Vertical open-bore MRI scanners generate significantly less radiofrequency heating around implanted leads: A study of deep brain stimulation implants in 1.2T OASIS scanners versus 1.5T horizontal systems

A deep brain stimulation-conditioned RF coil for 3T MRI

PURPOSE

To develop and test an MRI coil assembly for imaging deep brain stimulation (DBS) at 3 T with a reduced level of local specific absorption rate of RF fields near the implant.

METHODS

A mechanical rotatable linearly polarized birdcage transmitter outfitted with a 32-channel receive array was constructed. The coil performance and image quality were systematically evaluated using bench-level measurements and imaging performance tests, including SNR maps, array element noise correlation, and acceleration capabilities. Electromagnetic simulations and phantom experiments were performed with clinically relevant DBS device configurations to evaluate the reduction of specific absorption rate and temperature near the implant compared with a circular polarized body coil setup.

RESULTS

The linearly polarized birdcage coil features a block-shaped low electric field region to be co-aligned with the implanted DBS lead trajectory, while the close-fit receive array enables imaging with high SNR and enhanced encoding capabilities.

CONCLUSION

The 3T coil assembly, consisting of a rotating linear birdcage and a 32-channel close-fit receive array, showed DBS-conditioned imaging technology with substantially reduced heat generation at the DBS implants.

Enhancing fine-tuning efficiency and design optimization of an eight-channel 3T transmit array via equivalent circuit modeling and Eigenmode analysis

BACKGROUND

Radiofrequency (RF) transmit arrays play a crucial role in various MRI applications, offering enhanced field control and improved imaging capabilities. Designing and optimizing these arrays, particularly in high-field MRI settings, poses challenges related to coupling, resonance, and construction imperfections. Numerical electromagnetic simulation methods effectively aid in the initial design, but discrepancies between simulated and fabricated arrays often necessitate fine-tuning. Fine-tuning involves iteratively adjusting the array's lumped elements, a complex and time-consuming process that demands expertise and substantial experience. This process is particularly required for high-Q-factor arrays or those with decoupling circuitries, where the impact of construction variations and coupling between elements is more pronounced. In this context, our study introduces and validates an accelerated fine-tuning approach custom RF transmit arrays, leveraging the arrays equivalent circuit modeling and eigenmode analysis of the scattering (S) parameters.

PURPOSE

This study aims to streamline the fine-tuning process of lab-fabricated RF transmit arrays, specifically targeting an eight-channel degenerate birdcage coil designed for 3T MRI. The objective is to minimize the array's modal reflected power values and address challenges related to coupling and resonance.

METHODS

An eight-channel 3T transmit array is designed and simulated, optimizing capacitor values via the co-simulation strategy and eigenmode analysis. The resulting values are used in constructing a prototype. Experimental measurements of the fabricated coil's S-parameters and fitting them into an equivalent circuit model, enabling estimation of self/mutual-inductances and self/mutual-resistances of the fabricated coil. Capacitor adjustments in the equivalent circuit model minimize mismatches between experimental and simulated results.

RESULTS

The simulated eight-channel array, optimized for minimal normalized reflected power, exhibits excellent tuning and matching and an acceptable level of decoupling (|Snn|≤-23 dB and |Smn|≤-11 dB). However, the fabricated array displays deviations, including resonances at different frequencies and increased reflections. The proposed fine-tuning approach yields an updated set of capacitor values, improving resonance frequencies and reducing reflections. The fine-tuned array demonstrates comparable performance to the simulation (|Snn|≤-15 dB and |Smn|≤-9 dB), mitigating disparities caused by construction imperfections. The maximum error between the calculated and measured S-parameters is -7 dB.

CONCLUSION

This accelerated fine-tuning approach, integrating equivalent circuit modeling and eigenmode analysis, effectively optimizes the performance of fabricated transmit arrays. Demonstrated through the design and refinement of an eight-channel array, the method addresses construction-related disparities, showcasing its potential to enhance overall array performance. The approach holds promise for streamlining the design and optimization of complex RF coil systems, particularly for high Q-factor arrays and/or arrays with decoupling circuitry.

