Computational and experimental investigation of RF-induced heating for multiple orthopedic implants

Progress in Understanding Radiofrequency Heating and Burn Injuries for Safer MR Imaging

RF electromagnetic wave exposure during MRI scans induces heat and occasionally causes burn injuries to patients. Among all the types of physical injuries that have occurred during MRI examinations, RF burn injuries are the most common ones. The number of RF burn injuries increases as the static magnetic field of MRI systems increases because higher RFs lead to higher heating. The commonly believed mechanisms of RF burn injuries are the formation of a conductive loop by the patient's posture or cables, such as an electrocardiogram lead; however, the mechanisms of RF burn injuries that occur at the contact points, such as the bore wall and the elbow, remain unclear. A comprehensive understanding of RF heating is needed to address effective countermeasures against all RF burn injuries for safe MRI examinations. In this review, we summarize the occurrence of RF burn injury cases by categorizing RF burn injuries reported worldwide in recent decades. Safety standards and regulations governing RF heating that occurs during MRI examinations are presented, along with their theoretical and physiological backgrounds. The experimental assessment techniques for RF heating are then reviewed, and the development of numerical simulation techniques is explained. In addition, a comprehensive theoretical interpretation of RF burn injuries is presented. By including the results of recent experimental and numerical simulation studies on RF heating, this review describes the progress achieved in understanding RF heating from the standpoint of MRI burn injury prevention.

MR Conditionality of Abandoned Leads from Active Implantable Medical Devices at 1.5T

During Magnetic Resonance (MR) scans, abandoned leads from active implantable medical devices (AIMDs) can experience excessive heating near the lead-tip, depending on the types of termination applied to the proximal end. The influence of different proximal treatments, i.e., (i) freely exposed in the tissue, (ii) capped with metallic material, and (iii) capped with plastic material on the RF-induced heating are studied. Abandoned leads from a sacral neuromodulation (SNM) system were investigated in this study. The device models, i.e., the transfer functions, for different proximal treatments were developed. These models are then used to assess the in-vivo lead-tip heating inside four virtual human models (FATS, Duke, Ella, and Billie). The RF-induced heating from these abandoned leads with different proximal end treatments are compared with the lead-tip heating of the original AIMD system. The maximum lead-tip heating for abandoned leads using metal cap at the proximal end is lower than that from the original intact AIMD system. Abandoned leads with plastic cap treatment at the proximal end will lead to an average in-vivo temperature that is 3.5 times higher than that from the original intact AIMD system. Therefore, from this study and in terms of the RF-induced heating, the abandoned leads with metallic cap treatment at the proximal end can maintain the MR conditionality of the original AIMD system.Clinical Relevance- The different treatments of proximal end of the abandoned leads from AIMD are studied to ensure that MR Conditional AIMD leads remain MR Conditional when the leads are abandoned in the patients.

Fast Prediction of RF-induced Heating for Sacral Neuromodulation System Exposed to Multi-Channel 2 RF Field at 3T MRI

This paper presents a fast method to predict the radiofrequency (RF) induced heating for Sacral Neuromodulation System (SNM) under multi-channel 2 (MC-2) RF field of 3 Tesla (T) magnetic resonance imaging (MRI) system by using the artificial neural network (ANN). The raw computational model for the SNM was based on the transfer function approach. The MC-2 parallel transmission RF field at 3T MRI exposure was considered for 2 independent channels, which have an exposure space of -15 dB to 15 dB magnitude difference and -180 degrees to 170 degrees phase difference. A total number of 535,680 study cases that cover all possible shimming conditions and the corresponding temperature rises are collected from raw calculation data. The ANN was used as the surrogate model to predict the temperature rises against the incident electromagnetic field distributions. 40320 cases were used for training while the rest data sets were used for testing. The ANN can estimate the temperature rises for each human model in a small exposure sampling space. The testing performance of the ANN has a correlation coefficient higher than 0.99 and the mean absolute error was less than 0.12°C. It is demonstrated that the ANN can be used as an efficient tool for quick temperature rise estimation under MRI 3T shimming.

1.5 vs 3 Tesla Magnetic Resonance Imaging: A Review of Favorite Clinical Applications for Both Field Strengths-Part 1

ABSTRACT

Whole-body magnetic resonance imaging (MRI) systems with a field strength of 3 T have been offered by all leading manufacturers for approximately 2 decades and are increasingly used in clinical diagnostics despite higher costs. Technologically, MRI systems operating at 3 T have reached a high standard in recent years, as well as the 1.5-T devices that have been in use for a longer time. For modern MRI systems with 3 T, more complexity is required, especially for the magnet and the radiofrequency (RF) system (with multichannel transmission). Many clinical applications benefit greatly from the higher field strength due to the higher signal yield (eg, imaging of the brain or extremities), but there are also applications where the disadvantages of 3 T might outweigh the advantages (eg, lung imaging or examinations in the presence of implants). This review describes some technical features of modern 1.5-T and 3-T whole-body MRI systems, and reports on the experience of using both types of devices in different clinical settings, with all sections written by specialist radiologists in the respective fields.This first part of the review includes an overview of the general physicotechnical aspects of both field strengths and elaborates the special conditions of diffusion imaging. Many relevant aspects in the application areas of musculoskeletal imaging, abdominal imaging, and prostate diagnostics are discussed.

