Browsing by Subject "RF heating"

Now showing 1 - 8 of 8

Open Access
Measuring local RF heating in MRI: Simulating perfusion in a perfusionless phantom
(John Wiley & Sons, Inc., 2007) Akca, I. B.; Ferhanoglu, O.; Yeung, C. J.; Guney, S.; Tasci, T. O.; Atalar, Ergin
Purpose: To overcome conflicting methods of local RF heating measurements by proposing a simple technique for predicting in vivo temperature rise by using a gel phantom experiment. Materials and Methods: In vivo temperature measurements are difficult to conduct reproducibly; fluid phantoms introduce convection, and gel phantom lacks perfusion. In the proposed method the local temperature rise is measured in a gel phantom at a timepoint that the phantom temperature would be equal to the perfused body steady-state temperature value. The idea comes from the fact that the steady-state temperature rise in a perfused body is smaller than the steady-state temperature increase in a perfusionless phantom. Therefore, when measuring the temperature on a phantom there will be the timepoint that corresponds to the perfusion time constant of the body part. Results: The proposed method was tested with several phantom and in vivo experiments. Instead, an overall average of 30.8% error can be given as the amount of underestimation with the proposed method. This error is within the variability of in vivo experiments (45%). Conclusion: With the aid of this reliable temperature rise prediction the amount of power delivered by the scanner can be controlled, enabling safe MRI examinations of patients with implants. © 2007 Wiley-Liss, Inc.
Open Access
Modeling of radio-frequency induced currents on lead wires during MR imaging using a modified transmission line method
(Wiley-Blackwell Publishing, 2011-11-23) Acikel, V.; Atalar, Ergin
Purpose: Metallic implants may cause serious tissue heating during magnetic resonance (MR) imaging. This heating occurs due to the induced currents caused by the radio-frequency (RF) field. Much work has been done to date to understand the relationship between the RF field and the induced currents. Most of these studies, however, were based purely on experimental or numerical methods. This study has three main purposes: (1) to define the RF heating properties of an implant lead using two parameters; (2) to develop an analytical formulation that directly explains the relationship between RF fields and induced currents; and (3) to form a basis for analysis of complex cases. Methods: In this study, a lumped element model of the transmission line was modified to model leads of implants inside the body. Using this model, leads are defined using two parameters: impedance per unit length, Z, and effective wavenumber along the lead, k t. These two parameters were obtained by using methods that are similar to the transmission line theory. As long as these parameters are known for a lead, currents induced in the lead can be obtained no matter how complex the lead geometry is. The currents induced in bare wire, lossy wire, and insulated wire were calculated using this new method which is called the modified transmission line method or MoTLiM. First, the calculated induced currents under uniform electric field distribution were solved and compared with method-of-moments (MoM) calculations. In addition, MoTLiM results were compared with those of phantom experiments. For experimental verification, the flip angle distortion due to the induced currents was used. The flip angle distribution around a wire was both measured by using flip angle imaging methods and calculated using current distribution obtained from the MoTLiM. Finally, these results were compared and an error analysis was carried out. Results: Bare perfect electric, bare lossy, and insulated perfect electric conductor wires under uniform and linearly varying electric field exposure were solved, both for 1.5 T and 3 T scanners, using both the MoTLiM and MoM. The results are in agreement within 10 mean-square error. The flip angle distribution that was obtained from experiments was compared along the azimuthal paths with different distances from the wire. The highest mean-square error was 20 among compared cases. Conclusions: A novel method was developed to define the RF heating properties of implant leads with two parameters and analyze the induced currents on implant leads that are exposed to electromagnetic fields in a lossy medium during a magnetic resonance imaging (MRI) scan. Some simple cases are examined to explain the MoTLiM and a basis is formed for the analysis of complex cases. The method presented shows the direct relationship between the incident RF field and the induced currents. In addition, the MoTLiM reveals the RF heating properties of the implant leads in terms of the physical features of the lead and electrical properties of the medium.
