Absolute in vivo quantitation of myocardial activity

Quantitation of myocardial SPECT images corrected for attenuation underestimates the true radionuclide content due to partial volume errors. To measure radionuclide uptake more accurately, we have developed a technique to compensate these images for partial volume errors using coregistered Xray CT images. The CT image is used to define a template that approximates the geometrical extent of the myocardium. Once defined, the template is assigned unit activity and is mathematically projected using a realistic physical model of the radionuclide imaging process. These projections are then reconstructed and used to compensate the SPECT image for partial volume errors. The method was tested in a porcine model of myocardial perfusion using Tc-99m sestamibi. With attenuation correction alone, the in vivo activity concentration in the porcine myocardium had an error in the range -40% to -60% compared to the true activity concentration. By also correcting for partial volume errors, the in vivo activity concentration was determined to within 10% of the true value.