Analysis of the role of lead resistivity in specific absorption rate for deep brain stimulator leads at 3T MRI

Abstract

Magnetic resonance imaging (MRI) on patients with implanted deep brain stimulators (DBSs) can be hazardous because of the antenna-effect of leads exposed to the incident radio-frequency field. This study evaluated electromagnetic field and specific absorption rate (SAR) changes as a function of lead resistivity on an anatomically precise head model in a 3T system. The anatomical accuracy of our head model allowed for detailed modeling of the path of DBS leads between epidermis and the outer table. Our electromagnetic finite difference time domain (FDTD) analysis showed significant changes of 1 g and 10 g averaged SAR for the range of lead resistivity modeled, including highly conductive leads up to highly resistive leads. Antenna performance and whole-head SAR were sensitive to the presence of the DBS leads only within 10%, while changes of over one order of magnitude were observed for the peak 10 g averaged SAR, suggesting that local SAR values should be considered in DBS guidelines. With rho(lead) = rho(copper) , and the MRI coil driven to produce a whole-head SAR without leads of 3.2 W/kg, the 1 g averaged SAR was 1080 W/kg and the 10 g averaged SAR 120 W/kg at the tip of the DBS lead. Conversely, in the control case without leads, the 1 g and 10 g averaged SAR were 0.5 W/kg and 0.6 W/kg, respectively, in the same location. The SAR at the tip of lead was similar with electrically homogeneous and electrically heterogeneous models. Our results show that computational models can support the development of novel lead technology, properly balancing the requirements of SAR deposition at the tip of the lead and power dissipation of the system battery.

Fig. 1

Anatomically fine-grained head model used for this study. From left to right column: map of anatomical structures, mass density, electrical conductivity, and permittivity at 128 MHz. Axial (z = 215), coronal (y = 161), and sagittal (x = 162) planes are shown. The corresponding color code for each modeled structure is to the right of the image. Max scale values of σ and ε_r set to 1 and 75, respectively, for illustrative purposes (see Table I for full set of values).

Fig. 2

Geometrical model of the DBS implants. (A) 3D views of the head with the DBS implants placed in the RF coil modeled for the 3T study. (B) Coronal view of the head model with zoom of the implant inside the brain. The insulation was removed for illustrative purposes. (C) Sagittal view of head and zoom near one of the implants. Twenty-nine segments were used to model the leads and the insulation.

Fig. 3

(Top). Amplitude of magnetic field, electric field and induced currents without and with PEC DBS leads. The fields are shown for an axial slice (z = 167) immediately below the tip of the lead, where the maximum electric field was observed; a coronal slice (y = 160) and two sagittal slices (x = 129 and x = 193) containing the DBS leads.

Fig. 4

Profile of magnetic field (top) and induced currents with different lead resistivity on the axial slice adjacent and below the tip of the lead (i.e., y = 160 and z = 167).

Fig. 5

(Top) SAR_{1 g} with variable lead resistivity and without DBS leads along an axial slice (z = 167) immediately below the tip of the leads, a coronal slice (y = 160) and two sagittal slices (x = 129 and x = 193) containing the DBS leads (see also Fig. 2). The input power was normalized to obtain a SAR_w = 3.2 W/kg for the no-lead case.

Fig. 6

SAR values at the tip of the right and left lead, corresponding to (x, y, z) = (153,160,167) and (x, y, z) = (173,160,167), respectively. The computational model allowed estimating the asymmetric SAR changes between the two leads. The increasing lead resistivity allowed for a reduction of SAR_{1 g} and SAR_{10 g} below IEC guidelines for ρ > 0.002 Ωm.

Fig. 7

(Top) Amplitude of magnetic field, electric field and induced currents with PEC leads estimated with an electrically heterogeneous and homogeneous head model with PEC DBS leads along an axial slice (z = 170) containing the lead tip (see Fig. 2). (Bottom) Profile of induced currents with heterogeneous and homogeneous model on the axial slice adjacent and below the tip of the lead (i.e., y = 160 and z = 167).