Design and Evaluation of a Hybrid Radiofrequency Applicator for Magnetic Resonance Imaging and RF Induced Hyperthermia: Electromagnetic Field Simulations up to 14.0 Tesla and Proof-of-Concept at 7.0 Tesla

Abstract

This work demonstrates the feasibility of a hybrid radiofrequency (RF) applicator that supports magnetic resonance (MR) imaging and MR controlled targeted RF heating at ultrahigh magnetic fields (B0≥7.0T). For this purpose a virtual and an experimental configuration of an 8-channel transmit/receive (TX/RX) hybrid RF applicator was designed. For TX/RX bow tie antenna electric dipoles were employed. Electromagnetic field simulations (EMF) were performed to study RF heating versus RF wavelength (frequency range: 64 MHz (1.5T) to 600 MHz (14.0T)). The experimental version of the applicator was implemented at B0 = 7.0T. The applicators feasibility for targeted RF heating was evaluated in EMF simulations and in phantom studies. Temperature co-simulations were conducted in phantoms and in a human voxel model. Our results demonstrate that higher frequencies afford a reduction in the size of specific absorption rate (SAR) hotspots. At 7T (298 MHz) the hybrid applicator yielded a 50% iso-contour SAR (iso-SAR-50%) hotspot with a diameter of 43 mm. At 600 MHz an iso-SAR-50% hotspot of 26 mm in diameter was observed. RF power deposition per RF input power was found to increase with B0 which makes targeted RF heating more efficient at higher frequencies. The applicator was capable of generating deep-seated temperature hotspots in phantoms. The feasibility of 2D steering of a SAR/temperature hotspot to a target location was demonstrated by the induction of a focal temperature increase (ΔT = 8.1 K) in an off-center region of the phantom. Temperature simulations in the human brain performed at 298 MHz showed a maximum temperature increase to 48.6C for a deep-seated hotspot in the brain with a size of (19×23×32)mm3 iso-temperature-90%. The hybrid applicator provided imaging capabilities that facilitate high spatial resolution brain MRI. To conclude, this study outlines the technical underpinnings and demonstrates the basic feasibility of an 8-channel hybrid TX/RX applicator that supports MR imaging, MR thermometry and targeted RF heating in one device.

Magnetic resonance thermometry: Methodology, pitfalls and practical solutions

“…The average delivered power is determined by the RF pulse shape together with the applied repetition time and limited by the duty cycle and long term stability of the RFPA. At 7.0T with an integrated 8x1kW RFPA and ~30% RF cable losses to the RF coil plug, this would result in an average power of 560 W at a duty cycle of 10% (31). While UHF-MR systems equipped with 8x2kW or even 16x2kW are expected to become available soon, this would allow for an average RF power of 1120 W and 2240 W respectively, assuming similar duty cycle and RF losses.…”

“…The advantage of a pulsed RFPA used for RF hyperthermia application and MR thermometry is the ability to modify the imaging technique in order to perform RF hyperthermia and MRI simultaneously at the same frequency. This approach exhibits the merit of an artefact free operation without the need for additional hardware modifications (31). In order to reach sufficient RF power deposition levels in tissue high power RF pulses used for imaging can be applied, off-resonance pulses can be incorporated in the imaging technique or a combination of the two.…”

“…This justifies the use of birdcage type RF coils for transmission and loop RF coils for reception at lower fields (B0≤3.0T), while dipole antennas are well suited as transmit/receive elements for UHF MRI at B0≥7.0T (31,(119)(120)(121) and above (122). The usage of dipole or "dipole-like" antennas as a targeted RF heating device is very well recognized by the hyperthermia community (104,(126)(127)(128).…”

“…The second decade of development advanced the capabilities of MR thermometry towards more robust and reliable in vivo applications (22)(23)(24)(25). The future target of MR thermometry developments is clearly formed around novel methodology and hardware that supports accurate absolute temperature readings in vivo with the current developments already pushing into this direction (26)(27)(28)(29)(30)(31). These efforts are essential to benchmark thermal therapy efficiency, understand temperature related biophysical and physiological processes and to use these insights for defining and setting new landmarks for diagnostic and therapeutic capabilities.…”

Electrodynamics and radiofrequency antenna concepts for human magnetic resonance at 23.5 T (1 GHz) and beyond

“…To address this obstacle multi-element transceiver coil arrays have been developed. These developments took advantage of building blocks that include stripline elements (34)(35)(36)(37)(38), dielectric resonant antennae (39), loop elements (40)(41)(42)(43)(44)(45)(46) and electrical dipoles (47)(48)(49)(50)(51)(52)(53).…”