Effect of surgical modification of deep brain stimulation lead trajectories on radiofrequency heating during MRI at 3T: from phantom experiments to clinical implementation

OBJECTIVE

Radiofrequency (RF) tissue heating around deep brain stimulation (DBS) leads is a well-known safety risk during MRI, resulting in strict imaging guidelines and limited allowable protocols. The implanted lead's trajectory and orientation with respect to the MRI electric fields contribute to variations in the magnitude of RF heating across patients. Currently, there are no surgical requirements for implanting the extracranial portion of the DBS lead, resulting in substantial variations in clinical lead trajectories and consequently RF heating. Recent studies have shown that incorporating concentric loops in the extracranial lead trajectory can reduce RF heating. However, optimal positioning of the loops and the quantitative benefit of trajectory modification in terms of added safety margins during MRI remain unknown. In this study, the authors systematically evaluated the characteristics of DBS lead trajectories that minimize RF heating during 3T MRI to develop the best surgical practices for safe access to postoperative MRI, and they present the first surgical implementation of these modified trajectories.

METHODS

The authors performed experiments to assess the maximum temperature increase of 244 distinct lead trajectories. They investigated the effect of the position, number, and size of the concentric loops on the skull. Experiments were performed in an anthropomorphic phantom implanted with a commercial DBS system, and RF exposure was generated by applying a high specific absorption rate sequence (B1+rms = 2.7 µT). The authors conducted test-retest experiments to assess the reliability of measurements. Additionally, they evaluated the effect of imaging landmarks and perturbations to the DBS device configuration on the efficacy of low-heating trajectories. Finally, two neurosurgeons implanted the recommended modified trajectories in patients, and the authors characterized their RF heating in comparison with unmodified trajectories.

RESULTS

The maximum temperature increase ranged from 0.09°C to 7.34°C. The authors found that increasing the number of loops and positioning them closer to the surgical burr hole, particularly for the contralateral lead, substantially reduced RF heating. These trajectory modifications were easily incorporated during the surgical procedure and resulted in a threefold reduction in RF heating.

CONCLUSIONS

Surgically modifying the extracranial portion of the DBS lead trajectory can substantially reduce RF heating during 3T MRI. The authors' results indicate that simple adjustments to the lead's configuration, such as small, concentric loops near the burr hole, can be readily adopted during DBS lead implantation to improve patient safety during MRI.

Managing Patients With Unlabeled Passive Implants on MR Systems Operating Below 1.5 T

The standard of care for managing a patient with an implant is to identify the item and to assess the relative safety of scanning the patient. Because the 1.5 T MR system is the most prevalent scanner in the world and 3 T is the highest field strength in widespread use, implants typically have "MR Conditional" (i.e., an item with demonstrated safety in the MR environment within defined conditions) labeling at 1.5 and/or 3 T only. This presents challenges for a facility that has a scanner operating at a field strength below 1.5 T when encountering a patient with an implant, because scanning the patient is considered "off-label." In this case, the supervising physician is responsible for deciding whether to scan the patient based on the risks associated with the implant and the benefit of magnetic resonance imaging (MRI). For a passive implant, the MRI safety-related concerns are static magnetic field interactions (i.e., force and torque) and radiofrequency (RF) field-induced heating. The worldwide utilization of scanners operating below 1.5 T combined with the increasing incidence of patients with implants that need MRI creates circumstances that include patients potentially being subjected to unsafe imaging conditions or being denied access to MRI because physicians often lack the knowledge to perform an assessment of risk vs. benefit. Thus, physicians must have a complete understanding of the MRI-related safety issues that impact passive implants when managing patients with these products on scanners operating below 1.5 T. This monograph provides an overview of the various clinical MR systems operating below 1.5 T and discusses the MRI-related factors that influence safety for passive implants. Suggestions are provided for the management of patients with passive implants labeled MR Conditional at 1.5 and/or 3 T, referred to scanners operating below 1.5 T. The purpose of this information is to empower supervising physicians with the essential knowledge to perform MRI exams confidently and safely in patients with passive implants. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY: Stage 3.

Comparison of tight-fitting 7T parallel-transmit head array designs using excitation uniformity and local specific absorption rate metrics

PURPOSE

We model the performance of parallel transmission (pTx) arrays with 8, 16, 24, and 32 channels and varying loop sizes built on a close-fitting helmet for brain imaging at 7 T and compare their local specific absorption rate (SAR) and flip-angle performances to that of birdcage coil (used as a baseline) and cylindrical 8-channel and 16-channel pTx coils (single-row and dual-row).

