Datensatz

Zusammenfassung

In contrast to electrophysiological studies, the advantage of fMRI is that it allows simultaneous mapping of the functional organization of multiple cortical areas. FMRI of awake monkey has benefit of combining behavioral studies with BOLD-measurement to be used to precisely localize functional specific cortical areas for further invasive studies such as detailed electrophysiological single unit recordings. Although high magnetic field offers the benefit of increased signal-to-noise ratios and higher specificity, a drawback is the higher sensitivity to susceptibility gradients caused by the air-tissue interfaces. This can be particularly problematic in the lower floor of temporal lobe, because the large macroscopic susceptibility gradients near the ear canal result in distortion and loss of signal when the standard GE-EPI is used. For fMRI of such areas using spin-echo EPI (SE-EPI) is advantageous because it is less sensitive than GE-EPI to susceptibility artifacts, and does not suffer from signal dropout in these regions. Another advantage is that SE-EPI is less affected by frequency-changes in the main magnetic field, which are caused by movement of the animal. In this study, we compared SE-EPI and gradient-echo fMRI in the awake monkey (Macaca mulatta), using a vertical bore 7T MR system. A saddle coil optimized for temporal cortex was used to allow imaging of the major visual areas. The imaging parameters and slice orientation were optimized to minimize susceptibility effects. Resolution was typically 1.5x2x2mm, TE was 40 ms, TR was 1-2 s. In contrast to the GE-EPI images, which showed very large signal dropout in the temporal lobe, SE images showed minimal or no distortion or signal loss. Any remaining distortions were corrected using field-map correction to ensure matching of the functional map to the high-resolution T1-weighted anatomical images. Using movie- stimuli, we confirmed that reliable functional activation could be obtained with SE-EPI at high field, and we show robust activation in the temporal lobe and early visual areas. Using monkey faces, objects and fractal patterns we were also able to obtain functional activities in specific visual object sensitive areas in inferior temporal cortex. The reliability and specificity of the obtained activations with SE-EPI ensures the application of the method in our on-going visual perception studies.