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.
Highlights
Magnetic Resonance Imaging (MRI) is of proven diagnostic value with an ever growing number of applications that support interventional procedures and therapies [1,2,3,4]
While the radio frequency (RF) coils used for magnetic resonance (MR) imaging are commonly operated at a frequency of 64 MHz (1.5 T), RF transmission induced heating interventions are achieved with an applicator commonly driven at a frequency of 70–100 MHz [24]
At 128 MHz focal regions of specific absorption rate (SAR) increase were found which confirms results obtained for RF hyperthermia frequencies (f,140 MHz) used in a clinical setting
Summary
Magnetic Resonance Imaging (MRI) is of proven diagnostic value with an ever growing number of applications that support interventional procedures and therapies [1,2,3,4]. In current clinical RF hyperthermia practice RF coils are being used for imaging and MR thermometry (MRTh) for spatiotemporal monitoring of temperature and treatment efficacy [22,23]. Current clinical implementations require extra hardware retrofitted into the MR suite – notably antennas, amplifiers and frequency filters – which have the trait of driving costs, limiting patient comfort and ease of use and which bear the potential to induce imaging artifacts [25]
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