MRI as an imaging tool for in vivo noninvasive morphological and (partially) functional examination of injured spinal cord

Magnetic resonance imaging (MRI) is the state-of-the-art radiological tool used to visualize the spinal cord in both normal and pathological conditions. MRI enables visualization in nearly real time (within minutes), with submillimetric spatial resolution, and with high contrast between gray and white matter and surrounding cephalospinal fluid, and eventually ischemic, traumatic, and hemorrhagic lesions. The possibility of seeing the spinal cord directly changed the way surgeons and researchers approached spinal cord injury. A major drawback continues to be the limited size of the cord; major gray structures are visible, as well as major white matter tracts, but distinction between different tracts, and the vast majority of the vessels, including most of intramedullary vessels, are still not identifiable with current devices. Generally speaking, the spatial resolution and contrast of MRI are inversely related; in pathological conditions, one must choose to maximize one or the other. From the perspective of cellular damage, a step further was achieved by diffusion-weighted images, which achieve extremely high sensitivity for hyperacute cellular damage (at the cost of lowering spatial resolution). On the other hand, for axonal damage, diffusion-tensor images allowed researchers to produce synthetic images of white matter tracts (virtual tractography) and eventually to estimate the rate of fibers with coherent direction in a given voxel (fractional anisotropy). Also in this case, results come at a cost: the direct connection between signal acquisition and image generation is lost (images are interpolated, calculated a posteriori), which lowers spatial resolution for fractional anisotropy.