A retrofit to enable dynamic B 1 + steering for transmit arrays without multiple amplifiers

Rapid MR Scanner Independent B1 Field Measurement System for Phased Arrays

This paper demonstrates a rapid B1 field benchtop measurement system that is independent of an MR scanner and network analyzer. This system can be used to obtain radiofrequency (B1 field) strength distribution plots of multiple 2D slices (with an extension to 3D) of a liquid cylindrical phantom for multi-element phased arrays used in MRI. The system can be used in three modes- element, phased array, and multiple fixed point pattern measurement. These modes are demonstrated for a 7T 1H eight-channel dipole array and a corn-syrup based phantom. The system can measure complex phase and amplitude measurements from up to 8 elements in the first mode one or 8 different phase settings in the second mode at a rate of approximately 37 positions per minute, allowing a full 2D B1 mapping for 1303 points in 33.05 minutes. The scan patterns obtained using this setup are compared to the ones obtained using an HP network analyzer and simulations. This work can be extended to measure the E field, SAR and upon increasing the speed of measurement, could be used for applications such as Transmit SENSE. Clinical Relevance- This work benefits a faster and more widely accessible measurement system for phased array antennas for MRI. As phased arrays are becoming very important in MRI, the ability to assess individual element performance more rapidly and B1 shimming performance is important to aid in their further development.

Resistor-free and one-board-fits-all ratio adjustable power splitter for add-on RF shimming in high field MRI

Ratio adjustable power splitter (RAPS) circuits were recently proposed for add-on RF shimming. Previous RAPSs split the input RF signal with a Wilkinson splitter or 50-Ω-terminated hybrid coupler into two branches, delay these two signals with cable/microstrip line phase shifters, and recombine them with another hybrid coupler. They require resistors to provide high output isolation and a cable/microstrip line library to realize desired splitting ratios. Here we propose a novel resistor-free RAPS circuit in which the Wilkinson splitter/50-Ω-terminated hybrid is replaced with a resistor-free T-junction splitter. A novel sliding mechanism was employed to further combine the T-junction's output arms with subsequent phase shifters and realize a one-board-fits-all design. The resistor-free RAPS was theoretically analyzed, simulated, and validated on workbench and MRI experiments. The resistor-free RAPS's splitting ratio has a tan/cot dependence on the phase/length difference between the T-junction output arms. The ratio can be continuously adjusted to any value by sliding the input arm without additional cable/microstrip libraries, largely saving time and effort when determining the best RF weights in practice. The fabricated resistor-free RAPS has a compact size, excellent input impedance matching, and a low insertion loss. Potential safety concerns caused by unwanted power dissipation on RF resistors are eliminated. The simulation and MRI experiments demonstrated that the resistor-free RAPS functions well on a widely-used Tx coil.

Hybrid-pair ratio adjustable power splitters for add-on RF shimming and array-compressed parallel transmission

PURPOSE

A ratio adjustable power splitter (RAPS) circuit was recently proposed for add-on RF shimming and array-compressed parallel transmission. Here we propose a new RAPS circuit design based on off-the-shelf components for improved performance and manufacturability.

THEORY AND METHODS

The original RAPS used a pair of home-built Wilkinson splitter and hybrid coupler connected by a pair of connectorized coaxial cables. Here we propose a new hybrid-pair RAPS (or HP-RAPS) circuit that replaces the home-built circuits with two commercially available hybrid couplers and replaces connectorized cables with interchangeable microstrip lines. We derive the relation between the desired splitting ratio and the required phase shifts for HP-RAPS and investigate how to generate arbitrary splitting ratios using paired meandering and straight lines. Several HP-RAPSs with different splitting ratios were fabricated and tested on the workbench and MRI experiments.

RESULTS

The splitting ratio of an HP-RAPS circuit has a tan or cot dependence on the meandering line's additional length compared to the straight line. The fabricated HP-RAPSs exhibit accurate splitting ratios as expected (<4% deviations) and generate transmit fields that well agree with predicted fields. They also demonstrated a low insertion loss of 0.33 dB, high output isolation of -26 dB, and acceptable impedance matching of -16 dB.

CONCLUSION

A novel HP-RAPS circuit was developed and implemented. It is easy-to-fabricate/reproduce with minimal expertise. It also preserves the features of the original RAPS circuit (ratio-adjustable, small footprint, etc.) with lower insertion loss.