Peter Serano1,2, Maria Ida Iacono1, Leonardo M. Angelone1, and Sunder S. Rajan1
1U.S. Food and Drug Administration, Washington, DC, United States, 2Electrical and Computer Engineering, University of Maryland, College Park, MD, United States
Synopsis
In this study,
the authors present an analysis of a potentially overlooked clinical scenario,
namely overlapped stents separated with a layer of insulation. Electromagnetic and thermal simulations as well as measurements were performed
to test such configurations. The results show that implanted
medical devices that include gapped conductive structures, like overlapped
stents, can affect the location and magnitude of peak heating near the implant.Introduction
The
RF induced heating of elongated medical implants like stents is typically
exhibited by an increase in temperature in the tissue region around the
extremities of the device. As such, ASTM 2182-11a [1] recommends
measuring temperature increases at these locations. In this study, the authors
present an analysis of a potentially overlooked clinical scenario, namely overlapped
stents separated with a layer of insulation, which may both shift the location
and magnitude of peak heating near the implant. Electromagnetic and thermal
simulations as well as measurements were performed to test such configurations.
Methods
Four stent configurations were evaluated, each with an overall
length of 100mm and radius of 3.175mm. The first was a single 100mm stent. The
next three were configurations of two overlapping stents: 55mm stents with a 10mm
overlap, 70mm stents with a 40mm overlap, and 85mm stents with a 70mm overlap. Electromagnetic
and thermal simulations were performed using the ANSYS Workbench Platform with
ANSYS HFSS and Mechanical modules (ANSYS Inc, Canonsburg, PA). Test implants
were made from a sheet of copper mesh and PTFE heat-shrink tubing to create the
overlap. Implants were placed in a gel phantom in accordance with ASTM 2182-11a
and provided a circularly polarized 128MHz RF excitation via the MITS 3.0
system (Zurich Med Tech, Zurich, Switzerland). Temperature rise after 15 minutes
of exposure to a WBSAR=0.75W/kg was recorded using fluoroptic temperature
probes (Neoptix Inc, Quebec, Canada) at both the ends of the device as well as two
additional locations at the edges of the overlap.
Results and Discussion
The results show that the overlapped stent
configurations can affect both the location and magnitude of peak heating near
the implant. In the case of the two 55mm stents with a 10mm overlap, an
increase in the peak temperature compared with the single 100mm stent was observed
in both simulation and measurements along with a shift in the location of peak
heating from the end to the edge of the inner overlap. This effect is a result
of an increased magnitude of the E-field between the two gapped conductive structures.
Conclusion
In this study, the authors have demonstrated that implanted
medical devices that include gapped conductive structures, like overlapped
stents, can affect the location and magnitude of peak heating near the implant.
Acknowledgements
This project was supported in part by an appointment to the Research Participation Program at the Center for Devices and Radiological Health / Office of Science and Engineering Laboratories, U.S. Food and Drug Administration, administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and FDA.References
[1] American Society for Testing and Materials (ASTM) F2182 - 11a, “Standard test
method for measurement of radio frequency induced heating on or near passive
implants during magnetic resonance imaging.”