Effect of direct voltage induction by low-frequency security systems on neurostimulator lead.

Low-frequency (LF) security systems, such as antitheft electronic article surveillance (EAS) gates emit strong magnetic fields that could potentially interfere with neurostimulator operation. Some patients reported pain and shocking sensations near EAS gates, even after they turned off their pulse generator. To investigate the direct voltage induction of EAS systems on neurostimulator leads, we evaluated voltages induced by two EAS systems (14 kHz continuous wave or 58 kHz pulsed) on a 40 cm sacral neurostimulator lead formed in a circular loop attached to a pulse generator that was turned off. The lead and neurostimulator were mounted in a saline-filled rectangular phantom placed within electromagnetic fields emitted by EAS systems. The measured voltage waveforms were applied to computational models of spinal nerve axons to predict whether these voltages may evoke action potentials. Additional in vitro testing was performed on the semicircular lead geometry, to study the effect of lead geometry on EAS induced voltages. While standard neurostimulator testing per ISO 14708-3:2017 recommends electromagnetic compatibility testing with LF magnetic fields for induction of malfunctions of the active electronic circuitry while generating intended stimulating pulses, our results show that close to the EAS antenna frames, the induced voltage on the lead could be strong enough to evoke action potentials, even with the pulse generator turned off. This work suggests that patient reports of pain and shocking sensations when near EAS systems could also be correlated with the direct EAS-induced voltage on neurostimulator lead.

Parameters Affecting Worst-Case Gradient-Field Heating of Passive Conductive Implants.

BACKGROUND: Testing MRI gradient-induced heating of implanted medical devices is required by regulatory organizations and others. A gradient heating test of the ISO 10974 Technical Specification (TS) for active implants was adopted for this study of passive hip implants. All but one previous study of hip implants used nonuniform gradient exposure fields in clinical scanners and reported heating of less than 5 °C. This present study adapted methods of the TS, addressing the unmet need for identifying worst-case heating via exposures to uniform gradient fields. PURPOSE: To identify gradient-field parameters affecting maximum heating in vitro for a hip implant and a cylindrical titanium disk. STUDY TYPE: Computational simulations and experimental validation of induced heating. PHANTOM: Tissue-simulating gel. FIELD STRENGTH: 42 T/s RMS, sinusoidal, continuous B fields with high spatial uniformity ASSESSMENT: Hip implant heating at 1-10 kHz, via computational modeling, validated by limited point measurements. Experimental measurements of exposures of an implant at 42 T/s for 4, 6, and 9 kHz, analyzed at 50, 100, and 150 seconds. STATISTICAL TESTS: One sample student's t-test to assess difference between computational and experimental results. Experimental vs. computational results were not significantly different (p < 0.05). RESULTS: Maximum simulated temperature rise (10-minute exposure) was 10 °C at 1 kHz and 0.66 °C at 10 kHz. The ratio of the rise for 21 T/s vs. 42 T/s RMS was 4, after stabilizing at 50 seconds (dB/dt ratio squared). DATA CONCLUSIONS: Heating of an implant is proportional to the frequency of the B field and the implant's cross-sectional area and is greater for a thickness on the order of its skin depth. Testing with lower values of dB/dt RMS with lower cost amplifiers enables prediction of heating at higher values for dB/dt squared (per ISO TS) with identical frequency components and waveforms, once thermal equilibrium occurs. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 1.

Determining EMC Test Levels for Implantable Devices in Bipolar Lead Configuration.

