Estimation of T2 Relaxation Times of Downfield Peaks in Human Brain at 9.4 T
Saipavitra V. Murali Manohar1, Tamas Borbath1, Nicole Fichtner2,3, Ioannis Angelos Giapitzakis1, Daniel Zaldivar1, Roland Kreis3, and Anke Henning1,4

1Max Planck Institute for Biological Cybernetics, Tuebingen, Germany, 2Institute for Biomedical Engineering, UZH and ETH Zurich, Zurich, Switzerland, 3Depts. Radiology and Biomedical Research, University of Bern, Bern, Switzerland, 4Institute of Physics, Ernst-Moritz Arndt University Greifswald, Greifswald, Germany


T2 relaxations times for the downfield metabolites in human brain 1H MR spectra were estimated at 9.4 T. A possible new peak at 8.35 ppm with rapid T2 decay is reported. Due to the use of a non-water suppressed MRS method, the T2 of slowly exchanging peaks could be assessed. The shorter T2 relaxation times in the downfield compared to the upfield spectral areas leads us to suspect a macromolecular contribution, while also exchange effects may contribute to the short apparent T2s.


Previous studies demonstrated [1,2,3] that there is valuable additional information in the downfield part of 1H spectra of the human brain; however, it remains largely uncharacterized and unassigned. The aim of this study was to measure the T2 relaxation times of the downfield peaks at 9.4 T including that of exchangeable peaks, which would aid to better characterize the downfield area and peak assignment. This could be of great interest to both the MR Spectroscopy and the Chemical Exchange Saturation Transfer (CEST) communities.


The measurements were performed on a Siemens 9.4 T whole-body MRI scanner (Erlangen, Germany) using a home-built proton coil with 8-transmit and 16-receive channels [4]. Six healthy volunteers participated in the study after informed signed consent as approved by the local ethics board. A voxel (2x2x2 cm3) was chosen in the occipital lobe with mixed gray and white matter content. B0 shimming was performed using FASTESTMAP [5] and voxel-based power calibration was done [6]. An echo-time (TE) series (TE= 24, 32, 40, 52, 60 ms) was acquired using the non-water suppressed metabolite-cycled (MC)-semi-LASER sequence [7] (TR 6000 ms, 96 averages per TE). The RF frequency was centered in the downfield region at 7.3 ppm. The spectra from different volunteers were averaged for each TE response individually and the TE series was modeled using FitAID [8] with 14 narrow peaks and 4 broad line-shape components. All the components were modeled based on offset from [1]. The NAA peak was modeled in VesPA [9] using a spin echo sequence with ideal pulses for a starting TE = 6.5 ms for the TE series [10]. The averaged spectra were fit individually. Finally, the calculated areas were fit to exponential decays to retrieve T2 value estimates.


The decaying summed signal with increasing TE can be seen in Fig.1. The modeled peaks are indicated. In Fig.2, there is an additional broad peak at 8.35 ppm when compared to the MC-STEAM spectra acquired at 9.4 T previously [1]. The selected basis set models the spectra well, as indicated from the mostly flat fit residue (Fig.3). All modeled metabolites could be fit with an exponential decay function (Fig.4). Based on this exponential model, the T2 relaxation times and the R2 values of the fit are reported in Table 1.


This work gives an estimate of the T2 relaxation times of the downfield peaks at 9.4 T. The use of a non-water suppressed MRS method allowed assessment of the T2 relaxation times for exchangeable peaks [3]. The appearance of the peak at 8.35 ppm (Fig. 2) in the downfield human brain spectra is observed for the first time, potentially due to the spin-lock effect [11, 12] during the adiabatic pulses in MC-semiLASER or less exchange-sensitivity compared to the MC-STEAM sequence from [1]. Since this possible peak was estimated to have a very short T2 relaxation time of 5.9 ms (Table 1), we see it prominently for TE = 24 ms which is equivalent to a non-adiabatic spin-echo sequence of TE = 6.5 ms [10]. A tentative splitting of ~15 Hz is seen for the peak at 7.82 ppm when TE = 52-60 ms whereas the coupling constant of the NAA-NH group is 6.4 Hz [13]. A splitting at 7.82 ppm was reported also in [14]. Hence, a further investigation of high SNR spectra recorded at TE of 52 ms is required to fully understand the splitting at 7.82 ppm and distinguish possible other contributions. The correction for a T1-weighting bias [15] in the calculated T2 values is neglected as TR > ~8 T1, considering the T1 values reported in [1]. The apparent B0 dependency effects [16,17] are seen in the T2 values at 9.4 T when compared to the reported T2 values for the downfield peaks at 7 T in [2] . The T2 relaxation times in the downfield have values shorter than the values in the upfield [18], which lets us suspect macromolecular contributions in the downfield part as mentioned in [2,19], in addition to the exchange effects [1]. Further detailed exploration of the downfield range with more volunteers, individual volunteer-wise fitting and careful adjustment of first-order phase for finer results will be performed in the future.


This work was supported by the Horizon 2020 CDS-QUAMRI grant. The authors thank Johanna Dorst and Loreen Ruhm from Max Planck Institute for Biological Cybernetics, Tuebingen, for their help in this work.


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Fig.1: Summed spectra (from six healthy volunteers) of the downfield range for a TE series (24, 32, 40, 52 and 60 ms). Arrows point to the peaks modeled in the fitting of the spectra.

Fig. 2: Visual comparison of MC-STEAM (TE/TM/TR: 10/50/5000 ms) acquired in [1] and MC-semiLASER (TE/TR: 24/6000 ms) at 9.4 T; The new rapidly decaying peak at 8.35 ppm is seen in MC-semiLASER spectra possibly because of the spin-locking effect of the adiabatic pulses and the CPMG nature of the semiLASER sequence [11,20].

Fig. 3: The fit of downfield averaged spectra from six volunteers at TE = 32 ms; the spectrum is shown in black, the fit in blue and the residue in red. Similar mostly flat residues were achieved for the entire TE series.

Fig. 4: The exponential decay fit to all the modeled metabolites. The exponential fit coefficients give the T2 relaxation times. The goodness of the fit was estimated also as R2 values.

Table 1: T2 relaxation times of the downfield peaks at 9.4 T from averaged TE series and R-squared value showing the goodness of exponential decay fit; the T2 relaxation times from 7 T from averaged TE series reported in [2] is provided in the table for comparison: the apparent B0 effect and also the effect of possible exchange contribution [1] is seen on the T2 decay.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)