Summary: Accuracy in in vivo quantitation of brain func tion with positron emission tomography (PET) has often been limited by partial volume effects. This limitation be comes prominent in studies of aging and degenerative brain diseases where partial volume effects vary with dif ferent degrees of atrophy. The present study describes how the actual gray matter (GM) tracer concentration can be estimated using an algorithm that relates the regional fraction of GM to partial volume effects. The regional fraction of GM was determined by magnetic resonance imaging (MRI). The procedure is designated as GM PET. In computer simulations and phantom studies, the GM PET algorithm permitted a 100% recovery of the actual tracer concentration in neocortical GM and hippocam pus, irrespective of the GM volume. GM PET was apPositron emission tomography (PET) permits in vestigation of physiological and biochemical pro cesses in human brain in vivo, and has yielded new insights into both normal physiology and diseases (Kuhl et aI. , 1982;Foster, 1983; Wagner et aI. , 1983; Frost et aI., 1985;Phelps and Mazziotta, 1985;Frost, 1986; Yamaguchi et aI. , 1986; Yoshii et aI., Abbreviations used: AU, arbitrary units; FWHM, full width at half-maximum; OM, gray matter; MRI, magnetic resonance im aging; PET, positron emission tomography; RMSE, relative mean-squared error; ROI, region of interest; SPOR, spoiled grass; WM, white matter. 571plied in a test case of temporal lobe epilepsy revealing an increase in radiotracer activity in GM that was undetec ted in the PET image before correction for partial volume effects. In computer simulations, errors in the segmenta tion of GM and errors in registration of PET and MRI images resulted in less than 15% inaccuracy in the GM PET image. In conclusion, GM PET permits accurate de termination of the actual radiotracer concentration in hu man brain GM in vivo. The method differentiates whether a change in the apparent radiotracer concentration re flects solely an alteration in GM volume or rather a change in radiotracer concentration per unit volume of GM. Key Words: Brain gray matter-Positron emission tomography-Magnetic resonance imaging-Partial vol ume effects-Aging-Dementia-Brain atrophy.1988; Fowler, 1990; Frost and Wagner, 1990; Leen ders et aI., 1990;Martin et al., 1991; Mayberg et aI. , 1991). Nevertheless, a limitation of PET remains: its relatively poor spatial resolution. As a result, PET quantification, especially in structures smaller than two times the full width at half-maximum (FWHM) of the tomograph, is affected by partial volume effects (Hoffmann et aI. , 1979). Given that the in-plane FWHM of current PET instruments ranges from 2.6 mm (Valk et aI. , 1990) to about 14 mm, tracer activity in many brain structures, in cluding the neocortex, is often underestimated. In neocortex, gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) spaces are convo luted, and cannot be resolved using PET instrumen tation; a cortical PET signal thus reflects the aver age tracer concentr...

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We describe a fully-automatic 3D-segmentation technique for brain MR images. By