Certain low-frequency magnetic fields cause interference in implantable medical devices. Electromagnetic compatibility (EMC) standards prescribe injecting voltages into a device under evaluation to simplify testing while approximating or simulating real-world exposure situations to low-frequency magnetic fields. The EMC standard ISO 14117:2012, which covers implantable pacemakers and implantable cardioverter defibrillators (ICDs), specifies test levels for the bipolar configuration of sensing leads as being one-tenth of the levels for the unipolar configuration. The committee authoring this standard questioned this testing level difference and its clinical relevance. To evaluate this issue of EMC test levels, we performed both analytical calculations and computational modeling to determine a basis for this difference. Analytical calculations based upon Faraday's law determined the magnetically induced voltage in a 37.6-cm lead. Induced voltages were studied in a bipolar lead configuration with various spacing between a distal tip electrode and a ring electrode. Voltages induced in this bipolar lead configuration were compared with voltages induced in a unipolar lead configuration. Computational modeling of various lead configurations was performed using electromagnetic field simulation software. The two leads that were insulated, except for the distal and proximal tips, were immersed in a saline-conducting media. The leads were parallel and closely spaced to each other along their length. Both analytical calculations and computational modeling support continued use of a one-tenth amplitude reduction for testing pacemakers and ICDs in bipolar mode. The most recent edition of ISO 14117 includes rationale from this study.

Characterizing the 2.4 GHz Spectrum in a Hospital Environment: Modeling and Applicability to Coexistence Testing of Medical Devices.

The increasing use of shared, unlicensed spectrum bands by medical devices and nonmedical products highlights the need to address wireless coexistence to ensure medical device safety and effectiveness. This paper provides the first step to approximate the probability of a device coexisting in its intended environment by providing a generalized framework for modeling the environment. The application of this framework is shown through an 84-day spectrum survey of the 2.4-2.48 GHz industrial, scientific, and medical band in a hospital environment in the United States. A custom platform was used to monitor power flux spectral density and record received power. Channel utilization of three nonoverlapping channels of 20 MHz bandwidth-relative to IEEE 802.11 channels 1, 6, and 11-were calculated and fitted to a generalized extreme value distribution. Low channel utilization was observed (<10%) in the surveyed environment with sporadic occurrences of higher channel utilization (>50%). Reported findings can be complementary to wireless coexistence testing. This paper can provide input to the development of a consensus standard for wireless device coexistence test methods and a consensus document focused on wireless medical device coexistence risk management.

Comparison of Injected and Radiated EMC Testing of Active Implanted Cardiac Medical Devices at the Boundary Frequency of 450 MHz.

We compared testing via radiated vs. injected susceptibility methods specified in the ISO 14117 standard for electromagnetic compatibility of implantable cardiac medical devices. Injected testing is specified at and below 450 MHz and radiated testing is specified at and above this boundary frequency. Experimental and computational studies were performed to determine voltages induced in a model of an implant. The device under test (DUT) was a model with a metal case (5.0 x 4.5 x 3.4 cm high) containing a diode detector and a fiber optic link plus an insulated wire simulating a unipolar lead. We evaluated induced voltage in the model for radiated versus injected testing. The voltage from experimental measurements agreed with computed values within ±1.4 dB or less. Radiated testing with 12.25 Watts delivered to the dipole specified in the standard induced the same voltage as the injected test requirement (14 Vp-p). This is in strong contrast to the specified radiated test power of 120 mW or even the optional worst-case radiated test level of 8 Watts. Lateral displacement of the dipole by 2 cm changed the voltage induced in the DUT by 20% or less. For depth changes below saline surface from 5 mm to 10 mm voltage variations were 12.5%. There is poor agreement (by a factor of 9.9 or more) for radiated vs. injected testing at the border frequency of 450 MHz separating the two methods. Our computational model can also be used at other frequencies and for EMC studies of other types of active implants that use the ISO14117 standard's radiated test method, such as neuro-stimulators.

Evaluation of unintended electrical stimulation from MR gradient fields.

Exposure of patients with active implants (e.g. cardiac pacemakers and neurostimulators) to magnetic gradient fields (kHz range) during magnetic resonance imaging presents safety issues, such as unintended stimulation. Magnetically induced electric fields generate currents along the implant's lead, especially high at the distal tip. Experimental evaluation of the induced electric field was previously conducted. This study aimed to perform the same evaluation by means of computational methods, using two commercially available software packages (SemcadX and COMSOL Multiphysics). Electric field values were analyzed 1-3 mm from the distal tip. The effect of the two-electrode experimental probe was evaluated. The results were compared with previously published experimental data with reasonable agreement at locations more than 2-3 mm from the distal tip of the lead. The results were affected by the computational mesh size, with up to one order of magnitude difference for SEMCAD (resolution of 0.1 mm) compared to COMSOL (resolution of 0.5 mm). The results were also affected by the dimensions of the two-electrode probe, suggesting careful selection of the probe dimensions during experimental studies.

