Keywords: Safety, Safety, Deep brain stimulation, Active implantable medical devices, RF safety

Motivation: MRI of neurostimulators is severely constrained due to RF safety concerns.

Goal(s): Demonstrate that built-in sensors in commercial devices, such as a deep brain stimulator, can provide all necessary information to detect and improve RF safety. 

Approach: We investigated and utilized built-in impedance measurements of two commercial DBS systems for the detection and mitigation of RF-induced currents on the electrodes of a DBS lead. 

Results: Impedance measurements were correlated at various RF power levels. Temperature rise at the tip of DBS electrodes could be reduced to 0.02 K from 17.14 K at the same total powers (16.85±0.45 W).

Impact: Our demonstration of mitigation of RF-induced heating in active implants through built-in sensor measurements from a commercial DBS system indicated up to ~850× improvement in temperature rise proving the unmet value of sensors for MR imaging patients with active implants.

Keywords: Low-Field MRI, Safety, Medical Implants

Motivation: Radiofrequency-induced heating of elongated medical implants during MR imaging on newly introduced commercial 0.55T systems has not been thoroughly investigated.  

Goal(s): We aim to evaluate and compare the RF heating of elongated medical implants during MRI at 0.55T and 1.5T scanners.

Approach: Neurological and cardiac implant leads were routed along different trajectories inside a tissue mimicking gel phantom, and the temperature increase during MRI at 0.55T and 1.5T was measured at the lead tip.

Results: For certain implant configurations, RF heating at 0.55T MRI can be an order of magnitude higher than that at 1.5T.

Impact: Our findings show that unsafe levels of RF heating, exceeding those at higher field strengths, are possible on commercial 0.55T MRI systems for certain implant configurations. Therefore, extra caution should be taken during low-field MRI of patients with long implants.

Keywords: Safety, Parkinson's Disease, Deep Brain Stimulation

Motivation: Currently, patients with Deep Brain stimulation (DBS) implants cannot leverage the diagnostic potential of Magnetic Resonance Imaging (MRI) as traditional metal-based leads pose several safety concerns. 

Goal(s): We propose a new technology for manufacturing DBS microwires that ensures their safe operation with MRI up to 3T. 

Approach: Through the development of a metamaterial-based design, we have engineered microwires that effectively partially reflect RF-induced current, thereby reducing Specific Absorption Rate (SAR), tip heating, and associated artifacts.

Results: Our manufactured microwires demonstrated minimal tip heating in both 1.5T and 3T MRI scanners when compared to standard wires.

Impact: This innovative design facilitates safe MRI imaging for individuals with DBS implants, marking a pivotal advancement in the study of neural mechanisms involved in medically refractory pathological conditions, such as Parkinson’s disease.

Keywords: High-Field MRI, Segmentation, voxel models, ultra-high field, EM simulation

Motivation: Accurate human tissue models for simulation of RF power absorption are a key safety requirement for transmit coil development especially at ultra-high field.

Goal(s): To create individual voxel models of the human head and torso.

Approach: A pipeline for head and torso segmentation was developed based on a 3T multi-contrast protocol and tailored post-processing. The resulting voxel models were used for electromagnetic simulation of a self-developed Tx array at 9.4T.

Results: Strong agreement was found between measured and simulated B₁⁺ maps using the generated voxel model. Simulated worst-case SAR distributions differed significantly between individual and ‘off-the-shelf’ voxel models.

Impact: We present a pipeline for the creation of individual human tissue voxel models covering head and torso, which is based on multi-contrast MR image segmentation. This meets a central need in safety-related simulations of ultra-high field RF coil arrays.

Keywords: Safety, Safety, SAR, ultra-high field, pTX, pediatric

Motivation: Pediatric MRI neuroimaging has become common practice at 1.5 and 3 Tesla and we see the emergence of 7T and parallel transmission (pTX). However, today relevant local SAR prediction models are based on adult standards.

Goal(s): In this work, we study the local SAR at 7T with pTX for children population (6 to 14 years old) using RF electric field simulations.

Approach: We exploit a vast simulation database to build a pediatric local SAR prediction model and propose a methodology for its initial validation using convex optimization.

Results: A large simulation database (above 25) is desirable to combine RF safety and pTX performance.

Impact: This work aims to push pediatric MRI to ultra-high fields by developing safe SAR control for the children population. It also introduces a tool to determine appropriate safety margins to avoid excessive penalizing in the pTX pulse design.

