Electromagnetic Head-And-Neck Hyperthermia Applicator: Experimental Phantom Verification and FDTD Model

Abstract

Purpose: Experimental verification of the feasibility of focussed heating in the neck region by an array of two rings of six electromagnetic antennas. We further measured the dynamic specific absorption rate (SAR) steering possibilities of this setup and compared these SAR patterns to simulations. Methods and Materials: By a specially constructed laboratory prototype head and neck applicator, including a neck mimicking cylindrical muscle phantom, we performed SAR measurements by electric-field Schottky-diode sheet measurements and, using the power-pulse technique, by fiberoptic thermometry and infra-red (IR) thermography. Using phase-steering, we also steered the SAR distribution in radial and axial directions. All measured distributions where compared to predictions by a finite-difference time-domain (FDTD)-based electromagnetic simulator. Results: A central 50% iso-SAR focus of 35mm (±3mm) in diameter and around 100mm (±15mm) in length are obtained for all investigated settings. Further, this SAR focus can be steered towards the desired location in radial and axial direction with an accuracy of ∼5mm. SAR distributions as measured by all three experimental methods are well predicted by the simulations. Conclusions: We conclude that focussed heating in the neck is feasible and that this focus can effectively be steered in both radial and axial direction. For quality assurance (QA) measurements we conclude that the Schottky-diode sheet provides the best compromise between effort, speed and accuracy, though a more specific and improved design is warranted.

Magnetic resonance thermometry: Methodology, pitfalls and practical solutions

“…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). Recent reports demonstrated for the first time the feasibility of an integrated device that supports diagnostic MR, MR thermometry and localized RF hyperthermia using bow tie electric dipoles immersed in high permittivity dielectric (31).…”

Validation of MR thermometry: Method for temperature probe sensor registration accuracy in head and neck phantoms

“…The phantom was heated with phase-coherent RF radiation at 434 MHz using an MR-compatible laboratory prototype of the HYPERcollar RF hyperthermia applicator [29]. The heat applicator consists of two rings of six patch antennas optimised to give a minimum antenna reflection coefficient (S 11 ) at 434 MHz.…”

Clinical integration of software tool VEDO for adaptive and quantitative application of phased array hyperthermia in the head and neck

“…The validation of SEMCAD X was earlier extensively reported by Paulides et al [14,25], where SEMCAD X was shown to correctly predict the location and size of a SAR focus in a cylindrical phantom, using both lateral and longitudinal SAR steering. For this study, extra tests using the cylindrical phantom were carried out only on orientation to ensure that a focus shift from VEDO was properly converted into a focus shift in the phantom.…”

“…Different factors influence this mismatch between simulations and clinical practice. Firstly, although validation studies have been carried out on phantoms [25,28] and more recently in patients [29], there is still a lack of validation of the complete patient and HYPERcollar set-up. Inter-patient variation of EM and thermal properties exists and could be a source of error.…”