Brain-implanted conductors amplify radiofrequency fields in rodents: Advantages and risks.

Over the past few decades, daily exposure to radiofrequency (RF) fields has been increasing due to the rapid development of wireless and medical imaging technologies. Under extreme circumstances, exposure to very strong RF energy can lead to heating of body tissue, even resulting in tissue injury. The presence of implanted devices, moreover, can amplify RF effects on surrounding tissue. Therefore, it is important to understand the interactions of RF fields with tissue in the presence of implants, in order to establish appropriate wireless safety protocols, and also to extend the benefits of medical imaging to increasing numbers of people with implanted medical devices. This study explored the neurological effects of RF exposure in rodents implanted with neuronal recording electrodes. We exposed freely moving and anesthetized rats and mice to 950 MHz RF energy while monitoring their brain activity, temperature, and behavior. We found that RF exposure could induce fast onset firing of single neurons without heat injury. In addition, brain implants enhanced the effect of RF stimulation resulting in reversible behavioral changes. Using an optical temperature measurement system, we found greater than tenfold increase in brain temperature in the vicinity of the implant. On the one hand, our results underline the importance of careful safety assessment for brain-implanted devices, but on the other hand, we also show that metal implants may be used for neurostimulation if brain temperature can be kept within safe limits.

Towards an integrated radiofrequency safety concept for implant carriers in MRI based on sensor-equipped implants and parallel transmission.

To protect implant carriers in MRI from excessive radiofrequency (RF) heating it has previously been suggested to assess that hazard via sensors on the implant. Other work recommended parallel transmission (pTx) to actively mitigate implant-related heating. Here, both ideas are integrated into one comprehensive safety concept where native pTx safety (without implant) is ensured by state-of-the-art field simulations and the implant-specific hazard is quantified in situ using physical sensors. The concept is demonstrated by electromagnetic simulations performed on a human voxel model with a simplified spinal-cord implant in an eight-channel pTx body coil at 3 T . To integrate implant and native safety, the sensor signal must be calibrated in terms of an established safety metric (e.g., specific absorption rate [SAR]). Virtual experiments show that E -field and implant-current sensors are well suited for this purpose, while temperature sensors require some caution, and B 1 probes are inadequate. Based on an implant sensor matrix Q s , constructed in situ from sensor readings, and precomputed native SAR limits, a vector space of safe RF excitations is determined where both global (native) and local (implant-related) safety requirements are satisfied. Within this safe-excitation subspace, the solution with the best image quality in terms of B 1 + magnitude and homogeneity is then found by a straightforward optimization algorithm. In the investigated example, the optimized pTx shim provides a 3-fold higher mean B 1 + magnitude compared with circularly polarized excitation for a maximum implant-related temperature increase ∆ T imp ≤ 1 K . To date, sensor-equipped implants interfaced to a pTx scanner exist as demonstrator items in research labs, but commercial devices are not yet within sight. This paper aims to demonstrate the significant benefits of such an approach and how this could impact implant-related RF safety in MRI. Today, the responsibility for safe implant scanning lies with the implant manufacturer and the MRI operator; within the sensor concept, the MRI manufacturer would assume much of the operator's current responsibility.

Comparison of SAR distribution of hip and knee implantable devices in 1.5T conventional cylindrical-bore and 1.2T open-bore vertical MRI systems.

PURPOSE: There is increasing use of open-bore vertical MR systems that consist of two planar RF coils. A recent study showed that the RF-induced heating of a neuromodulation device was much lower in the open-bore system at the brain and the chest imaging landmarks. This study focused on the hip and knee implants and compared the specific absorption rate (SAR) distribution in human models in a 1.2T open-bore coil with that of a 1.5T conventional birdcage coil. METHODS: Computational modeling results were compared against the measurement values using a saline phantom. The differences in RF exposure were examined between a 1.2T open-bore coil and a 1.5T conventional birdcage coil using SAR in an anatomical human model. RESULTS: Modeling setups were validated. The body placed closed to the coil elements led to high SAR values in the birdcage system compared with the open-bore system. CONCLUSION: Our computational modeling showed that the 1.2T planar system demonstrated a lower intensity of SAR distribution adjacent to hip and knee implants compared with the 1.5T conventional birdcage system.

