J. Duncan and G. Gerig (Eds.): MICCAI 2005, LNCS 3750, pp. 684 – 691, 2005. © Springer-Verlag Berlin Heidelberg 2005 A New Method for SPECT Quantification of Targeted Radiotracers Uptake in the Myocardium Shimin Li, Lawrence W. Dobrucki, Albert J. Sinusas, and Yi-Hwa Liu Section of cardiovascular Medicine, Department of Internal Medicine, Yale University School of Medicine, New Haven, CT, USA Shimin.li@yale.edu Abstract. We developed a new method for absolute quantification of targeted radiotracers uptake in the myocardium using hybrid SPECT/CT and an external reference point source. A segmentation algorithm based on the level set was de- veloped to determine the endocardial edges from CT, which were subsequently applied to the physically co-registered SPECT. A 3-D Gaussian fitting method was applied for quantification of the external point source. The total targeted radiotracer activity in the myocardium was normalized to that in the point source to calculate the absolute uptake of targeted radiotracer in the myocar- dium. Preliminary validation was performed in rats with ischemia-induced an- giogenesis. The quantified in vivo radiotracer uptake was compared to the postmortem tissue radioactive well-counting of the myocardium. Our methods worked well for identification of the endocardial edges. Quantification of the focal uptake was consistent with the well-counting data. Our methods may have the potential of providing precise absolute quantification of targeted radiotracer uptake in the myocardium. 1 Introduction Recently substantial research was directed toward the developments of targeted mo- lecular imaging agents [1] for the cardiovascular system and radiotracer based imag- ing techniques for noninvasive visualization of the molecular processes in the myo- cardium. Radio-labeled agents targeted at the molecular processes often result in focal “hotspot” images, which are distinctly different from the perfusion images conven- tionally acquired in nuclear cardiology. Because the changes in the myocardial uptake of these targeted radiotracers are generally small, precisely tracking these subtle changes requires a sophisticated quantitative method to assess the absolute hotspot uptake in the myocardium. However, quantification of myocardial uptake of targeted radiotracers has not been extensively investigated previously in part due to the back- ground activity in the cardiac images. Quantification of hotspot focal uptake in the myocardium from single photon emission computerized tomography (SPECT) pre- sents another challenge due to the effects of image resolution, background activity [2, 3], object size, shape and voxel size in SPECT images [4]. Other confounding fac- tors such as photon attenuation and the partial volume effect may also cause errors in quantification of absolute hotspot tracer uptake [5] in the myocardium. On the other hand, it is difficult to obtain anatomical information from focal hotspot SPECT