An RF-gated wireless power transfer system for wireless MRI receive arrays

Stealth RF energy harvesting in MRI using selective shielding

PURPOSE

To utilize the transmit radiofrequency (RF) field in MRI as a power source, near or within the field of view but without affecting image quality or safety.

METHODS

Power harvesting is performed by RF induction in a resonant coil. Resulting RF field distortion in the subject is canceled by a selective shield that couples to the harvester while being transparent to the RF transmitter. Such shielding is designed with the help of electromagnetic simulation. A shielded harvester of 3 cm diameter is implemented, assessed on the bench, and tested in a 3T MRI system, recording power yield during typical scans.

RESULTS

The concept of selective shielding is confirmed by simulation. Bench tests show effective power harvesting in the presence of the shield. In the MRI system, it is confirmed that selective shielding virtually eliminates RF perturbation. In scans with the harvester immediately adjacent to a phantom, up to 100 mW of average power are harvested without affecting image quality.

CONCLUSION

Selective shielding enables stealthy RF harvesting which can be used to supply wireless power to on-body devices during MRI.

Detunable wireless Litzcage coil for human head MRI at 1.5 T

Inductively coupled radiofrequency (RF) coils are an inexpensive and simple method to realize wireless RF coils in magnetic resonance imaging (MRI), which can significantly ease the MRI scan setup and improve patient comfort because they do not require bulky components such as cables, baluns, preamplifiers, and connectors. However, volume-type wireless coils are typically operated in transmit/receive mode because detuning such coils is much more challenging due to their complex structure and multiple resonant modes. Meanwhile, adding too many detuning circuits to a wireless coil would decrease the coil's quality factor, impair the signal-to-noise ratio, and increase the cost. In this work, we proposed, constructed, and tested a novel wireless volume coil based on the Litzcage design for 1.5-T head imaging. Being an inductively coupled coil, it has a much simpler structure, resulting in a lighter weight and less bulky design. Despite its simpler structure, it exhibits comparable imaging performance with a commercial receive array, providing an alternative to conventional wired coils with a high cost and complex structure. The unique figure-of-8 conductor pattern within the rungs ensures that the proposed wireless Litzcage can be efficiently detuned with minimal detuning circuits.

A Semi-Blind Calibration and Compensation Method for Dynamic Range Recovery of Low-Power Pre-Amplifiers in MRI Receive Chains

To enable wireless MRI receive arrays, per-channel power consumption must be reduced by a significant factor. To address this, a low-power SiGe alternative to industry standard MRI pre-amplifier blocks has been proposed and its impact on imaging performance evaluated in a benchtop environment. The SiGe amplifier reduces power consumption 28x, but exhibits increased non-linearity and reduced dynamic range relative to industry standard amplifiers. This distorts the images, causing reduced contrast and a blurring of fine features. In conjunction with the amplifier, a semi-blind calibration and compensation framework has been proposed to remove artifacts caused by this non-linearity. Requiring the knowledge of the calibration signal bandwidth, the associated peak transmit powers, and the distorted baseband signals, a second non-linearity is constructed that when cascaded with the receive chain produces a linear response. This method was evaluated for both knee and phantom image datasets of peak input power -20dBm with a -40dBm peak input power image as reference. In the benchtop environment, industry standard amplifiers produced input normalized RMSEs of 0.0199 and 0.0310 for phantom and knee datasets, respectively. The low-power SiGe amplifier resulted in RMSEs of 0.0869 and 0.1130 which were reduced to 0.0158 and 0.0168 following compensation, for phantom and knee images respectively. The ability to effectively compensate for this reduced dynamic range encourages further investigation of low-power SiGe amplifiers for power limited MRI receive arrays.

Wireless, Battery-Free, and Fully Implantable Micro-Coil System for 7 T Brain MRI

An elegant solution for the concurrent transmission of data and power is essential for implantable wireless magnetic resonance imaging (MRI). This paper presents a self-tuned open interior microcoil (MC) antenna with three useful operating bands of 300 (7 T), 400, and 920 MHz, for blood vessel imaging, data telemetry, and efficient wireless transmission of power, respectively. The proposed open interior MC antenna contains two mirrorlike arms with diameters and lengths of 2.4 mm and 9.8 mm, respectively, to avoid blood flow blockage. To wirelessly show LED glow on a saline based phantom, the MC was fabricated on a flexible polyimide material and combined with a miniaturized rectifier and a micro-LED. Using a path gain, the power transfer efficiency (PTE) of the MC rotation was also analyzed. Additionally, the PTE was calculated for a range of distances between 25 and 60 mm, and a -27.1 dB PTE attained at a distance of of 30 mm. Based on the recommendations of the International Commission on Non-Ionizing Radiation Protection for human brain safety when exposed to radio-frequencies from external transmitter, a specific absorption rate analysis was analyzed. Measurements of the s-parameters were noted using a saline solution and blood vessel model to imitate a realistic human head. They were found to correlate reasonably with the simulated results.

