Proton spectroscopic metabolite signal relaxation times in preterm infants: A prerequisite for quantitative spectroscopy in infant brain

Abstract

Purpose: To determine relaxation times of metabolite signals in proton magnetic resonance (MR) spectra of immature brain, which allow a correction of relaxation that is necessary for a quantitative evaluation of spectra acquired with long TE. Proton MR spectra acquired with long TE allow a better definition of metabolites as N-acetyl aspartate (NAA) and lactate especially in children. Materials and Methods:Relaxation times were determined in the basal ganglia of 84 prematurely born infants at a postconceptional age of 37.8 Ϯ 2.2 (mean Ϯ SD) weeks. Metabolite resonances were investigated using the doublespin-echo volume selection method (PRESS) at 1.5 T. T1 was determined from intensity ratios of signals obtained with TRs of 1884 and 6000 msec, measured at 3 TEs (25 msec, 136 msec, 272 msec). T2 was determined from signal intensity ratios obtained with TEs of 136 msec and 272 msec, measured at 2 TR. Taking only long TEs reduced baseline distortions by macromolecules and lipids. For myo-inositol (MI), an apparent T2 for short TE was determined from the ratio of signals obtained with TE ϭ 25 msec and 136 msec. Intensities were determined by fitting a Lorentzian to the resonance, and by integration.Results: Relaxation times were as follows: trimethylaminecontaining compounds (Cho): T1 ϭ 1217 msec/T2 ϭ 273 msec; total creatine (Cr) at 3.9 ppm: 1010 msec/111 msec; Cr at 3.0 ppm: 1388 msec/224 msec; NAA: 1171 msec/499 msec; Lac: 1820 msec/1022 msec; MI: 1336 msec/173 msec; apparent T2 at short TE: 68 msec.Conclusion: T1 and T2 in the basal ganglia of premature infants do not differ much from previously published data from basal ganglia of older children and adults. T2 of Cho was lower than previous values. T2 of Cr at 3.9 ppm and Lac have been measured under different conditions before, and present values differ from these data. LOCALIZED IN VIVO nuclear magnetic resonance (NMR) spectroscopy allows investigation of specific metabolites in situ, and extends morphologic information provided by imaging modalities. Combined magnetic resonance imaging (MRI) and spectroscopy (MRS) has developed to a stage that enables routine studies of the neonatal brain in a clinical environment (1-4). For 1 H spectroscopy, protocols for absolute quantitation of metabolite concentrations have been proposed that utilize the tissue water signal as the internal concentration reference (5-7). In general, the determination of metabolite concentrations from MR spectra requires correction for relaxation or saturation. These effects can be minimized using short TE and long TR, but with common volume selection methods, there is a remaining error due to signal decay during the nonzero TE. Shorter TR may be necessary to allow sufficient averaging when measuring small volumes. In short TE, spectra signal area determination of metabolites is complicated-especially in small infants-due to macromolecules distorting the baseline, a high glutamine/ glutamate signal partially masking the small N-acetyl aspartate (NAA) signal, and lipids masking lactate (Lac...

Creatine, Glutamine plus Glutamate, and Macromolecules Are Decreased in the Central White Matter of Premature Neonates around Term

“…The lowest R 2 value was obtained for taurine in PN (apparent T 2 = 242 ± 94 ms, R 2 = 0.2334). Apparent T 2 values measured for tCr and ( myo -Ins+Gly) in PN and TN were in the same range of values measured at 1.5 T with the PRESS sequence in the basal ganglia of premature brain at 37.8 ± 2.2 weeks PCA [30] (tCr = 228 ms ± 12%, myo -Ins = 173 ms ± 67%). To our knowledge, T 2 values of Glx and taurine have not been previously measured in the neonatal brain at 1.5 T. Our T 2 values of Glx were twice lower than published values measured in the adult brain white matter at 3T [39], whereas the only comparative T 2 values available for taurine were obtained in the adult human brain at 7T and varied from 85 ± 10 to 120 ± 20 ms [41].…”

“…T 2 of tNAA, tCho or lactate), this model allowed the calculation of an apparent T 2 for metabolites known or supposed to have short T 2 (T 2 <250 ms) namely tCr, Glx, taurine and myo -Ins+Gly [30, 37–41]. The exponential regression was applied to each group dataset and not to each individual because there were too few data points available in this later case.…”

“…We hypothesized that some metabolic anomalies could have gone undetected at short echo time, since metabolite T 2 , and particularly the T 2 of choline-containing compounds had been reported to be shorter in the premature brain around term [30]. We therefore analyzed 1 H-MRS performed with the PRESS sequence at short and long TEs (30 and 135 ms).…”

Can magnetic resonance spectroscopy predict neurodevelopmental outcome in very low birth weight preterm infants?

“…Because MRS scans were performed after a series of MRI scans, it was not always possible to obtain spectra from all subjects who were not compliant for the full duration of the exam. Ratios, rather than absolute concentrations,34,35 were computed from the spectra. Owing to certain factors, such as magnetic field inhomogeneity over a large volume and patient motion, it was necessary to average signals over a large region of interest to obtain sufficiently high SNR for quantitative analysis.…”

Does visual cortex lactate increase following photic stimulation in migraine without aura patients? A functional 1H-MRS study

“… For the calculation of the different correction factors, we refer to a recent publication on quantitative 1 H-MRS in MwoA [11]. T 1 and T 2 relaxation times of Lac were derived from another study [30]. …”