METHODS

We use the co-simulation approach along with MATLAB scripting for batch-mode simulation of the coils. For each coil, we extracted B1 + maps and SAR matrices, which we compressed using the virtual observation points algorithm, and designed slice-selective RF shimming pTx pulses with multiple local SAR and peak power constraints to generate L-curves in the transverse, coronal, and sagittal orientations.

RESULTS

Helmet designs outperformed cylindrical pTx arrays at a constant number of channels in the flip-angle uniformity at a constant local SAR metric: up to 29% for 8-channel arrays, and up to 34% for 16-channel arrays, depending on the slice orientation. For all helmet arrays, increasing the loop diameter led to better local SAR versus flip-angle uniformity tradeoffs, although this effect was more pronounced for the 8-channel and 16-channel systems than the 24-channel and 32-channel systems, as the former have more limited degrees of freedom and therefore benefit more from loop-size optimization.

CONCLUSION

Helmet pTx arrays significantly outperformed cylindrical arrays with the same number of channels in local SAR and flip-angle uniformity metrics. This improvement was especially pronounced for non-transverse slice excitations. Loop diameter optimization for helmets appears to favor large loops, compatible with nearest-neighbor decoupling by overlap.

Modifying the trajectory of epicardial leads can substantially reduce MRI-induced RF heating in pediatric patients with a cardiac implantable electronic device at 1.5T

PURPOSE

After epicardial cardiac implantable electronic devices are implanted in pediatric patients, they become ineligible to receive MRI exams due to an elevated risk of RF heating. We investigated whether simple modifications in the trajectories of epicardial leads could substantially and reliably reduce RF heating during MRI at 1.5 T, with benefits extending to abandoned leads.

METHODS

Electromagnetic simulations were performed to assess RF heating of two common 35-cm epicardial lead trajectories exhibiting different degrees of coupling with MRI incident electric fields. Experiments in anthropomorphic phantoms implanted with commercial cardiac implantable electronic devices confirmed the findings. Both electromagnetic simulations and experimental measurements were performed using head-first and feet-first positioning and various landmarks. Transfer function approach was used to assess the performance of suggested modifications in realistic body models.

RESULTS

Simulations (head-first, chest landmark) of a 35-cm epicardial lead with a trajectory where the excess length of the lead was looped and placed on the inferior surface of the heart showed an 87-fold reduction in the 0.1 g-averaged specific absorption rate compared with the lead where the excess length was looped on the anterior surface. Repeated experiments with a commercial epicardial device confirmed this. For fully implanted systems following low-specific absorption rate trajectories, there was a 16-fold reduction in the average temperature rise and a 28-fold reduction for abandoned leads. The transfer function method predicted a 7-fold reduction in the RF heating in 336 realistic scenarios.

CONCLUSION

Surgical modification of epicardial lead trajectory can substantially reduce RF heating at 1.5 T, with benefits extending to abandoned leads.

Age Matters: A Comparative Study of RF Heating of Epicardial and Endocardial Electronic Devices in Pediatric and Adult Phantoms during Cardiothoracic MRI

This study focused on the potential risks of radiofrequency-induced heating of cardiac implantable electronic devices (CIEDs) in children and adults with epicardial and endocardial leads of varying lengths during cardiothoracic MRI scans. Infants and young children are the primary recipients of epicardial CIEDs, though the devices have not been approved as MR conditional by the FDA due to limited data, leading to pediatric hospitals either refusing the MRI service to most pediatric CIED patients or adopting a scan-all strategy based on results from adult studies. The study argues that risk-benefit decisions should be made on an individual basis. We used 120 clinically relevant epicardial and endocardial device configurations in adult and pediatric anthropomorphic phantoms to determine the temperature rise during RF exposure at 1.5 T. The results showed that there was significantly higher RF heating of epicardial leads than endocardial leads in the pediatric phantom, but not in the adult phantom. Additionally, body size and lead length significantly affected RF heating, with RF heating up to 12 °C observed in models based on younger children with short epicardial leads. The study provides evidence-based knowledge on RF-induced heating of CIEDs and highlights the importance of making individual risk-benefit decisions when assessing the potential risks of MRI scans in pediatric CIED patients.

