Keywords: Safety, Safety

Motivation: The effects of flexible receive-only coils on Specific Absorption Rate (SAR) are understudied and poorly understood.

Goal(s): The goal was to examine, in the pediatric case, SAR effects in the presence of a receive coil with various blocking impedance characteristics.

Approach: Finite Difference Time Domain analysis was applied to evaluate SAR for pediatric human models wrapped in a flexible 16-channel coil with three blocking impedance configurations.

Results: Results demonstrated that local and whole-body SAR decreased upon inclusion of a wrapped surface coil when blocking impedance was resistive or inductive and increased when capacitive.

Impact: The demonstration of SAR effects in the presence of flexible receive-only coils and the indication of SAR configurability via blocking impedance control informs coil design to lower SAR and facilitate safety in high thermal-risk applications such as pediatric imaging.

Keywords: Safety, RF Arrays & Systems

Motivation: 7T MRI enables ²³Na MRI due to enhanced sensitivity. Combined with proton imaging, functional and anatomical information can be acquired to characterise brain status.

Goal(s): Establishing safe RF power limits for an in-house built ¹H/²³Na loop/dipole array for subsequent in-vivo experiments.

Approach: Phantom MR results at 7T and electromagnetic simulations were compared to evaluate consistency between experiments and simulations. Human models were simulated to demonstrate SAR_10g levels for different patients.

Results: Simulated and experimental B₁⁺fields showed high correlation for individual magnitudes and phases. Worst-case SAR_10g for human models all remain within safety limits, demonstrating the clinical potential of our proposed coil.

Impact: Towards in-vivo RF coil use, we validated simulated RF models of the coil at ²³Na/¹H frequencies using measured B₁⁺field magnitude and phase maps. Safe RF power limits can therefore be established for research coils to proceed to in-vivo experiments.

Keywords: Safety, Safety, Electromagnetic Tissue Properties, SAR

Motivation: To enable real-time SAR monitoring in MRI examinations, particularly when numerical simulations are not feasible.

Goal(s): This work aimed to explore the feasibility of obtaining subject-specific image-based SAR maps directly from conventional MRI sequences in vivo.

Approach: The tissue electrical conductivities were characterized using an MRI phase-based electrical properties tomography approach, and the electric field distribution was measured by applying Ampère's law on the B1 field obtained from B1 mapping.

Results: Subject-specific image-based SAR maps can be obtained from conventional MRI data. Median electrical conductivity values and retrieved SAR maps qualitatively agreed with existing literature.

Impact: We analyze the feasibility of obtaining in vivo subject-specific image-based SAR maps directly from conventional MRI data. The electrical conductivity is obtained through electrical properties tomography (EPT) and the E-field is estimated from the B1 field.

Keywords: Safety, Safety

Motivation: In active implantable medical devices (AIMDs), electrical shorts may occur either between electrodes of two leads in multi-lead systems (inter-lead shorts), or within the AIMD header due to incomplete lead insertion (intra-lead shorts). 

Goal(s): Whether these shorts invalidate the originally-established MR safety condition for RF-induced heating?

Approach: The RF-induced heating for three shorted-lead cases were investigated. Device heating models (transfer functions) and RF-induced heating were measured for 1.5T MR systems. 

Results:  Though the intra-lead short case exhibited a similar transfer function (TF) and RF-induced heating to non-shorted leads, inter-lead shorts had significant impact on the RF-induced heating.

Impact: This research improves MRI safety for patients with active implantable medical devices (AIMDs) by studying the effects of electrical shorts.

Keywords: Safety, Safety

Motivation: UHF imaging is limited by both transmit uniformity and local SAR. Information on RF electric field is unavailable in conventional MR scan procedure.

Goal(s): This work aims to provide real-time RF electric field for joint optimization of imaging uniformity and peak local SAR.

Approach: A deep-learning method is proposed to predict the real-time EM field distribution using B1+ data obtained in routine prescan of the imaging procedure. The output field data is used in combined optimization of transmit uniformity and local SAR.

Results: In the torso imaging case, this method achieved both improvement of transmit field uniformity and reduction of peak local SAR.

Impact: This work studied the feasibility of machine-learning methods for RF field estimation and simultaneous optimization of transmit homogeneity and peak local SAR, aiming to reduce the estimation error of local SAR and increase the available maximum B1.

Keywords: Safety, Safety

Motivation: The RF-induced heating from different configurations of open impedance leads connected to a commercial Pulse Generator (IPG) was investigated. It was observed the RF-induced heating from open impedance leads can behave differently from an intact lead.

