Examination of methods to separate overlapping metabolites at 7T

Abstract

Purpose: Neurochemicals of interest quantified by MRS are often composites of overlapping signals. At higher field strengths (i.e., 7T), there is better separation of these signals. As the availability of higher field strengths is increasing, it is important to re-evaluate the separability of overlapping metabolite signals.

Methods: This study compares the ability of stimulated echo acquisition mode (STEAM-8; TE = 8 ms), short-TE semi-LASER (sLASER-34; TE = 34 ms), and long-TE semi-LASER (sLASER-105; TE = 105 ms) acquisitions to separate the commonly acquired neurochemicals at 7T (Glx, consisting of glutamate and glutamine; total N-acetyl aspartate, consisting of N-acetyl aspartate and N-acetylaspartylglutamate; total creatine, consisting of creatine and phosphocreatine; and total choline, consisting of choline, phosphocholine, and glycerophosphocholine).

Results: sLASER-34 produced the lowest fit errors for most neurochemicals; however, STEAM-8 had better within-subject reproducibility and required fewer subjects to detect a change between groups. However, this is dependent on the neurochemical of interest.

Conclusion: We recommend short-TE STEAM for separation of most standard neurochemicals at 7T over short-TE or long-TE sLASER.

Keywords: 7 T; STEAM; brain; high field; magnetic resonance spectroscopy; semi‐LASER.

FIGURE 1

(A) Example of voxel location in the parietal cortex. Mean (black) and SD (shaded gray) of spectra acquired using (B) STEAM‐8 (stimulated echo acquisition mode, TE = 8 ms), (C) sLASER‐34 (semi‐LASER, TE = 34 ms), and (D) sLASER‐105 (semi‐LASER, TE = 105 ms).

FIGURE 2

(A) Mean and spread of glutamate (Glu), glutamine (Gln), and Glu + Gln (Glx) data for each sequence. (B) Mean Cramér‐Rao lower bound (CRLB) values for Glu, Gln, and Glx, error bars represent standard deviation. (C) Coefficient of variation (CV) values for Glu, Gln, and Glx, error bars represent 95% confidence intervals, calculated using bootstrap resampling. *p < 0.05, **p < 0.01, ***p < 0.001. sLASER‐34, semi‐LASER, TE = 34 ms; sLASER‐105, semi‐LASER, TE = 105 ms; STEAM‐8, stimulated echo acquisition mode, TE = 8 ms.

FIGURE 3

(A) Mean and spread of N‐acetyl aspartate (NAA), N‐acetylaspartylglutamate (NAAG), and total N‐acetyl aspartate (tNAA; NAA + NAAG) data for each sequence. (B) Mean Cramér‐Rao lower bound (CRLB) values for NAA, NAAG and tNAA, error bars represent standard deviation. (C) Coefficient of variation (CV) values for NAA, NAAG and tNAA, error bars represent 95% confidence intervals calculated using bootstrap resampling. *p < 0.05, **p < 0.01, ***p < 0.001. sLASER‐34, semi‐LASER, TE = 34 ms; sLASER‐105, semi‐LASER, TE = 105 ms; STEAM‐8, stimulated echo acquisition mode, TE = 8 ms.

FIGURE 4

(A) Mean and spread of creatine (Cr), phosphocreatine (PCr), and total creatine (tCr; Cr + PCr) data for each sequence. (B) Mean Cramér‐Rao lower bound (CRLB) values for Cr, PCr, and tCr, error bars represent SD. (C) Coefficient of variation (CV) values for Cr, PCr, and tCr, error bars represent 95% confidence intervals calculated using bootstrap resampling. *p < 0.05, **p < 0.01, ***p < 0.001. sLASER‐34, semi‐LASER, TE = 34 ms; sLASER‐105, semi‐LASER, TE = 105 ms; STEAM‐8, stimulated echo acquisition mode, TE = 8 ms.

FIGURE 5

(A) Mean and spread of choline (Cho), glycerophosphocholine (GPC) and total choline (tCho; Cho + phosphocholine (PC) + GPC) data for each sequence. (B) Mean Cramér‐Rao lower bound (CRLB) values for Cho, GPC, and tCho, error bars represent SD. Mean Cho CRLB for sLASER‐34 not shown as > 500 (C) Coefficient of variation (CV) values for Cho, GPC and tCho, error bars represent 95% confidence intervals calculated using bootstrap resampling. *p < 0.05, **p < 0.01, ***p < 0.001. sLASER‐34, semi‐LASER, TE = 34 ms; sLASER‐105, semi‐LASER, TE = 105 ms; STEAM‐8, stimulated echo acquisition mode, TE = 8 ms.