Evaluation of B₁ inhomogeneity effect on DCE-MRI data analysis of brain tumor patients at 3T

Dynamic-contrast-enhanced (DCE) MRI data acquired using gradient echo based sequences is affected by errors in flip angle (FA) due to transmit B₁ inhomogeneity (B₁inh). The purpose of the study was to evaluate the effect of B₁inh on quantitative analysis of DCE-MRI data of human brain tumor patients and to evaluate the clinical significance of B₁inh correction of perfusion parameters (PPs) on tumor grading.

An MRI study was conducted on 35 glioma patients at 3T. The patients had histologically confirmed glioma with 23 high-grade (HG) and 12 low-grade (LG). Data for B₁-mapping, T₁-mapping and DCE-MRI were acquired. Relative B₁ maps (B_1rel) were generated using the saturated-double-angle method. T₁-maps were computed using the variable flip-angle method. Post-processing was performed for conversion of signal–intensity time (S(t)) curve to concentration–time (C(t)) curve followed by tracer kinetic analysis (K^trans, Ve, Vp, Kep) and first pass analysis (CBV, CBF) using the general tracer-kinetic model. DCE-MRI data was analyzed without and with B₁inh correction and errors in PPs were computed. Receiver-operating-characteristic (ROC) analysis was performed on HG and LG patients. Simulations were carried out to understand the effect of B₁ inhomogeneity on DCE-MRI data analysis in a systematic way. S(t) curves mimicking those in tumor tissue, were generated and FA errors were introduced followed by error analysis of PPs. Dependence of FA-based errors on the concentration of contrast agent and on the duration of DCE-MRI data was also studied. Simulations were also done to obtain K^trans of glioma patients at different B_1rel values and see whether grading is affected or not.

Current study shows that B_1rel value higher than nominal results in an overestimation of C(t) curves as well as derived PPs and vice versa. Moreover, at same B_1rel values, errors were large for larger values of C(t). Simulation results showed that grade of patients can change because of B₁inh.

B₁inh in the human brain at 3T-MRI can introduce substantial errors in PPs derived from DCE-MRI data that might affect the accuracy of tumor grading, particularly for border zone cases. These errors can be mitigated using B₁inh correction during DCE-MRI data analysis.