Goal(s): Heating in the distal end of open impedance leads need to be estimated.

Approach: Transfer functions and heating measurements were performed.

Results: Most open-impedance configurations led to increased electrode heating with the highest heating occurring when only a single channel of the eight-channel lead was left intact. Additionally, significant differences in heating were found depending on the location of the open impedance.

Impact: Patients with open impedance fractured leads could take MR scan with the help of this study.

Keywords: Safety, Safety, Parallel transmit (pTx); Spinal cord; Specific absorption rate;

Motivation: Reports highlight variability in 10g-averaged specific absorption rate (SAR_10g) when varying subjects and radiofrequency coil position, necessitating safety factors to prevent underestimation, leading to overconservative SAR constraints.

Goal(s): To provide a method for selecting appropriate body models to balance RF safety and sequence performance, while investigating SAR_10g variability in 7T spinal cord pTx MRI.

Approach: The impact of tissue properties, anatomy, body-mass index, and coil positioning on SAR_10g was evaluated according to multiple metrics.

Results: The mean absolute percentage error can complement the safety factor to decrease SAR_10g overestimation and guide the choice of human models used for SAR-monitoring.

Impact: Simulations of local-SAR in generic human models are widely used to ensure RF safety at 7T. We show that optimizing the choice of models used for SAR-monitoring may lead to less conservative SAR-constraints, which may improve sequence and pTx performance.

Keywords: Safety, Safety, RF-induced heating, Implantable Devices

Motivation: Patients with deep brain stimulation (DBS) devices are strictly prohibited from undergoing 7 Tesla magnetic resonance imaging (MRI) scans.

Goal(s): Mitigating radiofrequency (RF)-induced heating in DBS for 7 T MRI using a convenient approach.

Approach: By utilizing the electromagnetic characteristics arising from the near-field region of the RF coil, a novel method of rearranging the positions of the RF channels adjacent to the DBS lead using a geometrically adjustable RF array was proposed.

Results: Adjusting the position of channels adjacent to lead reduces the specific absorption rate and the extent of temperature increase around the electrode up to 34% and 23.8 %, respectively.

Impact: Our method involves strategically adjusting the position of RF coil near the lead, providing a practical solution to mitigate RF-induced heating in DBS. It eliminates the necessity for a new lead design or complex numerical optimization for incident electromagnetic field.

Keywords: Safety, In Silico, Numerical body position

Motivation: Use of commercial tools to reposition/pose numerical body models to match subject posture in MRI can result in significant non-anatomical distortions of the model.

Goal(s): Introduce approach to avoid non-anatomical distortions for a model in the seated position.

Approach: “Seated” versions of a body model were produced by: 1) using commercial software alone, and 2) strategically combining portions of the seated and original models offline followed by post-processing. SAR was calculated for both models in an open-bore double-donut MRI system.

Results: The Combined model avoided non-anatomical distortions seen when using the repositioning tool alone. These differences are also reflected in SAR distributions.

Impact: We introduce a method for avoiding significant non-anatomical distortions occurring in numerical body models when using commercial tools to reconfigure the posture/pose of the model. Use of this approach can also avoid related inaccuracies in calculated SAR and field distributions.

Keywords: Safety, Safety

Motivation: The TF validation process is time-consuming work. Here, we reduced the burden of TF approach using a low power tabletop E-field generator.

Goal(s): Using the TF approach to estimate the RF-induced heating of the stent tip at 3T using a E-field generator and compared predicted values against the estimated values.

Approach: TF of the stent was measured via piX method at 128MHz . TF was validated by exposing the device under diverse test fields using table-top E-field generator. The body coil was used to evaluate the TF approach. 

Results: The stent tip heating was estimated using TF approach with a low-power E-field generator. 

Impact: TF approach could aid estimating the RF-induced heating, not only active implantable device, but also elongated passive implantable device. A compact E-field generator can be used for TF validation. 

Keywords: Safety, Safety

Motivation: Real-time local SAR supervision is a major obstacle in pTx system especially since the computational demand scales proportionally with the number of RF channels.

Goal(s): To develop a safety supervision that monitors 32 transmit channels phase sensitive and perform local SAR calculation with a higher number of VOPs.

Approach: Directional couplers are placed in the transmit path, digitizers sample the RF signal, and a GPU performs the local SAR calculation.

Results: This study demonstrates real-time RF supervision for a 32-channel pTx transmit system with local SAR calculation using 600 VOPs on a single GPU.

