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

PURPOSE: To develop a technique for joint measurement of fat and water-specific longitudinal relaxation rates (R1f and R1w), 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.

THEORY AND

METHODS

All experiments were performed at 3 T. R1f and R1w 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.

RESULTS

Numerical simulations showed accurate quantification of R1f, R1w, 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.

CONCLUSION

We proposed an accurate method for simultaneous quantification of R1f, R1w, R2∗, and PDFF from a single acquisition with the ME-MP2RAGE sequence.