Multiple Sclerosis: MRI in Diagnosis, Management, and Monitoring

MRI has provided important insights into the pathophysiology of multiple sclerosis (MS).1 However, conventional MRI scans furnish only gross estimates of the nature and extent of tissue damage associated with MS,2 and the data correlate poorly with measures of concurrent disability in patients. Recent advances in MRI technology have improved the correlation of MRI findings with clinical status and have increased the utility of MRI data as surrogate markers in monitoring disease progression and response to therapy.3 Newer MRI techniques, such as magnetization-transfer (MT) MRI, diffusion-weighted MRI, and functional MRI (fMRI), as well as proton magnetic resonance spectroscopy (MRS) and measures of brain and spinal cord atrophy, may help further elucidate MS pathology2 and may provide opportunities for new treatment approaches.4 CONVENTIONAL MRI MRI usually is recommended for diagnosing MS to confirm clinical findings and evaluate patients for other pathologies. Typically, the body is exposed to an external magnetic field that causes protons to align in an orientation parallel or antiparallel to the external magnet. A radiofrequency (RF) pulse transfers energy to the protons, which resonate with the pulse, causing some of the protons to alter their orientation. The RF pulse is discontinued, and the protons relax, returning to their starting point. Measurements of this relaxation phase are used to create images, which vary depending on the tissue being scanned.4 Two relaxation times, T1 (longitudinal) and T2 (transverse), are important in using conventional magnetic resonance technology for the imaging of MS lesions. Contrast is influenced by the selected weighting of these relaxation times. T1 weighting uses a short delay between pulses, while T2 weighting uses a longer delay; this accentuates the differences in T2 relaxation time. Abnormalities seen with conventional MRI that are most often used to determine disease activity in patients with MS are hyperintense lesions visualized on T2-weighted images, hypointense lesions visualized on T1-weighted images, and gadolinium-enhanced (Gd+) hyperintense lesions visualized on postcontrast images (Table 1).2,5-8 T2-weighted images are used to assess edema and tissue destruction early in the inflammatory stage of MS. Later, when demyelination and gliosis occur,4 T2-weighted images are used most often to measure burden of disease, but they have limited sensitivity and specificity. There is generally a weak correlation between T2-weighted lesion load and concurrent clinical disability in patients with MS.9 Proton density (PD)-weighted scans, a type of T2 imaging, are obtained by minimizing T1 and T2 contrast effects5 and are used to distinguish periventricular lesions from the cerebrospinal fluid (CSF).4 Another widely used variant of T2 imaging is the "fluid attenuated inversion recovery" (FLAIR) sequence. FLAIR imaging suppresses the T2 hyperintensity of fluid and starkly differentiates the ventricles from periventricular white-matter lesions. FLAIR imaging also appears to make parenchymal hemisphere lesions stand out more prominently than does conventional T2; the ability to detect juxtacortical lesions is particularly improved. Because of concerns regarding the presence of artifacts, FLAIR imaging is not as useful as conventional T2 for evaluation of the spinal cord or posterior fossa. In these anatomic areas, PD or spin echo sequences are preferred. On T1-weighted scans, hypointense lesions that do not persist may indicate a reversible change, such as edema, whereas persistent lesions signify focal CNS damage, such as axonal loss and demyelination. Persistent hypointense T1-weighted images correlate more closely with disability than do T2-weighted images and are known as "black holes." These persistent T1 lesions may be useful markers of disease progression; however, further study is required to validate their usefulness. Gadolinium enhancement is used to estimate inflammation-induced permeability changes of the blood-brain barrier. The ability to distinguish between active and inactive lesions makes gadolinium enhancement the most clinically relevant MRI measure for ongoing inflammatory activity in patients