A printed Yagi-Uda antenna for application in magnetic resonance thermometry guided microwave hyperthermia applicators.

Biological studies and clinical trials show that addition of hyperthermia stimulates conventional cancer treatment modalities and significantly improves treatment outcome. This supra-additive stimulation can be optimized by adaptive hyperthermia to counteract strong and dynamic thermoregulation. The only clinically proven method for the 3D non-invasive temperature monitoring required is by magnetic resonance (MR) temperature imaging, but the currently available set of MR compatible hyperthermia applicators lack the degree of heat control required. In this work, we present the design and validation of a high-frequency (433 MHz ISM band) printed circuit board antenna with a very low MR-footprint. This design is ideally suited for use in a range of hyperthermia applicator configurations. Experiments emulating the clinical situation show excellent matching properties of the antenna over a 7.2% bandwidth (S 11 < -15 dB). Its strongly directional radiation properties minimize inter-element coupling for typical array configurations (S 21 < -23 dB). MR imaging distortion by the antenna was found negligible and MR temperature imaging in a homogeneous muscle phantom was highly correlated with gold-standard probe measurements (root mean square error: RMSE = 0.51 °C and R 2 = 0.99). This work paves the way for tailored MR imaging guided hyperthermia devices ranging from single antenna or incoherent antenna-arrays, to real-time adaptive hyperthermia with phased-arrays.

Laboratory prototype for experimental validation of MR-guided radiofrequency head and neck hyperthermia.

Clinical studies have established a strong benefit from adjuvant mild hyperthermia (HT) to radio- and chemotherapy for many tumor sites, including the head and neck (H&N). The recently developed HYPERcollar allows the application of local radiofrequency HT to tumors in the entire H&N. Treatment quality is optimized using electromagnetic and thermal simulators and, whenever placement risk is tolerable, assessed using invasively placed thermometers. To replace the current invasive procedure, we are investigating whether magnetic resonance (MR) thermometry can be exploited for continuous and 3D thermal dose assessment. In this work, we used our simulation tools to design an MR compatible laboratory prototype applicator. By simulations and measurements, we showed that the redesigned patch antennas are well matched to 50 Ω (S11<-10 dB). Simulations also show that, using 300 W input power, a maximum specific absorption rate (SAR) of 100 W kg(-1) and a temperature increase of 4.5 °C in 6 min is feasible at the center of a cylindrical fat/muscle phantom. Temperature measurements using the MR scanner confirmed the focused heating capabilities and MR compatibility of the setup. We conclude that the laboratory applicator provides the possibility for experimental assessment of the feasibility of hybrid MR-HT in the H&N region. This versatile design allows rigorous analysis of MR thermometry accuracy in increasingly complex phantoms that mimic patients' anatomies and thermodynamic characteristics.

Electromagnetic redesign of the HYPERcollar applicator: toward improved deep local head-and-neck hyperthermia.

Accumulating evidence shows that hyperthermia improves head-and-neck cancer treatment. Over the last decade, we introduced a radiofrequency applicator, named HYPERcollar, which enables local heating also of deep locations in this region. Based on clinical experience, we redesigned the HYPERcollar for improved comfort, reproducibility and operator handling. In the current study, we analyze the redesign from an electromagnetic point of view. We show that a higher number of antennas and their repositioning allow for a substantially improved treatment quality. Combined with the much better reproducibility of the water bolus, this will substantially minimize the risk of underexposure. All improvements combined enable a reduction of hot-spot prominence (hot-spot to target SAR quotient) by 32% at an average of 981 W, which drastically reduces the probability for system power to become a treatment limiting source. Moreover, the power deposited in the target selectively can be increased by more than twofold. Hence, we expect that the HYPERcollar redesign currently under construction allows us to double the clinically applied power to the target while reducing the hot-spots, resulting in higher temperatures and, consequently, better clinical outcome.