Rapid safety assessment and mitigation of radiofrequency induced implant heating using small root mean square sensors and the sensor matrix Qs.

PURPOSE: Rapid detection and mitigation of radiofrequency (RF)-induced implant heating during MRI based on small and low-cost embedded sensors. THEORY AND METHODS: A diode and a thermistor are embedded at the tip of an elongated mock implant. RF-induced voltages or temperature change measured by these root mean square (RMS) sensors are used to construct the sensor Q-Matrix (QS ). Hazard prediction, monitoring and parallel transmit (pTx)-based mitigation using these sensors is demonstrated in benchtop measurements at 300 MHz and within a 3T MRI. RESULTS: QS acquisition and mitigation can be performed in <20 ms demonstrating real-time capability. The acquisitions can be performed using safe low powers (<3 W) due to the high reading precision of the diode (126 µV) and thermistor (26 µK). The orthogonal projection method used for pTx mitigation was able to reduce the induced signals and temperatures in all 155 investigated locations. Using the QS approach in a pTx capable 3T MRI with either a two-channel body coil or an eight-channel head coil, RF-induced heating was successfully assessed, monitored and mitigated while the image quality outside the implant region was preserved. CONCLUSION: Small (<1.5 mm3 ) and low-cost (<1 €) RMS sensors embedded in an implant can provide all relevant information to predict, monitor and mitigate RF-induced heating in implants, while preserving image quality. The proposed pTx-based QS approach is independent of simulations or in vitro testing and therefore complements these existing safety assessments.

Radiofrequency-induced heating of broken and abandoned implant leads during magnetic resonance examinations.

PURPOSE: The risks of RF-induced heating of active implantable medical device (AIMD) leads during MR examinations must be well understood and realistically assessed. In this study, we evaluate the potential additional risks of broken and abandoned (cut) leads. METHODS: First, we defined a generic AIMD with a metallic implantable pulse generator (IPG) and a 100-cm long lead containing 1 or 2 wires. Next, we numerically estimated the deposited in vitro lead-tip power for an intact lead, as well as with wire breaks placed at 10 cm intervals. We studied the effect of the break size (wire gap width), as well as the presence of an intact wire parallel to the broken wire, and experimentally validated the numeric results for the configurations with maximum deposited in vitro lead-tip power. Finally, we performed a Tier 3 assessment of the deposited in vivo lead-tip power for the intact and broken lead in 4 high resolution virtual population anatomic models for over 54,000 MR examination scenarios. RESULTS: The enhancement of the deposited lead-tip power for the broken leads, compared to the intact lead, reached 30-fold in isoelectric exposure, and 16-fold in realistic clinical exposures. The presence of a nearby intact wire, or even a nearby broken wire, reduced this enhancement factor to <7-fold over the intact lead. CONCLUSION: Broken and abandoned leads can pose increased risk of RF-induced lead-tip heating to patients undergoing MR examinations. The potential enhancement of deposited in vivo lead-tip power depends on location and type of the wire break, lead design, and clinical routing of the lead, and should be carefully considered when performing risk assessment for MR examinations and MR conditional labeling.

Effect of Device Configuration and Patient's Body Composition on the RF Heating and Nonsusceptibility Artifact of Deep Brain Stimulation Implants During MRI at 1.5T and 3T.