Impact: This study demonstrates real-time RF supervision for a 32-channel pTx transmit system with single-GPU-based local SAR calculation using 600 virtual observation points.

Keywords: Safety, Simulations

Motivation: RF safety assessment requires accurate simulation results. FEM simulations offer superior performance for delicate structures while FDTD simulations are more efficient for realistic body models. 

Goal(s): Improve the accuracy of FDTD simulations for four-layered body coil models using a simplified, three layered, model combined with network co-simulation.

Approach: In Sim4Life, a body coil is simulated using lumped element capacitors instead of layered capacitor structures. Capacitor values are optimized by network co-simulation to minimize differences in the resulting E-field distribution and that of HFSS.  

Results: Improved E-field and B1+ field distributions are observed in common phantoms for whole-body SAR and implant local SAR assessment.

Impact: The proposed method can accurately simulate complex birdcage body coils with layered capacitor structures while also being able to include realistic body models. As such, the method combines the features of FDTD and FEM simulations.

Keywords: Safety, Safety, VOP, SAR

Motivation: The complexity of SAR calculation increases dramatically with the number of channels in a parallel transmit system.

Goal(s): The goal of this work is to improve VOP post processing and investigate the impact of post processing on the complexity of SAR calculation with increasing number of channels.

Approach: An existing post-processing algorithm was improved by introducing a new criterion for upper boundedness from the literature. The new algorithm was used to investigate the increase in the number of VOPs with the channel count when median relative overestimation was kept constant.

Results: The number of VOPs increases logarithmically with the number of channels.

Impact: VOP compression is important for SAR supervision and constraint RF-pulse calculation. Using an improved post-processing algorithm, we show that the increase in the number of VOPs when going to higher channel count N_ch can be reduced from N_ch^2.3 to log(N_ch).

Keywords: Safety, Safety, Modelling, MRI

Motivation: MRI RF safety evaluation requires simulations of many coils and body models, which is typically done using FDTD or FEM. These have complementary weaknesses, therefore we propose to combine them.

Goal(s): We propose a simulation process that combines FEM with FDTD via the Huygens’ principle.

Approach: We first compute the incident electric field created by the coil alone using FEM. Second, we compute field perturbations created by the body model using an FDTD Huygens’ implementation.

Results: The proposed method agrees well with the all-FDTD calculation at 64 MHz, but allows more efficient and flexible modelling of RF coils.

Impact: We propose a robust and flexible simulation method combining the strengths of FEM and FDTD via the Huygens principle that expedites coil and implant (ISO/TS10974 Tier 3) RF safety evaluations. 

Keywords: High-Field MRI, Parallel Imaging

Motivation: To improve signal-to-noise-ratio (SNR) in deep brain structures while using high-density receive arrays at 7T.

Goal(s): Compare SNR and g-factor performance of conventional transmit-only receive-only arrays with receive arrays combined with transceiver arrays.  

Approach: Transceive function was introduced on  8Tx63Rx and  16Tx96Rx 7T head coils, so that the number of receive channels during acquisition increased by 8 and 16, respectively. Three healthy volunteers were scanned, and SNR and g-factor maps were calculated for the different configurations. 

Results: The transceiver configuration provided about 16% increase in the central SNR. At higher acceleration factors, especially the 112-channel receive configuration provided improved g-factor performance.  

Impact: : High-density receive-only arrays provide high SNR close to the surface while maintaining the central SNR under sample noise dominant conditions. Transceiver loops surrounding the receive array enhances the central SNR at 7T, although not as high as reported at 10.5T.  

Keywords: Safety, Safety, Electroencephalography (EEG)

Motivation: Imperfections of B₀-field can reduce the MR image quality. To track the B₀ change during imaging, NMR field-probes inside the RF coil can be used. Insertion of the field-probes may lead to alteration of the B₁⁺ and pSAR.

Goal(s): To evaluate alterations of B₁⁺ field and pSAR of the 16-channel Tx-array in the presence of NMR field-probes at 9.4T.

Approach: To reach the goal, we simulated the Tx-part of the 16Tx32Rx array loaded by a phantom and human voxel model in the presence of 11 field-probes.

Results: Insertion of field-probes led to a small drop of B₁⁺ and a slight change of pSAR.

Impact: We showed that inserting field-probes doesn’t significantly alter the B₁⁺ field and pSAR of the 16Tx32Rx array coil at 9.4T. Therefore, field-probes can be safely used in-vivo to evaluate B₀.