Over the past 15 years the use of ultra-high field magnetic resonance imaging (MRI) at 7.0 Tesla (7T) has increased. These technological advances have offered new opportunities to uncover previously undetectable tissue injury in patients with multiple sclerosis (MS). Combined MRI-histopathologic studies confirmed the expected superior sensitivity and specificity to pathology of 7T-MRI compared to imaging at lower field strengths. Undesirably, though, those studies have also shown the limits of the technology.
Correlative investigations that focused on the detection and pathological characterization of cortical lesions (CLs) have provided an elegant demonstration of the advantage associated to 7T-MRI increased resolution. Specifically, post-mortem studies at 7T confirmed (1) the ability of higher resolution scans to disclose a larger CL burden; and (2) the notion that lesion size sets the limit for their visibility. However, those studies also demonstrated that much of the extensive MS-induced cortical pathology is invisible to ultra-high resolution 7T-MRI, indicating the limitation of currently available mechanisms of contrast in detecting cortical disease.
The insufficient sensitivity of T2-weighted scans at 7T has also been shown when assessing lesions with different degrees of myelin integrity. For example, areas of remyelination are known to contain a lower amount of (thinner) myelin. These areas are not visible using non quantitative MRI techniques, irrespective of the changes in contrast-to-noise ratio associated with 7T scans.
Validation studies exploiting the uniquely superb susceptibility-based contrast at 7T, have led to the discovery of a specific subset of chronic lesions. Such lesions are featured by a core of demyelination surrounded by a complete (or partial) rim of iron, to which susceptibility-based sequences at 7T are exclusively sensitive. Immunohistopathologic analyses demonstrated that this iron is located inside the perilesional microglia responsible for their slow expansion. Studies have also proven that tracking iron using MRI may lead to the identification of pathologies unrelated to activated microglia as well as to as healthy oligodendrocytes.
Cumulatively, pathology correlation and validation studies consolidate the notion that 7T MRI has the potential to better detect and more accurately characterize tissue injury than MRI at lower field strengths. Nevertheless, the discovery of new mechanisms of contrast remains critical so that quantitative and qualitative MRI methods at ultra-high resolution will continue to advance our insights in the yet concealed MS disease.