Recent Advances in Radio-Frequency Coil Technologies: Flexible, Wireless, and Integrated Coil Arrays

Radio-frequency (RF) coils are to magnetic resonance imaging (MRI) scanners what eyes are to the human body. Because of their critical importance, there have been constant innovations driving the rapid development of RF coil technologies. Over the past four decades, the breadth and depth of the RF coil technology evolution have far exceeded the space allowed for this review article. However, these past developments have laid the very foundation on which some of the recent technical breakthroughs are built upon. Here, we narrow our focus on some of the most recent RF coil advances, specifically, on flexible, wireless, and integrated coil arrays. To provide a detailed review, we discuss the theoretical underpinnings, experimental implementations, promising results, as well as future outlooks covering these exciting topics. These recent innovations have greatly improved patient comfort and ease of scan, while also increasing the signal-to-noise ratio, image resolution, temporal throughput, and diagnostic and treatment accuracy. Together with advances in other MRI subfields, they will undoubtedly continue to drive the field forward and lead us to an ever more exciting future. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 1.

A Novel Energy Harvesting Circuit for RF Surface Coils in the MRI System

RF surface coils are commonly used as receivers in Magnetic Resonance Imaging (MRI) systems to acquire sensitive signals from the human body. These coils rely on cables for power and transferring information from the patient to the computer for processing. Higher image quality and faster scan times are possible by using an array of surface coils, and there is a constant need for high-density surface coil arrays. Each array element utilizes at least three cables and, increasing the number of elements in the array also increases the number of cables, making the cable bundle bulkier. This makes the placement of cables complicated for the operators and may cause patient harm when improperly positioned. Wireless technologies can eliminate cables, and this paper proposes a novel design for harvesting the ambient RF energy present during the transmit phase of the MRI system operation. After introducing the surface coil's building blocks, the importance of the decoupler as a mechanism for safety and image quality is detailed. The paper presents the analysis and design of an RF energy harvesting circuit that couples to the decoupler circuit. Its performance is tested both in simulation and the Philips Ingenia 3.0 T MRI system. The results show that the circuit successfully harvests energy, up to 1 W, during the MRI's transmit phase without any adverse effects on the decoupler or surface coil. To make energy harvesting (EH) beneficial, a new GaN -based FET switch that consumes low power is also proposed.

Wireless Power Harvesting During MRI

We present methods to harvest wireless power directly from the MRI RF field. The system includes a harvester coil to capture RF energy and an RF-DC converter for rectification. Energy harvesting by the harvester coil is modeled as a function of the MRI B₁ RF field. Rectification is modeled using power-dependent large signal S-parameter simulation. A novel reference impedance-based modeling approach is leveraged to cascade models for linear inductive coupling and nonlinear diode rectification, and validated. The method permits independent optimization of harvester coils and RF-DC converters to maximize harvesting efficiency. Feasibility of this technique is demonstrated by implementing concurrent in-bore wireless power harvesting and MRI scanning on a clinical system. The effect of artifacts on image quality is also investigated.Clinical Relevance- In-bore wireless harvesting can provide power for medical accessories during MRI, with minimal system modification and cost.

Toward "plug and play" prospective motion correction for MRI by combining observations of the time varying gradient and static vector fields

PURPOSE

The efficacy of a Wireless Radio frequency triggered Acquisition Device (WRAD) is evaluated for high frequency (50 Hz) prospective motion correction in a 3-dimensional spoiled gradient echo pulse sequence.

METHODS

The device measures the rate of change in the gradient vector fields (slew) using a 3-dimensional assembly of Printed Circuit Board (PCB) inductors and the direction of the static magnetic field using a 3-axis Hall effect magnetometer. The slew vector encoding is highly efficient, because the Maxwell-term position encoding is observable, allowing overconstrained pose measurement using 3 sinusoidal gradient pulses lasting 880 μs. Since small offsets in the magnetometer can introduce bias into the pose estimates, sensor/system biases are tracked using a lightweight Kalman filter. The only calibration required is determining a geometric scaling factor for the pickup coils, which is specific to the device and will therefore be valid in any scanner.

RESULTS

The device was used to perform prospective motion correction in 3 subjects, resulting in an increase in Average Edge Strength (AES) for involuntary and deliberate motion.

CONCLUSIONS

The WRAD is simple to set up and use, with well-defined measurement variance. This could enable "plug and play" prospective motion correction if pulse sequence independence is achieved.

An MRI Compatible RF MEMs Controlled Wireless Power Transfer System

In magnetic resonance imaging (MRI), wearable wireless receive coil arrays are a key technology goal. An MRI compatible wireless power transfer (WPT) system will be needed to realize this technology. An MRI WPT system must withstand the extreme electromagnetic environment of the scanner and cannot degrade MRI image quality. Here, a WPT system is developed for operation in MRI scanners using new microelectromechanical RF switch (RF MEMs) technology. The WPT system includes a class-E power amplifier, RF MEMs automated impedance matching, a primary coil array employing RF MEMs power steering, and a flexible secondary coil with class E rectification. To adapt WPT technology to MRI, techniques are developed for operation at high magnetic field, and to mitigate the RF interactions between the scanner and WPT system. A major challenge was the identification and suppression of noise and harmonic interference, by gating, filtering, and rectifier topologies. The system can achieve 63% efficiency while exceeding 13 W delivery over a coil distance of 3.5 cm. For continuous WPT beyond 5W, added filters and full-wave class E rectification lowers harmonic generation at some cost to efficiency, while image SNR reaches about 32% of the ideal. RF-gated WPT, which interrupts power transfer in the MRI signal acquisition interval, achieves SNR performance to within 1 dB of the ideal. With further refinement, the inclusion of WPT technology in MRI scanners appears completely feasible.