BACKGROUND: Patients with deep brain stimulation (DBS) implants have limited access to MRI due to safety concerns associated with RF-induced heating. Currently, MRI in these patients is allowed in 1.5T horizontal bore scanners utilizing pulse sequences with reduced power. However, the use of 3T MRI in such patients is increasingly reported based on limited safety assessments. Here we present the results of comprehensive RF heating measurements for two commercially available DBS systems during MRI at 1.5T and 3T. PURPOSE: To assess the effect of imaging landmark, DBS lead configuration, and patient's body composition on RF heating of DBS leads during MRI at 1.5T and 3T. STUDY TYPE: Phantom and ex vivo study. POPULATION/SUBJECTS/PHANTOM/SPECIMEN/ANIMAL MODEL: Gel phantoms and cadaver brain. FIELD STRENGTH/SEQUENCE: 1.5T and 3T, T1 -weighted turbo spin echo. ASSESSMENT: RF heating was measured at the tips of DBS leads implanted in brain-mimicking gel. Image artifact was assessed in a cadaver brain implanted with an isolated DBS lead. STATISTICAL TESTS: Descriptive. RESULTS: We observed substantial fluctuation in RF heating, mainly affected by phantom composition and DBS lead configuration, ranging from 0.14°C to 23.73°C at 1.5T, and from 0.10°C to 7.39°C at 3T. The presence of subcutaneous fat substantially altered RF heating at the electrode tips (3.06°C < ∆T < 19.05° C). Introducing concentric loops in the extracranial portion of the lead at the surgical burr hole reduced RF heating by up to 89% at 1.5T and up to 98% at 3T compared to worst-case heating scenarios. DATA CONCLUSION: Device configuration and patient's body composition substantially altered the RF heating of DBS leads during MRI. Interestingly, certain lead trajectories consistently reduced RF heating and image artifact. Level of Evidence 1 Technical Efficacy Stage 1 J. MAGN. RESON. IMAGING 2021;53:599-610.

MRI-Related Heating of Implants and Devices: A Review.

During an MRI scan, the radiofrequency field from the scanner's transmit coil, but also the switched gradient fields, induce currents in any conductive object in the bore. This makes any metallic medical implant an additional risk for an MRI patient, because those currents can heat up the surrounding tissues to dangerous levels. This is one of the reasons why implants are, until today, considered a contraindication for MRI; for example, by scanner manufacturers. Due to the increasing prevalence of medical implants in our aging societies, such general exclusion is no longer acceptable. Also, it should be no longer needed, because of a much-improved safety-assessment methodology, in particular in the field of numerical simulations. The present article reviews existing literature on implant-related heating effects in MRI. Concepts for risk assessment and quantification are presented and also some first attempts towards an active safety management and risk mitigation. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 5.

A combined computational and experimental approach to assess the transfer function of real pacemaker leads for MR radiofrequency-induced heating.

OBJECTIVE: To propose and validate a variation of the classic techniques for the estimation of the transfer function (TF) of a real pacemaker (PM) lead. METHODS: The TF of three commercially available PM leads was measured by combining data from experimental measurements and numerical simulations generated by three sources: a) the experimental local SAR at the tip of the PM lead (single measurement point) exposed to a 64 MHz birdcage body coil; b) the experimental current distribution along the PM lead, obtained by directly injecting a 64 MHz signal inside the lead; c) the electric field (E-field) simulated with a computational model of the 64 MHz birdcage body coil adopted in the experimental measurement performed in a). The effect of the lead trajectory on the estimation of the TF was also estimated. RESULTS: The proposed methodology was validated by comparing the SAR obtained from the PM lead TF with experimental measurements: a maximum difference of 2.2 dB was observed. It was also shown that the estimation of the TF cannot be considered independent with the lead trajectory: a variation of the SAR estimation up to 3.4 dB was observed. CONCLUSION: For the three PM lead tested, the error in the SAR estimation is within the uncertainty level of SAR measurements (± 2 dB). Additionally, the estimation of the TF using the reciprocity principle is influenced by the particular lead trajectory adopted, even if the consequent variability in the SAR estimation is still close to the uncertainty level of SAR measurements.

Accelerating implant RF safety assessment using a low-rank inverse update method.

