Spinal fMRI

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Functional magnetic resonance imaging (fMRI) of the spinal cord (spinal fMRI) is an adaptation of the fMRI method that has been developed for use in the brain. Although the basic principles underlying the methods are the same, spinal fMRI requires a number of specific adaptations to accommodate the periodic motion of the spinal cord, the small cross-sectional dimensions (roughly 8 mm × 15 mm at the largest region) and length (~45 cm in adult humans) of the spinal cord, and the fact that the magnetic field that is used for MRI varies with position in the spinal cord because of magnetic susceptibility differences between bone and tissues.^[1] Spinal fMRI has been used to produce maps of neuronal activity at most levels of the spinal cord in response to various stimuli, such as touch, vibration, and thermal changes, and with motor tasks.^[1] Research applications of spinal fMRI to date include studies of normal sensory and motor function,^[2] and studies of the effects of trauma^[3] and multiple sclerosis^[4] on the spinal cord.

Two different data acquisition methods have been applied, one based on the established BOLD (blood-oxygen-level dependent) fMRI methods used in the brain, and the other based on SEEP (signal enhancement by extravascular water protons) contrast with essentially proton-density weighted spin-echo imaging (see MRI). The majority of the studies published to date are based on the SEEP contrast method. Methods demonstrated to overcome the challenges listed above include using a recording of the heart-beat to account for the related time course of spinal cord motion, acquiring image data with relatively high (~ 1–2 mm) spatial resolution to detect fine structural details, and acquiring images in thin contiguous sagittal slices to span a large extent of the spinal cord. Methods based on BOLD contrast have employed parallel imaging techniques to accelerate data acquisition, and imaging slices transverse to the spinal cord, in order to reduce the effects of spatial magnetic field distortions.^[5] Methods based on SEEP contrast have been developed specifically because they have low sensitivity to magnetic field distortions while maintaining sensitivity to changes in neuronal activity.