A Method of Temperature Measurement Using Low Field MRIs With a 3D Steady-state Free Precession Pulse Sequence

MRI can be used to monitor the temperature changes during a thermoablation treatment of tumors.3D steady-state free precession sequence(3D-SSFP) can be used for temperature mapping in the low field scanner.However,conventional SSFP method using single acquisition is highly sensitive to the inhomogeneity of the main magnetic field.In this study a method using multi-acquisition combination with high and low flip angles was proposed to improve the imaging tolerability to the inhomogeneity.The linear relationship between the temperature and the normalized signal was analyzed by simulation.The optimum flip angles were calculated to get the best tolerability to the inhomogeneity,and the system bias introduced by the uncertainty of relaxation time T_1 and T_2 was discussed.The results from simulation and experiments at 0.45 T permanent MRI scanner showed the feasibility of the proposed method.Comparing with single acquisition method,multi-acquisition combination method has equivalent temperature linearity,less sensitivity to the heterogeneity of the main magnetic field and less system bias.The imaging time for each temperature map with the new method is 6 s,which is suitable for the clinical applications.