Open Access
Modeling RF heating of active implantable medical devices during MRI using safety index
(2007) Irak, Halise
Magnetic Resonance Imaging (MRI) is known as a safe imaging modality that can be hazardous for patients with active implantable medical devices, such as a pacemakers or deep brain stimulators. The primary reason for that is the radio frequency (RF) heating at the tips of the implant leads. In the past, this problem has been analyzed with phantom, animal and human experiments. The amount of temperature rise at the lead tip of these implants, however, has not been theoretically analyzed. In this thesis, a simple approximate formula for the safety index of implants, which is the temperature increase at the implant lead tip per unit deposited power in the tissue without the implant in place, was derived. For that purpose, an analytical quadrature birdcage coil model was developed and the longitudinal incident electric field distribution inside the body was formularized as follows: ER H z () R = −ω µ − in which ω is the angular frequency, µ is the magnetic permeability of the tissue, H- is the left hand rotating component of the RF magnetic field and R is the radial distance from the center of the body. This formula was examined by simulations and phantom experiments. The analytical, simulation and experimental results of that model are in good agreement.Then, depending on the quadrature birdcage coil model safety index (SI) formula for active implants with short leads was derived as shown below: 2 max 2 peak 1 ( ) 2 j t b T SI Rl Ae f Dv SAR c R θ α ∆ == + where ∆Tmax is the maximum temperature increase in the tissue, SARpeak is the maximum deposited power in the body when there is no implant in the body, α is the diffusivity of the tissue, ct isthe heat capacity of the tissue, Rb is radius of the body, R is the radial distance from the center of the body, l is the length of the implant lead, A is the area of the curvature of the lead, θ is the angle that curvature of the implant makes with the radial axis, and f(Dv) is the perfusion correction factor, which is function of the diameter of the electrode and perfusion. The safety index formula was tested by simulations. Simulation results showed that the theoretical safety index formula approximates and identifies the RF heating problem of active implants with short leads accurately. The safety index formula derived in this thesis is valid for only short wires. However, the formulation for long wires is currently under investigation. Despite the fact that the results obtained for short leads can not be generalized for the safety of patients with active implants, it is believed that this study is the first step towards safety of these patients. Using safety index as a measure of safety is very beneficial to ensure the safety of patients with active implants. Because, it uses the MR scanner-estimated deposited power that does not take the existence of the implant in the patient body into account. This formulation is the first study illustrating the advantage of the safety index metric for RF heating studies of active implants.
Open Access
MRI compatible lead designs for implantable medical devices
(2007) Ermeydan, Ahmet
It is currently estimated that 600,000 cardiac pacemakers are implanted per year worldwide. It is expected that the usage of other stimulators such as deep brain stimulators (DBS) will reach this number in a short period. On the other hand, 2,000,000 MRI examinations are carried out each year worldwide and usage of MRI is expected to increase. Unfortunately, people with metallic implants have significant risks in the MRI scanners. It is known that radio frequency and gradient fields of the MRI scanners may induce harmful currents on the implant leads. Radio frequency pulses may cause excessive heating and burns. In addition to this, time-varying gradient magnetic field induced currents on the leads can cause nerve stimulation. In case of cardiac pacemakers, this nerve stimulation may cause cardiac arrest. In this thesis, novel MRI compatible lead designs were proposed. Lead designs are presented to ensure safe magnetic resonance scanning of patients with active metallic implants such as pacemakers, neurostimulators, and implantable cardio defibrillators. Semiconductor components such as transistors and diodes are used to prevent these undesired induced currents on the implant leads. Circuits on the implants are designed such that while the induction of currents is prevented, the desired signal transmission in between the implanted pulse generator and the body part is maintained. The designs were tested by using experiments and computer stimulation. It was seen that the new techniques are effective in making MRI safe implantable devices. Benefits and problems of each design will be discussed in this text. It is believed that using this or similar techniques, patients with the implants will be able to be examined safely in MRI scanners.
Open Access
Patient’s body composition can significantly affect RF power deposition in the tissue around DBS implants: ramifications for lead management strategies and MRI field-shaping techniques
(Institute of Physics Publishing Ltd., 2021-01-13) Bhusal, B.; Keil, B.; Rosenow, J.; Kazemivalipour, Ehsan; Golestanirad, L.
Patients with active implants such as deep brain stimulation (DBS) devices have limited access to magnetic resonance imaging (MRI) due to risks associated with RF heating of implants in MRI environment. With an aging population and increased prevalence of neurodegenerative disease, the indication for MRI exams in patients with such implants increases as well. In response to this growing need, many groups have investigated strategies to mitigate RF heating of DBS implants during MRI. These efforts fall into two main categories: MRI field-shaping methods, where the electric field of the MRI RF coil is modified to reduce the interaction with implanted leads, and lead management techniques where surgical modifications in the trajectory reduces the coupling with RF fields. Studies that characterize these techniques, however, have relied either on simulations with homogenous body models, or experiments with box-shaped single-material phantoms. It is well established, however, that the shape and heterogeneity of human body affects the distribution of RF electric fields, which by proxy, alters the heating of an implant inside the body. In this contribution, we applied numerical simulations and phantom experiments to examine the degree to which variations in patient's body composition affects RF power deposition. We then assessed effectiveness of RF-heating mitigation strategies under variant patient body compositions. Our results demonstrated that patient's body composition substantially alters RF power deposition in the tissue around implanted leads. However, both techniques based on MRI field-shaping and DBS lead management performed well under variant body types.
