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.

Safety of MRI in patients with retained cardiac leads.

PURPOSE: To evaluate the safety of MRI in patients with fragmented retained leads (FRLs) through numerical simulation and phantom experiments. METHODS: Electromagnetic and thermal simulations were performed to determine the worst-case RF heating of 10 patient-derived FRL models during MRI at 1.5 T and 3 T and at imaging landmarks corresponding to head, chest, and abdomen. RF heating measurements were performed in phantoms implanted with reconstructed FRL models that produced highest heating in numerical simulations. The potential for unintended tissue stimulation was assessed through a conservative estimation of the electric field induced in the tissue due to gradient-induced voltages developed along the length of FRLs. RESULTS: In simulations under conservative approach, RF exposure at B1+ ≤ 2 µT generated cumulative equivalent minutes (CEM)43 < 40 at all imaging landmarks at both 1.5 T and 3 T, indicating no thermal damage for acquisition times (TAs) < 10 min. In experiments, the maximum temperature rise when FRLs were positioned at the location of maximum electric field exposure was measured to be 2.4°C at 3 T and 2.1°C at 1.5 T. Electric fields induced in the tissue due to gradient-induced voltages remained below the threshold for cardiac tissue stimulation in all cases. CONCLUSIONS: Simulation and experimental results indicate that patients with FRLs can be scanned safely at both 1.5 T and 3 T with most clinical pulse sequences.

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, Etan. A dataset of 260 unique patient-derived and artificial DBS lead trajectories was constructed, and the 1 g-averaged SAR, 1gSARmax, at the lead-tip during 1.5 T MRI was determined by EM simulations. Etan 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 Etan could effectively predict 1gSARmax 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.

Predicting RF Heating of Conductive Leads During Magnetic Resonance Imaging at 1.5 T: A Machine Learning Approach.

The number of patients with active implantable medical devices continues to rise in the United States and around the world. It is estimated that 50-75% of patients with conductive implants will need magnetic resonance imaging (MRI) in their lifetime. A major risk of performing MRI in patients with elongated conductive implants is the radiofrequency (RF) heating of the tissue surrounding the implant's tip due to the antenna effect. Currently, applying full-wave electromagnetic simulation is the standard way to predict the interaction of MRI RF fields with the human body in the presence of conductive implants; however, these simulations are notoriously extensive in terms of memory requirement and computational time. Here we present a proof-of-concept simulation study to demonstrate the feasibility of applying machine learning to predict MRI-induced power deposition in the tissue surrounding conductive wires. We generated 600 clinically relevant trajectories of leads as observed in patients with cardiac conductive implants and trained a feedforward neural network to predict the 1g-averaged SAR at the lead tips knowing only the background field of MRI RF coil and coordinates of points along the lead trajectory. Training of the network was completed in 11.54 seconds and predictions were made within a second with R2 = 0.87 and Root Mean Squared Error (RMSE) = 14.5 W/kg. Our results suggest that machine learning could provide a promising approach for safety assessment of MRI in patients with conductive leads.Clinical Relevance- Machine learning can potentially allow real-time assessment of MRI RF safety in patients with conductive leads when only the knowledge of lead's trajectory (image-based) and MRI RF coil features (vendor-specific) are in hand.

On the accuracy of Tier 4 simulations to predict RF heating of wire implants during magnetic resonance imaging at 1.5 T.

