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Effect of device configuration and patient’s body composition on RF heating of deep brain stimulation (DBS) implants during MRI

Bhumi Bhusal¹, Joshua Rosenow², Bach Nguyen¹, Jasmine Vu¹, Noah Pasquinelli¹, and Laleh Golestanirad¹
¹Northwestern University, Chicago, IL, United States, ²Northwestern Medicine, Chicago, IL, United States

Synopsis

Off-label use of MRI in patients with DBS implants has been reported at 3T, relying only on safety tests in ASTM phantoms and with limited configurations. We constructed a multi-compartment anthropomorphic phantom consisting of brain, skull and body tissue, and measured RF heating of a fully implanted DBS system with various configurations during MRI at 1.5T. We found an 89-fold variation in RF heating depending on DBS device configuration. Simulating subcutaneous fat (as happens in obese patients) by adding a thin layer of oil to cover the upper face of the IPG increased RF heating by 3-fold in some configurations.

Introduction

Off-label use of MRI in patients with deep brain stimulation (DBS) implants is increasingly reported at 3T [1, 2], relying on safety assessments in ASTM-like phantoms (i.e., box shaped, single material) and limited number of DBS device configurations. This is alarming for two reasons: first, as RF heating is highly sensitive to implant’s configuration and its orientation with respect to MRI electric field [3-16], and as trajectory of DBS leads is substantially variant among patients [9], basing safety inferences on few tested configurations can expose large number of patients to unreasonable risks. Second, recent studies have reported up to 10-fold discrepancy in calculated SAR around implants in the ASTM box versus human body models [17].Here we report results of RF heating of a fully implanted DBS device with eight different clinically relevant configurations implanted in a multi-compartment anthropomorphic phantom consisting of brain-mimicking tissue, skull, and human-shaped head and torso. We observed an 89-fold variation in RF heating (from 0.14°C to 12.5°C) based on body composition and DBS device configuration. Our results suggest that a paradigm shift toward using multi-tissue anthropomorphic phantoms is necessary in MRI RF safety assessments.

Methods

Phantom design and construction: An anthropomorphic phantom was designed and constructed based on CT images of a patient with a deep brain stimulation implant. Images were segmented using thresholding, smoothing and Island removal functions in 3D slicer (3D Slicer 4.10). The segmented silhouette of the patient was used to design phantom’s outer shell which was then caudally extruded and capped at the abdominal end using Rhino 3D CAD tool (Rhino 6.0). The skull was splitted into two halves connecting at a coronal plane allowing the skull to be reopened and refilled with different brain-mimicking tissue material. Phantom parts were 3D printed in ABS plastic and coated with acrylic for waterproofing (Figure 1). We also designed and 3D printed grids and stands that allowed DBS implanted pulse generator (IPG) and extension cables to be precisely positioned at locations analogous to clinical practice.
DBS device configurations: A Medtronic DBS system consisting of the lead (model 3387), extension (model 3708660), and IPG (Activa PC-37601) was positioned inside the phantom following eight clinically relevant configurations. Two flouroptic temperature probes were attached to contacts 0 (most distal contact) and 2 of the lead and inserted into the skull at the entry point and penetration angle analogous to clinical approach to reach right subhtalamic nucleus. The skull was filled with agar gel doped with NaCl with the conductivity and relative permittivity measured with a network analyzer to be 0.40 S/m and 78. The IPG was positioned at the location of left pectoral muscle, mimicking contralateral DBS configuration (Figure 2). The extra cranial medium was either filled with saline alone, with conductivity matched to muscle (0.50 S/m), or saline topped with a thin layer of oil (σ = 0 S/m, ε_r = 3) to cover the upper face of the IPG. The latter happens in obese patients who have more subcutaneous fat. For each scenario, DBS lead and extension were routed along one of typical trajectories illustrated in Figure 3.
RF exposure: Imaging was performed in a Siemens Aera 1.5T MRI scanner using a T1-TSE sequence (TR=814 ms, TE=7.3 ms, B1 rms= 4.1 µT). For each body composition and DBS configuration, we measured temperature rise during 4 minutes of RF exposure for the phantom located at three different imaging landmarks corresponding to head, shoulder and lower chest imaging.

Results

A total of 48 measurements were performed. The Measured temperature increase at the most distal contact (contact-0) has been presented in the plots (Figure 5). The temperature increase recorded by probe connected to the contact-2 were close to the values at contact-0 (within 10%). RF heating was maximum for the phantom positioned at head landmark (2.56±3.03 °C), followed by shoulder landmark (1.18±1.08°C) and lower chest landmark (0.44±0.49°C). Addition of subcutaneous fat significantly increased RF heating (>3-fold) for lead trajectories where no extracranial loop was present and the excess length of extension was instead looped around the IPG. Introduction of concentric loops simultaneously at the surgical burr hole and on the temporal bone reduced RF heating by 55% on average over all body compositions and imaging landmarks. The effect was extreme in the case of a no-loop trajectory in a patient with subcutaneous fat (0.1°C vs 12°C).

Discussions and Conclusions

The absence of obvious adverse effects in patients with electronic implants who have undergone off-label MRI have resulted in a more lenient approach toward MRI RF safety based on limited data. This, can jeopardize patient’s safety, as it only takes one unfortunate combination of factors to cause irreversible thermal damage. We show that variations in DBS device configuration and patient’s body composition introduces extreme fluctuations in RF heating. These results strongly suggest that benefits of off-label MRI in patients with conductive implants be evaluated against realistic risks in consideration of patients’ biological factors and possible device configurations. We are now in the process of evaluating these effects during MRI at 3T and in multi-compartment phantoms with shapes and range of tissue conductivities representing different body types.

Acknowledgements

This work has been supported by NIH grants R00EB021320, R03EB025344, and R03EB024705.

References

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Figures

Figure 1: (A-C) image segmentation, model construction, and 3D printed anthropomorphic phantom. Although the 3D-printed bone structures are non-conductive, our simulations show that the error caused by replacing the low-conductivity bone (with non-conductive material) is negligible in terms of SAR at electrodes. The skull is filled with NaCl doped agar gel at desired conductivity and firmness to supports DBS leads. A hole at the base of skull ensures electric connection of inside and outside media. (D) The container is filled with a saline topped with oil to mimic subcutaneous fat.

Figure 2: (A&B) High concentration of agar was used to creat a semi-solid gel that supported DBS leads in place and prevented convection. C the placement of temperature probes around the contact 0 and 2 of DBS lead. (D&E) Positioning of the lead-probe system into the skull. (F) Fully assembled phantom. Dotted lines represent imaging landmarks at head, shoulder and lower chest.

Figure 3: The position of DBS lead on the skull alongside schematics showing the full trajectory of DBS device. Different lead trajectories (e.g., mediolateral and anteroposterior) and different positioning of extracranial loops on the skull were examined. Introducing loops in the extracranial trajectory of DBS leads has shown to reduce RF heating [16, 18, 19], however, the optimum positioning of these loops for maximal reduction of RF heating were yet to be determined.

Figure 4: (A) Before each experiment, the integrity of DBS device was examined using the Medtronic patient programmer by measuring the inter-electrode and the electrode-IPG impedances to verify the proper connection of DBS system. B Full setup in the 1.5T scanner, the body part of phantom filled with saline. C same setup with 7 liter of saline replaced by vegetable oil to mimic subcutaneous fat.

Figure 5: Bar Plots for the measured temperature increases at the most distal contact (contact-0) of dbs implant for different configurations. The observed 12 degree temperature rise in brain tissue can cause irreversible thermal damage which may not be immediately detectable in the patient, but cause long term chronic complications.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)

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