Solving the Time- and Frequency-Multiplexed Problem of Constrained Radiofrequency Induced Hyperthermia

Abstract

Targeted radiofrequency (RF) heating induced hyperthermia has a wide range of applications, ranging from adjunct anti-cancer treatment to localized release of drugs. Focal RF heating is usually approached using time-consuming nonconvex optimization procedures or approximations, which significantly hampers its application. To address this limitation, this work presents an algorithm that recasts the problem as a semidefinite program and quickly solves it to global optimality, even for very large (human voxel) models. The target region and a desired RF power deposition pattern as well as constraints can be freely defined on a voxel level, and the optimum application RF frequencies and time-multiplexed RF excitations are automatically determined. 2D and 3D example applications conducted for test objects containing pure water (rtarget = 19 mm, frequency range: 500–2000 MHz) and for human brain models including brain tumors of various size (r1 = 20 mm, r2 = 30 mm, frequency range 100–1000 MHz) and locations (center, off-center, disjoint) demonstrate the applicability and capabilities of the proposed approach. Due to its high performance, the algorithm can solve typical clinical problems in a few seconds, making the presented approach ideally suited for interactive hyperthermia treatment planning, thermal dose and safety management, and the design, rapid evaluation, and comparison of RF applicator configurations.

Design, Implementation, Evaluation and Application of a 32-Channel Radio Frequency Signal Generator for Thermal Magnetic Resonance Based Anti-Cancer Treatment

“…HT devices are increasingly capable of the personalized radio frequency (RF)-induced heating of target tissue volumes guided by sophisticated treatment planning procedures and thermal dose control [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. Thermal Magnetic Resonance (ThermalMR) is an HT variant that accommodates RF-induced heating [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ], temperature mapping using MR thermometry (MRT) [ 26 , 27 , 28 , 29 ], anatomic and functional imaging and the option for x-nuclei MR imaging (MRI) in a single, multi-purpose RF applicator.…”

“…Thus, the RF signal source is the key component for facilitating appropriate frequency, amplitude and phase settings of the RF signals. The radiation pattern of the single RF transmit element, the RF channel count and the RF frequency of the RF applicator are of high relevance for ensuring a patient and problem-oriented adaptation of the size, uniformity and location of the RF energy deposition in the target region [ 19 , 21 , 22 , 24 , 30 ]. The (re)design of multi-channel RF applicator configurations showed more than twofold enhancement of the RF power focusing capability by increasing the number of RF antennae from 12 to 20 [ 31 , 32 ].…”

“…Higher RF frequencies facilitate focusing EM energy onto small targets. Numerical simulations and evaluation studies investigated the optimal RF frequency [ 24 ]. For regional hyperthermia improvement of the RF power absorption in the target region versus regions outside the target was demonstrated when increasing the RF frequency from 100 MHz to 150 MHz and 200 MHz [ 35 , 36 ].…”

“…Time-multiplexed beamforming, a mixed frequency approach and multi-frequency SAR focusing provide other directions into optimization of RF heating performance [ 38 , 39 , 40 ]. Recently, an iterative multiplexed vector field shaping (MVFS) approach was introduced to solve the time- and frequency multiplexed problem of constrained RF-induced hyperthermia [ 24 ]. This work underlined the need of wideband signal generators by demonstrating the contribution of distinct frequencies to the RF heating and by showing that these frequencies and contributions depend on the target geometry.…”

Controlled Release of Therapeutics from Thermoresponsive Nanogels: A Thermal Magnetic Resonance Feasibility Study

“…For example, for brain tissue, a hotspot size as small as (6 × 9) mm 2 was reported for an RF frequency of f = 297 MHz [ 29 , 30 ]. On the other hand, large heating volumes (~500 mL) can also be achieved by manipulating the phase, power, and frequency of different channels of the array [ 31 , 32 ]. To apply heat into a target site, controlled manipulation of temperature is required while concomitantly characterizing its outcome in vivo.…”