First heating measurements of endovascular stents in magnetic particle imaging

Magnetic particle imaging (MPI) is a 3D imaging method which visualizes the spatial distribution of superparamagnetic iron oxide nanoparticles (SPIOs) with oscillating and static magnetic fields. It is a quantitative method as the intensity of its signal is proportional to the concentration of the SPIOs. Due to its quantitativeness, high temporal resolution and kidney-safe tracers it is proposed to be a very promising noninvasive method for cardiovascular imaging. In this regard MPI in the presence of endovascular stents and assessment of stent patency with MPI has to be evaluated. The purpose of this study was to evaluate the heating of endovascular stents due to the alternating magnetic fields as a safety aspect of MPI. Twenty one commercially available endovascular stents of different sizes (diameter: 3, 3.5, 4, 5, 6, 7, 8, 10 mm, length: 11-99 mm) and materials (stainless steel, nitinol, platinum-chromium, cobalt-chromium) were evaluated. They were implanted in silicone tubes matching the stent diameter and placed at the center of the bore of a preclinical MPI scanner (Bruker-Biospin, Ettlingen, Germany). The temperature was measured with fiber optic thermometers over a scan duration of 431 s. A temperature change of ≥ 0.1 K (absolute accuracy of the temperature measurement) was defined as heating of the material. Nine stents showed no heating. Twelve stents showed heating of at least 0.1 K. Only five stents showed heating of more than 3.0 K, all equal or larger than 6 mm. The largest temperature difference was 12.4 K, corresponding to the stent with the highest diameter (10 mm). The only predictor of the heating behavior was the stent diameter. The stent material had no measurable effect on the heating behavior. With few exceptions, the safe use of endovascular stents in MPI is possible. It has to be further evaluated if the increase of temperature of stents with a larger diameter is still measurable under (blood) flow-conditions and thus relevant in vivo.