Keywords: Safety, Safety

Motivation: To develop a reliable method to predict heating around deep brain stimulation implants as a safety assessment prior to scanning patients.

Goal(s): To investigate the accuracy of a heating prediction workflow in-vivo.

Approach: We surgically inserted a DBS electrode into swine brains and predicted the heating around DBS contacts, with and without perfusion. Our heating prediction workflow uses a new MR-based current measurement, proposed for this work, as well as quasi-static electromagnetic and thermal simulations to predict heating around the electrical contacts of DBS electrodes.

Results: The predicted temperature around electrical contacts agreed with the measurements (NRMSE ≤ 0.09). 

Impact: Our workflow predicts heating around the electrical contacts in-vivo without complex modeling and simulations. The results demonstrate the reliability of the workflow to assess heating risk before scanning patients with DBS implants.

Keywords: Safety, Safety

Motivation: RF-induced heating for the partially-in and partially-out (PIPO) medical implants is the primary concern for patient’s safety under MRI. 

Goal(s): Winding the external portion of the PIPO medical electrodes is proposed to reduce the RF heating under 1.5T MRI. 

Approach: One commercial PIPO medical electrode and an insulated solid wire were used to demonstrate this concept. RF heating is measured inside the ASTM phantom under a 1.5T RF birdcage coil and evaluated from the in-vivo simulations using the transfer function (TF) approach. 

Results: The results showed a significant reduction of RF heating in the commercial lead and simplified wire.

Impact: The proposed method can reduce RF-induced heating significantly, mitigating the risk of tissue damage and improving the quality of care for patients with partially-in and partially-out (PIPO) medical devices under 1.5T MRI.

Keywords: Safety, Safety, Low Field, RF-induced heating, Interventional

Motivation: RF-induced heating of interventional devices is reduced at lower magnetic fields, but dangerous heating conditions can still occur during MR-guided interventions.

Goal(s): To investigate the effect of body coil drive mode on RF-induced heating of interventional devices at 0.55T and 1.5T.

Approach: Numerical simulations, E-field, transfer function and temperature measurements are used to evaluate the dependency of RF-induced heating of commonly used interventional devices on the phase and amplitude settings of body coil ports.

Results: RF-induced heating can be reduced using individually adapted body coil drive modes.

Impact: Dual-drive body coil technology can be used to generate implant-friendly electromagnetic fields, which can enable safe use of clinical intravascular devices during MR-guided interventions at 0.55T and 1.5T.

Keywords: Safety, Safety

Motivation: MRI of (biodegradable) passively conducting implants is challenged by potential elevation of RF power deposition (SAR) in the vicinity of an implant. Accidental implant fractures or fissures due to dynamic degradation of biodegradable implants alter the implants’ structure. This mechanical impact induces changes in the electromagnetic response of the system versus an intact implant. 

Goal(s): Recognizing this clinical and patient safety challenge, this work first examines the SAR magnification caused by implant fracture

Approach: The efficacy of an optimized parallel excitation vectors deduced from a multi-objective genetic algorithm is demonstrated.

Results: Reduction of SAR magnification in fractured implant using the optimized excitation vector.

Impact: Amplification of RF power deposition in MRI of fractured metallic implants constitutes a patient safety hazard. This risk can be mitigated with parallel transmission using GA-driven excitation. This approach provides a viable clinical alternative for MRI monitoring of implantation sites.

Keywords: Safety, Safety, dental MRI, RF safety, intraoral coil, simulation

Motivation: In dental applications intraoral coils (IOCs) offer higher SNR than external surface coils, but the details of RF-induced heating of an intraoral coil dependent on many factors such as coil size, shape, incident E-field, surrounding tissue, and acquisition protocol.

Goal(s): This study investigates the effect of anatomical model complexity on SAR simulations for IOCs.

Approach: Numerical simulations were compared to electric field mapping and temperature measurements to evaluate RF-induced heating of various IOCs.

Results: RF heating and SAR simulations of IOCs can be performed with limited tissue models, but the required complexity of the tissue model depends on the coil type.

Impact: The complexity of numerical simulations can be reduced to limited tissues while maintaining accuracy of SAR estimation to determine the safety of intraoral coils through hotspot detection.

Keywords: Safety, Safety, Deep Brain Stimulation; Parallel Transmission

Motivation: Bilateral deep brain stimulation (DBS) surgery would benefit from safe intraoperative 3T MRI using conventional scanners.

