fMRI of the temporal lobe of the awake macaque at 7T

The temporal lobe of the primate brain is thought to be important in high-level object recognition and learning. In contrast to electrophysiological studies, fMRI has the advantage that it allows simultaneous mapping of the functional organization of multiple cortical areas. However, there are few fMRI studies of the inferior temporal lobe in the non-human primate, because gradient-echo echo-planar-imaging (GE-EPI), which is commonly used, suffers from susceptibility related signal losses due to the ear canal and the cancellous nature of the temporal bone. The large macroscopic susceptibility gradients caused by air-tissue interfaces result in distortion and reduced signal-to-noise in affected areas. At high magnetic fields this is especially problematic, because in addition to increases in the BOLD signal, the susceptibility artifacts also increase. In areas of high susceptibility, using spin-echo EPI (SE-EPI) may be advantageous because it is less sensitive than GE-EPI to susceptibility artifacts, and does not suffer from signal dropout in these regions. In this study, we compare SE-EPI and gradient-echo fMRI in the awake monkey (Macaca mulatta), using a vertical bore 7T MR system. An 8 cm surface coil was positioned over the monkey’s ear, which covers one hemisphere, allowing 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 losses. Using movies as a stimulus, reliable functional activation was obtained in the inferior temporal lobe (as well as in other visual areas). The reliability and specificity of the obtained activations with SE-EPI ensures the application of the method in our on-going visual perception and learning studies