MR-oximetry with fat DESPOT

The R 1 relaxation rate of fat is a promising marker of tissue oxygenation. Existing techniques to map fat R 1 in MR-oximetry offer limited spatial coverage, require long scan times, or pulse sequences that are not readily available on clinical scanners. This work addresses these limitations with a 3D voxel-wise fat R 1 mapping technique for MR-oximetry based on a variable flip angle (VFA) approach at 3 T. Varying levels of dissolved oxygen ( O 2 ) were generated in a phantom consisting of vials of safflower oil emulsion, used to approximate human fat. Joint voxel-wise mapping of fat and water R 1 was performed with a two-compartment VFA model fitted to multi-echo gradient-echo magnitude data acquired at four flip angles, referred to as Fat DESPOT. Global R 1 was also calculated. Variations of fat, water, and global R 1 were investigated as a function of the partial pressure of O 2 ( pO 2 ). Inversion-prepared stimulated echo magnetic resonance spectroscopy was used as the reference technique for R 1 measurements. Fat R 1 from Fat DESPOT was more sensitive than water R 1 and global R 1 to variations in pO 2, consistent with previous studies performed with different R 1 mapping techniques. Fat R 1 sensitivity to pO 2 variations with Fat DESPOT (median O 2 relaxivity r 1, O2 = 1.57× 10 −3  s −1  mmHg −1 ) was comparable to spectroscopy-based measurements for methylene, the main fat resonance (median r 1, O2 = 1.80 × 10 −3  s −1  mmHg −1 ). Fat and water R 1 can be measured on a voxel-wise basis using a two-component fit to multi-echo 3D VFA magnitude data in a clinically acceptable scan time. Fat and water R 1 measured with Fat DESPOT were sensitive to variations in pO 2. These observations suggest an approach to 3D in vivo MR oximetry.