Radiated radiofrequency immunity testing of automated external defibrillators--modifications of applicable standards are needed.

BACKGROUND: We studied the worst-case radiated radiofrequency (RF) susceptibility of automated external defibrillators (AEDs) based on the electromagnetic compatibility (EMC) requirements of a current standard for cardiac defibrillators, IEC 60601-2-4. Square wave modulation was used to mimic cardiac physiological frequencies of 1-3 Hz. Deviations from the IEC standard were a lower frequency limit of 30 MHz to explore frequencies where the patient-connected leads could resonate. Also testing up to 20 V/m was performed. We tested AEDs with ventricular fibrillation (V-Fib) and normal sinus rhythm signals on the patient leads to enable testing for false negatives (inappropriate "no shock advised" by the AED). METHODS: We performed radiated exposures in a 10 meter anechoic chamber using two broadband antennas to generate E fields in the 30-2500 MHz frequency range at 1% frequency steps. An AED patient simulator was housed in a shielded box and delivered normal and fibrillation waveforms to the AED's patient leads. We developed a technique to screen ECG waveforms stored in each AED for electromagnetic interference at all frequencies without waiting for the long cycle times between analyses (normally 20 to over 200 s). RESULTS: Five of the seven AEDs tested were susceptible to RF interference, primarily at frequencies below 80 MHz. Some induced errors could cause AEDs to malfunction and effectively inhibit operator prompts to deliver a shock to a patient experiencing lethal fibrillation. Failures occurred in some AEDs exposed to E fields between 3 V/m and 20 V/m, in the 38 - 50 MHz range. These occurred when the patient simulator was delivering a V-Fib waveform to the AED. Also, we found it is not possible to test modern battery-only-operated AEDs for EMI using a patient simulator if the IEC 60601-2-4 defibrillator standard's simulated patient load is used. CONCLUSIONS: AEDs experienced potentially life-threatening false-negative failures from radiated RF, primarily below the lower frequency limit of present AED standards. Field strengths causing failures were at levels as low as 3 V/m at frequencies below 80 MHz where resonance of the patient leads and the AED input circuitry occurred. This plus problems with the standard's' prescribed patient load make changes to the standard necessary.

In-vitro mapping of E-fields induced near pacemaker leads by simulated MR gradient fields.

BACKGROUND: Magnetic resonance imaging (MRI) of patients with implanted cardiac pacemakers is generally contraindicated but some clinicians condone scanning certain patients. We assessed the risk of inducing unintended cardiac stimulation by measuring electric fields (E) induced near lead tips by a simulated MRI gradient system. The objectives of this study are to map magnetically induced E near distal tips of leads in a saline tank to determine the spatial distribution and magnitude of E and compare them with E induced by a pacemaker pulse generator (PG). METHODS: We mapped magnetically induced E with 0.1 mm resolution as close as 1 mm from lead tips. We used probes with two straight electrodes (e.g. wire diameter of 0.2 mm separated by 0.9 mm). We generated magnetic flux density (B) with a Helmholtz coil throughout 0.6% saline in a 24 cm diameter tank with (dB/dt) of 1 T/sec (1 kHz sinusoidal waveform). Separately, we measured E near the tip of leads when connected to a PG set to a unipolar mode. Measurements were non-invasive (not altering the leads or PG under study). RESULTS: When scaled to 30 T/s (a clinically relevant value), magnetically-induced E exceeded the E produced by a PG. The magnetically-induced E only occurred when B was coincident with or within 15 msec of implantable pacemaker's pulse. CONCLUSIONS: Potentially hazardous situations are possible during an MR scan due to gradient fields. Unintended stimulation can be induced via abandoned leads and leads connected to a pulse generator with loss of hermetic seal at the connector. Also, pacemaker-dependent patients can receive drastically altered pacing pulses.

