Purpose: To optimize the homogeneity and efficiency of the B(1) magnetic field of a four-ring birdcage head coil that is double-tuned at the Larmor frequencies of both (31)P and (1)H and optimized to acquire magnetic resonance spectroscopy (MRS) data at 3T for the study of infants.
Materials and methods: We developed a finite difference time domain (FDTD) tool in-house to iteratively compute and seek the range of geometric and electromagnetic parameters of a dual-tuned, four-ring birdcage coil that would produce the desired resonance patterns, optimize homogeneity of the B(1)-field, and maximize efficiency of the coil. To demonstrate the validity of our computational results, we constructed three RF coils: one dual-tuned coil that was based on the calculated optimized parameters and two single-tuned coils that had dimensions similar to those of the dual-tuned coil, but tuned at the Larmor frequencies of both (31)P and (1)H, respectively. We then tested and compared the performances of the dual-tuned coil and single-tuned coils at both of these frequencies.
Results: We found that a dual-tuned, four-ring birdcage coil with a diameter of 180 mm, an inner birdcage length of 100-300 mm, and an outer birdcage length of 25-100 mm produces the desired resonance patterns. For the use of this coil with human infants, optimization of the homogeneity of the B(1) field, combined with improved coil efficiency, yielded a dual-tuned birdcage coil with diameter of 180mm, an inner birdcage length of 150 mm, an outer birdcage length of 25 mm, and corresponding inner and outer capacitances of 17.2 pF and 7.6 pF, respectively. The experimental results from a constructed coil having the same parameters with the modeled coil agreed well with the computational results from the modeled coil. This optimized design overcame the deficiencies of existing dual-tuned, four-ring birdcage coils.
Conclusion: The homogeneity and efficiency of the B(1) field for (31)P/(1)H dual-tuned, four-ring birdcage coils can be optimized well using our FDTD tool, especially at high static magnetic fields (B(0)).