Multi-Segment Leads To Reduce RF Heating in MRI: A Computational Evaluation at 1.5T and 3T

Implanted neurostimulators are currently in widespread use and allow patients to receive therapeutic nerve stimulation for a variety of conditions. Such devices often make use of long leads extending from the device to the relevant nerve to deliver their stimulation. These leads carry a significant radiofrequency (RF) safety concern for patients who also receive magnetic resonance imaging (MRI) scans. The incident RF energy from the MRI body coil can couple with the lead and produce dangerous levels of heating at the tip of the lead during a scan. Recent studies have shown one useful approach to mitigate this heating is to vary the conductivity of the wire along its length to decrease the coupling of the incoming RF energy from the MRI coil with the long lead. In this study, we adopt a similar approach and extend it by segmenting a long cylindrical lead model into two sections of differing conductivities and assessing the maximum 1g specific absorption rate (SAR) at the lead tip at both 64 MHz and 127 MHz. We also evaluated the effect of insulation thickness as well as conductivity of the phantom on the maximum 1g SAR. An 11-fold reduction in the SAR was achieved when using high conductivity ratios between the two wire segments for the 127 MHz coil and a 2-fold reduction was seen for the 64 MHz coil.Clinical relevance- Design of an implantable segmented lead has potential to mitigate RF heating concerns and open a wider patient population to both 1.5T and 3T MRI scans.

RF-induced heating of capped and uncapped abandoned epicardial leads during MRI at 1.5 T and 3 T

There is a paucity of data regarding the safety of magnetic resonance imaging (MRI) in patients with abandoned epicardial leads. Few studies have reported temperature rises up to 76 °C during MRI at 1.5 T in gel phantoms implanted with epicardial leads; however, lead trajectories used in these experiments were not clinically relevant. This work reports patient-specific RF heating of both capped and uncapped abandoned epicardial lead configurations during MRI at both 1.5 T and 3 T field strengths. We found that leads routed along realistic, patient-derived trajectories generated substantially lower RF heating than the previously reported worst-case phantom experiments. We also found that MRI at the head imaging landmark leads to substantially lower RF heating compared to MRI at the chest or abdomen landmarks at both 1.5 T and 3 T. Our results suggest that patients with abandoned epicardial leads may safely undergo MRI for head imaging, but caution is warranted during chest and abdominal imaging.Clinical Relevance- Patients with abandoned epicardial leads may safely undergo MRI for head imaging, but caution is warranted during chest and abdominal imaging.

Implants talk to each-other: RF heating changes when two DBS leads are present simultaneously during MRI

Deep brain stimulation (DBS) has proven to be an effective treatment for Parkinson's disease and other brain disorders. The procedure often involves implanting two elongated leads aimed at specific brain nuclei in both the left and right hemispheres. However, evaluating the safety of MRI in patients with such implants has only been done on an individual lead basis, ignoring the possibility of crosstalk between the leads. This study evaluates the impact of crosstalk on power deposition at the lead tip through numerical simulation and phantom experiments. We used CT images to obtain patient-specific lead trajectories and compared the power deposition at the lead tip in cases with bilateral and unilateral DBS implants. Our results indicate that the RF power deposition at the lead tip can vary by up to 6-fold when two DBS leads are present together compared to when only one lead is present. Experimental measurements in a simplified case of two lead-only DBS systems confirmed the existence of crosstalk.Clinical Relevance-Our results indicate that RF heating of implanted leads during MRI can be affected by the presence of another lead in the body, which may increase or decrease the power deposition in the tissue depending on the position and configuration of the leads.