Keywords: Safety, Safety, Cardiac stimulation

Motivation: Our previous modeling of gradient-induced cardiac stimulation (CS) in two body models indicated that the regulatory IEC 60601-2-33 CS limit overestimated CS thresholds by 9- to 46-fold.

Goal(s): To investigate the expected variance of CS thresholds across a healthy population.

Approach: We deploy our validated cardiac magnetostimulation modeling approach in six body models with varying shape/BMI/age and a commercial gradient system.

Results: Predicted CS thresholds vary up to 2-fold across body models. Worst-case CS thresholds are 7X greater than the IEC CS limit and 4X greater than experimental PNS limits.

Impact: Our modeling allows investigation of the variability of CS thresholds across the population, which is not accessible experimentally. This knowledge is critical to obtain a robust estimate of safe, but not overly restrictive cardiac safety limits for MRI gradients.

Keywords: Gradients, Gradients, peripheral nerve stimulation, subject-specific PNS prediction, electric field calculation

Motivation: Given the large population variability in PNS thresholds (~3-fold), rapid and accurately prediction of PNS thresholds for individual subjects would be valuable.

Goal(s): To apply our E-field-based PNS prediction method to individual subjects, and to test the hypothesis that we can predict an individual subject’s PNS threshold with reasonable accuracy.

Approach: Subject-specific body models were fit to an individual’s anatomy based on a localizer. E-field calculations yielded E_max and therefore PNS threshold. We compared to measured PNS thresholds using 3 different head gradient coils and 7 different gradient directions.

Results: Individual subject PNS thresholds values can be predicted to an accuracy of ~35%.

Impact: There would be significant advantages in being able to predict PNS thresholds rapidly and accurately for individual subjects. This would allow much more effective use of high-performance gradient hardware, benefitting the subset of the population with high PNS thresholds.

Keywords: Safety, Safety

Motivation: Investigating optimised RF heating and B₁⁺ field disturbance with sEEG electrodes placement, covering epileptogenic zones in five configurations, using electromagnetic simulations with 16-channel parallel-transmit coil array at 3T for simultaneous sEEG-MRI.

Goal(s): The challenge is reducing RF heating from implanted electrodes.

Approach: We investigated five electrode placements covering epileptogenic zones to control RF heating and local signal increases using computational field simulations on a realistic computational human model.

Results: Implanting electrodes in the left hemisphere improved RF uniformity. Shimming optimisation, integrating B₁⁺ and 0.1g SAR, showed negligible SAR differences from electrode-free setups. Lower SAR correlated with reduced RF strength, suggesting a necessary trade-off.

Impact: Our study advances simultaneous EEG/fMRI for drug-resistant epilepsy, optimising electrode safety to better localise seizure onset zones and refine surgical approaches, while ensuring patient safety. It allows for more precise epileptogenic zone resection, potentially advancing therapeutic outcomes.

Keywords: Safety, Safety

Motivation: Higher SNR promised at higher field strengths can be traded for higher spatial resolution, which is essential for imaging infant brains.

Goal(s): To assess and ensure safe operation of a commercially-available RF head coil at 7T for infant subjects.

Approach: We developed an EM model of the coil and experimentally validated it. Utilizing EM simulations, we calculated local and head SARs for an infant model to determine the safe operation limits.

Results: We showed that the head SAR limit is reached before the local SAR limit. We acquired structural and functional MRI data from an infant's brain at 7T.

Impact: Assessing and ensuring the safe operation of 7T MRI scanners for infant subjects can pave the way to exploring the early stages of human brain development, which is hardly possible at lower field strengths due to lower spatiotemporal resolution.

Keywords: Safety, Safety, specific absorption rate; deep learning; parallel transmit; convolutional neural network; subject-specific SAR assessment; ultra-high field MRI

Motivation: The methods presented for on-line local SAR evaluation require access to geometric design details of the transmit coil which are not always available.

Goal(s): Evaluate the generalization capabilities of deep learning-based methods when they are used to assess the local SAR distribution for coils not included in the training data.

Approach: We built a diverse synthetic dataset four different coils and trained a neural network: using only samples from each coil, and using samples from all coils except one.

Results: Including a reasonably wide variety of coils in the training process enables local SAR assessment without knowing the design details of the coil.

Impact: The lack of access to design details of the coil makes it challenging to transition the more advanced local SAR assessment methods into clinical practice. Training with a diverse set of coils could enable local SAR assessment without coil information.