FIGURE 6 In the axial view the anterior commissure (AC) posterior commissure (PC) appear as thin white lines connecting white matter between hemispheres. In the sagittal view the AC is a conspicuous white, slightly elliptical structure, and the PC is at the elbow between the pineal body (pb) and superior colliculus (sc). See also Plate 81.

Individual Average

FIGURE 7 Axial section views of spatially normalized MRI and 0-15 PET images from individual and averages from 12 subjects. The average MR image has better definitation of the interhemispheric fissure but less cortical detail. The average PET shows a significant improvement in signal-to-noise ratio.

(Fig. 7, PET). The spatial precision for activation sites is not necessarily limited by the system FWHM, since activation loci are often calculated as a weighted centroid in a thresholded region of interest, and this can provide millimeter-level precision [16]. However, the low spatial resolution inherent in PET imaging systems reduces activation peak intensities in small regions, thereby reducing sensitivity to their detection.

For imaging systems with higher spatial resolution, more accurate spatial normalization is necessary to preserve resolution in group averages. For example, the blurring introduced by averaging globally spatially normalized images has a FWHM ranging from approximately 1 mm to 1 cm. When applied to PET images with a system FWHM « 1 cm, the net FWHM ranges from approximately 1.05-1.41 cm, since independent resolution components in linear systems add in quadrature [12,30]. The FWHM of an average PET image is just 5-41% larger than that of a single image, mostly because of the PET system's FWHM. This provides an overall improvement of the signal-to-noise ratio in an average PET image (Fig. 7, PET). However, when averaging globally spatially normalized highresolution MR images, with system FWHM « 1 mm, the net FWHM ranges from approximately 1.41 to 10.5 mm, better than PET but much worse than that of single MR images. The mid-sagittal plane in an average MR image appears to be well resolved as well as other central structures (Fig. 7, MRI). However, in regions with high anatomical variability, the FWHM can increase by a factor of 10 and is dominated by the blurring effect of group averaging. The group average MR image has a lower contrast-to-noise ratio than individual MR images in these highly blurred areas, especially in the cortex (Fig. 7, MRI). To obtain better resolution in group-averaged MR images, the FWHM component due to spatial normalization must be made smaller, and this can only be done using higher-resolution regional spatial normalization methods. The goal is to have the FWHM component of spatial resolution due to group averaging similar to the FWHM of the imaging system. Two major obstacles in attaining this goal are low object contrast of critical features, even when resolution is high, and incomplete feature correspondence across brains.

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