Magnetic Resonance Imaging (MRI) access remains conditional to patients with conductive medical implants, as RF heating generated around the implant during scanning may cause tissue burns. Experiments have been traditionally used to assess this heating, but they are time-consuming and expensive, and in many cases cannot faithfully replicate the in-vivo scenario. Alternatively, ISO TS 10974 outlines a four-tier RF heating assessment approach based on a combination of experiments and full-wave electromagnetic (EM) simulations with varying degrees of complexity. From these, Tier 4 approach relies entirely on EM simulations. There are, however, very few studies validating such numerical models against direct thermal measurements. In this work, we evaluated the agreement between simulated and measured RF heating around wire implants during RF exposure at 63.6 MHz (proton imaging at 1.5 T). Heating was assessed around wire implants with 25 unique trajectories within an ASTM phantom. The root mean square percentage error (RMSPE) of simulated vs. measured RF heating remained <1.6% despite the wide range of observed heating (0.2 °C-53 °C). Our results suggest that good agreement can be achieved between experiments and simulations as long as important experimental features such as characteristics of the MRI RF coil, implant's geometry, position, and trajectory, as well as electric and thermal properties of gel are closely mimicked in simulations.Clinical Relevance- This work validates the application of full-wave EM simulations for modeling and predicting RF heating of conductive wires in an MRI environment, providing researchers with a validated tool to assess MRI safety in patients with implants.

Radiofrequency heating of retained cardiac leads during magnetic resonance imaging at 1.5 T and 3 T.

Patients with cardiovascular implantable electronic devices (CIEDs) are often prevented from receiving magnetic resonance imaging (MRI) due to risks associated with radiofrequency (RF) heating of tissue around the implanted leads. Although MR-conditional CIEDs are available, the safety labeling of such devices does not extend to patients with fragmented retained leads (FRLs), where segments of the leads are left in the tissue after the original device is extracted. Unlike intact and isolated leads of CIEDs, FRLs are often bare conductive lead fragments in direct contact with the tissue. No experimental work has been reported that assess RF heating of FRL during MRI thus far. In this work, we performed phantom experiments to measure RF heating of 4 patient-derived FRL models in a gel-based ASTM-like phantom during RF exposure at 64 MHz (proton imaging at 1.5 T) and 123 MHz (proton imaging at 3 T). We found FRL models to generate negligible temperature rise in the gel (∆T<1.84 °C) during a 10-minute scan at both 1.5 T and 3 T. These results are in agreement with previous simulation studies and suggest MRI may be performed safely in patients with fragmented retained leads.

Origin of Free Energy Barriers of Decarboxylation and the Reverse Process of CO2 Capture in Dimethylformamide and in Water.

In aqueous solution, biological decarboxylation reactions proceed irreversibly to completion, whereas the reverse carboxylation processes are typically powered by the hydrolysis of ATP. The exchange of the carboxylate of ring-substituted arylacetates with isotope-labeled CO2 in polar aprotic solvents reported recently suggests a dramatic change in the partition of reaction pathways. Yet, there is little experimental data pertinent to the kinetic barriers for protonation and thermodynamic data on CO2 capture by the carbanions of decarboxylation reactions. Employing a combined quantum mechanical and molecular mechanical simulation approach, we investigated the decarboxylation reactions of a series of organic carboxylate compounds in aqueous and in dimethylformamide solutions, revealing that the reverse carboxylation barriers in solution are fully induced by solvent effects. A linear Bell-Evans-Polanyi relationship was found between the rates of decarboxylation and the Gibbs energies of reaction, indicating diminishing recombination barriers in DMF. In contrast, protonation of the carbanions by the DMF solvent has large free energy barriers, rendering the competing exchange of isotope-labeled CO2 reversible in DMF. The finding of an intricate interplay of carbanion stability and solute-solvent interaction in decarboxylation and carboxylation could be useful to designing novel materials for CO2 capture.

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.

Effect of Biophysical Model Complexity on Predictions of Volume of Tissue Activated (VTA) during Deep Brain Stimulation.

Deep brain stimulation (DBS) has evolved to an important treatment for several drug-resistant neurological and psychiatric disorders, such as epilepsy, Parkinson's disease, essential tremor and dystonia. Despite general effectiveness of DBS, however, its mechanisms of action are not completely understood. Simulations are commonly used to predict the volume of tissue activated (VTA) around DBS electrodes, which in turn helps interpreting clinical outcomes and understand therapeutic mechanisms. Computational models are commonly used to visualize the extend of volume of activated tissue (VTA) for different stimulation schemes, which in turn helps interpreting and understanding the outcomes. The degree of model complexity, however, can affect the predicted VTA. In this work we investigate the effect of volume conductor model complexity on the predicted VTA, when the VTA is estimated from activation function field metrics. Our results can help clinicians to decide what level of model complexity is suitable for their specific need.

