Advanced Radio Frequency Applicators for Thermal Magnetic Resonance Theranostics of Brain Tumors

SIMPLE SUMMARY: Localized thermal therapy has been reported to have clinical benefits as a potent sensitizer of chemo and radiotherapy for various cancers, and for facilitating targeted drug delivery. Thermal magnetic resonance (ThermalMR) integrates targeted radiofrequency (RF) induced heating in the hyperthermia (HT) range, together with diagnostic MRI and in vivo non-invasive temperature mapping within a single RF applicator for thermal theranostics. The potential of dipole antenna arrays for ThermalMR and hyperthermia RF applicators is well recognized, but the additional value of loop elements remains to be investigated. Therefore, we designed circular and elliptical ThermalMR RF applicator arrays with circular 360° and horse-shoe shaped (arc = 270°) coverage of the human head, combining loop antennas and self-grounded bow-tie (SGBT) dipole antennas in a hybrid design. We investigated the performance of these designs in terms of the MRI transmission B(1)(+) field, RF power deposition to optimize targeted RF heating, using electromagnetic field (EMF) and temperature simulations performed on a virtual patient with a clinically realistic intracranial brain tumor. ThermalMR RF applicators with the hybrid loop+SGBT dipole design showed superior MRI performance and targeted RF heating inside the tumor, while preserving healthy tissue. ABSTRACT: Thermal Magnetic Resonance (ThermalMR) is a theranostic concept that combines diagnostic magnetic resonance imaging (MRI) with targeted thermal therapy in the hyperthermia (HT) range using a radiofrequency (RF) applicator in an integrated system. ThermalMR adds a therapeutic dimension to a diagnostic MRI device. Focused, targeted RF heating of deep-seated brain tumors, accurate non-invasive temperature monitoring and high-resolution MRI are specific requirements of ThermalMR that can be addressed with novel concepts in RF applicator design. This work examines hybrid RF applicator arrays combining loop and self-grounded bow-tie (SGBT) dipole antennas for ThermalMR of brain tumors, at magnetic field strengths of 7.0 T, 9.4 T and 10.5 T. These high-density RF arrays improve the feasible transmission channel count, and provide additional degrees of freedom for RF shimming not afforded by using dipole antennas only, for superior thermal therapy and MRI diagnostics. These improvements are especially relevant for ThermalMR theranostics of deep-seated brain tumors because of the small surface area of the head. ThermalMR RF applicators with the hybrid loop+SGBT dipole design outperformed applicators using dipole-only and loop-only designs, with superior MRI performance and targeted RF heating. Array variants with a horse-shoe configuration covering an arc (270°) around the head avoiding the eyes performed better than designs with 360° coverage, with a 1.3 °C higher temperature rise inside the tumor while sparing healthy tissue. Our EMF and temperature simulations performed on a virtual patient with a clinically realistic intracranial tumor provide a technical foundation for implementation of advanced RF applicators tailored for ThermalMR theranostics of brain tumors.