Once the diagnosis is established, brain MRI is recommended for follow-up and for surveillance of drug safety. Spinal cord and optic nerve MRIs are desirable but optional in the follow-up of pwMS and are not required for drug surveillance. Spinal cord MRIs are required at follow-up for patients whose progression cannot be explained by brain MRI features, or who manifest with recurrent spinal cord symptoms, or have spinal cord comorbidities. In these cases, spinal cord MRI also may assist with treatment decisions. Similarly, optic nerve MRI is necessary during follow-up only when optic nerve comorbidities are suspected or when there is progression or reoccurrence of optic nerve–related symptoms.
Brain MRIs are recommended for monitoring drug effect yearly (or at longer intervals, after a few years of disease stability). Conversely, a repeat brain MRI is advised after 6 months if nonsymptomatic radiological disease activity is discovered on surveillance scans.
Abbreviated but more frequent serial brain MRI protocols (eg, every 3 to 4 months) are recommended for pwMS treated with natalizumab and at high risk of developing progressive multifocal leukoencephalopathy (eg, pwMS who are John Cunningham virus [JCV]–positive, and have been treated with natalizumabfor ≥ 18 months, have a JCV antibody index > 0.9, or have a history of immunosuppression). A similar approach is recommended for carryover cases, such as those with high JCV antibody index who are switched to other immunosuppressive treatments.
MRI Field, Scan Resolution, and Coverage
Both 1.5-Tesla (1.5-T) and 3-T scans are believed to be equally effective in imaging pwMS, providing that the 1.5-T scans are good quality. Although imaging at < 1.5 T is not recommended due to suboptimal disease detection, the use of scanners > 3 T is equally discouraged outside the supervision of trained investigators. Signal-to-noise ratio and resolution are key factors impacting scan quality, and their optimization is prioritized over the number of sequences in the updated 2021 consensus recommendations. For brain imaging, a resolution of 1 mm3 isotropic is preferred for 3-dimensional (3D) imaging and slice thickness ≤ 3 mm without gap (≤ 5 mm with 10-30% gaps for diffusion-weighted imaging only) is recommended for 2D sequences. Images should cover the entire brain and as much of the cervical spine as possible; images should be prescribed axial for 2D or reformatted axial oblique for 3D using the subcallosal plane as reference. For spine imaging, sites should aim at an in-plane resolution of 1 mm2; using sagittal slices ≤ 3 mm thick and axial slices ≤ 5 mm thick, both with no gap. Scans should cover the entire cervical and thoracolumbar region inclusive of the conus. For the optic nerve images, slices should be ≤ 2 or 3 mm thick with an in-plane resolution of 1 mm2. Images should be aligned to the orientation of the optic nerve and chiasms, both of which should be entirely covered.
Postgadolinium Images Use
The discovery of the higher sensitivity of post-gadolinium (Gd) T1-weighted (T1-w) MRI relative to high iodine (88.1 g I) computed tomography scans in demonstrating contrast-enhancing MS lesions has revolutionized the way clinicians diagnose and monitor this disease.11 However, in recent years the role of postcontrast MRI has been debated, considering the potential safety concerns secondary to Gd tissue deposition. For this reason, an intentionally more judicious use of postcontrast MRI is proposed by the consensus recommendations. At disease diagnosis, the use of Gd is advisable to (1) show disease dissemination in time; (2) differentiate the diagnosis based on the Gd pattern; (3) predict short-term disease activity; and (4) characterize activity in the setting of progression. When monitoring pwMS, the use of Gd may be useful in the first year of follow-up, particularly if in the setting of low potency medications or for patients for whom the detection of one or more active lesions would lead to a change in disease-modifying agents. Gd also should be used to first, confirm a clinical exacerbation (if needed); second, further characterize a lesion suggestive of progressive multifocal encephalopathy or monitor this disease over time; and third, monitor lesion burden change in patients with large confluent lesions, the count of which otherwise may be difficult.
MRI During Pregnancy and Lactation
The consensus recommendations state that Gd contrast–enhanced MRI is not absolutely contraindicated during pregnancy, although its use should be limited to strictly necessary situations, particularly those involving differential diagnosis, such as cerebral venous thrombosis or monitoring of possibly enlarging lesion burden. The use of Gd is not contraindicated during lactation, as only a small proportion (< 0.4%) passes into the breast milk, leading to an exposure to < 1% of the permitted Gd dose for neonates.12,13
Harmonizing MRI Reports
The consensus recommendations propose reporting the exact lesion count on T2-weighted (T2-w) images when lesions are < 20, or specifying if the number of T2 lesions is between 20 and 50, between 50 and 100, or uncountable, eg, confluent large lesions. Similarly, for the spinal cord, the consensus recommendations propose reporting the exact lesion count on T2-w images when lesions are < 10, or otherwise report that > 10 lesions are seen.
The VHA workgroup proposed reporting a mild, moderate, or severe T2-lesion burden for a T2-lesion count < 20, between 20 and 50, and > 50, respectively. For follow-up MRIs, notation should be made if there is any change in lesion number, indicating the number of new lesions whenever possible. At each timepoint, the presence of active lesions on postcontrast images should be accurately defined.