The reason for this poor correlation is not clear. However, one possibility is that the pathologic processes being represented by T2-weighted lesions of the brain are quite diverse. Inflammation, demyelination, edema, gliosis, and axonal loss are all processes that can be represented by T2-weighted images of the MS brain, yet only axonal loss and chronic demyelination would be expected to result in permanent accumulation of neurological disability. As a clinical tool, the Gd-enhancing lesion allows one to distinguish between acute and chronic T2 lesions, with the acute lesion demonstrating enhancement following Gd administration. The pattern of enhancement is also quite variable and can appear as either a homogeneous or a ring-enhancing lesion.
Many factors can affect the level of Gd enhancement in an individual patient.
This measure has also been used as an outcome measure in MS clinical trials, although the correlation between Gd-enhancing lesions and subsequent clinical disability is also weak. The correlation between Gd-enhancing lesions and clinical relapses is somewhat better, but it is clear that new techniques are needed to improve the correlation between the clinical and MRI measures of MS disease activity. Improving the correlation between MRI measures and clinical progression is still a major hurdle for MS investigators. T1 black holes are areas that appear hypointense on T1-weighted images and usually have extensive tissue destruction. When one considers the history of an MS lesion, the T1 black hole usually represents the end stage of the process, when significant demyelination, axonal loss, and reactive gliosis have occurred.