Attenuation correction of SPECT using X-ray CT on an emission-transmission CT system: Myocardial perfusion assessment

We present experimental results for attenuation correction of emission tomography through use of an attenuation map derived from X-ray CT data. Both the X-ray and gamma-ray data were obtained on a prototype emission-transmission imaging system ("X-SPECT") which incorporates a third-generation X-ray CT scanner and a single-headed SPECT scanner. The two commercial imaging devices are juxtaposed such that the CT table can move the patient directly into the SPECT scanner after the X-ray scan. This enables systematic registration of SPECT and X-ray CT images. To calibrate attenuation map creation, a set of CT scans for a specific phantom geometry and X-ray technique were acquired, with varying attenuating materials in each scan. A piecewise linear fit was applied to the CT numbers and known attenuation coefficients at 140 keV, and the result is used to create attenuation maps at 140 keV from given CT images. This technique has been applied to small pigs in myocardial perfusion studies. Correcting for attenuation increased the image-derived myocardial radionuclide concentration values from 16% of the values observed for excised myocardium to 35% (ignoring background). Adding an additional correction for partial volume effects increases the image-derived value to 93% of the expected value (ignoring background). Precision, however, remains limited.