In high-field magnetic resonance imaging (MRI) systems, B0 fields of 7 and 9.4 T, the RF field shows greater inhomogeneity compared to clinical MRI systems with B0 fields of 1.5 and 3.0 T. In multichannel RF coils, the magnitude and phase of the input to each coil element can be controlled independently to reduce the nonuniformity of the RF field. The convex optimization technique has been used to obtain the optimum excitation parameters with iterative solutions for homogeneity in a selected region of interest. The pseudoinverse method has also been used to find a solution. The simulation results for 9.4-and 7-T MRI systems are discussed in detail for the head model. Variation of the simulation results in a 9.4-T system with the number of RF coil elements for different positions of the regions of interest in a spherical phantom are also discussed. Experimental results were obtained in a phantom in the 9.4-T system and are compared to the simulation results and the specific absorption rate has been evaluated.
Bibliographical noteFunding Information:
Manuscript received September 22, 2011; revised February 7, 2012; May 14, 2012; accepted June 30, 2012. Date of publication August 23, 2012; date of current version November 22, 2012. This work was supported in part by National Institutes of Health under Grant NIH R01-EB006835, in part by the Biotechnology Research Center under Grant BTRC P41-RR008079, in part by the Keck Foundation, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology under Grant 2011-0014776. Asterisk indicates corresponding author.
- Convex optimization
- RF B field
- high-field MRI
- magnetic resonance imaging (MRI)
- parallel excitation
- transmission line head coil