Calculation of MRI-induced heating of an implanted medical lead wire with an electric field transfer function.

PURPOSE: 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. MATERIALS AND METHODS: 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. RESULTS: 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. CONCLUSION: 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.

Reduction of magnetic resonance imaging-related heating in deep brain stimulation leads using a lead management device.

OBJECTIVE: 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. METHODS: 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. RESULTS: 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). CONCLUSION: 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.

Thresholds for 60 Hz magnetic field stimulation of peripheral nerves in human subjects.

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.

Neurostimulation systems: assessment of magnetic field interactions associated with 1.5- and 3-Tesla MR systems.

PURPOSE: To evaluate magnetic field interactions at 1.5- and 3-Tesla for implantable pulse generators (IPGs) and radiofrequency (RF) receivers used for implantable neurostimulation systems. MATERIALS AND METHODS: Measurements of magnetically induced displacement force and torque were determined for 10 devices (seven IPGs, three RF receivers) used for neurostimulation systems. Displacement force and torque were assessed at various positions in 1.5- and 3-Tesla MR systems using standardized techniques. RESULTS: Four IPGs exhibited force ratios (magnetic attraction force/device weight) greater than 1.0, with the overall magnitude of the force ratio increasing significantly when comparing the 1.5-Tesla to the 3-Tesla MR system. Of the seven IPGs tested, one exhibited a torque ratio (magnetic induced torque/product of the device weight and length) greater than 1.0. The RF receivers displayed relatively strong magnetic field interactions at both 1.5- and 3-Tesla, exhibiting force and torque ratios greater than 1.0. CONCLUSIONS: The neurostimulation implants tested exhibited varying degrees of magnetic field interactions, with four of the seven IPGs and the three RF receivers exhibiting at least one MR-induced force or torque value greater than the effect of gravity. These findings have important implications for patients with these implants who are referred for MRI examinations.

Peripheral nerve stimulation by gradient switching fields in magnetic resonance imaging.

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.

Implantable microstimulator: magnetic resonance safety at 1.5 Tesla.

RATIONALE AND OBJECTIVE: 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. METHODS: 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. RESULTS: 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). CONCLUSION: 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.

Evaluation of specific absorption rate as a dosimeter of MRI-related implant heating.

PURPOSE: To compare the magnetic resonance imaging (MRI)-related heating per unit of whole body averaged specific absorption rate (SAR) of a conductive implant exposed to two different 1.5-Tesla/64 MHz MR systems. MATERIALS AND METHODS: Temperature changes at the electrode contacts of a deep brain stimulation lead were measured using fluoroptic thermometry. The leads were placed in a typical surgical implant configuration within a gel-filled phantom of the human head and torso. MRI was performed using two different transmit/receive body coils on two different generation 1.5-Tesla MR systems from the same manufacturer. Temperature changes were normalized to whole body averaged SAR values and compared between the two scanners. RESULTS: Depending on the landmark location, the normalized temperature change for the implant was significantly higher on one MR system compared to the other (P < 0.001). CONCLUSION: The findings revealed marked differences across two MR systems in the level of radiofrequency (RF)-induced temperature changes per unit of whole body SAR for a conductive implant. Thus, these data suggest that using SAR to guide MR safety recommendations for neurostimulation systems or other similar implants across different MR systems is unreliable and, therefore, potentially dangerous. Better, more universal, measures are required in order to ensure patient safety.

MR imaging-related heating of deep brain stimulation electrodes: in vitro study.

BACKGROUND AND PURPOSE: Recent work has shown a potential for excessive heating of deep brain stimulation electrodes during MR imaging. This in vitro study investigates the relationship between electrode heating and the specific absorption rate (SAR) of several MR images. METHODS: In vitro testing was performed by using a 1.5-T MR imaging system and a head transmit-receive coil, with bilateral deep brain stimulation systems positioned in a gel saline-filled phantom, and temperature monitoring with a fluoroptic thermometry system. Standardized fast spin-echo sequences were performed over a range of high, medium, and low SAR values. Several additional, clinically important MR imaging techniques, including 3D magnetization prepared rapid acquisition gradient-echo imaging, echo-planar imaging, quantitative magnetization transfer imaging, and magnetization transfer-suppressed MR angiography, were also tested by using typical parameters. RESULTS: A significant, highly linear relationship between SAR and electrode heating was found, with the temperature elevation being approximately 0.9 times the local SAR value. Minor temperature elevations, <1 degrees C, were found with the fast spin-echo, magnetization prepared rapid acquisition gradient-echo, and echo-planar clinical imaging sequences. The high dB/dt echo-planar imaging sequence had no significant heating independent of SAR considerations. Sequences with magnetization transfer pulses produced temperature elevations in the 1.0 to 2.0 degrees C range, which was less than theoretically predicted for the relatively high SAR values. CONCLUSION: A potential exists for excessive MR imaging-related heating in patients with deep brain stimulation electrodes; however, the temperature increases are linearly related to SAR values. Clinical imaging sequences that are associated with tolerable temperature elevations in the

Simulation of human respiration in fMRI with a mechanical model.

