g), then compared against in vitro 99mTc activity concentration measured from the excised myocardium. In addition, the coregistered x-ray CT image was used to determine anatomical boundaries for quantitation of myocardial regions with low perfusion. RESULTS: The accuracy of the quantitative measurement of in vivo activity concentration in the porcine myocardium was improved by object-specific attenuation correction. However, an additional correction for partial volume errors was required to retrieve the true activity concentration from the reconstructed SPECT images. CONCLUSION: Accurate absolute SPECT quantitation required object-specific correction for attenuation and partial volume effects. Additional anatomical information from the x-ray CT image was helpful in defining regions of interest for quantitation of the SPECT images.

Characterization and correction of pulse pile-up in simultaneous emission-transmission computed tomography.

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.