Goal(s): Our goal was to develop a method to determine patient-specific safety constraints to scan a bilateral lead configuration with minimal RF-heating, by minimizing RF-induced currents at maximal B₁.

Approach: We scanned a phantom with two leads in different configurations, measured local heating and determined the transimpedance. 

Results: We could determine the phantom-specific safety constraints and find a low-SAR sequence meeting the constraints, after optimizing the two channel RF-shimming to minimize RF-heating by minimizing RF-induced currents at maximal B₁.

Impact: The proposed method can be used to minimize heating of a bilateral lead configuration and estimate patient-specific safety constraints, but further research has to be conducted to be able to use it during deep brain stimulation surgery.

Keywords: Safety, Safety

Motivation: Electrically short fragmented leads are sometimes left behind in the human body after the extraction of an active implantable medical device (AIMD). When a new AIMD is implanted near the fragmented leads, the radiofrequency (RF)-induced heating of the newly implanted AIMD could be altered. 

Goal(s): Develop a method to evaluate the RF-induced heating of AIMD with fragmented leads in the vicinity.

Approach: We propose to use the electric field distribution which includes the presence of the fragmented leads with the AIMD transfer function model for RF-induced heating evaluation.

Results: The proposed method can effectively predict the RF-induced heating for AIMDs near fragmented leads.

Impact: It was observed that the RF-induced heating for AIMD system can be altered by the nearby fragmented leads.

Keywords: RF Arrays & Systems, RF Arrays & Systems

Motivation: To improve the design of pTx head coils intended to produce uniform RF fields while controlling local SAR levels for patients with DBS implants.

Goal(s): To evaluate the effect of extended coil length and increased number of rows on performance using simulations.

Approach: Comparison of SAR and RF homogeneity in 3-row and 4-row pTx head coil configurations using FDTD simulations on a human head model with an implanted DBS lead. 

Results: The 4-row coil offered the best SAR control for the whole brain, with length having a greater impact on local SAR than the number of rows.

Impact: The research indicates that the length of multi-row pTx coils affects local SAR in MRI for DBS patients, enhancing efficiency and safety, and broadening imaging possibilities, potentially advancing MRI-compatible devices and personalized imaging techniques.

Keywords: Multimodal, Multimodal, EEG, fMRI

Motivation: Evaluate the impact of EPI read-out gradient slew-rate on EEG amp heating, for EEG-fMRI experiments at 7T.

Goal(s): Identify slew-rates that will minimize heating of EEG amps, for EEG-fMRI experiments at 7T.

Approach: Test a range of EPI slew-rates while measuring the temperature of the EEG amp, during phantom tests at 7T.

Results: EPI read-out slew-rates of 120, 88, 61 and 46 T/m/s were tested and the slew-rates of 61 and 46 T/m/s were found to induce minimal heating of the EEG amp during EEG-fMRI experiments at 7T.

Impact: These results should help neuroscientists doing EEG-fMRI studies at 7T, to conduct their experiments without damaging the EEG amps.

Keywords: Safety, Safety

Motivation: To reduce radiofrequency (RF)-induced heating of abandoned implantable cardiac leads equipped with plastic caps, which remarkably increase heat generation around the lead tip. 

Goal(s): RF-induced heating with various electrical property of the cap was investigated to find optimal condition.

Approach: Tissue heating around the lead wire with caps of electrical conductivity ranging over 10^-6 to 10⁺⁶ S/m were compared in the ASTM phantom model by FDTD simulation, and validated by experiments. 

Results: Tissue heating around the lead tip clearly decreased and became similar to the uncapped case, when the conductivity was higher than 10^-1 S/m, which was approximately the same as the media. 

Impact: Simulations and experiments on abandoned cardiac leads revealed that the "electrically conductive cap" dramatically reduces RF-induced heating around the lead tip. Such cap may protect the lead from body fluid infiltration into the insulation material while also contribute to MR-safety.

Keywords: Safety, Safety, Low-Field MRI, B1+RMS, SAR

Motivation: The primary RF exposure metric used for MR Conditional labeling of medical devices is whole-body(wb) average SAR, which is typically overestimated by MRI scanners, leading to potentially overly restrictive labels.

Goal(s): The purpose of this study was to determine if B₁⁺RMS as an RF exposure limit can help prevent overly restrictive MR Conditional labels.

Approach: In-vivo RF-induced heating simulations in a 0.55 T MRI scanner were performed to compare potential labeling at the scanner-reported B₁⁺RMS and wbSAR with the driving voltage maximized.

