Single-scan R₂(*) measurement with macroscopic field inhomogeneity correction

Accurate quantification of R₂(*) (=1/T₂(*)) is important for many applications in neuroimaging. However, R₂(*) measurements made using conventional multi-echo gradient echo imaging are hampered by macroscopic field inhomogeneities. Several methods for compensation of macroscopic field inhomogeneities have been introduced, most of them requiring increased scan time. In this paper, an R₂(*) estimation process using a modified multi-echo gradient echo sequence that includes bipolar compensation gradients and does not require additional data acquisition time is proposed. A post-processing algorithm based on an excitation-profile weighted signal model is used. The optimal amount of compensation gradients and performance was investigated by numerical simulation and phantom tests. Multi-slice R₂(*) maps were obtained from 11 human volunteers at 3T. Simulation results demonstrated that this method successfully removes the effects of macroscopic field inhomogeneities of up to ± 300μT/m within an error range of ± 8%. ROI analysis revealed R₂(*) values of 30.4 ± 3.0s⁻¹ (substantia nigra), 25.8 ± 2.7s⁻¹ (red nuclei), 23.2 ± 1.0s⁻¹ (genu), 20.8 ± 1.2s⁻¹ (putamen), 34.2 ± 3.4s⁻¹ (globus pallidus), and 21.8 ± 1.4s⁻¹ (splenium) using the proposed method, with statistically significant differences compared to conventional method in the regions of the substantia nigra, red nucleus, genu, putamen, and globus pallidus (p<0.05). Our proposed scheme allows for fast single-scan multi-slice R₂(*) measurement and includes compensation for the effects of macroscopic field inhomogeneity.