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 absolute radionuclide uptake, the authors have developed a technique to compensate these images for partial volume errors using coregistered X-ray 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. The in vivo activity concentration in the porcine myocardium had an error in the range -40% to -60% with attenuation correction alone. By also correcting for partial volume errors, absolute quantitation with an accuracy error of 20% or better was achieved.