Results: Results depicted a 7-13x decrease in maximum temperature rise when the RF exposure is limited by B1+RMS instead of wbSAR.

Impact: The use of B₁⁺RMS to limit RF exposure instead of SAR has significant potential to prevent unnecessarily restrictive MR Conditional labels for medical devices, especially in lower magnetic field strength MRI systems that are incapable of achieving high SAR levels.

Keywords: Safety, Safety, temperature rise

Motivation: Evaluating induced temperature rise (ΔT) in human tissue at a hot spot.

Goal(s): Comprehensive analysis of ΔT by examining cases where SAR_{0.1 g} (SAR, specifically averaged over a mass of 0.1 gram of human tissue) remains constant but the spatial distribution of SAR inside and outside the averaging volume is different.

Approach: Analytical solutions were obtained for estimating ΔT as a result of the SAR distribution exhibiting spherical symmetry with both uniform and Gaussian distributions.

Results: Using SAR_0.1g without consideration of the distribution of SAR within and outside the enclosed volume cannot be a reliable tool for estimation of ΔT.

Impact: We have obtained analytical results on the effect of the SAR distribution on the conservatively estimated ΔT in different RF exposure scenarios.

Keywords: Safety, Safety, VOP, SAR

Motivation: Compression of SAR matrices can take very long for large data sets and large channel counts when using non-clustering algorithms that show the highest compression efficiency.

Goal(s): The goal of this study was to develop an algorithm that performs the compression faster while maintaining the compression efficiency.

Approach: We use a hybrid method that combines different algorithms to form a hybrid algorithm with greatly increased calculation speed.

Results: The new compression algorithm outperforms the older non-clustering compression algorithms at all VOP counts while maintaining the compression efficiency.

Impact: VOP compression is important for local SAR supervision and constraint pulse calculation in parallel transmission. We propose a new algorithm for non-clustered compression that greatly increases calculation speed, which is important especially at large channel counts.

Keywords: High-Field MRI, Safety

Motivation: Acoustic noise can be a severe physiological barrier for acquisitions at high field.

Goal(s): To isolate the main source of acoustic noise in the NexGen 7T scanner and investigate ways to reduce it.

Approach: Vibration and acoustic noise measurements were performed. Sponge window seal materials were stuck on the RF coil to attempt altering its vibrations and decrease acoustic noise.

Results: The experimental data is consistent with noise induced by eddy-currents on the RF shield. Altering its vibrations allowed decreasing sound level by up to 10 dB at some EPI echo-spacings.

Impact: Identifying the main source of acoustic noise shall open new research avenues to mitigate acoustic noise 

Keywords: Shims, Shims, High-Field MRI, Simulations, Safety, New Devices

Motivation: The safety concerns of heating and torque in ultra-high field MRI easily become a problem in implementing local devices. As our designed brain shim coil with dense wire pattern can improve the B₀ homogeneity significantly, it is important to ensure safety before utilization. 

Goal(s): The study aims to minimize the power dissipation and torque in the designed shim coil. 

Approach: The study introduced regularization for power dissipation and torque, employs the wire interpolation method to enhance power efficiency, and simulates the impact of varying the number of turns in shim coils. 

Results: We found the optimal design parameters balancing safety and performance.  

Impact: We introduced a series of methods to design a safe and efficient shim coil for the brain in ultra-high field. The torque and heat results provide insight into the performance of the local shim coil.  

Keywords: High-Field MRI, High-Field MRI, RF coil testing hardware

Motivation: Parallel transmission (pTx) is a very useful method for minimizing peak SAR and RF field inhomogeneity at UHF. To simplify the design, individual elements of pTx RF arrays are connected directly to scanner plugs and not easily accessible. In addition, in the case of a Tx-only/Rx-only (ToRo) setup, safety regulations require testing the Tx-only array without Rx-array.

Goal(s): To develop testing hardware and procedure for adjusting and evaluating pTx 16-channel Tx/32-channel Rx 9.4T array coils.

Approach: Electronic hardware and procedure was designed, constructed, and tested.

Results: The hardware was constructed and successfully used in development of the pTx 16Tx/32Rx human head ToRo-array coil.

Impact: We developed testing hardware and procedure for pTx-coils and used them in development of the human head ToRo-array. The developed hardware allows testing any pTx 9.4T RF coil with Rx-channel count up to 32 and Tx-channel count up to 16.