Off-resonance and detuned surface coils for B₁ inhomogeneity in 7-Tesla MRI

Abstract

A problem with high-field MRI is the lack of B1 homogeneity, particularly signal cancellation in the outer parts of the head. Here we attempt to correct this by adding surface coils. To adjust the mutual coupling, we vary the resonance properties of the added coil. A new agar-based head phantom was built, and two surface coils were built and tuned. The surface coils were placed in various configurations against the phantom to modify the B1 field with their presence, while images were taken using a 16-rung birdcage coil to transmit and receive. Trials were taken with various spacings between the surface coil and the phantom, while the resonance of the surface coil was either shifted in frequency by changing the voltage across a varactor diode, or detuned using a resonant detuning circuit. It was discovered that with a 1 cm spacing and a surface coil tuned just above resonance, SNR near the surface coil could be improved by upwards of 400%, with the trade-off of a reduced signal in other areas on the periphery of the head. Other configurations could achieve better B1 homogeneity at the expense of reduced SNR throughout the head. Future studies will explore the possibility of using more than one surface coil to improve SNR in more places on the periphery of the head.