Measuring water content using T2 relaxation at 3 T: Phantom validations and simulations

Purpose: In vivo measurement of water content would be very useful for evaluating microstructural tissue changes, such as edema, that occur in neurological diseases. Careful assessment of the T-2 relaxation decay curve can provide simultaneous measurements of total water content (TWC) and myelin water fraction, a marker for myelin which is also relevant in brain pathology. This work validates a T-2 relaxation based method for TWC measurement at 3 T using phantoms and simulations. Methods: A phantom consisting of tubes with known water concentrations was scanned using 3 T MRI. T-2 relaxation data was collected with both gradient echo spin-echo (GRASE) and spin echo sequences, while an inversion recovery experiment provided T-1 relaxation data. Voxel-wise T-2 distributions were calculated by fitting the T-2 relaxation data with a non-negative least squares algorithm that incorporated a correction for errors in flip angle due to B-1(+) inhomogeneity. TWC was calculated as the sum of the signal in the T-2 distribution, corrected for T-1 relaxation, relative to that of a tube containing 100% water. TWC from GRASE was compared to that of spin echo in order to test if the accuracy of the TWC measurement was impacted by using additional gradient echoes to fill k-space. Simulations were performed to determine theoretical errors in TWC. Results: Measured TWC strongly correlated to actual TWC (R = 0.997, p = 9 x 10(-8), mean discrepancy = 1.8%). Accuracy of GRASE and spin echo TWC measurements did not significantly differ. Simulations indicated a mean systematic TWC error of 0.07% and random error of 0.8%, and revealed that the technique performs well in the presence of B-1(+) inhomogeneity. Conclusion: This work demonstrates that, using the T-2 relaxation decay curve, TWC can be measured to within 3% accuracy at 3 T. Given that T-2 relaxation can provide accurate estimates of both TWC and myelin water fraction, multi-echo T-2 measurement should be considered a multifaceted approach for assessing pathology and evaluating therapy of central nervous system diseases. (C) 2015 Elsevier Inc. All rights reserved.