500 MHz Inductive Birdcage RF Coil for Brain MRI: Design, Implementation and Validation.

Abstract

We present a 500 MHz inductive birdcage RF resonator for imaging the human brain in an 11.7 T MRI scanner. A homogenous circularly polarized transmit field ($\text{B}_{1}^{+}$) was generated by transmitting power to the resonator through four couplers driven in differential mode and with an incremental 90-degree phase delay. A detailed mechanical and electrical model of the hardware, loaded with different phantoms, was generated and its performance simulated using a finite-difference time-domain method. The head-size inductively coupled birdcage presented a fundamental mode at 500 MHz. MR thermometry maps were in good agreement with heating profiles estimated from simulated SAR maps Conclusion: The model of the hardware was validated through both bench and MRI measurements. This validation is important for future analysis of radiofrequency safety and performance at ultra-high field MRI through the prediction of SAR and $\text{B}_{1}^{+}$ profiles across different human brain models at various positions inside the coil.