MRI-based multiscale model for electromagnetic analysis in the human head with implanted DBS

Abstract

Deep brain stimulation (DBS) is an established procedure for the treatment of movement and affective disorders. Patients with DBS may benefit from magnetic resonance imaging (MRI) to evaluate injuries or comorbidities. However, the MRI radio-frequency (RF) energy may cause excessive tissue heating particularly near the electrode. This paper studies how the accuracy of numerical modeling of the RF field inside a DBS patient varies with spatial resolution and corresponding anatomical detail of the volume surrounding the electrodes. A multiscale model (MS) was created by an atlas-based segmentation using a 1 mm(3) head model (mRes) refined in the basal ganglia by a 200 μ m(2) ex-vivo dataset. Four DBS electrodes targeting the left globus pallidus internus were modeled. Electromagnetic simulations at 128 MHz showed that the peak of the electric field of the MS doubled (18.7 kV/m versus 9.33 kV/m) and shifted 6.4 mm compared to the mRes model. Additionally, the MS had a sixfold increase over the mRes model in peak-specific absorption rate (SAR of 43.9 kW/kg versus 7 kW/kg). The results suggest that submillimetric resolution and improved anatomical detail in the model may increase the accuracy of computed electric field and local SAR around the tip of the implant.

Figure 1

Workflow of the procedure for multi-scale (MS) model generation (e). Step 1: the original MRI datasets (a) μRes and (b) mRes were first rigidly registered. Step 2: the contours of the target nucleus (i.e., GPi) and the surrounding major basal ganglia nuclei were segmented on the registered μRes dataset to generate the μRes model (c). Segmentation and generation of the mRes head model (d) were described in [21]. Step 3: each segmented structure in the μRes model—that is, the caudate (μC), the putamen (μP), and the globus pallidus (μGP, i.e., combined μGPe and μGPi)—was registered with its corresponding structure in the mRes head model (mC, mP, and mGP, resp.) using a non-rigid version of the iterative closest point (ICP). The μRes model structures were propagated on the mRes model (label propagation) and the resulting dataset was a multi-scale (MS) model (e) enhanced in the basal ganglia (yellow square).

Figure 2

Axial (a), coronal (b), and sagittal (c) views of the deformed grids at three different resolution levels—that is, control point spacing of 10 mm³ (NR10) (top row), 5 mm³ (NR5) (middle row), and 3 mm³ (NR3) (bottom row)—resulting from the caudate coarse-to-fine non-rigid (NR) registration.

Figure 3

Electrical model: (a) model of the electrodes and (b) a zoomed view of the microresolution mesh around the electrode. Caudate, putamen, GPe, and GPi were labeled as grey matter (σ = 0.58 S/m, ε _r = 73.51). The DBS is made of platinum/iridium conductor wire and electrodes (σ = 4 · 10⁶ S/m) with 80 A urethane insulation (σ = 10⁻¹⁰ S/m, ε _r = 3) [20]. The high resolution of the model allowed to outline and precisely characterize both geometrically and electrically small anatomical details such as the white matter (σ = 0.34 S/m, ε _r = 52.53) between the GPe and the GPi (i.e., the internal medullary lamina IML) and between the GPe and the putamen (i.e., the external medullary lamina EML) and the caudoputaminal bridges (grey matter).

Figure 4

Segmentation obtained via label propagation on the sagittal (S), coronal (C), axial (A), and three-dimensional (3D) views of the 1 mm³ resolution MRIs.

Figure 5

Axial, coronal, and sagittal mappings of the magnitude of the electric field ||E|| around the electrode for the 1 mm³ (mRes) and multi-scale (MS) models.

Figure 6

1 mm³ (mRes) and multi-scale (MS) whole-head mapping of the electric field along the axial, coronal, and sagittal slices where the maximum electric field was observed.

Figure 7

(a) Coronal map of the electric field in proximity of the electrode and difference of the electric field ||E|| between 1 mm³ (mRes) and multi-scale (MS) along the right (and left) profile of the electrode from point Ra (La) to Rc (Lc) through point Rb (Lb) (radiological convention, i.e., left is right). Coronal views are shown since the distal part of the DBS is contained in one single coronal plane. (b) Global difference of the electric field ||E|| between mRes and MS configurations in specific positions along two different layers: the grey matter (GM) and the skin (S) layer.

Figure 8

From left to right: anatomical model, raw SAR (0 dB = 43915.78 W/kg), SAR_1 g (0 dB = 362.79 W/kg), and SAR_10 g (0 dB = 57.90 W/kg) distributions along the coronal slice where the peak is located for the mRes (top) and the MS model (bottom).