In magnetic resonance imaging (MRI) environment, adding more receiving channels requires a a significant increase in the number of connectors and cables, causing discomfort to the patient. Powering coils or sensor devices inside the MRI scanner could minimize this problem. The challenge here is the ability to recover energy from a 4.7T MRI scanner without impacting MR images.
While increasing the receiver channels and the number of sensors inside the MRI scanner, the number of cables and connectors required is also increased. The need of wireless powering methods and autonomous devices is a real challenge. Energy harvesting in MRI scanners presents a very promising solution for cable-free and devices energy self-sufficient. The aim is to harvest energy from RF-pulses during the MRI imaging sequences. The high-power field B1 in MRI provides the capability to harvest power inside the MRI bore. Preliminary studies of Energy Harvesting until 11mW inside a Bruker AVANCE III MRI spectrometer (400 MHz) have been previously demonstrated1 (J.Hofflin). In this paper, with a high efficiency diode rectifier and a harvesting loop, we are able to harvest energy inside a 4.7T MRI scanner in order to power a preamplifier of receiving arrays or heart and respiration sensors, all with a minimal impact on the MR images.
A 2 cm diameter loop antenna, impedance matched to a target load of 50Ω, with a RF rectifier of Schottky diodes bridge and voltage doubler, is used as Energy Harvesting System (EHS). The proposed harvester is shown in Figure 1. The rectifier performs RF-DC conversion and uses Schottky diodes for low voltage drop and fast switching. The rectifier is needed to convert the RF input power into DC power. For maximum power transfer between the antenna and the rectifier, a matching network is designed, simulated, manufactured and measured. A large storage capacitor of 10 μF is used at the output of the RF rectifier in order to provide a continuous power to the load (preamplifier or sensors) when the system is switched on and off. The integration of the EHS with a 4.7T scanner is demonstrated.
Otherwise, in order to implement the energy harvester in the MRI scanner, it is important to minimize noise and harmonic interference that can be added to the MR images. The most convenient solution is the separation distance of 7cm between the coils (the MRI coils and the harvesting loop). It is the most effective way until now to maintain the image quality obtained without the harvesting system. We present here our first prototype tests demonstrating that the EHS can coexist in a 4.7T scanner with a minimal impact on the image quality. The simulated results presented in this work were obtained with ADS and CST Software. The MRI experiments were realized on a Bruker MRI Biospec 4.7T (200 MHz).
In MR system, different pulse sequences of B1 field provide different amounts of power. A high-impedance oscilloscope measured the DC power obtained from the rectifier. Figure 2 shows results of Flash sequences with a long TR of 150ms, 10ms TE, increasing flip angle and number of slices. Figure 3 presents the energy harvesting results for a Flash sequence, with 10ms TR. With shorter TR, smaller load resistors can be used, increasing the harvested energy. Figure 4 shows images with TR of 150ms and flip angle of 60°. Hence, harvesting energy from the B1 field generates an RF current in the harvesting coil at the MRI frequency, with a minimal impact on the MR images. The results with and without the EH system are displayed.
As the harvesting energy is also performed in the MRI frequency, the loop antenna and the rectifier could generate noise that induces local distortion in B1 field, leading to degradation of the MR image. In this case, the most critical issue is to harvest energy and maintain the SNR of the obtained images with the energy harvester. Otherwise, the efficiency of the energy harvesting system depends on the coil design, the rectifier design and the resistor load. The design of the MRI harvester is also constrained by MRI compatibility requirements, as well as the coil size.
The proof of concept of harvesting energy using a loop antenna, bridge and voltage doubler rectifiers is validated in 4.7T MRI Scanner. We were able to harvest energy by using a receiving coil with a high efficiency rectifier. Preamplifiers could be powered without external power supply inside the MRI scanner by reducing the long BNC cables, which are causing a power loss between the coils and the power amplifier. The harvested power is highly dependent on the pulse sequence RF duty-cycle. Otherwise, the system is recommended to be compatible with the MRI environment for a minimal impact on the MR images.