Water proton T1 measurements in brain tissue at 7, 3, and 1.5 T using IR-EPI, IR-TSE, and MPRAGE:: results and optimization

Method This paper presents methods of measuring the longitudinal relaxation time using inversion recovery turbo spin echo (IR-TSE) and magnetization-prepared rapid gradient echo (MPRAGE) sequences, comparing and optimizing these sequences, reporting T (1) values for water protons measured from brain tissue at 1.5, 3, and 7T. T (1) was measured in cortical grey matter and white matter using the IR-TSE, MPRAGE, and inversion recovery echo planar imaging (IR-EPI) pulse sequences. Results In four subjects the T (1) of white and grey matter were found to be 646 +/- 32 and 1,197 +/- 134ms at 1.5T, 838 +/- 50 and 1,607 +/- 112ms at 3T, and 1,126 +/- 97, and 1,939 +/- 149ms at 7T with the MPRAGE sequence. The T (1) of the putamen was found to be 1,084 +/- 63ms at 1.5T, 1,332 +/- 68ms at 3T, and 1,644 +/- 167ms at 7T. The T (1) of the caudate head was found to be 1,109 +/- 66ms at 1.5T, 1,395 +/- 49ms at 3T, and 1,684 +/- 76ms at 7T. Discussion There was a trend for the IR-TSE sequence to underestimate T (1) in vivo. The sequence parameters for the IR-TSE and MPRAGE sequences were also optimized in terms of the signal-to-noise ratio (SNR) in the fitted T (1). The optimal sequence for IR-TSE in terms of SNR in the fitted T (1) was found to have five readouts at TIs of 120, 260, 563, 1,221, 2,647, 5,736ms and TR of 7 s. The optimal pulse sequence for MPRAGE with readout flip angle = 8 degrees was found to have five readouts at TIs of 160, 398, 988, 2,455, and 6,102ms and a TR of 9 s. Further optimization including the readout flip angle suggests that the flip angle should be increased, beyond levels that are acceptable in terms of power deposition and point-spread function.