[Show abstract][Hide abstract] ABSTRACT: The in-vitro and in-vivo temperature rises during MRI for a sample passive implant in the form of a metal rod of 8 mm diameter and 118 mm length in the right humerus were calculated with FDTD and the heat equation. The temperature rise in a phantom test was calculated for a background medium with electric properties specified in ASTM F2182-09 and for a medium with electrical properties of the cancellous bone that surrounds the implant. Scaled to local background SAR, in-vitro rises are greater for the bone. For a patient in the MRI coil with landmark in the torso and a whole body SAR of 2 W/kg, the calculated temperature rise after six minutes of RF power deposition was 1.3°C for 64 MHz and 2.4°C for 128 MHz. The numerical methods presented here could be extended to determine the temperature rises that would occur in the phantom and in the patient for other implants.
General Assembly and Scientific Symposium, 2011 XXXth URSI; 09/2011
[Show abstract][Hide abstract] ABSTRACT: In this paper, sophisticated eddy-current techniques incorporating model-based inverse methods were successfully demonstrated to measure the thickness and remaining-life of high-temperature coatings. To further assure the performance of these inverse methods, several estimation metrics including Fisher Information, Cramer-Rao Lower Bound (CRLB), covariance, and singular value decomposition (SVD) are introduced. The connections and utility of these metrics are illustrated in the design of eddy current methods for estimating layer thickness, conductivity and probe liftoff.
[Show abstract][Hide abstract] ABSTRACT: Medical implants in the form of linear conductive structures partially insulated along their length are especially prone to induced heating when subjected to the radiofrequency field used during magnetic resonance imaging (MRI). Leads or similar structures are often implanted near the skin and we have analyzed such implants when the implantation depth is varied in steps from 3 mm to 9 mm or more. Current, electric field, and induced temperature rise distributions in tissue have been obtained. The results have been validated by laboratory measurements.
Progress In Electromagnetics Research C 01/2010; 13:195-202. DOI:10.2528/PIERC10041805 · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Magnetic Resonance Imaging (MRI) is a common diagnostic medical procedure. Although it is often safe to perform MRI on an individual that has an orthopaedic device, potential safety issues do exist. Implants fabricated from ferromagnetic materials may experience displacement and torque in the large magnetic field present in MRI units. Under certain circumstances, alternating electrical currents may cause heating of the implant and surrounding tissue irrespective of the magnetic characteristics of the implant. Also, the possibility of substantial image artifacts may compromise image quality. In 2008, the Food and Drug Administration (FDA) released a guidance document entitled, Establishing Safety and Compatibility of Passive Implants in the Magnetic Resonance Environment. This document requires orthopaedic device manufacturers to establish the safety of their devices from a magnetic force, magnetic torque, radio frequency induced heating, and image artifacts perspective. This study will discuss the results of MRI compatibility testing performed on the Zimmer Natural Nail tibial nail system in a 1.5 and 3.0 Tesla MR environment.
Materials and Processes 2009 Conference and Exposition American Society for Metals; 08/2009
[Show abstract][Hide abstract] ABSTRACT: We have investigated the scattering of the magnetic resonance imaging (MRI) RF electromagnetic field by implants for vagus nerve stimulation (VNS) therapy using the finite element method to perform full 3-D realistic simulations. For an implanted VNS lead, we calculated the RF scattered field and the heat equation was solved using an FEM tool to find the temperature rise. Current distributions in the twin-strand lead, specific absorption rate (SAR) and temperature rise distributions are presented.
Antennas and Propagation Society International Symposium, 2008. AP-S 2008. IEEE; 08/2008
[Show abstract][Hide abstract] ABSTRACT: To develop and demonstrate a method to calculate the temperature rise that is induced by the radio frequency (RF) field in MRI at the electrode of an implanted medical lead.
The electric field near the electrode is calculated by integrating the product of the tangential electric field and a transfer function along the length of the lead. The transfer function is numerically calculated with the method of moments. Transfer functions were calculated at 64 MHz for different lengths of model implants in the form of bare wires and insulated wires with 1 cm of wire exposed at one or both ends.
Heating at the electrode depends on the magnitude and the phase distribution of the transfer function and the incident electric field along the length of the lead. For a uniform electric field, the electrode heating is maximized for a lead length of approximately one-half a wavelength when the lead is terminated open. The heating can be greater for a worst-case phase distribution of the incident field.