In vitro investigation of pacemaker lead heating induced by magnetic resonance imaging: role of implant geometry.

PURPOSE: To evaluate the effect of the geometry of implantable pacemakers (PMs) on lead heating induced by magnetic resonance imaging (MRI). MATERIALS AND METHODS: In vitro experiments were conducted with two different setups, using fluoroptic probes to measure the temperature increase. The first experiment consisted of a rectangular box filled with a gelled saline and a pacemaker with its leads. This box was exposed in an MRI birdcage coil to a sinusoidal 64-MHz field with a calibrated whole-body specific absorption rate (WB-SAR) of 1 W/kg. The highest SAR and temperature increase (3000 W/kg, 12 degrees C) occurred for the implant configuration having the largest area. The second experimental setup consisted of a human-shaped torso filled with gelled saline. In this setup the PM and its lead were exposed to a real MRI scanner, using clinical sequences with WB-SAR up to 2 W/kg. RESULTS: We found that higher heating occurs for configurations with longer exposed lead lengths and that right chest PMs showed the highest temperature and local SAR (11.9 degrees C, 2345 W/kg), whereas the left chest PMs were less heated (6.3 degrees C, 1362 W/kg). Implant geometry, exposed lead length, and lead area must be considered in the wide variation of temperature increases induced by MRI. CONCLUSIONS: The amount of MRI-induced lead tip heating depends strongly on implant geometry, particularly the lead area, exposed lead length, and position of the implant in the phantom. Critical lead tip heating was found for the longer leads. Therefore, to minimize MRI-induced lead tip heating, the PM lead should be as short as possible.

Complexity of MRI induced heating on metallic leads: experimental measurements of 374 configurations.

BACKGROUND: MRI induced heating on PM leads is a very complex issue. The widely varying results described in literature suggest that there are many factors that influence the degree of heating and that not always are adequately addressed by existing testing methods. METHODS: We present a wide database of experimental measurements of the heating of metallic wires and PM leads in a 1.5 T RF coil. The aim of these measurements is to systematically quantify the contribution of some potential factors involved in the MRI induced heating: the length and the geometric structure of the lead; the implant location within the body and the lead path; the shape of the phantom used to simulate the human trunk and its relative position inside the RF coil. RESULTS: We found that the several factors are the primary influence on heating at the tip. Closer locations of the leads to the edge of the phantom and to the edge of the coil produce maximum heating. The lead length is the other crucial factor, whereas the implant area does not seem to have a major role in the induced temperature increase. Also the lead structure and the geometry of the phantom revealed to be elements that can significantly modify the amount of heating. CONCLUSION: Our findings highlight the factors that have significant effects on MRI induced heating of implanted wires and leads. These factors must be taken into account by those who plan to study or model MRI heating of implants. Also our data should help those who wish to develop guidelines for defining safe medical implants for MRI patients. In addition, our database of the entire set of measurements can help those who wish to validate their numerical models of implants that may be exposed to MRI systems.

Low frequency magnetic emissions and resulting induced voltages in a pacemaker by iPod portable music players.

