Simultaneous quantitative susceptibility mapping (QSM) and R 2 * for high iron concentration quantification with 3D ultrashort echo time sequences: An echo dependence study

Purpose To evaluate the echo dependence of 3D ultrashort echo time (TE) quantitative susceptibility mapping (3D UTE‐QSM) and effective transverse relaxation rate ( R 2 * ) measurement in the setting of high concentrations of iron oxide nanoparticles. Methods A phantom study with iron concentrations ranging from 2 to 22 mM was performed using a 3D UTE Cones sequence. Simultaneous QSM processing with morphology‐enabled dipole inversion (MEDI) andR 2 *single exponential fitting was conducted offline with the acquired 3D UTE data. The dependence of UTE‐QSM andR 2 *on echo spacing (ΔTE) and first TE (TE 1 ) was investigated. Results A linear relationship was observed between UTE‐QSM measurement and iron concentration up to 22 mM only, with the minimal TE 1 of 0.032 ms and ΔTE of less than 0.1 ms. A linear relationship was observed betweenR 2 *and iron concentration up to 22 mM only when TE 1 was less than 0.132 ms and ΔTE was less than 1.2 ms. UTE‐QSM with MEDI processing showed strong dependence on ΔTE and TE 1 , especially at high iron concentrations. Conclusion UTE‐QSM is more sensitive thanR 2 *measurement to TE selection. Both an ultrashort TE 1 and a small ΔTE are needed to achieve accurate QSM for high iron concentrations. Magn Reson Med 79:2315–2322, 2018. © 2018 International Society for Magnetic Resonance in Medicine.