Standalone RF Self-Interference Cancellation System for In-Vivo Simultaneous Transmit and Receive (STAR) MRI.

Demonstrated is a standalone RF self-interference canceller for simultaneous transmit and receive (STAR) magnetic resonance imaging (MRI) at 1.5T. Standalone STAR cancels the leakage signal directly coupled between transmit and receive RF coils. A cancellation signal, introduced by tapping the input of a transmit coil with a power divider, is manipulated with voltage-controlled attenuators and phase shifters to match the leakage signal in amplitude, 180° out of phase, to exhibit high isolation between the transmitter and receiver. The cancellation signal is initially generated by a voltage-controlled oscillator (VCO); therefore, it does not require any external RF or synchronization signals from the MRI console for calibration. The system employs a field programmable gate array (FPGA) with an on-board analog to digital converter (ADC) to calibrate the cancellation signal by tapping the receive signal, which contains the leakage signal. Once calibrated, the VCO is disabled and the transmit signal path switches to the MRI console for STAR MR imaging. To compensate for the changes of parameters in RF sequences after the automatic calibration and to further improve isolation, a wireless user board that uses an ESP32 microcontroller was built to communicate with the FPGA for final fine-tuning of the output state. The standalone STAR system achieved 74.2 dB of isolation with a 94 second calibration time. With such high isolation, in-vivo MR images were obtained with approximately 40 mW of RF peak power.

Flexible Metamaterial Wrap for Improved Head Imaging at 3 T MRI With Low-Cost and Easy Fabrication Method.

Magnetic resonance imaging (MRI) requires spatial uniformity of the radiofrequency (RF) field inside the subject for maximum signal-to-noise ratio (SNR) and image contrast. Bulky high permittivity dielectric pads (HPDPs) focus magnetic fields into the region of interest (ROI) and increase RF field uniformity when placed between the patient and RF coils in the MR scanner. Metamaterials could replace HPDPs and reduce system bulkiness, but those in the literature often require a complicated fabrication process and cannot conform to patient body shape. Proposed is a flexible metamaterial for brain imaging made with a scalable fabrication process using conductive paint and a plastic laminate substrate. The effects of single and double-sided placement of the metamaterial around a human head phantom were investigated in a 3 T scanner. When two metamaterial sheets were wrapped around a head phantom (double-sided placement), the total average signal in the resulting image increased by 10.14% compared to placing a single metamaterial sheet underneath the phantom (single-sided placement). The difference between the maximum and minimum signal intensity values decreased by 57% in six different ROIs with double-sided placement compared to single-sided placement.