displays numerically-calculated PRESS spectra of mI (brown, thin) and mI+Gly (blue, thick) vs. the first and second subecho times TE1 and TE2 between 50 – 110 ms, for a concentration ratio of [mI]/[Gly] = 5. The spectral pattern and signal intensity of mI depended on the subecho times due to the J coupling effects, while the Gly peak was the same at all echo times, ignoring T2 relaxation effects. For TE1 and TE2 in 50 – 70 ms, the maximum amplitude of the mI multiplet appeared at the Gly ~3.55 ppm resonance, making it difficult to resolve Gly from the mI background signal. The mI multiplet intensity at ~3.55 was decreased at total echo time ~ 160 ms. At (TE1, TE2) = (60, 100) ms, the maximum amplitude of mI was shifted to 3.62 ppm and as a result, the spectral difference between mI and mI+Gly became pronounced. The mI multiplet was dependent on TE1 and TE2 for a constant total echo time. When TE1 was greater than TE2, the mI peak amplitude at 3.62 ppm became similar to that at 3.55 ppm and eventually the mI multiplet exhibited a uniform low intensity between 3.52 – 3.62 ppm at (TE1, TE2) = (110, 50) ms. A Gly peak may be easily detectable at this condition, but mI may not be measured reliably due to its relatively low intensity. Thus, (TE1, TE2) = (60, 100) ms may be optimal for measuring both Gly and mI and was selected for the present study.
FIG. 1 Calculated PRESS spectra of mI+Gly (thick/blue) and mI (thin/brown), at 3T, are displayed vs. subecho times TE1 and TE2 for 50 – 110 ms with 10 ms increments. The spectra are scaled with a concentration ratio of [Gly]:[mI] = 1:5. The spectral (more ...)
presents the phantom validation for the performance of the optimized PRESS echo time. A STEAM sequence with (TE, TM) = (18
) ms, whereby the mI signal modulation due to J coupling effects was negligible, reproduced closely a 90°-acquired spectrum. For phantom-1, a STEAM spectrum showed a mI multiplet at ~3.55 ppm with amplitude 31% with respect to the Cr 3.03-ppm singlet at linewidth of 3 Hz. Following the PRESS (TE1
) = (60, 100) ms, the mI signal between 3.45 – 3.65 ppm was reduced to 5% relative to the Cr peak. The lineshape of this mI multiplet agreed well with the simulation result in . For phantom-2, which contained Gly at [mI]/[Gly] = 5, Gly was not readily detectable in a STEAM spectrum due to the large mI signal. However, the small Gly singlet at 3.55 ppm was clearly revealed in the optimized-PRESS spectrum. The composite signal pattern of Gly and mI agreed well with the calculated spectra in . An LCModel analysis of this phantom spectrum gave a concentration ratio of [Cr]:[mI]:[Gly] = 7:4.8:1. The MRS-measured concentration ratio reproduced closely the prepared concentration ratio (i.e.
, 7:5:1). The measured ratio of mI was slightly lower than the prepared value most likely due to the mI T2
shorter than the Cr T2
in the phantom solution (0.8 and 1.6 s, respectively, as measured from phantom-1). The mI multiplet at 4.06 ppm was smaller in the PRESS spectra than in the STEAM spectra, mainly due to the effects of J evolution. The Glu multiplet was also reduced in the PRESS spectrum due to the J coupling effects, giving a C4-proton multiplet at 2.35 ppm with amplitude ~10% with respect to the Cr 3.03 ppm peak.
FIG. 2 In vitro spectra at PRESS (TE1, TE2) = (60, 100) ms, obtained from Phantom-1 with mI (25 mM) and Cr (40 mM), and Phantom-2 with Gly, mI, Cr, NAA and Glu at a concentration ratio of 1:5:8:10:10, at 3T, are shown together with short-TE STEAM spectra. Singlet (more ...)
presents signal-to-noise ratio (SNR) dependence of Gly estimation by the optimized PRESS sequence for the healthy brain. Spectra are shown for various number of signal averages (NSA) in the left column and their LCModel fitting results (concentration estimates and CRLB) are plotted vs. NSA in the right column for Gly, mI, tCr, NAA and GPCPC. With TR = 2 s and voxel size = 8 ml, the predominant signals of Cr, NAA and GPCPC was measurable with CRLB less than 5% even at NSA = 8, but the concentration estimates were not constant until NSA = 30. The dependence of the concentration estimates and CRLB on NSA was more pronounced for mI and Gly. The CRLB of mI was less than 20% for NSA ≥ 8. The mI concentration estimates were varied at small NSAs and became constant after NSA ~ 80. For Gly, CRLB less than 20% was achievable only when NSA was greater than 50. After this, Gly CRLB was further decreased with increasing NSA and eventually became ≤ 8 at NSA > 110. present LCModel fitting results from a basis set with or without Gly, respectively. While the fit with Gly in the basis function gave noise-level residuals at ~3.55 ppm, a fit without Gly produced large residuals at the Gly resonance, indicating that a peak at 3.55 ppm is primarily attributed to Gly.