Device Configuration and Patient's Body Composition Significantly Affect RF Heating of Deep Brain Stimulation Implants During MRI: An Experimental Study at 1.5T and 3T.

Patients with deep brain stimulation (DBS) devices have limited access to magnetic resonance imaging (MRI) due to safety concerns associated with RF heating generated around the implant. The problem of predicting RF heating of conductive leads is complex with a large parameter space and several interplaying factors. Recently however, off-label use of MRI in patients with DBS devices has been reported based on limited safety assessments, raising the concern that potentially dangerous scenarios may have been overlooked. In this work, we present results of a systematic assessment of RF heating of a commercial DBS device during MRI at 1.5T and 3T, taking into account the effect of device configuration, imaging landmark, and patient's body composition. Ninety-six (96) RF heating measurements were performed using anthropomorphic phantoms implanted with a full DBS system. We evaluated eight clinically relevant device configurations, implanted in phantoms with different material compositions, and imaged at three different landmarks (head, shoulder, and lower chest) in 1.5 T and 3T scanners. We observed a substantial fluctuation in the RF heating depending on phantom's composition and device configuration. RF heating in the brain-mimicking gel varied from 0.1°C to 12°C during 1.5 T MRI and from <0.1°C to 4.5°C during 3T MRI. We also observed that certain device configurations consistently reduced RF heating across different phantom compositions, imaging landmarks, and MRI transmit frequencies.

Evaluating Accuracy of Numerical Simulations in Predicting Heating of Wire Implants During MRI at 1.5 T.

Patients with long conductive implants such as deep brain stimulation (DBS) leads are often denied access to magnetic resonance imaging (MRI) exams due to safety concerns associated with radiofrequency (RF) heating of implants. Experimental temperature measurements in tissue-mimicking gel phantoms under MRI RF exposure conditions are common practices to predict in-vivo heating in the tissue surrounding wire implants. Such experiments are both expensive-as they require access to MRI units-and time-consuming due to complex implant setups. Recently, full-wave numerical simulations, which include realistic MRI RF coil models and human phantoms, are suggested as an alternative to experiments. There is however, little literature available on the accuracy of such numerical models against direct thermal measurements. This study aimed to evaluate the agreement between simulations and measurements of temperature rise at the tips of wire implants exposed to RF exposure at 64 MHz (1.5 T) for different implant trajectories typically encountered in patients with DBS leads. Heating was assessed in seven patient-derived lead configurations using both simulations and RF heating measurements during imaging of an anthropomorphic head phantom with implanted wires. We found substantial variation in RF heating as a function of lead trajectory; there was a 9.5-fold and 9-fold increase in temperature rise from ID1 to ID7 during simulations and experimental measurements, respectively. There was a strong correlation (r2 = 0.74) between simulated and measured temperatures for different lead trajectories. The maximum difference between simulated and measured temperature was 0.26 °C with simulations overestimating the temperature rise.

The effect of simulation strategies on prediction of power deposition in the tissue around electronic implants during magnetic resonance imaging.

