Segmentation of hippocampal subregion via 5-Tesla magnetic resonance imaging: a comparative study

Small voxel sizes in two-dimensional (2D) hippocampal magnetic resonance imaging (MRI) facilitate subfield segmentation. However, thinner slices remain challenging despite the advancements in in-plane resolution due to the tradeoffs in image quality. A higher signal-to-noise ratio (SNR) at 5 Tesla (T) compared to 3 T could enable thinner slices; however, the effects of changes in tissue contrast and inhomogeneous B1 variation may be weaken the SNR and contrast-to-noise ratio (CNR). However, the effectiveness of current automatic segmentation tools in handling 5-T images, particularly for thinner slices, remains to be fully elucidated. This study thus aimed to assess hippocampus image quality improvement from 3 to 5 T and to evaluate the autosegmentation of the hippocampal subregion on 5 T with different slice thicknesses.

This prospective study included 28 healthy participants (14 females, with a mean age of 31.8±8.96 years). T2-weighted series with 2-, 1-, and 0.7-mm slice thicknesses were acquired on 3 T and 5T. CNR, SNR, and visual scores of the molecular layer were compared. Hippocampal subregions were segmented with 5-T data using FreeSurfer and HippUnfold toolboxes. Dice coefficients of stratum radiatum, lacunosum, and moleculare (SRLM) were compared between FreeSurfer and HippUnfold. Fleiss' Kappa, the Kruskal-Wallis test, and the pairwise Wilcoxon signed-rank test were used for statistical analyses, with a P value <0.05 being considered statistically significant.

SNR, CNR, and visual scores were higher on 5 T than on 3 T at the same slice thickness. The SNR of the 1-mm T2 images on 5 T was comparable to that of 2-mm images on 3 T, and the CNR of the 0.7-mm images on 5 T was comparable to that of the 2-mm images on 3 T. With a reduction in slice thickness, the Dice coefficients of SRLM between HippUnfold and manual segmentation increased, while those between FreeSurfer and manual segmentation decreased.

The 5-T magnetic resonance system offers higher SNR and CNR values for hippocampal imaging as compared to 3-T imaging. A slice thickness of 0.7-1 mm is recommended for 2D T2-weighted imaging. However, in the selection of automatic segmentation tools for hippocampal subregions, caution is advised if the slice thickness is less than 2 mm.