Low-field MRI's Spark on Implant Safety: A Closer Look at Radiofrequency Heating

Advances in low-field magnetic resonance imaging (MRI) are making imaging more accessible without significant losses in image quality. In addition to being more cost-effective and easier to place without as much needed infrastructure, it has been publicized that the lower field strengths make MRI safer for patients with implants. To test this claim, we conducted a total of 368 simulations with wires of various lengths and geometries in a gel phantom during radiofrequency (RF) exposure at 23 MHz and 63.6 MHz (corresponding to MRI at 0.55 T and 1.5 T). Our results showed that heating in the gel around wire tips could be higher in certain cases at 0.55 T. To examine the impact on real patients, we simulated two models of patients with deep brain stimulation (DBS) implants of different lengths. These simulations provide quantitative evidence that low-field MRI is not always safer, and this paper serves to illustrate some of the basic principles involved in RF heating of elongated implants in MRI environments.Clinical Relevance- This paper illustrates the physical concepts of resonance and inductive coupling in RF heating during MRI scanning with implants through systematic simulations and discusses the impact of these principles in practice.

Rapid prediction of MRI-induced RF heating of active implantable medical devices using machine learning

The interaction between an active implantable medical device and magnetic resonance imaging (MRI) radiofrequency (RF) fields can cause excessive tissue heating. Existing methods for predicting RF heating in the presence of an implant rely on either extensive phantom experiments or electromagnetic (EM) simulations with varying degrees of approximation of the MR environment, the patient, or the implant. On the contrary, fast MR thermometry techniques can provide a reliable real-time map of temperature rise in the tissue in the vicinity of conductive implants. In this proof-of-concept study, we examined whether a machine learning (ML) based model could predict the temperature increase in the tissue near the tip of an implanted lead after several minutes of RF exposure based on only a few seconds of experimentally measured temperature values. We performed phantom experiments with a commercial deep brain stimulation (DBS) system to train a fully connected feedforward neural network (NN) to predict temperature rise after ~3 minutes of scanning at a 3 T scanner using only data from the first 5 seconds. The NN effectively predicted ΔTmax-R2 = 0.99 for predictions in the test dataset. Our model also showed potential in predicting RF heating for other various scenarios, including a DBS system at a different field strength (1.5 T MRI, R2 = 0.87), different field polarization (1.2 T vertical MRI, R2 = 0.79), and an unseen implant (cardiac leads at 1.5 T MRI, R2 = 0.91). Our results indicate great potential for the application of ML in combination with fast MR thermometry techniques for rapid prediction of RF heating for implants in various MR environments.Clinical Relevance- Machine learning-based algorithms can potentially enable rapid prediction of MRI-induced RF heating in the presence of unknown AIMDs in various MR environments.

Optimizing the trajectory of deep brain stimulation leads reduces RF heating during MRI at 3 T: Characteristics and clinical translation

Radiofrequency (RF) induced tissue heating around deep brain stimulation (DBS) leads is a well-known safety risk during magnetic resonance imaging (MRI), hindering routine protocols for patients. Known factors that contribute to variations in the magnitude of RF heating across patients include the implanted lead's trajectory and its orientation with respect to the MRI electric fields. Currently, there are no consistent requirements for surgically implanting the extracranial portion of the DBS lead. Recent studies have shown that incorporating concentric loops in the extracranial trajectory of the lead can reduce RF heating, but the optimal positioning of the loop is unknown. In this study, we evaluated RF heating of 77 unique lead trajectories to determine how different characteristics of the trajectory affect RF heating during MRI at 3 T. We performed phantom experiments with commercial DBS systems from two manufacturers to determine how consistently modifying the lead trajectory mitigates RF heating. We also presented the first surgical implementation of these modified trajectories in patients. Low-heating trajectories included small concentric loops near the surgical burr hole which were readily implemented during the surgical procedure; these trajectories generated nearly a 2-fold reduction in RF heating compared to unmodified trajectories.Clinical Relevance- Surgically modifying the DBS lead trajectory can be a cost-effective strategy for reducing RF-induced heating during MRI at 3 T.

RF-induced heating of interventional devices at 23.66 MHz

OBJECTIVE

Low-field MRI systems are expected to cause less RF heating in conventional interventional devices due to lower Larmor frequency. We systematically evaluate RF-induced heating of commonly used intravascular devices at the Larmor frequency of a 0.55 T system (23.66 MHz) with a focus on the effect of patient size, target organ, and device position on maximum temperature rise.

