Improved Methods for Motion-Compensating and Event-Related Spinal Functional Magnetic Resonance Imaging (fMRI)
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Functional magnetic resonance imaging (fMRI) has become a widely used technique for non-invasive brain mapping, and methods have now evolved to allow fMRI of the spinal cord (spinal fMRI) as well. With the goal of improving spinal fMRI, the studies presented herein have investigated potential sources of noise that might limit its sensitivity and reliability. For example, multiple studies had previously suggested that the majority of structured physiological noise, such as spinal cord motion and cerebrospinal fluid (CSF) flow, appeared to be synchronous with cardiac pulsations. Therefore, we measured cardiac-related spinal cord motion at various levels along the cord, finding that peak anterior-posterior spinal cord displacements often exceeded 0.5 mm throughout the cervical and upper-thoracic regions. On the other hand, we found that cord motion throughout the lower-thoracic, lumbar and sacral levels was consistently small. Based on these findings, we concluded that cord motion is likely to be a significant source of error in spinal fMRI throughout superior, but not inferior, cord regions. Since all motion measurements were acquired at 24 phases of the cardiac cycle, this also allowed us to determine, and subsequently model, the main components of cardiac-related spinal cord motion. By then including these terms in a general linear model (GLM) analysis and reanalyzing 100 previously acquired cervical spinal fMRI datasets, we showed that the sensitivity and specificity were improved by 15-20 % and 5-6 %, respectively, over previous spinal fMRI methods. To push the limits of these improvements, we then carried out the first event-related spinal fMRI study, consistently observing spinal cord responses to 1 s applications of 22 °C thermal stimulation. By measuring these responses at many different phases, we were also able, for the first time, to characterize the impulse response function of SEEP (signal enhancement by extravascular water protons) contrast in the human cervical spinal cord.