Automated T2 relaxometry of the hippocampus for temporal lobe epilepsy

Abstract

Objective: Hippocampal sclerosis (HS), the most common cause of refractory temporal lobe epilepsy, is associated with hippocampal volume loss and increased T2 signal. These can be identified on quantitative imaging with hippocampal volumetry and T2 relaxometry. Although hippocampal segmentation for volumetry has been automated, T2 relaxometry currently involves subjective and time-consuming manual delineation of regions of interest. In this work, we develop and validate an automated technique for hippocampal T2 relaxometry.

Methods: Fifty patients with unilateral or bilateral HS and 50 healthy controls underwent T₁ -weighted and dual-echo fast recovery fast spin echo scans. Hippocampi were automatically segmented using a multi-atlas-based segmentation algorithm (STEPS) and a template database. Voxelwise T2 maps were determined using a monoexponential fit. The hippocampal segmentations were registered to the T2 maps and eroded to reduce partial volume effect. Voxels with T2 >170 msec excluded to minimize cerebrospinal fluid (CSF) contamination. Manual determination of T2 values was performed twice in each subject. Twenty controls underwent repeat scans to assess interscan reproducibility.

Results: Hippocampal T2 values were reliably determined using the automated method. There was a significant ipsilateral increase in T2 values in HS (p < 0.001), and a smaller but significant contralateral increase. The combination of hippocampal volumes and T2 values separated the groups well. There was a strong correlation between automated and manual methods for hippocampal T2 measurement (0.917 left, 0.896 right, both p < 0.001). Interscan reproducibility was superior for automated compared to manual measurements.

Significance: Automated hippocampal segmentation can be reliably extended to the determination of hippocampal T2 values, and a combination of hippocampal volumes and T2 values can separate subjects with HS from healthy controls. There is good agreement with manual measurements, and the technique is more reproducible on repeat scans than manual measurement. This protocol can be readily introduced into a clinical workflow for the assessment of patients with focal epilepsy.

Keywords: Hippocampus; Magnetic resonance imaging; T2 relaxometry; Temporal lobe epilepsy.

Figure 1

Example of a patient with left hippocampal sclerosis. Structural T₁‐weighted scan (A) with automated hippocampal segmentation (B). The segmentation is registered to the dual‐echo PD/T₂‐weighted scan, eroded, and CSF excluded (C), and the mean T2 value determined on the voxelwise T2 map (D). Manual determination of T2 values relies of manually drawn elliptic ROIs (E).

Figure 2

Scatterplot of hippocampal volumes corrected for intracranial volume (left) and mean hippocampal T2 (right) of both hippocampi obtained by the automated method in the different groups.

Figure 3

Box plots of hippocampal volumes corrected for intracranial volume and mean hippocampal T2 obtained by the automated method in the different groups.

Figure 4

Scatter plot of hippocampal volumes corrected for intracranial volume and mean hippocampal T2 of the left and right hippocampi, respectively, showing good separation between the different groups.

Figure 5

Comparison of automated and manual hippocampal T2 determination (top) and of intrarater reliability of manual determination on the same scans (bottom).