Fast mapping of the T₂ relaxation time of cerebral metabolites using proton echo‐planar spectroscopic imaging (PEPSI)

Abstract

Metabolite T 2 is necessary for accurate quantification of the absolute concentration of metabolites using long-echo-time (TE) acquisition schemes. However, lengthy data acquisition times pose a major challenge to mapping metabolite T 2 . In this study we used proton echo-planar spectroscopic imaging (PEPSI) at 3T to obtain fast T 2 maps of three major cerebral metabolites: N-acetyl-aspartate (NAA), creatine (Cre), and choline (Cho). We showed that PEPSI spectra matched T 2 values obtained using single-voxel spectroscopy (SVS). Data acquisition for 2D metabolite maps with a voxel volume of 0.95 ml (32 ؋ 32 image matrix) can be completed in 25 min using five TEs and eight averages. A sufficient spectral signal-to-noise ratio (SNR) for T 2 estimation was validated by high Pearson's correlation coefficients between logarithmic MR signals and TEs (R 2 ‫؍‬ 0.98, 0.97, and 0.95 for NAA, Cre, and Cho, respectively). In agreement with previous studies, we found that the T 2 values of NAA, but not Cre and Cho, were significantly different between gray matter (GM) and white matter (WM; P < 0.001). The difference between the T 2 estimates of the PEPSI and SVS scans was less than 9%. Consistent spatial distributions of T 2 were found in six healthy subjects, and disagreement among subjects was less than 10%. In summary, the PEPSI technique is a robust method to obtain fast mapping of metabolite Key words: proton echo-planar spectroscopic imaging; singlevoxel spectroscopy; T 2 relaxation time; cerebral metabolites; gray/white matter difference Estimation of the relaxation times of metabolites is necessary for accurate quantification of metabolite concentrations using long-echo-time (TE) acquisition schemes (1-3). Given the T2 relaxation time, metabolite signals acquired at different TEs can be extrapolated to obtain the signal at TE ϭ 0 and thus estimate the concentration of the metabolite (4). Differences in T2 decay are negligible only for short-TE methods (with TE below 10 ms). In several pathological conditions, such as edema and ischemic stroke (5-8), interactions between cerebral metabolites and macromolecules and proteins are known to modify the biochemical environments of the metabolites, which in turn alters the motion-sensitive T2 relaxation time. Accurate estimation of metabolite T2 is therefore especially important in clinical applications of magnetic resonance spectroscopy (MRS) to ascertain whether changes in metabolite MR signals are derived from fluctuations in the metabolite concentration or from changes in the metabolite relaxation time (2,9). In addition to providing information about metabolite concentration, T2 values may give complementary information about metabolite behavior, as has been suggested by studies of brain tumor (9), ischemic stroke (5-7,10), virus infection (11), drug abuse (12), and other neurological disorders (13-15).The T2 relaxation times of metabolites can be measured by collecting multiple spectra over a range of TEs and then fitting the MR signals as a function of TE (1-3,5,10). Se...

Regional metabolite T2 in the healthy rhesus macaque brain at 7T

“…2 time, over an extensive (25%) of the macaque brain at spatial resolution that scales as the respective brain volumes. The results reveal that these T 2 values are, as expected, much shorter than the 180 -250 ms reported for these metabolites at 3T in the human brain (9,10). Comparing with single-voxel human T 2 values at 7T reported by Michaeli et al (15) shows that these T 2 values are in agreement with the macaque's, i.e., 109 vs. 114 ms for Cr and 158 vs. 169 ms for NAA (Cho T 2 was not reported in that study).…”

“…Although T 1 -and T 2 -weighting can be reduced with long repetition and short echo-times (TR ӷ T 1 and TE Ӷ T 2 ) intermediate-and long-TE spectra are often preferred for their flatter baseline, decreased lipid contamination, and simpler peak structure (4). Consequently, reliable estimation of the metabolites' T 2 values is needed for their accurate quantification, especially at high fields (9,10).…”

Brain Metabolite Proton T2 Mapping at 3.0 T in Relapsing-Remitting Multiple Sclerosis

“…This weighting will decrease proportionally for shorter TEs ( 17 ). Furthermore, the similar T2 distributions among patients, refl ected by overlapping histograms, are also similar in their full width at half maximum values to that of their healthy contemporaries (17)(18)(19).…”

“…While the patients' mean T2 values are in the ranges of 221-343 msec, 137-178 msec, and 187-248 msec reported in controls for NAA, Cr, and Cho, respectively (17)(18)(19)36,41 ), the most appropriate comparison is with values obtained by using the same protocol and instrument. This revealed all patient T2 values as shorter than those of their healthy contemporaries, suggesting that MS pathologic features alter the molecular environment factors that infl uence T2 and that the second scenario (ie, that of similar inter-and intrapatient T2 values that differ from controls) is operative.…”

“…Because longer-TE spectra are often preferred over shorter-TE spectra owing to their fl atter baseline, simpler peak structure, and milder demands on the hardware ( 16 ), the accuracy of their quantifi cation depends on reliable knowledge of T2 values ( 17,18 ). Surprisingly, although T2 distributions are well characterized in controls (17)(18)(19), they are rarely measured in MS patients and, when reported, are noted only in the voxel of interest (20)(21)(22). Absent these T2 values or the time to obtain them in every image, subjects quantitative implicit assumptions: That one set of T2 values is applicable anywhere in the brain; and that this set of T2 values is also appropriate for all subjects.…”