PET quantitation and imaging of the non-pure positron-emitting iodine isotope 124I.

A series of PET studies using phantoms is presented to characterize the imaging and quantitative performance of the positron-emitting iodine isotope 124I. Measurements were performed on the 2D-PET scanner GE 4096+ as well as on the Siemens PET scanner HRR+ operated in both 2D and 3D modes. No specific correction was applied for the gamma-rays emitted together with the positrons. As compared to 18F, in studies with 124I there is a small loss of image resolution and contrast, and an increase in background. The quantitative results varied between different scanners and various acquisition as well as reconstruction modes, with an average relative difference of -6 +/- 13% (mean+/-SD) in respect of the phantom radioactivity as measured with gamma-ray spectroscopy. We conclude that quantitation of a radiopharmaceutical labelled with 124I is feasible and may be improved by the development of specific corrections.

Integrating electrophysiology and neuroimaging of spatial selective attention to simple isolated visual stimuli.

Visual-spatial attention involves modulations of activity in human visual cortex as indexed by electrophysiological and functional neuroimaging measures. Prior studies investigating the time course and functional anatomy of spatial attention mechanisms in visual cortex have used higher-order discrimination tasks with complex stimuli (e.g. symbol matching in bilateral stimulus arrays, or letter discrimination), or simple detection tasks but in the presence of complex distracting information (e.g. luminance detection with superimposed symbols as distractors). Here we tested the hypothesis that short-latency modulations of incoming sensory signals in extrastriate visual cortex reflect an early spatially specific attentional mechanism. We sought evidence of attentional modulations of sensory input processing for simple, isolated stimuli requiring only an elementary discrimination (i.e. size discrimination). As in prior studies using complex symbols, we observed attention-related changes in regional cerebral blood flow in extrastriate visual cortex that were associated with changes in event-related potentials at a specific latency range. These findings support the idea that early in cortical processing, spatially-specific attentional selection mechanisms can modulate incoming sensory signals based on their spatial location and perhaps independently of higher-order stimulus form.

On two methods of statistical image analysis.

The computerized brain atlas (CBA) and statistical parametric mapping (SPM) are two procedures for voxel-based statistical evaluation of PET activation studies. Each includes spatial standardization of image volumes, computation of a statistic, and evaluation of its significance. In addition, smoothing and correcting for differences of global means are commonly performed in SPM before statistical analysis. We report a comparison of methods in an analysis of regional cerebral blood flow (rCBF) in 10 human volunteers and 10 simulated activations. For the human studies, CBA or linear SPM standarization methods were followed by smoothing and computation of a statistic with the paired t-test of CBA or general linear model of SPM. No standardization, linear, and nonlinear SPM standardization were applied to the simulations. Significance of the statistic was evaluated using the cluster-size method common to SPM and CBA. SPM employs the theory of Gaussian random fields to estimate the cluster size distributions; simulations described in the Appendix provided empirical distributions derived from t-maps. The quantities evaluated were number and size of functional regions (FRs), maximum statistic, average resting rCBF, and percentage change. For the simulations, the efficiency of signal detection and rate of false positives could be evaluated as well as the distributions of statistics and cluster size in the absence of signal. The similarity of the results yielded by similar methods of analysis for the human studies and the simulated activations substantiates the robustness of the methods for selecting functional regions. However, the analysis of simulated activations demonstrated that quantitative evaluation of significance of a functional region encounters important obstacles at every stage of the analysis.