Modeling of a multi-scale electromagnetic problem: pacemaker lead heating in MRI

Conference poster, 2012

Modeling of so-called multi-scale problems is challenging due to large differences between the length scales of the problem, which can be associated with large computational costs. For example, modeling of pacemaker lead heating by a 1.5 T magnetic resonance imaging (MRI) system requires length scales differing by a factor 1000 to be resolved. Lead heating is the main cause of the contraindication of pacemakers for MRI [1]. The human body sets the size of the largest scale whereas the helically shaped conductors of the pacemaker lead constitute the smallest scale. To this date, modeling of these length scales and the heterogeneous body tissue has not been performed concurrently. Neufeld et al. [2] exploited the finite-difference time-domain technique (FDTD) and modeled the heating caused by a single helix. However, the associated computational cost prevented them from examining more complex leads. Nevertheless, modern pacing leads often include two helically shaped conductors, consisting of several filars each, whose winding scheme has been shown experimentally to have significant impact on the heating [3]. Therefore, we devote special attention to the multi-scale part of the problem and exploit the frequency-domain method of moments to model an MRI radio frequency coil and a homogeneous human body phantom with an implanted pacemaker system. The pacemaker lead consists of two helically shaped conductors modeled as thin wires, insulation, and electrodes modeled by surfaces. We exploit the model to assess the effect on the heating by different factors, such as the presence of a pacemaker unit. Figure 1 shows the amplification of the absolute value of the electric field with respect to the fields in an empty phantom. Furthermore, we study the accuracy of the thin-wire approximation for densely wound helices by comparing it to a surface discretization of the helix. Figure 2 shows the maximum value of the induced current on a straight helix which is illuminated by a plane wave polarized along the helix axis. The wave impinges at a straight angle to the helix axis. [1] Götte et al. Magnetic resonance imaging, pacemakers and implantable cardioverter-defibrillators: current situation and clinical perspective. Netherlands heart journal, 18(1):31–7, January 2010. [2] Neufeld et al. Measurement, simulation and uncertainty assessment of implant heating during MRI. Physics in medicine and biology, 54(13):4151–69, July 2009. [3] Bottomley et al. Designing passive MRI-safe implantable conducting leads with electrodes. Medical Physics, 37(7):3828–3843, 2010.