Functional magnetic resonance imaging study of word recognition in normal elders.

Four healthy, cognitively intact elders participated. Subjects underwent fMRI scanning while performing a word recognition task with an easy condition (low demand) and a difficult condition titrated to each subject's ability (titrated demand). Relative to low, titrated demand was associated with increased activation of the left medial frontal (cluster level P <.002), right superior temporal (P <.007) and right superior parietal cortices (P<.001). Increased activation of bilateral cortical areas by elders during the more challenging titrated demand compared with low demand may indicate recruitment of additional brain regions, enabling subjects to maintain performance despite increasing difficulty. Alternatively, the bilateral activation on this word recognition task may reflect compensatory use of right hemisphere networks.

Image distortion in MRI-based polymer gel dosimetry of gamma knife stereotactic radiosurgery systems.

We have studied effects of MR (magnetic resonance) image distortion on polymer gel dosimetry of Gamma Knife stereotactic radiosurgery systems. MR images of BANG polymer gel phantoms were acquired by using a Hahn spin-echo sequence and a fast 3D imaging GRASS sequence. Image artifacts were studied by varying the directions of frequency encoding and the receiver bandwidth. The phantoms were also CT (computed tomography) scanned. The studies showed that the measured dose distributions are shifted by 1.8+/-0.5 mm (2 pixels) in the frequency encoding direction. The magnitude of the shift is inversely proportional to the receiver bandwidth in agreement with theory. Comparison of MRI with CT showed the same image shift. We concluded that the discrepancy is caused by MR image distortion due to a difference in susceptibility effects between the phantom and the fiducial markers of the Leksell localization box.