Other Inflammatory/Infectious Diseases

34 Other Inflammatory/Infectious Diseases

At autopsy spinal cord involvement is seen in essentially all cases of MS. Cord lesions are also part of the consensus criteria for the MRI diagnosis of MS (see Chapter 18). Furthermore, isolated spinal involvement may occur in up to a quarter of cases, and suggestive lesions on spinal MRI are much more specific for MS than are hyperintensities on T2WI of the brain. Thus MRIs of the brain, cervical, and thoracic spine comprise a complete imaging evaluation of MS. MRI of the lumbar spine—where the spinal cord is absent—need not be performed. Figure 34.1A demonstrates an FSE T2WI of an MS plaque in the cervical spine with a focal area of high SI (black arrow) extending longitudinally within the cord. As is typical for MS, this lesion exerts little mass effect and it spans less than two vertebral body segments in length. Occasionally, edema above and below a plaque may result in a flame-like appearance on sagittal images. In Figs. 34.1A,B,C the lesion on sagittal MRI (A) is more subtle in appearance than in the axial GRE (B) and FSE T2WI (C). The axial plane is preferred to sagittal imaging for the detection of MS in the spinal cord due in part to partial volume averaging—a term referring to the fact that the SI of a voxel on MRI represents an average of SI over a volume of actual tissue. Thus, in a longitudinally extending MS plaque of thin width, the SI of a voxel in the sagittal plane may contain SI contributions from both the plaque itself and also from normal cord, the average of the two contributing to a inconspicuous appearance of plaque on the final image. Lesions are typically not well-visualized on T1WI due to relative isointensity to the cord. In Figs. 34.1B,C, lesion conspicuity is also slightly greater on (C) the axial FSE (black arrow) than on the (B