Abstracts

Rationale: Presurgical assessment of pathology is crucial for patients with mesial temporal lobe epilepsy (MTLE). Surgical resection can be curative for patients with a clear hippocampal lesion identified by MRI. Presently available diagnostic MRI fails to identify a lesion in about 30% of patients, although about half of these “MRI-negative” patients do have a lesion discovered post-surgically. Surgery on these patients has a poorer post-surgical outcome. Previously developed MRI approaches are not sufficiently sensitive or truly quantitative. The intrinsic physical parameter which changes linearly with the extent of underlying pathology is the transverse relaxation rate, R2. The R2 of a brain volume element (voxel) represents the value integrated over that voxel. Thus, changes in R2 reflect more accurately and with higher sensitivity the T2-weighted (T2w) signal change previously shown to reflect underlying cellular alterations in hippocampal pathology. The identification of subtle or early structural abnormalities, presently undetected by MRI but potentially quantifiable by our method, could help guide the neurosurgeon in selection of tissue for resection. This should lead to improved treatment of MTLE patients. Methods: Feasibility was tested in 4 subjects: three with right MTLE (later confirmed by intracranial EEG and post-surgical histology) and one normal control. Standard epilepsy protocol MRI revealed pathology in the right hippocampus in 2 of the 3 patients, while the third patient’s and control subject’s MRI were normal. A modified fast-spin echo-based FLAIR sequence was used with multiple TEs and R2-maps were calculated. Single-slice coronal hippocampal R2-maps were generated. For comparison, conventional FLAIR images were also acquired and T2-maps generated for the same slices. For each voxel imaged, a single number was derived from the R2-map: the “percent-pathology”, which reflects the percent of epileptogenic cell mass in that voxel. A percent-pathology map (PPM) was then generated for each slice. From the PPMs, pathology-scores were derived for each hippocampus. Asymmetry indices were generated for these scores as well as for two alternative MRI methods (T2 relaxometry and T2w signal intensity (T2wSI)).Results: The ratios of each method’s result for the abnormal hippocampus in the MRI-negative patient to the ipsilateral hippocampus in the normal control subject were 1.27 by T2-relaxometry, 1.52 by T2wSI, and 2.62 by our method. Asymmetry indices showed a 3-12% difference between the pathological and normal hippocampi by the T2wSI approach, 11-18% by T2 relaxometry, and 24-61% by our method. The normal subject had minimal asymmetry by all methods.Conclusions: Quantification of pathology in MTLE using our method is feasible and warrants a larger study to assess its clinical utility. If a larger study does confirm that this method provides the means for noninvasive detection and quantification of epileptogenic tissue, it could help guide neurosurgeons in selection of tissue for resection and should lead to improved treatment of MTLE patients.