Safety and Effectiveness Evaluation of Medical Therapeutic and Diagnostic Devices Using EM Fields
Currently, there are a number of promising applications of electromagnetic fields in medical diagnosis and therapy. Some, such as non-invasive monitoring of blood glucose concentration in patients with diabetes or temperature monitoring during hyperthermia, are in the phase of feasibility studies. The early diagnosis of breast tumors or the classification of cerebrovascular events using microwave imaging and machine learning algorithms have already led to the emergence of start-ups, and prototypes are already being tested on patients. Microwave hyperthermia and ablation or transcranial magnetic stimulation, on the other hand, have been in clinical practice for several decades and have helped many patients. For all the mentioned applications and a whole range of others, information on the distribution of the electromagnetic field or the induced temperature in the treated area - part of the patient's body is needed for the design and optimization of the respective devices, and for some even for the device's function itself (microwave imaging algorithms, hyperthermia treatment planning). An analytical solution to Maxwell's equations or the Bioheat equation does not exist due to the high complexity of the anatomy of the human body and the fact that individual tissues show different values of dielectric and thermal parameters. Measuring the distribution of EM fields and temperature can be time-consuming and expensive, and at the same time, current measurement methods affect the measured quantities or even lack sufficient temporal and spatial resolution. Even if it is possible to create patient phantoms that are accurate in terms of anatomy and dielectric parameters, it is usually no longer possible to make them accurate in terms of thermal parameters at the same time. In addition to the mentioned development and optimization of the devices, numerical simulations can also be used to study their safety and effectiveness. Already at the very beginning of the design and development, they can provide important information, from the range of input impedances of the electrodes or antennas, through the minimum necessary output power of the amplifiers, the appropriate number of antenna elements to the optimal working frequency. For the purposes of certification, they can be used in preclinical testing of a medical device prototype according to MDR 2017/745. In this contribution, we will deal with determining the basic parameters of a four-channel radiofrequency hyperthermia regional system. Such systems are used for heating deep-seated tumors in the pelvic region using the constructive interference of monofrequency electromagnetic waves emitted from individual antenna elements. In order for the constructive interference to occur precisely in the tumor and at the same time to avoid unnecessary thermal stress on healthy tissues, so-called treatment planning is carried out. It searches for complex amplitudes of antenna signals using global optimization methods. Methods of calculating a) electromagnetic fields from individual antenna elements, b) reflection and transmission coefficients, c) electromagnetic fields after optimizing the amplitudes and phases of antenna signals and d) the corresponding temperature profile will be presented. In these calculations, an anatomically, dielectrically and thermally faithful model of the patient will be used.