The transfer function is proposed as an efficient method to calculate MRI-induced heating at an electrode of a medical lead. Measured temperature rises of a model implant in a phantom were in good agreement with the rises predicted by the transfer function. The transfer function could be numerically or experimentally determined.
Journal of Magnetic Resonance Imaging 11/2007; 26(5):1278-85. DOI:10.1002/jmri.21159 · 2.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper discusses the cultivation and growth of one successful engineering service learning partnership and how these types of K-16 relationships may influence interest, motivation, and learning in the fields of engineering, science, mathematics, technology, and education. Recommendations for sustaining long term relationships are offered and the mutually beneficial academic and professional benefits of this type of educational methodology are explored by the authors who serve as joint instructional leaders for the Happy Hollow Elementary School and Engineering Projects in Community Service partnership
Frontiers in Education, 2005. FIE '05. Proceedings 35th Annual Conference; 11/2005
[Show abstract][Hide abstract] ABSTRACT: One of the most crucial magnetic resonance (MR) safety concerns is related to excessive heating of metallic implants by the radio frequency (RF) magnetic field. In this study, heating by the MR imaging (MRI) RF magnetic field of bare and insulated wires was evaluated to model the heating of medical lead wires. Currents induced in the wire were calculated using the method of moments. The electric field in the tissue surrounding the wire was calculated to determine the power deposition. From the power, the temperature rise was calculated using the bioheat equation. For bare wires, the calculated and maximal temperature rise, which is about 28°C, occurred for a length of 20 cm. For lengths exceeding 20 cm, temperature rises for the insulated wires were greater, and the resonance length exceeded 40 cm.
[Show abstract][Hide abstract] ABSTRACT: A patient in MRI is exposed to the time varying gradient and radio-frequency (RF) fields. Measurements of RF-induced temperature rise in implant in phantoms are used to predict the in-vivo temperature rise. This paper focuses on the numerical computation of power deposition inside such a phantom. The finite-difference time domain (FDTD) was used for the calculations. The phantom was placed inside a whole body bird cage coil. For landmarks in the torso of the phantom, the power deposition is concentrated near the edges. The whole phantom average specific absorption rate (SAR) for a mean square field intensity of 1(μT)<sup>2</sup> ranged from 0.032 W/kg for the eyes landmark to 0.151 W/kg for the waist landmark. The SAR increased with increasing conductivity in the range 0.2 to 0.8 S/m. The distribution of the measured temperature rise versus lateral position at the landmark was consistent with the calculation. The electric field is tangential to the phantom edges but has significant elliptical polarization near the corners.
[Show abstract][Hide abstract] ABSTRACT: To evaluate the ability of a lead management device to reduce magnetic resonance imaging (MRI)-related heating of deep brain stimulation (DBS) leads and thereby to decrease the risks of exposing patients with these implants to MRI procedures.
Experiments were performed using the Activa series (Medtronic, Inc., Minneapolis, MN) DBS systems in an in vitro, gelled-saline head and torso phantom. Temperature change was recorded using fluoroptic thermometry during MRI performed using a transmit-and-receive radiofrequency body coil at 1.5 T and a transmit-and-receive radiofrequency head coil at 3 T. A cranial model placed in the phantom was used to test a custom-designed burr hole device that permitted the placement of small-diameter, concentric loops around the burr hole at the DBS lead as it exited the cranium.
A total of 41 scans were performed, with absolute temperature changes ranging from 0.8 to 10.3 degrees C. Depending on the MRI system tested and the side of the phantom on which the hardware was placed, loop placement resulted in reductions in temperature rise of 41 to 74%. The effect was linearly related to the number of loops formed (P < 0.01) over the range tested (0-2.75 loops).
Small, concentric loops placed around the burr hole seem to reduce MRI-related heating for these implants. Although the mechanism is still not fully understood, a device such as that used in the present study could permit a wider range of clinical scanning sequences to be used at 1.5 and 3 T in patients with DBS implants, in addition to increasing the margin of safety for the patient.