BACKGROUND: Recently, malfunctioning of a cardiac pacemaker electromagnetic, caused by electromagnetic interference (EMI) by fields emitted by personal portable music players was highly publicized around the world. A clinical study of one patient was performed and two types of interference were observed when the clinicians placed a pacemaker programming head and an iPod were placed adjacent to the patient's implanted pacemaker. The authors concluded that "Warning labels may be needed to avoid close contact between pacemakers and iPods". We performed an in-vitro study to evaluate these claims of EMI and present our findings of no-effects" in this paper. METHODS: We performed in-vitro evaluations of the low frequency magnetic field emissions from various models of the Apple Inc. iPod music player. We measured magnetic field emissions with a 3-coil sensor (diameter of 3.5 cm) placed within 1 cm of the surface of the player. Highly localized fields were observed (only existing in a one square cm area). We also measured the voltages induced inside an 'instrumented-can' pacemaker with two standard unipolar leads. Each iPod was placed in the air, 2.7 cm above the pacemaker case. The pacemaker case and leads were placed in a saline filled torso simulator per pacemaker electromagnetic compatibility standard ANSI/AAMI PC69:2000. Voltages inside the can were measured. RESULTS: Emissions were strongest ( approximately 0.2 muT pp) near a few localized points on the cases of the two iPods with hard drives. Emissions consisted of 100 kHz sinusoidal signal with lower frequency (20 msec wide) pulsed amplitude modulation. Voltages induced in the iPods were below the noise level of our instruments (0.5 mV pp in the 0 - 1 kHz band or 2 mV pp in the 0 - 5 MHz bandwidth. CONCLUSION: Our measurements of the magnitude and the spatial distribution of low frequency magnetic flux density emissions by 4 different models of iPod portable music players. Levels of less than 0.2 muT exist very close (1 cm) from the case. The measured voltages induced inside an 'instrumented-can' pacemaker were below the noise level of our instruments. Based on the observations of our in-vitro study we conclude that no interference effects can occur in pacemakers exposed to the iPod devices we tested.

Magnetic-resonance-induced heating of implantable leads.

In this study a methodological approach for measuring temperature and local absorption rate (SAR) on thin metallic structures, such as pacemaker (PM) leads, is provided. First preliminary experiments were performed to evaluate the error in temperature and SAR measurements made by fluoroptic temperature probes when the temperature probe is in different contact configuration with the PM lead tip. Our results show how the position of temperature probes affects the temperature and SAR value measured at the lead tip. The transversal contact between the thermal sensor and the lead tip is the configuration which leads to the highest values for temperature and SAR. In the second part of this paper we describe two physical models of a human trunk and an experimental set-up to investigate the influence of the implant geometry and of the lead path on the heating and the local SAR deposition. Experiments reveled that the implant location and configuration are crucial elements for the heat generation at the lead tip.

Calculation of induced current densities and specific absorption rates (SAR) for pregnant women exposed to hand-held metal detectors.

The finite difference time domain (FDTD) method in combination with a well established frequency scaling method was used to calculate the internal fields and current densities induced in a simple model of a pregnant woman and her foetus, when exposed to hand-held metal detectors. The pregnant woman and foetus were modelled using a simple semi-heterogeneous model in 10 mm resolution, consisting of three different types of tissue. The model is based on the scanned shape of a pregnant woman in the 34th gestational week. Nine different representative models of hand-held metal detectors operating in the frequency range from 8 kHz to 2 MHz were evaluated. The metal detectors were placed directly on the abdomen of the computational model with a spacing of 1 cm. Both the induced current density and the specific absorption rate (SAR) are well below the recommended limits for exposure of the general public published in the ICNIRP Guidelines and the IEEE C95.1 Standard. The highest current density is 8.3 mA m(-2) and the highest SAR is 26.5 microW kg(-1). Compared to the limits for the induced current density recommended in the ICNIRP Guidelines, a minimum safety factor of 3 exists. Compared to the IEEE C95. 1 Standard, a safety factor of 60 000 for the specific absorption rate was found. Based on the very low specific absorption rate and an induced current density below the recommended exposure limits, significant temperature rise or nerve stimulation in the pregnant woman or in the foetus can be excluded.

Method for evaluating optical characteristics of endoscopes for recording fluorescence-related cardiac electrical activity.

Nondestructive methods were used to evaluate marketed fiber-optic endoscopes (intended for simple viewing) for fluorescence recording. Our application is for optical recording from the heart. For one angioscope, we measured a focal length of 0.33 mm, a field of view of 45 degrees, an aperture of 0.26 mm, and an efficiency of 43%. We calculated that the angioscope would give a signal-to-noise ratio of 1.0 for a cardiac action potential, if its field of view were divided into a nine-pixel array (for safe continuous illumination). Our methods are useful in designing and evaluating fluorescence fiber-optic systems with superior signal quality and spatial resolution.