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Abstract:
Purpose/Introduction: Determining T1 relaxation times of macromolecular (MM) resonances aid us to understand their behavior better. Very few attempts have been made to estimate the T1 relaxation times of MM resonances. T1 relaxation time of MM baseline as a whole [1] or for individual MM resonance at 0.916 ppm [2] have been previously reported. T1 relaxation times for individual MM resonances have been estimated in our previous work [3] for a grey matter rich region in the occipital lobe. This work extends to estimate the T1 relaxation times in both GM rich and WM rich voxel as well as to report the T1 values in pure GM and WM voxels. Subjects and Methods: 11 inversion time (TI1 and TI2) combinations (Fig. 1) were determined by performing Bloch simulations for a double inversion recovery (DIR) sequence and considering those combinations for which the metabolite contributions were minimal. A metabolite-cycled semiLASER sequence was used to localize voxels in the occipital lobe and left parietal lobe preceded by a DIR technique [4] (TE/TR: 24, 8000 ms; NEX: 32). MP2RAGE images [5] were acquired to calculate the tissue volume fractions. Data were acquired from 11 and 6 healthy volunteeers in GM and WM rich regions respectively. The raw data were pre-processed and the fitted in LCModel-v6.3-1L [6] using simulated Voigt lines. Residual subtraction from the MM spectra was performed by fitting sharper Voigt lines at the observed shifts in LCModel. The LCModel concentrations were then fit to the DIR signal equation and the T1 relaxation times were calculated. Furthermore, by using a linear relationship of T1 time variations across tissue composition [7], a system of linear equations were simultaneously solved to estimate the T1 relaxation times for pure GM and WM voxels. Results: Fig. 1 shows Bloch simulation for 2150/600 ms where metabolite contribution is minimal and MM contribution is considerably higher. A similar metabolite nulling was achieved for almost all TI combinations covering a range of magnetizations for MMS except 1050/238 ms which is an MM null point. The MM spectra DIR series is shown in Fig. 2 for GM rich voxel. Fig. 3 also shows a curve fitting for the DIR signal equation for MM at 0.916 ppm. Fig. 3 tabulates the T1 relaxation times of individual MM peaks for GM rich, WM rich, pure GM and pure WM voxels. Discussion/Conclusion: A novel DIR technique was developed for acquiring DIR series MM spectra. The T1 relaxation times of individual MM resonances for both GM rich and WM rich regions in the human brain at 9.4 T are reported. Further solving a system of linear equations, T1 values for pure GM and WM voxels are reported additionally.
