Water proton T 1 measurements in brain tissue at 7, 3, and 1.5T using IR-EPI, IR-TSE, and MPRAGE: results and optimization

Abstract

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° 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.