Mapping R₁ of fat, R₁ of water, joint R₂^⁎, and proton density fat fraction at 3 T using the multi-echo MP2RAGE...

To develop a technique for joint measurement of fat and water-specific longitudinal relaxation rates (R_1f and R_1w), effective transverse relaxation rate (R₂^∗), and proton density fat fraction (PDFF) combining the Multi-Echo Magnetization Prepared Two Rapid Acquisition of Gradient Echoes (ME-MP2RAGE) sequence and fat-water separation.

All experiments were performed at 3 T. R_1f and R_1w were calculated with fat-specific and water-specific MP2RAGE signals. R₂^∗ and PDFF maps were obtained from fat-water separation applied to the second RAGE block. Sequence parameters optimization was performed via Cramér-Rao lower bounds theory, and we designed four protocols with different combinations of number of echoes and readout gradient schemes (I: 3 echoes unipolar, II: 6 echoes unipolar, III: 6 echoes bipolar, and IV: 10 echoes bipolar). We tested and validated these protocols with numerical simulations, phantom, and in vivo experiments. In phantoms, we compared ME-MP2RAGE measurements with inversion recovery spin-echo (IR-SE) global R₁ and 3D Fast Low Angle Shot (3D FLASH) R₂^∗ and PDFF. In vivo, we scanned the lower leg and neck of a healthy volunteer.

Numerical simulations showed accurate quantification of R_1f, R_1w, and R₂^∗ with mean relative bias <3% and PDFF with mean bias <0.003 using protocol ME-MP2RAGE IV (10 echoes bipolar). Phantom experiments showed excellent agreement with IR-SE and 3D FLASH measurements. In vivo, measurements in the lower leg and neck were consistent with literature values.

We proposed an accurate method for simultaneous quantification of R_1f, R_1w, R₂^∗, and PDFF from a single acquisition with the ME-MP2RAGE sequence.