Quantitative MR for epilepsy: a clinical and research tool?

An explosion in the use of quantitative magnetic resonance (MR) for the investigation of epilepsy has taken place during the last 5 years. Assessment of structural brain changes with MRbased volumetrics ( 1-7) or T2 relaxometry (81 0) is an ideal model for integrating research with clinical decision making. These techniques have furthered our understanding not only of brain dysfunction (in this case hippocampal sclerosis) but also of brain function (in this case the hippocampus). Quantitative techniques have found a place in the preoperative assessment of hippocampal sclerosis because of their increased sensitivity over visual inspection (11). Most quantitative MR investigations for epilepsy have concentrated on measuring brain volumes rather than relaxation times. Using this method, the degree of hippocampal atrophy can be quantified and compared with other variables for both research and clinical purposes. Hippocampal atrophy already has been shown to correlate with hippocampal sclerosis, lateralization of the electroencephalographic abnormalities, degree of hippocampal neuronal loss, verbal memory performance, and postoperative seizure control (1-7, 12-14). The critical clinical question is whether these techniques offer enough additional information over visual inspection to warrant the increased investment in personnel, training, time, equipment, and normative data development. With current imaging techniques, volume measurements detect hippocampal sclerosis in about 80% to 100% (1-7, 13) of cases compared with 80% to 90% (1, 7, 11, 12, 15-18) with visual assessment. (These values are somewhat inflated because of the methods used, such as retrospective analysis, narrow inclusion criteria, and inconsistent bases for diagnosis.) In studies directly comparing these two methods, volumetrics has a small but increased sensitivity (7, 11). There is debate as to the need for volume quantitation in routine clinical work (11). We agree with others that visual assessment is very accurate if multiple features of hippocampal sclerosis are evaluated, such as hippocampal atrophy, morphometric findings, and signal changes (12, 16, 18). We have not compared visual inspection with volume measurements at our institution. However, we detected hippocampal sclerosis visually in 50 (88%) of 57 patients with pathologic verification. Ten additional patients with no significant hippocampal neuronal loss were all correctly categorized as not having hippocampal sclerosis. The problem with visual inspection is that it requires a well-trained observer to assess the hippocampus correctly when confronted with head rotation or subtle changes (15). We believe volumetric measurements have a clinical role because they can correct for rotation, are reproducible, and are slightly more sensitive than visual evaluation. The manuscript by Grunewald et al (8) in this issue of AJNR, as well as a related paper by Jackson et al (9), describes a different quantitative technique for studying epilepsy. They found T2 relaxometry to be a reliable and sensitive method for detecting hippocampal sclerosis (for T2 values greater than 116 milliseconds). An older study by Matsuda et al also used T2 values to uncover hippocampal sclerosis (10). Although these findings are intriguing, one might ask why we should be interested in this quantitative technique for determining hippocampal sclerosis, when a reliable, sensitive, and specific method already exists? The answer lies with the biological factors that are the basis for the MR findings . The underlying mechanism for T2 prolongation may be independent of the mechanism producing the atrophic changes. This could have important implications for the following clinical and research questions: 1. Can T2 values help identify the additional 5% to 15% of patients with hippocampal