Development and validation of a monitor for detecting metal implants with heating risk within the application field of a magnetic stimulator

Abstract

Pelvic floor therapy with Functional Magnetic Stimulation (FES) is becoming more and more popular in different groups of patients. More generally the field of FMS applications extends over a multitude of different possibilities, starting with the transcranial magnetic stimulation for migraine treatment to strengthening various muscles via neuromuscular activation. Magnetic stimulation has key advantages over conventional electrical therapies. These advantages are mostly because of the ease of application, where the treatment can be done through everyday clothing and the avoidance of possible skin irritations or even burns due to high current densities in the electrode contact surface and a highly effective treatment with less sensible discomfort. An important limitation of magnetic stimulation strong impulse fields is the potential heating of metal parts by induced eddy currents. This heating can be dangerous if the surrounding tissues cannot compensate this heating with natural mechanisms. Due to the increasing number of hip implants, a safe detection of the implants is becoming more important. This thesis describes the development of the prototype for a metal monitor/detector developed by the Medical University of Vienna, in cooperation with gbo Medizintechnik AG, for PonteMed GmbH. The PelviPower Magnetic Field Trainer is a device for the muscular strengthening of the pelvic and the surrounding muscles. The system consists of a magnetic coil embedded in a seat surface. The electrical induction leads to a depolarization of nerves and thus to action potentials that result in muscle contractions. To ensure the safety of patients, it is particularly important to identify any implants in the field range before starting a treatment in order to avoid possible damage of tissue around these objects. The evaluated metal samples were a flat cylinder of iron, a cobalt-chrome sphere shaped hip head, a titanium and a titanium-aluminum alloy acetabulum prosthesis. For the identification of dangerous regions in which a metal can harm surrounding tissue, metal samples were exposed to the magnetic field, the temperature rise was monitored and the power density calculated from these results. Wolfs claims (Wolf, 2008) that a power density above 40mW/cm² could become dangerous for the surrounding tissue. Accordingly, the regions of interest were determined where the power density is above this critical power density mark. Because of varying magnetic remanence effects in the core of the therapy coil, recalibration is necessary at the beginning of new application sessions. After that, the system tests for magnetic impedance changes, potentially cause by metal objects within the regions of interest. If a metal is detected, the process stops and a warning signal is displayed on the control screen. In summary, all distances and positions where a critical power density occurred in a specific sample could reliably be detected in the test series. Furthermore, all samples, exception iron at one specific position, were detected with an additional 20mm safety margin between the maximum detectable distance and the distance where the power density is below the critical mark.