FIG. 3 (a) In vivo brain spectra are presented vs. number of signal averages (NSA). Vertical dotted lines are drawn at 3.55 and 3.62 ppm. (b) Voxel (2×2×2 mm3) positioning in the occipital cortex. (c) Concentration estimates and (d) LCModel spectral (more ...)
displays in vivo
brain spectra and the LCModel fits from a healthy volunteer and a GBM patient, together with the individual signals of Gly, mI, Lac, Ala, Glu and Gln. The GBM patient was scanned for two regions where the T2w-FLAIR intensity was enhanced. The in vivo
spectra were well reproduced by the fits, with small residuals between 1.0 – 4.1 ppm. The residuals between 3.45 – 3.65 ppm were equivalent to the noise levels, indicating that the signals were primarily attributed to Gly and mI. The spectra from the GBM patient both exhibited spectral patterns different from the normal-brain spectral pattern. Moreover, the GBM spectra were quite different from each other. shows metabolite concentrations of tCr, NAA, GPCPC, Gly, mI, Lac and Ala, estimated with Eq. 
. The Gly and mI concentrations in the normal brain were estimated to be 0.6±0.1 and 4.3±0.4 (mean±SD, n=5), respectively. The Gly signal in was much larger than in . The Gly level in this tumor mass was estimated as 3.3 mM, approximately 6-fold higher than the mean Gly level in the healthy brain. The mI level in this region was measured to be lower compared to the normal brain tissue. In contrast, the spectrum from another tumor mass, , exhibited a normal Gly level and a substantially increased mI level. The mI level in this tumor mass was estimated to be 12.3 mM, ~3-fold higher than the normal level. Lac and Ala were also measurable in the tumor spectra, but undetectable in the normal brain. The Lac and Ala concentrations were estimated to be ~3 mM and ~1 mM for both tumor regions, with mean CRLBs of ~8% and ~18%, respectively. Further, Glu and Gln levels were altered in a tumor mass (), whilst the other tumor mass () showed no difference compared to normal tissues. For tCr, NAA and GPCPC, the signals exhibited a well-known tumor spectral pattern, i.e.
, a GPCPC signal greater than tCr and NAA signals. The tCr level in the tumor mass in the parietal region () was estimated to be 11.1 mM, ~1.4 times higher compared to the normal brain. The tumor mass for the spectrum in showed T1
w-MRI enhancement following a gadolinium injection, but the tumor mass for did not, as observed in a regular clinical scan in this patient.
FIG. 4 In vivo brain spectra from (a) a healthy volunteer and (b, c) a multi-focal GBM patient are shown together with the LCModel fits and individual signals of Gly, mI, Lac, Ala, Glu and Gln. Voxel positioning (2×2×2 cm3) is shown in the images. (more ...)
Table 1 Metabolite concentrations of 5 normal subjects (mean±SD) and eight GBM patients are tabulated. The concentrations were estimated using short-TE water signals as a normalization reference and assuming a normal-brain tCr concentration at 8 mM. The (more ...)
Among the 12 GBM patients enrolled in the present study, Gly levels were elevated in 8 patients. displays in vivo brain spectra from a normal subject () and seven patients with GBM (), following the normalization with respect to the short-TE water signals. Compared to the normal-tissue spectrum, spectra from tumors all exhibited abnormal spectral patterns, which include increases in Gly, GPCPC, Lac, Ala and lipids (Lip) and decreases in tCr and NAA. The Gly levels in were higher than the normal level (by factors of 3.1, 3.8, 8.3, 1.9 and 2.0, respectively). The GBM patients of exhibited normal Gly levels but abnormal concentrations for other metabolites. Increased GPCPC/tCr and GPCPC/NAA ratios may be representative abnormalities in metabolic profiles in GBM, as shown in . The concentrations of tCr and NAA appeared to be lower than normal in most of the spectra from GBM patients, leading to elevation of GPCPC/tCr and GPCPC/NAA ratios. However, for patients for , the increases in these concentration ratios resulted from decreased tCr and NAA levels, with the GPCPC level about the same as or slightly smaller than the normal level. In , a large negative doublet of the Lac CH3 protons was detected at 1.31 ppm. The Lac level was estimated to be 12.6 mM, much higher than the normal level (< 1 mM). In , large positive signals appeared at 1.3 ppm, due to increased Lip levels, but the spectral patterns at the resonance were slightly different between the two spectra due to the signals of Lac and Ala. In , relatively small negative signals of Lac and Ala were overlapped with the large Lip signal, but with their negative polarity, the Lac and Ala signals were well differentiated from Lip signals using the LCModel spectral fitting, giving Lac and Ala levels at 4.9 and 1.7 mM, respectively, with CRLB of 9% for both. The Lac and Ala levels were not measurable in due to the insufficient SNR and the presence of a large, broad lipid signal. In , for which the Gly levels were estimated to be 2-fold higher than the normal level, the spectral patterns at 3.5 – 3.65 ppm were different due to different mI concentrations in these patients. The mI concentration was similar to the normal level in , but lower in . In these two spectra, Lac-Lip composite signals were resolved, giving Lac levels as 7.0 and 5.4 mM, respectively. The Ala levels were negligible in these patients. In the GBM patients for , Lac and Ala signals were both pronounced, indicating their concentrations much higher than normal.
FIG. 5 In vivo brain spectra from a normal subject (a) and 7 patients with GBM are shown on top of LCModel fits, together with voxel positioning (size 2×2×2 cm3). Spectra were all acquired with PRESS (TE1, TE2) = (60, 100) ms, TR = 2 s, and NSA (more ...)