Obtaining functional magnetic resonance images of the brain is a challenging measurement process having a low characteristic signal-to-noise ratio. Images contain various forms of noise, including those induced by physiologic processes. One of the prevalent disturbances is hypothesized to result from susceptibility fluctuations caused by abdominal volume changes during respiration. To test this hypothesis and characterize the contribution of respiration noise to both magnitude and phase images, a mechanical model of a respiring human was constructed. The model was tested by comparing data from the model with that of a resting human. Power spectrum analyses show that the model induces both phase and magnitude disturbances similar to those in the human. The disturbances are directly related to the frequency of the respiration, with the noise most prevalent in the phase images. Though magnitude image noise is hard to identify in the human, the manikin demonstrates the presence of this disturbance. The construction of the manikin rules out motion as the primary source of the observed fluctuations and variation of the electrical properties of the manikin also indicates that signal fluctuations are not primarily due to eddy currents. Therefore, the changes are most probably induced by bulk susceptibility changes correlating with respiration.

Comparison of the threshold for peripheral nerve stimulation during gradient switching in whole body MR systems.

PURPOSE: To compare thresholds for peripheral nerve stimulation from gradient switching in whole body magnetic resonance (MR) equipment of different design. MATERIALS AND METHODS: Threshold data obtained in three experiments were reformatted into a single joint data set describing thresholds for anterio-posterior (AP) gradient orientation and Echo Planar Imaging (EPI) waveforms with bipolar ramp times between 0.07 and 1.2 ms. Reformatting included the use of: a) the rate of change of the maximum field in the patient space as a measure of gradient output, b) lowest observable thresholds, c) lognormal distribution of thresholds, and d) equal standard deviation (SD) of all samples. RESULTS: The joint data fit a hyperbolic threshold function. The residues were not significantly different between experiments. CONCLUSION: Then expressed in appropriate format, the thresholds for peripheral nerve stimulation in volunteers for whole body MR equipment can be described with a hyperbolic threshold curve with rheobase 18.8 +/- 0.6 Tesla/second and chronaxie 0.36 +/- 0.02 milliseconds.

Neurostimulation systems for deep brain stimulation: in vitro evaluation of magnetic resonance imaging-related heating at 1.5 tesla.

PURPOSE: To assess magnetic resonance imaging (MRI)-related heating for a neurostimulation system (Activa Tremor Control System, Medtronic, Minneapolis, MN) used for chronic deep brain stimulation (DBS). MATERIALS AND METHODS: Different configurations were evaluated for bilateral neurostimulators (Soletra Model 7426), extensions, and leads to assess worst-case and clinically relevant positioning scenarios. In vitro testing was performed using a 1.5-T/64-MHz MR system and a gel-filled phantom designed to approximate the head and upper torso of a human subject. MRI was conducted using the transmit/receive body and transmit/receive head radio frequency (RF) coils. Various levels of RF energy were applied with the transmit/receive body (whole-body averaged specific absorption rate (SAR); range, 0.98-3.90 W/kg) and transmit/receive head (whole-body averaged SAR; range, 0.07-0.24 W/kg) coils. A fluoroptic thermometry system was used to record temperatures at multiple locations before (1 minute) and during (15 minutes) MRI. RESULTS: Using the body RF coil, the highest temperature changes ranged from 2.5 degrees-25.3 degrees C. Using the head RF coil, the highest temperature changes ranged from 2.3 degrees-7.1 degrees C.Thus, these findings indicated that substantial heating occurs under certain conditions, while others produce relatively minor, physiologically inconsequential temperature increases. CONCLUSION: The temperature increases were dependent on the type of RF coil, level of SAR used, and how the lead wires were positioned. Notably, the use of clinically relevant positioning techniques for the neurostimulation system and low SARs commonly used for imaging the brain generated little heating. Based on this information, MR safety guidelines are provided. These observations are restricted to the tested neurostimulation system.