Numerical simulations are increasingly employed in safety assessment of high-field magnetic resonance imaging (MRI) in patients with conductive medical implants such as those with deep brain stimulation (DBS) devices. Performing numerical simulations with realistic patient models and implant geometry is the preferred method as it provides the most accurate results; however, in many cases such an approach is infeasible due to limitation of computational resources. The difficulties in reconstructing realistic patient and device models and obtaining accurate electrical properties of tissue have compelled researchers to adopt compromises, either to exceedingly simplify implant structure and geometry, or the complexity of the body model. This study examines the effect of variations in anatomical details of the human body model and implant geometry on predicted values of specific absorption rate (SAR) values during MRI in a patient with a DBS implant. We used a patient-derived model of a fully implanted DBS implant and performed numerical simulations to calculate the maximum SAR during MRI at 1.5T (64 MHz) and 3T (127 MHz). We then assessed the effect of uncertainties in dielectric properties of tissue, complexity of body model, truncation of body/DBS model, and DBS lead geometry on SAR. Our results showed that 40% variation in the conductivity of individual tissues in a heterogeneous body model caused a peak of 7% variation in maximum SAR value at 64 MHz, and 10.6% variation in SAR at 127 MHz. SAR predictions from a homogeneous body model with a conductivity range of [Formula: see text] could cover the full range of SAR variations predicted by the heterogeneous body model. Truncation of body model below the implanted pulse generator changed the predicted SAR by 16% at 1.5T and 32% at 3T while saving 250% and 148% in computational time and memory allocation, respectively. In contrast, variation in DBS lead geometry significantly changed the SAR by up to 51% at 64 MHz and 67% at 127 MHz. These results suggest that the error introduced by simplifying the implant's geometry could negate the benefit of using a realistic body model, should such model be used at the expense of oversimplifying implant geometry.

The Effect of Cross-Linking with Citric Acid on the Properties of Agar/Fish Gelatin Films.

The aim of this work was to assess the effect of fish gelatin-citric acid nucleophilic substitution and agar-citric acid esterification reactions on the properties of agar/fish gelatin films. Since temperature is an important cross-linking parameter, films were treated at 90 °C and 105 °C and film properties were compared to those of non-cured films. It was observed that temperature favored the aforementioned reactions, which induced physical and morphological changes. In this regard, darker films with a rougher surface were obtained for the films with a higher cross-linking degree. While mechanical properties were slightly modified, the barrier properties were enhanced due to the reactions that occurred. Therefore, these agar/fish gelatin films cross-linked through two different reactions can be considered to be promising materials as active films for different purposes, such as active packaging or pharmaceutical applications.

Mixed iota and kappa carrageenan gels in the presence of both calcium and potassium ions.

The effect was studied of adding both KCl and CaCl2 on gelation of solutions of ι-carrageenan, κ-carrageenan and mixtures of both types. The gel temperature (Tg) of ι-car was found to be determined by the CaCl2 concentration and Tg of κ-car by the KCl concentration. At a given salt concentration, ι-car was stiffest with pure CaCl2, but κ-car gels and mixed carrageenan gels were stiffer when both KCl and CaCl2 were present. Gelation of κ-car increased the turbidity of mixed carrageenan gels in the presence of KCl or CaCl2, but when both salts were present it led to a drop of the turbidity. In mixed salt, K+ induces formation of a homogeneous κ-car network that causes the mixed network to become more homogeneous. Rheological and structural properties of carrageenan gels can be tuned for a given polymer and salt concentration by adding both KCl and CaCl2 to κ-car/ι-car mixtures.

Inhibition and Promotion of Heat-Induced Gelation of Whey Proteins in the Presence of Calcium by Addition of Sodium Caseinate.

Heat-induced aggregation and gelation of aqueous solutions of whey protein isolate (WPI) in the presence of sodium caseinate (SC) and CaCl2 was studied at pH 6.6. The effect of adding SC (0-100 g/L) on the structure of the aggregates and the gels was investigated by light scattering and confocal laser scanning microscopy at different CaCl2 concentration ([CaCl2] = 0-30 mM). The gelation process was studied by oscillatory shear rheology. At the whey protein concentrations studied here (34 and 60 g/L), no gels were formed in the absence of CaCl2 and SC. However, WPI solutions gelled above a critical CaCl2 concentration that increased with increasing SC concentration. In the absence of CaCl2, WPI gels were formed only above a critical SC concentration. The critical SC concentration needed to induce WPI gelation decreased weakly when CaCl2 was added. In an intermediate range of CaCl2 concentrations, gels were formed both at low and high SC concentrations, but not at intermediate SC concentrations. Finally, at high CaCl2 concentrations gels were formed at all SC concentrations. The gelation rate and the gel structure of the gels formed at low and high casein concentrations were very different. The effect of SC on the thermal gelation of WPI was interpreted by competition for Ca2+, a chaperon effect, and microphase separation.

pH-responsive water-in-water Pickering emulsions.