Magnetic resonance conditionality of abandoned leads from active implantable medical devices at 1.5 T

PURPOSE

During MR scans, abandoned leads from active implantable medical devices (AIMDs) can experience excessive heating at the lead tip, depending on the type of termination applied to the proximal contacts (proximal end treatment). The influence of different proximal end treatments (ie, [1] freely exposed in the tissue, [2] terminated with metal in contact with the tissue, or [3] capped with plastic, and thereby fully insulated, on the RF-induced lead-tip heating) are studied. A technique to ensure that MR Conditional AIMD leads remain MR Conditional even when abandoned is recommended.

METHODS

Abandoned leads from three MR Conditional AIMDs ([1] a sacral neuromodulation system, [2] a cardiac rhythm management pacemaker system, and [3] a deep brain stimulator system) were investigated in this study. The computational lead models (ie, the transfer functions) for different proximal end treatments were measured and used to assess the in vivo lead-tip heating for four virtual human models (FATS, Duke, Ella, and Billie) and compared with the lead-tip heating of the complete MR Conditional AIMD system.

RESULT

The average and maximum lead-tip heating for abandoned leads proximally capped with metal is always lower than that from the complete AIMD system. Abandoned leads proximally insulated could lead to an average in vivo temperature rise up to 3.5 times higher than that from the complete AIMD system.

CONCLUSION

For the three investigated AIMDs under 1.5T MR scanning, our results indicate that RF-induced lead-tip heating of abandoned leads strongly depends on the proximal lead termination. A metallic cap applied to the proximal termination of the tested leads could significantly reduce the RF-induced lead-tip heating.

Body-loop related MRI radiofrequency-induced heating hazards: Observations, characterizations, and recommendations

PURPOSE

To assess RF-induced heating hazards in 1.5T MR systems caused by body-loop postures.

METHODS

Twelve advanced high-resolution anatomically correct human body models with different body-loop postures are created based on poseable human adult male models. Numerical simulations are performed to assess the radiofrequency (RF)-induced heating of these 12 models at 11 landmarks. A customized phantom is developed to validate the numerical simulations and quantitatively analyze factors affecting the RF-induced heating, eg, the contact area, the loop size, and the loading position. The RF-induced heating inside three differently posed phantoms is measured.

RESULTS

The RF-induced heating from the body-loop postures can be up to 11 times higher than that from the original posture. The RF-induced heating increases with increasing body-loop size and decreasing contact area. The magnetic flux increases when the body-loop center and the RF coil isocenter are close to each other, leading to increased RF-induced heating. An air gap created in the body loop or generating a polarized magnetic field parallel to the body loop can reduce the heating by a factor of three at least. Experimental measurements are provided, validating the correctness of the numerical results.

CONCLUSION

Safe patient posture during MR examinations is recommended with the use of insulation materials to prevent loop formation and consequently avoiding high RF-induced heating. If body loops cannot be avoided, the body loop should be placed outside the RF transmitting coil. In addition, linear polarization with magnetic fields parallel to the body loop can be used to circumvent high RF-induced heating.

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.

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.

Wire-based sternal closure: MRI-related heating at 1.5 T/64 MHz and 3 T/128 MHz based on simulation and experimental phantom study

PURPOSE

The paper investigates factors that affect the RF-induced heating for commonly used wire-based sternal closure under 1.5 T and 3 T MRI systems and clarifies the heating mechanisms.

METHODS

Numerical simulations based on the finite-difference time-domain method and experimental measurements in ASTM (American Society for Testing and Materials) phantom were used in the study. Various configurations of the wire-based sternal closure in the phantom were studied based on parameter sweeps to understand key factors related to the RF-induced heating. In vivo simulations were further performed to explore the RF-induced heating in computational human phantoms for clinically relevant scenarios.

RESULTS

The wire-based sternal closure can lead to peak 1-g averaged spatial absorption ratio of 106.3 W/kg and 75.2 W/kg in phantom and peak 1-g averaged specific absorption rate of 32.1 W/kg and 62.1 W/kg in computational human models near the device at 1.5 T and 3 T, respectively. In phantom, the simulated maximum temperature rises for 15-minute RF exposure are 9.4°C at 1.5 T and 5.8°C at 3 T. Generally, the RF-induced heating will be higher when the electrical length of the device is close to the resonant length or when multiple components are spaced closely along the longitudinal direction.

CONCLUSION

The RF-induced heating related to wire-based sternal closure can be significant due to the antenna effect and capacitive mutual coupling effect related to the specific geometries of devices.