PURPOSE: Patients who have medical metallic implants, e.g. orthopaedic implants and pacemakers, often cannot undergo an MRI exam. One of the largest risks is tissue heating due to the radio frequency (RF) fields. The RF safety assessment of implants is computationally demanding. This is due to the large dimensions of the transmit coil compared to the very detailed geometry of an implant. METHODS: In this work, we explore a faster computational method for the RF safety assessment of implants that exploits the small geometry. The method requires the RF field without an implant as a basis and calculates the perturbation that the implant induces. The inputs for this method are the incident fields and a library matrix that contains the RF field response of every edge an implant can occupy. Through a low-rank inverse update, using the Sherman-Woodbury-Morrison matrix identity, the EM response of arbitrary implants can be computed within seconds. We compare the solution from full-wave simulations with the results from the presented method, for two implant geometries. RESULTS: From the comparison, we found that the resulting electric and magnetic fields are numerically equivalent (maximum error of 1.35%). However, the computation was between 171 to 2478 times faster than the corresponding GPU accelerated full-wave simulation. CONCLUSIONS: The presented method enables for rapid and efficient evaluation of the RF fields near implants and might enable situation-specific scanning conditions.

Parallel transmission medical implant safety testbed: Real-time mitigation of RF induced tip heating using time-domain E-field sensors.

PURPOSE: To implement a modular, flexible, open-source hardware configuration for parallel transmission (pTx) experiments on medical implant safety and to demonstrate real-time mitigation strategies for radio frequency (RF) induced implant heating based on sensor measurements. METHODS: The hardware comprises a home-built 8-channel pTx system (scalable to 32-channels), wideband power amplifiers and a positioning system with submillimeter precision. The orthogonal projection (OP) method is used to mitigate RF induced tip heating and to maintain sufficient B1+ for imaging. Experiments are performed at 297MHz and inside a clinical 3T MRI using 8-channel pTx RF coils, a guidewire substitute inside a phantom with attached thermistor and time-domain E-field probes. RESULTS: Repeatability and precision are ~3% for E-field measurements including guidewire repositioning, ~3% for temperature slopes and an ~6% root-mean-square deviation between B1+ measurements and simulations. Real-time pTx mitigation with the OP mode reduces the E-fields everywhere within the investigated area with a maximum reduction factor of 26 compared to the circularly polarized mode. Tip heating was measured with ~100 µK resolution and ~14 Hz sampling frequency and showed substantial reduction for the OP vs CP mode. CONCLUSION: The pTx medical implant safety testbed presents a much-needed flexible and modular hardware configuration for the in-vitro assessment of implant safety, covering all field strengths from 0.5-7 T. Sensor based real-time mitigation strategies utilizing pTx and the OP method allow to substantially reduce RF induced implant heating while maintaining sufficient image quality without the need for a priori knowledge based on simulations or in-vitro testing.

Efficient and Reliable Assessment of the Maximum Local Tissue Temperature Increase at the Electrodes of Medical Implants under MRI Exposure.

Standard risk evaluations posed by medical implants during magnetic resonance imaging (MRI) includes (i) the assessment of the total local electromagnetic (EM) power (P) absorbed in the vicinity of the electrodes and (ii) the translation of P into a local in vivo tissue temperature increase ∆T (P2∆T) in animal experiments or simulations. We investigated the implant/tissue modeling requirements and associated uncertainties by applying full-wave EM and linear bioheat solvers to different implant models, incident field conditions, electrode configurations, and tissue models. Results show that the magnitude of the power is predominately determined by the lead, while the power distribution, and the P2∆T conversion, is determined by the electrode and surrounding tissues. P2∆T is strongly dependent on the size of the electrode, tissue type in contact with the electrode, and tissue inhomogeneity (factor of >2 each) but less on the modeling of the lead (<±10%) and incident field distribution along the lead (<±20%). This was confirmed by means of full-wave simulations performed with detailed high-resolution anatomical phantoms exposed to two commonly used MRI clinical scenarios (64 and 128 MHz), resulting in differences of less than 6%. For the determination of P2∆T, only the electrode and surrounding tissues must be modeled in great detail, whereas the lead can be modeled as a computationally efficient simplified structure exposed to a uniform field. The separate assessments of lead and electrode reduce the overall computational effort by several orders of magnitude. The errors introduced by this simplification can be considered by uncertainty terms. Bioelectromagnetics. 2019;40:422-433. © 2019 Bioelectromagnetics Society.

