Hippocampal MRS and subfield volumetry at 7T detects dysfunction not specific to seizure focus

Abstract

Ultra high-field 7T MRI offers sensitivity to localize hippocampal pathology in temporal lobe epilepsy (TLE), but has rarely been evaluated in patients with normal-appearing clinical MRI. We applied multimodal 7T MRI to assess if focal subfield atrophy and deviations in brain metabolites characterize epileptic hippocampi. Twelve pre-surgical TLE patients (7 MRI-negative) and age-matched healthy volunteers were scanned at 7T. Hippocampal subfields were manually segmented from 600μm isotropic resolution susceptibility-weighted images. Hippocampal metabolite spectra were acquired to determine absolute concentrations of glutamate, glutamine, myo-inositol, NAA, creatine and choline. We performed case-controls analyses, using permutation testing, to identify abnormalities in hippocampal imaging measures in individual patients, for evaluation against clinical evidence of seizure lateralisation and neuropsychological memory test scores. Volume analyses identified hippocampal subfield atrophy in 9/12 patients (75%), commonly affecting CA3. 7/8 patients had altered metabolite concentrations, most showing reduced glutamine levels (62.5%). However, neither volume nor metabolite deviations consistently lateralized the epileptogenic hippocampus. Rather, lower subiculum volumes and glutamine concentrations correlated with impaired verbal memory performance. Hippocampal subfield and metabolic abnormalities detected at 7T appear to reflect pathophysiological processes beyond epileptogenesis. Despite limited diagnostic contributions, these markers show promise to help elucidate mnemonic processing in TLE.

Hippocampal Subfields in Acute and Remitted Depression—an Ultra-High Field Magnetic Resonance Imaging Study

“…Of note, most studies included in a meta-analysis reported bilateral hippocampal volumes between 4794 mm 3 and 8298 mm 3 (Colle et al, 2018). A similar variation exists for subfields, for example, right CA1 volumes in healthy subjects between 34 and 1635 mm 3 are reported (Sone et al, 2016; Voets et al, 2017).…”

Dissociable age and memory relationships with hippocampal subfield volumes in vivo:Data from the Irish Longitudinal Study on Ageing (TILDA)

“…As predicted, our results revealed roles for CA1-CA4 and GC-DG, in addition to molecular layer, in the encoding and subsequent retrieval of novel verbal word lists. Several recent studies have provided piecemeal evidence of associations between subfield volumes and verbal episodic memory, implicating subiculum in immediate verbal recall 45 , and CA1 46 , subiculum 46,47 , and presubiculum 45 in delayed verbal recall. Our current IR findings implicate each of the CA subfields and GC-DG, supporting their previously demonstrated roles in encoding of novel stimuli/environments 12,34 , and in verbal and visuo-spatial episodic memory 17,48 .…”

Neurochemical correlates of scene processing in the precuneus/posterior cingulate cortex: A multimodal fMRI and 1H‐MRS study

“…Due to its location and size, it is technically challenging to acquire high quality 1 H‐MRS spectra in the hippocampal region (Geurts et al, ; Kreis, ; Venkatraman et al, ). Future studies using ultra‐high field 1 H‐MRS (e.g., Voets, Hodgetts, Sen, Adcock, & Emir, ) are now required.…”

“…Future studies using ultra-high field 1 H-MRS (e.g., Voets, Hodgetts, Sen, Adcock, & Emir, 2017) are now required.…”