Optimized multi-echo gradient-echo magnetic resonance imaging for gray and white matter segmentation in the lumbosacral cord at 3 T

Abstract

Atrophy in the spinal cord (SC), gray (GM) and white matter (WM) is typically measured in-vivo by image segmentation on multi-echo gradient-echo magnetic resonance images. The aim of this study was to establish an acquisition and analysis protocol for optimal SC and GM segmentation in the lumbosacral cord at 3 T. Ten healthy volunteers underwent imaging of the lumbosacral cord using a 3D spoiled multi-echo gradient-echo sequence (Siemens FLASH, with 5 echoes and 8 repetitions) on a Siemens Prisma 3 T scanner. Optimal numbers of successive echoes and signal averages were investigated comparing signal-to-noise (SNR) and contrast-to-noise ratio (CNR) values as well as qualitative ratings for segmentability by experts. The combination of 5 successive echoes yielded the highest CNR between WM and cerebrospinal fluid and the highest rating for SC segmentability. The combination of 3 and 4 successive echoes yielded the highest CNR between GM and WM and the highest rating for GM segmentability in the lumbosacral enlargement and conus medullaris, respectively. For segmenting the SC and GM in the same image, we suggest combining 3 successive echoes. For SC or GM segmentation only, we recommend combining 5 or 3 successive echoes, respectively. Six signal averages yielded good contrast for reliable SC and GM segmentation in all subjects. Clinical applications could benefit from these recommendations as they allow for accurate SC and GM segmentation in the lumbosacral cord.

Improved Inter-Subject Alignment of the Lumbosacral Cord for Group-Level In Vivo Gray and White Matter Assessments: A Scan-Rescan MRI Study at 3T

“…We demonstrated excellent intra-rater reliability for CSA measurements, when using the previously optimized ME-GRE imaging protocol (Büeler et al, 2022), with intra-rater CV values below 4%, 5%, and 6% for SC, GM, and WM CSA, respectively, at and within a 15 mm radius of the LSE landmark. Notably, the inter-rater CV values for CSA measurements were approximately twice as high as the corresponding intra-rater CV values.…”

“…The higher values for SC CSA, in comparison to GM and WM CSA, reflect the raters’ experience that SC segmentation is an "easier" task than GM segmentation. This is due to the fact that the contrast between the GM and WM is typically lower than between the WM and cerebrospinal fluid on ME-GRE images, and GM exhibits more ambiguity owing to its irregular shape (Büeler et al, 2022). The reliability of CSA measurements depends on the confidence with which manual segmentation is performed, referred to as "segmentability".…”

“…as it has been demonstrated to be the optimal combination for segmenting the SC and GM within the same image (Büeler et al, 2022). The resulting combined echo was then averaged across all repetitions.…”

“…The reliability of CSA measurements depends on the confidence with which manual segmentation is performed, referred to as "segmentability". SC and GM segmentability is influenced by various factors, including the rater's experience, the subject being investigated, the imaging hardware, pulse sequence, and sequence parameters (Büeler et al, 2022). Consequently, the comparability of reliability values across studies is limited by variations in these factors.…”

“…Furthermore, advanced MRI techniques, such as diffusion MRI, have provided complimentary information on WM integrity (Ciccarelli et al, 2007; Cohen-Adad et al, 2011, 2013; Combes et al, 2022; David et al, 2021, 2022; David, et al, 2019b; Huber et al, 2018; Vallotton et al, 2021). In contrast, there have been only a few studies investigating the conus medullaris (CM) extending caudally from the LSE (Büeler et al, 2022; Yiannakas et al, 2019). Imaging the entire lumbosacral cord is of particular interest, as tissue damage may also occur (Konno et al, 1986) or even originate within the CM (Panicker et al, 2020).…”