MATERIALS AND METHODS

To assess RF-induced heating, high-resolution measurements of the electric field, temperature, and transfer function were combined. Realistic device trajectories were derived from vascular models to evaluate the variation of the temperature increase as a function of the device trajectory. At a low-field RF test bench, the effects of patient size and positioning, target organ (liver and heart) and body coil type were measured for six commonly used interventional devices (two guidewires, two catheters, an applicator and a biopsy needle).

RESULTS

Electric field mapping shows that the hotspots are not necessarily localized at the device tip. Of all procedures, the liver catheterizations showed the lowest heating, and a modification of the transmit body coil could further reduce the temperature increase. For common commercial needles no significant heating was measured at the needle tip. Comparable local SAR values were found in the temperature measurements and the TF-based calculations.

CONCLUSION

At low fields, interventions with shorter insertion lengths such as hepatic catheterizations result in less RF-induced heating than coronary interventions. The maximum temperature increase depends on body coil design.

Application of Machine learning to predict RF heating of cardiac leads during magnetic resonance imaging at 1.5 T and 3 T: A simulation study

Predicting magnetic resonance imaging (MRI)-induced heating of elongated conductive implants, such as leads in cardiovascular implantable electronic devices, is essential to assessing patient safety. Phantom experiments have traditionally been used to estimate radio-frequency (RF) heating of implants, but they are time-consuming. Recently, machine learning has shown promise for fast prediction of RF heating of orthopaedic implants when the implant position within the MRI RF coil was predetermined. We explored whether deep learning could be applied to predict RF heating of conductive leads with variable positions and orientations during MRI at 1.5 T and 3 T. Models of 600 cardiac leads with clinically relevant trajectories were generated, and electromagnetic simulations were performed to calculate the maximum of the 1 g-averaged specific absorption rate (SAR) of RF energy at the tips of lead models during MRI at 1.5 T and 3 T. Neural networks were trained to predict the maximum SAR at the lead tip from the knowledge of the coordinates of points along the lead trajectory. Despite the large range of SAR values (∼230 W/kg to ∼ 3200 W/kg and ∼ 10 W/kg to ∼ 3300 W/kg), the root- mean-square error of the predicted vs ground truth SAR remained at 223 W/kg and 206 W/kg, with the R² scores of 0.89 and 0.85 on the testing set for 1.5 T and 3 T models, respectively. The results suggest that machine learning is a promising approach for fast assessment of RF heating of lead-like implants when only the knowledge of the lead geometry and MRI RF coil features are in hand.

Artifacts Can Be Deceiving: The Actual Location of Deep Brain Stimulation Electrodes Differs from the Artifact Seen on Magnetic Resonance Images

INTRODUCTION

Deep brain stimulation (DBS) is a common treatment for a variety of neurological and psychiatric disorders. Recent studies have highlighted the role of neuroimaging in localizing the position of electrode contacts relative to target brain areas in order to optimize DBS programming. Among different imaging methods, postoperative magnetic resonance imaging (MRI) has been widely used for DBS electrode localization; however, the geometrical distortion induced by the lead limits its accuracy. In this work, we investigated to what degree the difference between the actual location of the lead's tip and the location of the tip estimated from the MRI artifact varies depending on the MRI sequence parameters such as acquisition plane and phase encoding direction, as well as the lead's extracranial configuration. Accordingly, an imaging technique to increase the accuracy of lead localization was devised and discussed.

METHODS

We designed and constructed an anthropomorphic phantom with an implanted DBS system following 18 clinically relevant configurations. The phantom was scanned at a Siemens 1.5 Tesla Aera scanner using a T1MPRAGE sequence optimized for clinical use and a T1TSE sequence optimized for research purposes. We varied slice acquisition plane and phase encoding direction and calculated the distance between the caudal tip of the DBS lead MRI artifact and the actual tip of the lead, as estimated from MRI reference markers.

RESULTS

Imaging parameters and lead configuration substantially altered the difference in the depth of the lead within its MRI artifact on the scale of several millimeters - with a difference as large as 4.99 mm. The actual tip of the DBS lead was found to be consistently more rostral than the tip estimated from the MR image artifact. The smallest difference between the tip of the DBS lead and the tip of the MRI artifact using the clinically relevant sequence (i.e., T1MPRAGE) was found with the sagittal acquisition plane and anterior-posterior phase encoding direction.

