I think (I might be wrong) the brain might be a good target because there is not much motion. To get fast readings from an MRI you probably need the stronger magnets. MRI works not by capturing spatial variation directly like a camera, but collects a time series of aggregate magnetic field readings while a gradient field is used to create changing spatial variation. In essence, the time series gives the k-space of the image, which is then Fourier transformed into the original image. Consequently, the body must remain fairly still while this k-space time series is constructed. The heart, for instance, is notoriously hard to image.