Open Access
Reduction of implant RF heating through modification of transmit coil electric field
(WILEY, 2010-12-08) Eryaman, Y.; Akin, B.; Atalar, Ergin
In this work, we demonstrate the possibility to modify the electric-field distribution of a radio frequency (RF) coil to generate electric field-free zones in the body without significantly altering the transmit sensitivity. Because implant heating is directly related to the electric-field distribution, implant-friendly RF transmit coils can be obtained by this approach. We propose a linear birdcage transmit coil with a zero electric-field plane as an example of such implant-friendly coils. When the zero electric-field plane coincides with the implant position, implant heating is reduced, as we demonstrated by the phantom experiments. By feeding RF pulses with identical phases and shapes but different amplitudes to the two orthogonal ports of the coil, the position of the zero electric-field plane can also be adjusted. Although implant heating is reduced with this method, a linear birdcage coil results in a whole-volume average specific absorption rate that is twice that of a quadrature birdcage coil. To solve this issue, we propose alternative methods to design implant-friendly RF coils with optimized electromagnetic fields and reduced whole-volume average specific absorption rate. With these methods, the transmit field was modified to reduce RF heating of implants and obtain uniform transmit sensitivity.
Open Access
RF heating of deep brain stimulation implants in open-bore vertical MRI systems: a simulation study with realistic device configurations
(International Society for Magnetic Resonance in Medicine, 2020) Golestanirad, L.; Kazemivalipour, Ehsan; Lampman, D.; Habara, H.; Atalar, Ergin; Rosenow, J.; Pilitsis, J.; Kirsch, J.
Purpose Patients with deep brain stimulation (DBS) implants benefit highly from MRI, however, access to MRI is restricted for these patients because of safety hazards associated with RF heating of the implant. To date, all MRI studies on RF heating of medical implants have been performed in horizontal closed‐bore systems. Vertical MRI scanners have a fundamentally different distribution of electric and magnetic fields and are now available at 1.2T, capable of high‐resolution structural and functional MRI. This work presents the first simulation study of RF heating of DBS implants in high‐field vertical scanners. Methods We performed finite element electromagnetic simulations to calculate specific absorption rate (SAR) at tips of DBS leads during MRI in a commercially available 1.2T vertical coil compared to a 1.5T horizontal scanner. Both isolated leads and fully implanted systems were included. Results We found 10‐ to 30‐fold reduction in SAR implication at tips of isolated DBS leads, and up to 19‐fold SAR reduction at tips of leads in fully implanted systems in vertical coils compared to horizontal birdcage coils. Conclusions If confirmed in larger patient cohorts and verified experimentally, this result can open the door to plethora of structural and functional MRI applications to guide, interpret, and advance DBS therapy.
Open Access
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
(John Wiley & Sons, Inc., 2021-04-07) Kazemivalipour, Ehsan; Bhusal, B.; Vu, J.; Lin, S.; Nguyen, B. T.; Kirsch, J.; Nowac, E.; Pilitsis, J.; Rosenow, J.; Atalar, Ergin; Golestanirad, L.
Purpose Patients with active implants such as deep brain stimulation (DBS) devices are often denied access to MRI due to safety concerns associated with the radiofrequency (RF) heating of their electrodes. The majority of studies on RF heating of conductive implants have been performed in horizontal close-bore MRI scanners. Vertical MRI scanners which have a 90° rotated transmit coil generate fundamentally different electric and magnetic field distributions, yet very little is known about RF heating of implants in this class of scanners. We performed numerical simulations as well as phantom experiments to compare RF heating of DBS implants in a 1.2T vertical scanner (OASIS, Hitachi) compared to a 1.5T horizontal scanner (Aera, Siemens). Methods Simulations were performed on 90 lead models created from post-operative CT images of patients with DBS implants. Experiments were performed with wires and commercial DBS devices implanted in an anthropomorphic phantom. Results We found significant reduction of 0.1 g-averaged specific absorption rate (30-fold, P < 1 × 10−5) and RF heating (9-fold, P < .026) in the 1.2T vertical scanner compared to the 1.5T conventional scanner. Conclusion Vertical MRI scanners appear to generate lower RF heating around DBS leads, providing potentially heightened safety or the flexibility to use sequences with higher power levels than on conventional systems.