SPATIAL DATA-ANALYSIS IN THE QUANTITATIVE ASSESSMENT OF CEREBRAL WHITE-MATTER PATHOLOGY ON MRI IN HIV-INFECTION

This study was carried out using MRI (proton density - and T2-weighted ) on 16 HIV-negative controls, 9 symptom-free HIV-positive patients an d 25 with CDC IV HIV disease. The studies from this last group had pre viously been allocated by a radiologist to the following categories: 8 with focal mass lesions and normal-appearing white matter; 9 with dif fuse encephalopathy (high signal on T2-weighted images, affecting most or all of the white matter) and 8 with patchy encephalopathy (high si gnal affecting only one or two areas within the white matter). Moran's 1, a statistic of spatial autocorrelation, was calculated for the gre y-scale values of a sampled pixel array from a central white matter re gion of each of the images. All values of Moran's 1 calculated in this study showed a large positive excess over the expected value under ra ndomisation, indicating highly significant positive autocorrelation in the spatial arrangement of the grey-scale values. On T2-weighted imag es a statistically significant increase in the mean value of Moran's 1 , compared with controls, was found in the diffuse encephalopathy grou p, indicating that quantifiable changes in the spatial autocorrelation of pixel data can be related to recognised qualitative changes in the appearance of white matter in subjects with HIV disease. A lesser, bu t significant, rise in the mean value of Moran's 1 was also found in t he focal mass lesion group, suggesting that changes in spatial autocor relation may indicate pathological change in advance of qualitative MR I changes.