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.

Conflict of interest statement

The authors declare no competing interests.

© 2022. The Author(s).

Figures

Figure 1

(A) Sagittal T2-weighted turbo spin echo acquisition in the lower spine used for subsequent prescription of the high-resolution axial acquisition. (B) Corresponding axial slices acquired with the 3D multi-echo gradient-echo sequence ( Siemens FLASH) in the caudal-rostral direction (slices 1–20). Highlighted are the slice in the lumbosacral enlargement (LSE) with the largest cord cross-sectional spinal cord area (defined as the "LSE slice" and shown in light blue in A and B; here: slice 15), and the most caudal slice in the conus medullaris (CM) where the gray matter still has the characteristic butterfly shape (defined as the "CM slice" and shown in red in A and B; here: slice 9). A saturation band, displayed as yellow shaded area in A, was placed anterior to the spine to suppress signal and possible artifacts arising from abdominal peristalsis.

Figure 2

Visual representation of echoes, echo combinations, signal averages, and image segmentations. The 3D spoiled multi-echo gradient echo sequence (Siemens FLASH) consisted of 5 echoes and was acquired with 8 individual repetitions. For each subject, a series of images was created by successively combining echoes (echo 1, 1–2, 1–3, 1–4, 1–5) and averaging across repetitions (number of signal averages (NSA): 1, 2, 3, …, 8), resulting in a total of 72 images. (A) Image series of individual echoes (NSA = 8) for a representative slice in the lumbosacral enlargement (LSE). (B) Image series of increasing number of combined echoes in the same slice as in (A) (NSA = 8). (C) Image series with increasing NSA (3 combined echoes). (D) Spinal cord (SC) and gray matter (GM) were segmented manually in each slice (here a representative LSE and conus medullaris slice are shown). A mask of cerebrospinal fluid (CSF) was drawn anterior to the SC. White matter (WM) mask was obtained by subtracting GM from the SC mask.

Figure 3

Quantitative comparison of the individual and combined echoes (8 signal averages) in terms of contrast-to-noise ratio (CNR) and contrast between white matter (WM) and cerebrospinal fluid (CSF) and betwen gray matter (GM) and WM, and signal-to-noise ratio (SNR) of WM and GM. Values are displayed for individual echoes (1, 2, 3, 4, and 5) and combined echoes (1–2, 1–3, 1–4 and 1–5). All measures are displayed separately for the lumbosacral enlargement (LSE) in blue and conus medullaris (CM) in red. In all subplots, error bars represent standard deviation across participants (n = 10).

Figure 4

Quantitative comparison of the number of signal averages (NSA) (3 combined echoes) in terms of contrast-to-noise ratio (CNR) and contrast between white matter (WM) and cerebrospinal fluid (CSF) and between gray matter (GM) and WM, and signal-to-noise ratio (SNR) of WM and GM. All measures are displayed separately for the lumbosacral enlargement (LSE) in blue and conus medullaris (CM) in red. In all subplots, error bars represent standard deviation across participants (n = 10).

Figure 5

Qualitative comparison of the individual and combined echoes (8 signal averages). Shown are ranking scores assigned to individual echoes (1, 2, 3, 4, 5) and combined echoes (1–2, 1–3, 1–4, 1–5) and averaged across all participants (n = 10) and raters (n = 5). Echoes/echo combinations were ranked separately for spinal cord (SC) only, gray matter (GM) only and joint SC and GM segmentability. The analysis was performed in a representative slice at the lumbosacral enlargement (LSE) in blue and conus medullaris (CM) in red. Ranking score ranges from 1 (lowest score) to 9 (highest score). Error bars represent standard deviation across participants.