Abstracts

RATIONALE: Magnetisation Transfer Imaging (MTI) allows the visualisation of protons tightly bound to macromolecular structures, such as proteins and lipid membranes, and which are essentially invisible to conventional MRI because of their very short relaxation times. The exchange of magnetisation between bound protons and free water is represented by the magnetisation transfer ratio (MTR) which provides a quantitative measure of macromolecular structural integrity. Our objective was to test the hypothesis that MTI would identify areas of reduced MTR in patients with either epileptogenic malformations of cortical development (MCD), acquired lesions or normal conventional MRI (MRI-negative patients).
METHODS: 30 healthy controls and 55 patients with localisation-related epilepsy (15 with MCD, 10 with acquired lesions, and 30 MRI-negative patients) were scanned with conventional MRI and MTI using a 1.5T GE scanner. MTR maps were calculated and normalised into Talairach space. Each patient was then individually compared to the 30 control subjects on a voxel-by-voxel basis using statistical parametric mapping (SPM99) and significant differences in MTR were detected at a threshold of p[lt]0.001 (corrected for multiple comparisons: p[lt]0.05).
RESULTS: Significant reductions of MTR were seen in 13/15 patients with MCD and all 10 with acquired lesions. In all of these patients, the areas of reduced MTR concurred with abnormalities identified on the conventional MRI. In addition, in the MCD group, there were 11 areas, and in the acquired lesion group there were 2 areas, of reduced MTR in tissue that was normal on conventional MRI. Ten of the 30 MRI-negative patients demonstrated significant reductions in MTR. In all, these areas concurred with epileptiform EEG activity. Group analyses were also performed on MRI-negative patients with electroclinical evidence of temporal lobe epilepsy (TLE) (8 patients with left TLE, 3 with right TLE). Compared to the 30 control subjects these groups had significant reductions of MTR within each respective temporal lobe.
CONCLUSIONS: MTI analysed using SPM was sensitive in patients with MCD and acquired cerebral damage, in some cases detecting abnormalities in cerebral tissue previously thought to be normal on conventional MR imaging. This finding is of importance should surgical management be considered. In addition, the technique identified a clinically concordant abnormality in 33% of individual MRI-negative patients. This could be caused by either aetiological factors, such as occult dysgenesis, or as a result of chronic seizures, for example atrophy and neuronal loss. MTI is a promising imaging method for investigating localisation-related epilepsy.
Support: Action Research and National Society for Epilepsy