MAGNETIC-RESONANCE-IMAGING OF MULTIPLE-SCLEROSIS LESIONS

Magnetic resonance imaging represents voxels (volume elements) of the body placed in a magnet, by their magnetization determined under vario us acquisition conditions weighting the contrast of the image by the d ensity of free water protons and their relaxation times T1 and T2. Thu s, the sensitivity in depicting lesions is high but pathological speci fity is poor. Efforts are made to increase the diagnosis powerfulness of M.R.I. in multiple sclerosis: a careful correlation with the clinic al presentation and the use of better M.R.I. criteria increase the spe cificity 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 th e detection of lesions. Under specific conditions M.R.I. can be used t o monitor the evolution of M.S. Acute phase monitoring focuses on chan ges in disease activity, new, recurring, enlarging gadolinium (Gd) enh ancing lesions, and chronic phase monitoring appreciate the burden of the disease. However M.R.I. is always considered as a secondary outcom e in the phase IN trials because insufficient correlations with the cl inical disability In the neurological daily practice conventional M.R. I. is of poor interest in the follow up of individual M.S. patients co nsidering the weakness of prognosis value and the problems in the meas urement of the lesions load which is emphasized in the methodology of the clinical trials. Nevertheless, there is a continuing search for te chniques which correlate better with clinical measures of the disease such as the quantification of ''black holes'' on T1 w images or the ce rebral and spinal atrophy. New techniques allow to weight the signal b y 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 (functi onal M.R.I.). The basic aspects of the pathological lesions in M.S., e dema, membrane disruption, demyelination, gliosis, cellular infiltrati on and axonal loss can be studied more precisely by these new M.R. tec hniques which should better describe the actual clinical impact of the destructive process. in the last year the importance of axonal loss h as simultaneously been confirmed by M.R. spectroscopy and pathological findings. However, magnetization transfer imaging, M.R. diffusion ima ging and functional M.R.I. are intensively under investigation for a b etter analysis of these different factors conditioning the reversibili ty of the patient disability.