Design of radio-frequency coils for a 50 mT mobile breast magnetic resonance imaging prototype

Breast cancer has become the most common cancer among women worldwide. Magnetic resonance imaging (MRI) can compensate for the limitations of x-ray breast scanning and ultrasound in the early detection of patients with dense breasts. However, limited by the density of MRI units per million population and the cost of high-field MRI systems, breast MRI is not widely used in breast cancer screening. Ultra-low field MRI can potentially promote the application of MRI for breast detection, but specialized breast radio-frequency (RF) coils are required.

The RF coils with a resonant frequency of 2.131 MHz include a high uniformity transmit coil with a low depth-to-width ratio and a four-channel phased-array receive coil, which are designed specifically for a 50 mT runway-type lightweight magnet with a vertical static magnetic field to achieve mobile breast MRI.

The transmit coil adopts the variant solenoid structure. To ensure the RF field uniformity within the target area is greater than 95%, the transmit coil's dimension parameters were calculated using a nonlinear programming algorithm. Due to the main magnetic field being a vertical field, the receive coil employs a four-channel phased-array to achieve high sensitivity, and the overlapping area between the channels was optimized to achieve decoupling by simulating the minimum mutual inductance. Finally, the RF coils' performance was verified through a phantom experiment on the self-developed mobile 50 mT breast MRI prototype.

The non-uniformity of the RF field of the modified solenoid coil in the target area can reach 3.7%. The adjacent receive coils have a minimum mutual inductance of 0.232 nH, as the overlapping area is 16.12%. The performance results indicated that the designed transmit coil has a low return loss (RL) of -44.4 dB and a high Q value of 46.55. The S12 value between adjacent channels of the receive coil is below -25 dB, and between non-adjacent channels is about -12 dB. The imaging ability of the designed RF coils was validated by the phantom experiments on the 50 mT breast MRI prototype.

The designed transmit and receive coils in this paper can meet the requirements of the 50 mT mobile MRI with a vertical static magnetic field, which is expected to promote the application of ultra-low field MRI technology in breast cancer screening.