The structure and stability of water-in-water emulsions was investigated in the presence of spherical, pH-sensitive microgels. The emulsions were formed by mixing aqueous solutions of dextran and PEO. The microgels consisted of cross-linked, synthetic polymers with a radius that steeply increased from 60 to 220 nm with increasing pH within a narrow range around 7.0. At all pH values between 5.0 and 7.5, the microgels were preferentially situated at the interface, but only in a narrow range between pH 7.0 and 7.5, the emulsions were stable for at least 1 week. The droplet size was visualized with confocal laser scanning microscopy and was found to be smallest in the stable pH range. Emulsions could be stabilized or destabilized by small changes of the pH. Addition of small amounts of salt led to a shift of the pH range where the emulsions were stable. The effects of varying the microgel concentration and the polymer composition were investigated.

The effect of protein aggregate morphology on phase separation in mixtures with polysaccharides.

The morphology of aggregates formed by heating the globular protein ß-lactoglobulin (ß-lg) changes with the addition of a small amount of CaCl2, from small strands to larger spherical aggregates (microgels). We investigated the effect of this morphological transition on the structure of mixtures of ß-lg aggregates with the polysaccharide κ-carrageenan (κ-car), using confocal laser scanning microscopy and dynamic light scattering. The change in the morphology of the ß-lg aggregates strongly reduced the κ-car concentration at which the system phase separated. As a consequence a dramatic change in the structure of the mixtures occurred over a narrow range of the CaCl2 concentration. Phase separation leads to the formation of micron-sized protein rich domains that have a tendency to stick together in large flocs. There is a big difference between the protein concentrations in the two phases, but the κ-car concentration is only weakly lower in the protein rich phase. A comparison is made between mixtures prepared at room temperature, after separately heating ß-lg, and heated mixtures of native ß-lg and κ-car. The micro-phase separated structure of the two systems is similar, but the aggregates disperse upon dilution in the former case, while they are covalently bound within the domains in the latter case. Other, more subtle, differences were also observed. The results explain the very high sensitivity of the structure of ß-lg/κ-car mixtures to calcium ions.

Synergistic effects of mixed salt on the gelation of κ-carrageenan.

The effect of the addition of calcium or sodium ions on the potassium induced gelation of κ-carrageenan (κ-car) is investigated using oscillatory shear rheology and turbidimetry. Both the gelation kinetics and the steady state shear moduli are investigated. Gelation in mixed salt solutions is compared with that in pure potassium and calcium solutions. It is shown that the elastic shear modulus increases with increasing pure KCl concentration, but decreases with increasing pure CaCl2 concentration. In mixed salts, gelation of κ-car is induced by potassium and addition of CaCl2 leads to an increase of the elastic modulus with increasing CaCl2 concentration. κ-Car gelled at low mixed salt concentrations for which it remained liquid in pure salt. At equivalent ionic strengths, the effect of adding NaCl on potassium induced gelation is much weaker. In pure KCl solutions, κ-car gels are transparent, but in pure CaCl2 they become increasingly turbid with increasing CaCl2 concentration. The turbidity of gels formed in mixed salts is intermediate.

Stabilization of water-in-water emulsions by addition of protein particles.

The effect of the addition of protein particles was investigated on the stability of water-in-water emulsions formed by mixing aqueous dextran and poly (ethylene oxide) solutions. Protein particles with hydrodynamic radii ranging from 15 to 320 nm were produced by heating globular proteins in controlled conditions. The structure of the emulsions was visualized with confocal laser scanning microscopy using different fluorescent probes to label the dextran phase and the protein particles. It is shown that contrary to native proteins, protein particles adsorb at the interface and can form a monolayer that inhibits fusion of emulsion droplets. In this way, water-in-water emulsions could be stabilized for a period of weeks. The effect of the polymer composition and the protein particle size and concentration was investigated.