MRI of Deep Brain Stimulation Devices May Be Scalable to 3T

For patients implanted with a deep brain stimulation (DBS) device, neuroimaging is important for target verification and postoperative monitoring of functional changes in affected brain networks. Magnetic resonance imaging (MRI) is an excellent method, but the potential for interactions between implanted leads and the radiofrequency (RF) fields of MRI scanners poses a safety hazard.

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In previous work, Lawrence L. Wald, PhD, director of the NMR Core at the Athinoula A. Martinos Center for Biomedical Imaging at Massachusetts General Hospital, and colleagues demonstrated the feasibility of patient-adjustable, reconfigurable MRI in this setting. In modeling studies, use of a rotating transmitter coil drastically reduced the specific absorption rate (SAR), a surrogate measure of RF heating, during MRI at 1.5T.

However, there is a strong incentive for imaging DBS devices at 3T. The stronger magnet can better estimate the boundaries of the subthalamic nucleus and globus pallidus (currently the main targets of DBS devices) and better detect intracranial hemorrhage. In NeuroImage, the researchers present evidence that their reconfigurable MRI technology is scalable to 3T.

Simulations

To assess the performance of the rotating coil system at 3T, the researchers constructed models of bilateral DBS systems from postoperative computed tomography images of 13 patients. Twelve patients had isolated leads and one patient had a fully implanted DBS system with bilateral internal pulse generators.

For each model, 64 coil position simulations were performed to evaluate the SAR reduction performance of a rotating 3T head coil that is currently under construction. Compared with a conventional circularly polarized body coil, the average SAR-reduction efficiency with the rotating coil was 83% for unilateral leads and 59% for bilateral leads.

Surgical Lead Management

Using computational models, the research team has been investigating the standardized placement of the extracranial portions of bilateral leads to ensure optimal performance of the rotating coil, as previously published in NeuroImage. The team's current research is the first to report extracranial lead management in a patient.

The 74-year-old woman was receiving a fully implanted bilateral lead DBS device for Parkinson's disease. During implantation, the modifications were to:

• Incorporate concentric loops close to the surgical bur hole to further reduce SAR amplification at the tips of both leads, through the cancelation of induced voltages. A video demonstrating the formation and positioning of the loop is available in a supplement to the article
• Align and overlap the right and left leads and extension cables, to allow the rotating coil to maximally contain both leads in its low electric field region. The goal was to minimize the SAR at the tips of both leads simultaneously

The SAR-reduction efficiency for this patient was 95%, substantially higher than the 59% in the patient models, where there had been no specific instruction about where to place the extracranial portions of leads.

During eight MRI examinations, the researchers measured the temperature in the tissue around this patient's leads, which is a better measure of RF safety than SAR is. The temperature rise remained below 1 °C for all imaging sequences, 15-fold lower than during MRI modeling with a circularly polarized body coil.

Thus, the adapted technology and suggestions for lead management show promise for allowing safe MRI at 3T for patients with deep brain stimulation implants.

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