DISCUSSION/CONCLUSION

The actual tip of an implanted DBS lead is located up to several millimeters rostral to the tip of the lead's artifact on postoperative MR images. This distance depends on the MRI sequence parameters and the DBS system's extracranial trajectory. MRI parameters may be altered to improve this localization.

A comparative study of RF heating of deep brain stimulation devices in vertical vs. horizontal MRI systems

The majority of studies that assess magnetic resonance imaging (MRI) induced radiofrequency (RF) heating of the tissue when active electronic implants are present have been performed in horizontal, closed-bore MRI systems. Vertical, open-bore MRI systems have a 90° rotated magnet and a fundamentally different RF coil geometry, thus generating a substantially different RF field distribution inside the body. Little is known about the RF heating of elongated implants such as deep brain stimulation (DBS) devices in this class of scanners. Here, we conducted the first large-scale experimental study investigating whether RF heating was significantly different in a 1.2 T vertical field MRI scanner (Oasis, Fujifilm Healthcare) compared to a 1.5 T horizontal field MRI scanner (Aera, Siemens Healthineers). A commercial DBS device mimicking 30 realistic patient-derived lead trajectories extracted from postoperative computed tomography images of patients who underwent DBS surgery at our institution was implanted in a multi-material, anthropomorphic phantom. RF heating around the DBS lead was measured during four minutes of high-SAR RF exposure. Additionally, we performed electromagnetic simulations with leads of various internal structures to examine this effect on RF heating. When controlling for RMS B1+, the temperature increase around the DBS lead-tip was significantly lower in the vertical scanner compared to the horizontal scanner (0.33 ± 0.24°C vs. 4.19 ± 2.29°C). Electromagnetic simulations demonstrated up to a 17-fold reduction in the maximum of 0.1g-averaged SAR in the tissue surrounding the lead-tip in the vertical scanner compared to the horizontal scanner. Results were consistent across leads with straight and helical internal wires. Radiofrequency heating and power deposition around the DBS lead-tip were substantially lower in the 1.2 T vertical scanner compared to the 1.5 T horizontal scanner. Simulations with different lead structures suggest that the results may extend to leads from other manufacturers.

A workflow for predicting temperature increase at the electrical contacts of deep brain stimulation electrodes undergoing MRI

Purpose

The purpose of this study is to present a workflow for predicting the radiofrequency (RF) heating around the contacts of a deep brain stimulation (DBS) lead during an MRI scan.

Methods

The induced RF current on the DBS lead accumulates electric charge on the metallic contacts, which may cause a high local specific absorption rate (SAR), and therefore, heating. The accumulated charge was modeled by imposing a voltage boundary condition on the contacts in a quasi‐static electromagnetic (EM) simulation allowing thermal simulations to be performed with the resulting SAR distributions. Estimating SAR and temperature increases from a lead in vivo through EM simulation is not practical given anatomic differences and variations in lead geometry. To overcome this limitation, a new parameter, transimpedance, was defined to characterize a given lead. By combining the transimpedance, which can be measured in a single calibration scan, along with MR‐based current measurements of the lead in a unique orientation and anatomy, local heating can be estimated. Heating determined with this approach was compared with results from heating studies of a commercial DBS electrode in a gel phantom with different lead configurations to validate the proposed method.

Results

Using data from a single calibration experiment, the transimpedance of a commercial DBS electrode (directional lead, Infinity DBS system, Abbott Laboratories, Chicago, IL) was determined to be 88 Ω. Heating predictions using the DBS transimpedance and rapidly acquired MR‐based current measurements in 26 different lead configurations resulted in a <23% (on average 11.3%) normalized root‐mean‐square error compared to experimental heating measurements during RF scans.

Conclusion

In this study, a workflow consisting of an MR‐based current measurement on the DBS lead and simple quasi‐static EM/thermal simulations to predict the temperature increase around a DBS electrode undergoing an MRI scan is proposed and validated using a commercial DBS electrode.

