[MRI visualization of multiple sclerosis lesions].

Abstract

Magnetic resonance imaging represents voxels (volume elements) of the body placed in a magnet, by their magnetization determined under various acquisition conditions weighting the contrast of the image by the density of free water protons and their relaxation times T1 and T2. Thus, the sensitivity in depicting lesions is high but pathological specificity is poor. Efforts are made to increase the diagnosis powerfulness of M.R.I. in multiple sclerosis: a careful correlation with the clinical presentation and the use of better M.R.I. criteria increase the specificity of the conventional T2 sequences. New sequences such as fast spin echo (F.S.E.), turbo spin echo (T.S.E.) or derived from inversion recovery (F.L.A.I.R.: fluid attenuated inversion recovery) improve the detection of lesions. Under specific conditions M.R.I. can be used to monitor the evolution of M.S. Acute phase monitoring focuses on changes in disease activity, new, recurring, enlarging, gadolinium (Gd) enhancing lesions, and chronic phase monitoring appreciate the burden of the disease. However M.R.I. is always considered as a secondary outcome in the phase III trials because insufficient correlations with the clinical disability. In the neurological daily practice conventional M.R.I. is of poor interest in the follow up of individual M.S. patients considering the weakness of prognosis value and the problems in the measurement of the lesions load which is emphasized in the methodology of the clinical trials. Nevertheless, there is a continuing search for techniques which correlate better with clinical measures of the disease such as the quantification of "black holes" on T1 w images or the cerebral and spinal atrophy. New techniques allow to weight the signal by the movement (diffusion imaging), by the complexity of the molecular architecture (magnetization transfer imaging), by the chemical shift (chemical shift imaging) or by the local status of oxygenation (functional M.R.I.). The basic aspects of the pathological lesions in M.S., edema, membrane disruption, demyelination, gliosis, cellular infiltration and axonal loss can be studied more precisely by these new M.R. techniques which should better describe the actual clinical impact of the destructive process. In the last year the importance of axonal loss has simultaneously been confirmed by M. R. spectroscopy and pathological findings. However, magnetization transfer imaging, M.R. diffusion imaging and functional M.R.I. are intensively under investigation for a better analysis of these different factors conditioning the reversibility of the patient disability.