Clear and Consistent Imaging of Hippocampal Internal Architecture With High Resolution Multiple Image Co-registration and Averaging (HR-MICRA)

Abstract

Magnetic resonance imaging of hippocampal internal architecture (HIA) at 3T is challenging. HIA is defined by layers of gray and white matter that are less than 1 mm thick in the coronal plane. To visualize HIA, conventional MRI approaches have relied on sequences with high in-plane resolution (≤0.5 mm) but comparatively thick slices (2–5 mm). However, thicker slices are prone to volume averaging effects that result in loss of HIA clarity and blurring of the borders of the hippocampal subfields in up to 61% of slices as has been reported. In this work we describe an approach to hippocampal imaging that provides consistently high HIA clarity using a commonly available sequence and post-processing techniques that is flexible and may be applicable to any MRI platform. We refer to this approach as High Resolution Multiple Image Co-registration and Averaging (HR-MICRA). This approach uses a variable flip angle turbo spin echo sequence to repeatedly acquire a whole brain T2w image volume with high resolution in three dimensions in a relatively short amount of time, and then co-register the volumes to correct for movement and average the repeated scans to improve SNR. We compared the averages of 4, 9, and 16 individual scans in 20 healthy controls using a published HIA clarity rating scale. In the body of the hippocampus, the proportion of slices with good or excellent HIA clarity was 90%, 83%, and 67% for the 16x, 9x, and 4x HR-MICRA images, respectively. Using the 4x HR-MICRA images as a baseline, the 9x HR-MICRA images were 2.6 times and 16x HR-MICRA images were 3.2 times more likely to have high HIA ratings (p < 0.001) across all hippocampal segments (head, body, and tail). The thin slices of the HR-MICRA images allow reformatting in any plane with clear visualization of hippocampal dentation in the sagittal plane. Clear and consistent visualization of HIA will allow application of this technique to future hippocampal structure research, as well as more precise manual or automated segmentation.

Loss of hippocampal dentation in hippocampal sclerosis and its relationship to memory dysfunction

“…However, in this work, we have used a method that evaluates only the number of dentes on the inferior aspect of the CA1/subiculum region. The primary reason for this difference is that the previous study used a specific high‐resolution T2w acquisition and processing scheme (HR‐MICRA), 38 which allowed for imaging hippocampal internal architecture and hippocampal subfields (particularly SRLM (i.e) Stratum Radiatum, Lacunosum, and Moleculare) more precisely with excellent contrast variation, which would be impossible to visualize with common 2D approaches or T1w MPRAGE images. The image resolution of HR‐MICRA sequence is .5 mm × .5 mm × .75 mm.…”

“…Of note, this approach to quantification of dentation is different from our prior published work, which utilized high-resolution 3D T2w images generated using the HR-MICRA method. 38 Those images were acquired with different tissue contrast and at a much higher resolution, which allowed clear visualization of the hippocampal layers and subfields and assessment of the depth and contour of dentes. This allowed for scoring of the prominence of dentation as well as the quantity.…”

Development of an objective method to quantify hippocampal dentation

“…The dentation score was formulated to account for the quantity and prominence of dentation separately by assigning a score of 0 to 2 for each parameter based on defined visual characteristics for the anterior and posterior sections of each hippocampi (Beattie et al, 2017 ). This subjective method was applied to a cohort of healthy individuals using ultra high‐resolution T2w images acquired with a time‐consuming method (HR‐MICRA) that provides excellent differentiation of the gray and white matter layers of the hippocampal internal structure (Ver Hoef et al, 2021 ), which is essential for applying the scoring system. Results showed a positive correlation between left‐sided dentation and episodic verbal learning memory, and both left and right‐sided dentation were significantly correlated with visual memory scores.…”

Recent Advances in Neuroimaging of Epilepsy

“…Thus, although morphometry is an important part of volumetric studies, some morphometry studies extend beyond assessing regional volumetric changes to visualize and quantify changes in specific shape patterns and internal architecture of brain structures (e.g., dentation of the hippocampus). Visualization and quantification of fine cortical anomalies and changes in structural patterns of hippocampal nuclei and subnuclei has been made possible through optimization of T1-or T2-weighted imaging at 3 T (3 T) and 7 T MRI and/or introduction of new data collection schemes resulting in improved spatial resolution of the collected images [17]. Specifically, morphometric analysis of hippocampal dentations of the inferior surface has shown that greater dentation relates to better episodic memory performance in healthy controls [16].…”

“…Similar methods have recently been utilized to quantify smooth or mildly irregular hippocampi in TLE, suggesting that asymmetry in dentation may serve as a hallmark of TLE phenotypes [18]. Additionally, subsequent reconstruction of anatomical information acquired during T2-weighted imaging is susceptible to artificial smoothing during initial processing of the MRI data at 3 T [17]. However, the increased spatial resolution of 7 T imaging combined with automated segmentation methods can more accurately and precisely estimate internal architecture of the hippocampi [19].Thus, new understanding about the role of hippocampal subfields and dentation patterns in patient profiles and as predictors of surgical outcomes may be possible [20].…”