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 da ta The X-ray and gamma-ray data were obtained from a third-generation X-ray CT scanner and a single-headed SPECT scanner juxtaposed to enabl e systematic registration of SPECT and X-ray CT images. CT measurement s were obtained from known materials at a given X-ray technique, then related to attenuation coefficients at 140 keV. The resulting calibrat ion data were used to generate attenuation maps for the coregistered S PECT data. In addition, phantom studies provided recovery coefficients as a function of object size to compensate for partial volume effects in the coregistered radionuclide data. This technique has been applie d to a torso phantom, yielding attenuation maps with an average accura cy error < 1%. Reconstruction of the SPECT image without attenuation c orrection underestimated the regional myocardial activity errors with an average accuracy error of -90%. Reconstruction with attenuation cor rection reduced the average accuracy error to -37%, but addition of a partial volume correction overestimated the regional myocardial activi ty concentration by an average error of 94%. The technique has also be en applied to small pigs in myocardial perfusion studies by correlatin g the image-derived myocardial activity concentrations with those obta ined directly from excised tissue samples. Correcting for attenuation increased the slope of the regression line from 0.16 to 0.36 (compared with an ideal slope of 1.00.) Adding an additional correction for par tial volume effects improved the slope of the regression line to 0.93. The absolute precision error (standard error of the estimate) obtaine d without correction (2.14 mu Ci/ml) was degraded by attenuation corre ction (4.23 mu Ci/ml) and was also degraded by adding a correction for partial volume errors (10.89 mu Ci/ml), although the relative precisi on error without correction (31.2%) was improved by attenuation correc tion to 27.6% when the images were reconstructed with attenuation corr ection alone or with a combination of attenuation correction and compe nsation for partial volume effects.