Quantitative analysis of temporal lobe white matter T2 relaxation time in temporal lobe epilepsy

Abstract

The objective of this study was to assess temporal lobe white matter (WM) quantitatively using T2 relaxometry in patients with pharmacologically intractable temporal lobe epilepsy (TLE). T2 relaxometry was performed using a dual-echo sequence with 23 contiguous oblique coronal slices in 56 consecutive TLE patients and in 30 healthy subjects. Averages of six slices were chosen to calculate T2 relaxation time in the temporal lobe WM (WM-T2) and the hippocampus (Hippo-T2). Twenty-seven patients had unilateral hippocampal atrophy (HA), and twenty-nine patients had normal hippocampal volumes (NV) on volumetric MRI. Mean WM-T2 was increased ipsilateral to the seizure focus in TLE patients with HA and those with NV (P < 0.001). Contralateral mean WM-T2 was increased in left and right TLE with HA (P < 0.001) and in right TLE with NV (P = 0.001). There was a positive correlation between WM-T2 and Hippo-T2. Individual analysis showed a prolongation of WM-T2 in about 70% of TLE patients with HA and NV. In half of the patients, WM-T2 increase was bilateral and symmetric. However, in 33% of patients with NV and bilateral symmetric increase in Hippo-T2, WM-T2 provided a correct lateralization of the seizure focus. Regardless of the pattern of T2 abnormalities, that is, bilateral symmetric or ipsilateral, the majority of patients with HA became seizure-free after surgery, while those with NV did not have a favorable outcome. In patients with NV, WM-T2 measurement may provide additional lateralizing information compared to Hippo-T2.

Introduction

Hippocampal sclerosis is the histopathological hallmark of medically intractable temporal lobe epilepsy (TLE), the most common human epileptic syndrome (Engel et al., 1997). Visual inspection of magnetic resonance imaging (MRI) of TLE patients enables in vivo detection of obvious hippocampal atrophy or altered signal intensity related to hippocampal sclerosis Cascino et al., 1991, Jackson et al., 1990. However, novel quantitative techniques such as MRI volumetry and T2 relaxometry are being increasingly used to reliably identify hippocampal neuronal loss and gliosis Bernasconi et al., 2000, Briellmann et al., 2002, Jack et al., 1990, Jackson et al., 1993, Van Paesschen et al., 1995.

T2 relaxometry allows quantitative determination of T2-weighted intensity changes. An approximation of the T2 relaxation behavior may be obtained by a variety of methods, for example, using 16-echo times (Jackson et al., 1993) or 2-echo times Bernasconi et al., 2000, Duncan et al., 1996 sequences. A dual-echo approach gives reliable estimates of the T2 relaxation process and is easily amenable to whole brain examination (Duncan et al., 1996). In TLE, T2 relaxometry has allowed detection of subtle hippocampal pathology that was not recognized by visual inspection Bernasconi et al., 2000, Jackson et al., 1994.

In addition to hippocampal sclerosis, temporal lobe white matter (WM) abnormalities have been described in neuropathological studies in TLE Falconer et al., 1964, Margerison and Corsellis, 1966. Falconer et al. (1964) suggested that the sclerotic process affecting the mesial structures affects the temporal lobe widely, leading to a generalized atrophy and gliosis of both the cortex and the WM. In their observations of specimens obtained from temporal lobectomies, these authors (Falconer et al., 1964) reported gliosis of the WM in both patients with clear-cut hippocampal sclerosis and those without. Recent studies examining the WM in resected brain tissue from TLE patients have shown varying degrees of gliosis Mitchell et al., 1999, Swartz et al., 1992, microdysgenesis, and demyelination Choi et al., 1999, Hardiman et al., 1988, Kasper et al., 1999, Kasper et al., 2003, Theodore et al., 1990, Zentner et al., 1995.

Besides temporal lobe WM atrophy Breier et al., 1996, Coste et al., 2002, Lee et al., 1998, Marsh et al., 1997, T2-weighted intensity increases in the temporal pole are described as part of the spectrum of MRI features found in hippocampal sclerosis Choi et al., 1999, Coste et al., 2002, Meiners et al., 1994, Mitchell et al., 1999, Mitchell et al., 2003. These abnormalities are often seen on the side of hippocampal atrophy, and there are indications that they may extend beyond the temporal pole in some cases Mitchell et al., 2003, Ryvlin et al., 2002. In addition, it is unclear whether these changes might occur in TLE patients without hippocampal atrophy on MRI (Andermann, 2002).

The purpose of this study was to assess temporal lobe WM T2-weighted intensity using T2 relaxometry in patients with pharmacologically intractable TLE. We chose the temporal stem as our region of interest (ROI) representing the temporal lobe WM as it contains major projection fibers connecting the temporal lobe to the rest of the brain Naidich et al., 1987, Sedat and Duvernoy, 1990. In addition, this region is relatively free of partial volume effects and flow artifacts, allowing more accurate T2 relaxation time estimates.