Anatomical Model Uncertainty for RF Safety Evaluation of Metallic Implants Under MRI Exposure.

The Virtual Population (ViP) phantoms have been used in many dosimetry studies, yet, to date, anatomical phantom uncertainty in radiofrequency (RF) research has largely been neglected. The objective of this study is to gain insight, for the first time, regarding the uncertainty in RF-induced fields during magnetic resonance imaging associated with tissue assignment and segmentation quality and consistency in anatomical phantoms by evaluating the differences between two generations of ViP phantoms, ViP1.x and ViP3.0. The RF-induced 10g-average electric (E-) fields, tangential E-fields distribution along active implantable medical devices (AIMD) routings, and estimated AIMD heating were compared for five phantoms that are part of both ViP1.x and ViP3.0. The results demonstrated that differences exceeded 3 dB (-29%, +41%) for local quantities and 1 dB (±12% for field, ±25% for power) for integrated and volume-averaged quantities (e.g., estimated AIMD-heating and 10 g-average E-fields), while the variation across different ViP phantoms of the same generation can exceed 10 dB (-68% and +217% for field, -90% and +900% for power). In conclusion, the anatomical phantom uncertainty associated with tissue assignment and segmentation quality/consistency is larger than previously assumed, i.e., 0.6 dB or ±15% (k = 1) for AIMD heating. Further, multiple phantoms based on different volunteers covering the target population are required for quantitative analysis of dosimetric endpoints, e.g., AIMD heating, which depend on patient anatomy. Phantoms with the highest fidelity in tissue assignment and segmentation should be used, as these ensure the lowest uncertainty and possible underestimation of exposure. To verify that the uncertainty decreases monotonically with improved phantom quality, the evaluation of differences between phantom generations should be repeated for any improvement in segmentation. Bioelectromagnetics. 2019;40:458-471. © 2019 Bioelectromagnetics Society.

Offline impedance measurements for detection and mitigation of dangerous implant interactions: an RF safety prescreen.

PURPOSE: The concept of a "radiofrequency safety prescreen" is investigated, wherein dangerous interactions between radiofrequency fields used in MRI, and conductive implants in patients are detected through impedance changes in the radiofrequency coil. THEORY: The behavior of coupled oscillators is reviewed, and the resulting, observable impedance changes are discussed. METHODS: A birdcage coil is loaded with a static head phantom and a wire phantom with a wire close to its resonant length, the shape, position, and orientation of which can be changed. Interactions are probed with a current sensor and network analyzer. RESULTS: Impedance spectra show dramatic, unmistakable splitting in cases of strong coupling, and strong correlation is observed between induced current and scattering parameters. CONCLUSIONS: The feasibility of a new, low-power prescreening technique has been demonstrated in a simple phantom experiment, which can unambiguously detect resonant interactions between an implanted wire and an imaging coil. A new technique has also been presented which can detect parallel transmit null modes for the wire.

Parallel transmit pulse design for patients with deep brain stimulation implants.

PURPOSE: Specific absorption rate (SAR) amplification around active implantable medical devices during diagnostic MRI procedures poses a potential risk for patient safety. In this study, we present a parallel transmit (pTx) strategy that can be used to safely scan patients with deep brain stimulation (DBS) implants. METHODS: We performed electromagnetic simulations at 3T using a uniform phantom and a multitissue realistic head model with a generic DBS implant. Our strategy is based on using implant-friendly modes, which are defined as the modes of an array that reduce the local SAR around the DBS lead tip. These modes are used in a spokes pulse design algorithm in order to produce highly uniform magnitude least-squares flip angle excitations. RESULTS: Local SAR (1 g) at the lead tip is reduced below 0.1 W/kg compared with 31.2 W/kg, which is obtained by a simple quadrature birdcage excitation without any sort of SAR mitigation. For the multitissue realistic head model, peak 10 g local SAR and global SAR are obtained as 4.52 W/kg and 0.48 W/kg, respectively. A uniform axial flip angle is also obtained (NRMSE <3%). CONCLUSION: Parallel transmit arrays can be used to generate implant-friendly modes and to reduce SAR around DBS implants while constraining peak local SAR and global SAR and maximizing flip angle homogeneity.