[Show abstract][Hide abstract] ABSTRACT: There are three principal magnetic fields in magnetic resonance imaging (MRI) that may interact with medical implants. The static field will induce force and torque on ferromagnetic objects. The pulsed gradients are of audio frequency and the implant may concentrate the induced currents, with a potential for nerve stimulation or electrical inference. The currents induced in the body by the radio frequency (RF) field may also be concentrated by an implant, resulting in potentially dangerous heating of surrounding tissues. This paper presents basic information about MRI interactions with implants with an emphasis on RF-induced heating of leads used for deep brain stimulation (DBS). The temperature rise at the electrodes was measured in vitro as a function of the overall length of a DBS lead at an RF frequency of 64 MHz. The maximal temperature rise occurred for an overall length of 41 cm. The method of moments was used to calculate the current induced in the lead. From the induced currents, the RF power deposition near the electrodes was calculated and the heat equation was used to model the temperature rise. The calculated temperature rises as a function of lead length were in good agreement with the measured values.
IEEE Transactions on Device and Materials Reliability 10/2005; 5(3-5):467 - 480. DOI:10.1109/TDMR.2005.859033 · 1.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The goal of the research reported here is to narrow the range of uncertainty about peripheral nerve stimulation (PNS) thresholds associated with whole body magnetic field exposures at 50/60 Hz. This involved combining PNS thresholds measured in human subjects exposed to pulsed magnetic gradient fields with calculations of electric fields induced in detailed anatomical models of the body by that same exposure system. PNS thresholds at power frequencies (50/60 Hz) can be predicted from these data due to the wide range of pulse durations (70 mus to 1 ms), the length of the pulse trains (several tens of ms), and the exposure of a large part of the body to the magnetic field. These data together with the calculations of the rheobase electric field exceeded in 1% (E(1%)) of two anatomical body models, lead to a median PNS detection threshold of 47.9 +/- 4.4 mT for a uniform 60 Hz magnetic field exposure coronal to the body. The threshold for the most sensitive 1% of the population is about 27.8 mT. These values are lower than PNS thresholds produced by magnetic fields with sagittal and vertical orientations or nonuniform exposures.
[Show abstract][Hide abstract] ABSTRACT: A method and apparatus for detection of broken bars in the rotor of a squirrel cage induction motor are described. The test method is easy to implement, no electrical connections need be made to the rotor, and it is not necessary to turn the rotor. The effect of the broken bar is modeled as an image current in a direction opposite to the current if it were intact and a corresponding returning image current in the other bars. Sense coils encompassing 180 degrees are examined, the inner coil is wound in the slots near the gap and the outer coil is on the outside of the stator. The main signal induced in the sense coils is due to the image currents associated with the broken bar. The signal induced in the coils is greatest when the broken bar is near the edge of the sense coils. A test apparatus with 8 poles and 24 slots was constructed and results of measurements on a rotor with 17 intact and one broken bar are presented, The inner and outer sense coils yield complementary information on the state of the rotor.
[Show abstract][Hide abstract] ABSTRACT: A heterogeneous model of the human body and the scalar potential finite difference method are used to compute electric fields induced in tissue by magnetic field exposures. Two types of coils are considered that simulate exposure to gradient switching fields during magnetic resonance imaging (MRI). These coils producing coronal (y axis) and axial (z axis) magnetic fields have previously been used in experiments with humans. The computed fields can, therefore, be directly compared to human response data. The computed electric fields in subcutaneous fat and skin corresponding to peripheral nerve stimulation (PNS) thresholds in humans in simulated MRI experiments range from 3.8 to 5.8 V/m for the fields exceeded in 0.5% of tissue volume (skin and fat of the torso). The threshold depends on coil type and position along the body, and on the anatomy and resolution of the human body model. The computed values are in agreement with previously established thresholds for neural stimulation.
[Show abstract][Hide abstract] ABSTRACT: Ex vivo testing is necessary to characterize implants to determine if it is safe for the patient to undergo a magnetic resonance imaging (MRI) examination. Therefore, the objective of this study was to evaluate MR safety for an implantable microstimulator in association with a 1.5 Tesla MR system.
A microstimulator (RF BION, Alfred E. Mann Foundation for Scientific Research, Valencia, CA) was evaluated for magnetic field interactions and MRI-related heating. The functional aspects of this implant were assessed immediately before and after exposure to MRI (15 different pulse sequences). Artifacts were also characterized.
Magnetic field interactions exhibited by the microstimulator will not pose a hazard after a suitable postimplantation period has elapsed. Temperature changes will not pose a risk. The function of the microstimulator was unaffected by MRI. Artifacts will only create a problem if the area of interest is in proximity to this implant (largest artifact area: T1-weighted spin echo, 2291 mm2; gradient echo, 3310 mm2).
The overall findings indicated that it is safe for a patient with the microstimulator to undergo MRI at 1.5 Tesla by following specific safety guidelines described herein.