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A comparative study of MRI-induced RF heating in pediatric and adult populations with epicardial and endocardial implantable electronic devices

Patients with congenital heart defects, inherited arrhythmia syndromes, and congenital disorders of cardiac conduction often receive a cardiac implantable electronic device (CIED). At least 75% of patients with CIEDs will need magnetic resonance imaging (MRI) during their lifetime. In 2011, the US Food and Drug Administration approved the first MR-conditional CIEDs for patients with endocardial systems, in which leads are passed through the vein and affixed to the endocardium. The majority of children, however, receive an epicardial CIED, where leads are directly sewn to the epicardium. Unfortunately, an epicardial CIED is a relative contraindication to MRI due to the unknown risk of RF heating. In this work, we performed anthropomorphic phantom experiments to investigate differences in RF heating between endocardial and epicardial leads in both pediatric and adult-sized phantoms, where adult endocardial CIED was the control. Clinical Relevance-This work provides a quantitative comparison of MRI RF heating of epicardial and endocardial leads in pediatric and adult populations.

The Position and Orientation of the Pulse Generator Affects MRI RF Heating of Epicardial Leads in Children

Infants and children with congenital heart defects often receive a cardiac implantable electronic device (CIED). Because transvenous access to the heart is difficult in patients with small veins, the majority of young children receive epicardial CIEDs. Unfortunately, however, once an epicardial CIED is placed, patients are no longer eligible to receive magnetic resonance imaging (MRI) exams due to the unknown risk of MRI-induced radiofrequency (RF) heating of the device. Although many studies have assessed the role of device configuration in RF heating of endocardial CIEDs in adults, such case for epicardial devices in pediatric patients is relatively unexplored. In this study, we evaluated the variation in RF heating of an epicardial lead due to changes in the lateral position and orientation of the implantable pulse generator (IPG). We found that changing the orientation and position of the IPG resulted in a five-fold variation in the RF heating at the lead's tip. Maximum heating was observed when the IPG was moved to a left lateral abdominal position of patient, and minimum heating was observed when the IPG was positioned directly under the heart. Clinical Relevance- This study examines the role of device configuration on MRI-induced RF heating of an epicardial CIED in a pediatric phantom. Results could help pediatric cardiac surgeons to modify device implantation to reduce future risks of MRI in patients.

Intraoperative MR Imaging during Glioma Resection

One of the major issues in the surgical treatment of gliomas is the concern about maximizing the extent of resection while minimizing neurological impairment. Thus, surgical planning by carefully observing the relationship between the glioma infiltration area and eloquent area of the connecting fibers is crucial. Neurosurgeons usually detect an eloquent area by functional MRI and identify a connecting fiber by diffusion tensor imaging. However, during surgery, the accuracy of neuronavigation can be decreased due to brain shift, but the positional information may be updated by intraoperative MRI and the next steps can be planned accordingly. In addition, various intraoperative modalities may be used to guide surgery, including neurophysiological monitoring that provides real-time information (e.g., awake surgery, motor-evoked potentials, and sensory evoked potential); photodynamic diagnosis, which can identify high-grade glioma cells; and other imaging techniques that provide anatomical information during the surgery. In this review, we present the historical and current context of the intraoperative MRI and some related approaches for an audience active in the technical, clinical, and research areas of radiology, as well as mention important aspects regarding safety and types of devices.

Machine learning-based prediction of MRI-induced power absorption in the tissue in patients with simplified deep brain stimulation lead models

Interaction of an active electronic implant such as a deep brain stimulation (DBS) system and MRI RF fields can induce excessive tissue heating, limiting MRI accessibility. Efforts to quantify RF heating mostly rely on electromagnetic (EM) simulations to assess individualized specific absorption rate (SAR), but such simulations require extensive computational resources. Here, we investigate if a predictive model using machine learning (ML) can predict the local SAR in the tissue around tips of implanted leads from the distribution of the tangential component of the MRI incident electric field, E_tan. A dataset of 260 unique patient-derived and artificial DBS lead trajectories was constructed, and the 1 g-averaged SAR, 1gSAR_max, at the lead-tip during 1.5 T MRI was determined by EM simulations. E_tan values along each lead's trajectory and the simulated SAR values were used to train and test the ML algorithm. The resulting predictions of the ML algorithm indicated that the distribution of E_tan could effectively predict 1gSAR_max at the DBS lead-tip (R = 0.82). Our results indicate that ML has the potential to provide a fast method for predicting MR-induced power absorption in the tissue around tips of implanted leads such as those in active electronic medical devices.