Accuracy and safety of freehand vs. end-on fluoroscopic guided drill-hole placement in canine cadaveric thoracic, lumbar and sacral vertebrae.

Objective: To develop and evaluate the safety and accuracy of an open, end-on fluoroscopic guided (EOFG) drill hole position technique in canine cadaveric spinal surgery, in comparison to a traditional free-hand (FH) drilling technique. Study design: Cadaveric comparison study. Animals: Canine cadaveric vertebral columns (n = 4). Methods: Computed tomography (CT) scans were performed for in-silico planning. Ideal implant purchase depth and angulations were determined from previously published data. Plans for end-on fluoroscopic guided drill holes included angled reconstructions in thick slab mode to mimic fluoroscopic images. Following surgical preparation of T8 to S2, holes were drilled by one of two experienced surgeons randomized evenly by operated side, surgeon, and technique. C-arm fluoroscopy was utilized for the end-on technique. CT was repeated after the procedures. Safety was determined categorically using a modified Zdichavsky classification and "optimal" placement was compared between techniques. Continuous data for drill-hole accuracy was calculated as angle and depth deviations from the planned trajectories. Data sets were analyzed at both univariable and multivariable levels with logistic regression analysis. Results: Drill hole safety was categorized as optimal (modified Zdichavsky classification 1) in 51/60 (85%) of drill holes using EOFG and 33/60 (55%) using FH (P < 0.001) techniques. There were no "unsafe" holes (modified Zdichavsky classification 3a). Optimal drill hole placement was significantly associated with the EOFG technique and use of the largest cadaver, and was significantly less likely within the thoracic region. Mean angle and depth deviations were significantly lower with the EOFG technique. Angle deviations were significantly lower for EOFG in the lumbar region, whereas bone purchase deviations were significantly lower for EOFG in both the thoracic and lumbar regions. The mean time taken to drill the hole was significantly longer for the EOFG technique. Conclusion: Optimal drill hole placement was significantly more likely with the EOFG technique and improved the accuracy of bone purchase in the thoracic region. Clinical significance: The EOFG technique shows promise for translation into a clinically setting, potentially improving implant purchase and therefore stabilizing construct strength, whilst potentially reducing the likelihood of neurovascular injury and need for surgical revision.

The Safety of Micro-Implants for the Brain.

Technological advancements in electronics and micromachining now allow the development of discrete wireless brain implantable micro-devices. Applications of such devices include stimulation or sensing and could enable direct placement near regions of interest within the brain without the need for electrode leads or separate battery compartments that are at increased risk of breakage and infection. Clinical use of leadless brain implants is accompanied by novel risks, such as migration of the implant. Additionally, the encapsulation material of the implants plays an important role in mitigating unwanted tissue reactions. These risks have the potential to cause harm or reduce the service of life of the implant. In the present study, we have assessed post-implantation tissue reaction and migration of borosilicate glass-encapsulated micro-implants within the cortex of the brain. Twenty borosilicate glass-encapsulated devices (2 × 3.5 × 20 mm) were implanted into the parenchyma of 10 sheep for 6 months. Radiographs were taken directly post-surgery and at 3 and 6 months. Subsequently, sheep were euthanized, and GFAP and IBA-1 histological analysis was performed. The migration of the implants was tracked by reference to two stainless steel screws placed in the skull. We found no significant difference in fluoroscopy intensity of GFAP and a small difference in IBA-1 between implanted tissue and control. There was no glial scar formation found at the site of the implant's track wall. Furthermore, we observed movement of up to 4.6 mm in a subset of implants in the first 3 months of implantation and no movement in any implant during the 3-6-month period of implantation. Subsequent histological analysis revealed no evidence of a migration track or tissue damage. We conclude that the implantation of this discrete micro-implant within the brain does not present additional risk due to migration.