SPIN-SPIN RELAXATION IN EXPERIMENTAL ALLERGIC ENCEPHALOMYELITIS - ANALYSIS OF CPMG DATA USING A NONLINEAR LEAST-SQUARES METHOD AND LINEAR INVERSE-THEORY

We have used the CPMG pulse sequence to measure proton T2 values and w ater content in spinal cord and brain samples from Hartley guinea pigs inoculated to produce experimental allergic encephalomyelitis (EAE). Relaxation data were fitted using minuit, a non-linear curve fitting r outine. Three exponentials provided the best fit to spinal cord data ( 10 ms (13%), 76 ms (57%), 215 ms (30%)) and two exponentials for brain tissue (10 ms (4%), 92 ms (96%)). Least squares algorithms were also used to analyse the spinal cord data in terms of discrete and smooth d istributions of relaxation times. The discrete least squares solutions consisted of three to five isolated spikes between 0.010 and 0.300 s. This type of solution was difficult to interpret in terms of water re servoirs. Smooth solutions consisted of two broad peaks, a small peak with a T2 near 0.010 s and a larger peak near 0.100 s. The integral ra tio of the larger to the smaller peak was 7.092 +/-782 for normal tiss ue, and increased to a maximum of 16 with increasing parenchymal cellu lar infiltration and demyelination. The short T2 peak has been assigne d to water in the hydration layers of the myelin sheath. The width of the longer T2 peak was sensitive to tissue heterogeneity. The least sq uares and smooth distribution analysis models could be used to disting uish samples with extensive parenchymal infiltration from normal tissu e, even though only a maximum of 60% of the tissue was affected. The s hort T2 component could provide a direct method of measuring intact my elin, which would have a profound effect on the understanding of the e volution of pathology in multiple sclerosis.