Surgically implanted neurostimulators and electrodes may be utilized to provide functional electrical stimulation of the affected site. However, these devices may be associated with considerable surgical morbidity and expense. As such, there has been an on-going effort to develop technology that combines the reliability of using an implanted device with a low morbidity and low cost procedure. This effort has yielded a miniaturized, implantable device designed for functional electrical stimulation.

In 1988, Heetderks first demonstrated the feasibility of using a millimeter-sized, neural prosthetic implant. Over the years, this so-called "microstimulator" evolved to its present form. The microstimulator now exists as a relatively small, wireless, digitally controlled stimulator that is implanted using a minimally invasive procedure to provide electrical pulses to a muscle or nerve. This device receives power and command signals by inductive coupling from an externally worn coil that generates a radiofrequency magnetic field. The microstimulator is currently undergoing clinical trials to assess its therapeutic effect on a variety of neurological disorders including urinary incontinence, shoulder subluxation, drop foot, ventilator-dependant respiratory deficiencies, and sleep apnea.

RF BION Microstimulator
The implantable RF BION Microstimulator (Alfred E. Mann Foundation for Scientific Research, Valencia, CA and Advanced Bionics Corporation, Valencia, CA) is a wireless device designed for functional electrical stimulation of the peripheral nervous system. This hermetically-sealed implant is a small, lightweight, cylindrical-shaped device (length, 16.6-mm; diameter-2.4 mm; mass, 0.265-g) made of a ceramic tube closed on each end by titanium caps and contains components made from titanium, gold, copper, ferrite, platinum, iridium, silicon, zirconium, and tantalum. The active electrodes are welded on each end cap: an iridium disk on the cathodal side and a platinum-iridium eyelet on the anodal side.

The microstimulator receives power and digital commands via a 2-MHz radiofrequency magnetic field link generated from an external coil that is worn by the patient. The RF Bion Microstimulator produces asymmetric, biphasic, capacitively coupled constant-current pulses. Stimulation circuitry and a receiving multi-turn loop antenna are contained within the microstimulator. The antenna is wound around two pieces of ferrite, cut from a cylinder of radius 0.74-mm. Because of the small size of this microstimulator, it may be implanted through a specially designed, trocar-based 12- or 14-gauge implant tool or via a small surgical opening for placement near a nerve or at the motor unit of a muscle. A suture passed through one of the end caps allows the microstimulator to be maintained acutely in a properly implanted position.

MRI and the RF BION Microstimulator
Safety information for the use of a magnetic resonance imaging (MRI) procedure (i.e., imaging, angiography, functional imaging, spectroscopy, etc.) in a patient with the RF BION Microstimulator is highly specific to the type of MR system and conditions used to determine safety criteria for this implant. Safety information for MR procedures described herein pertains to the use of MR systems operating with static magnetic fields of 1.5-Tesla, gradient magnetic fields of 20-Tesla/second or less, and a whole body averaged specific absorption rate (SAR) of 2.0 W/kg or less for 15-minutes of imaging (i.e., per pulse sequence).

Warning
The effects on the RF BION Microstimulator of performing MRI procedures using other MR systems and other conditions have not been determined.

The following information describes testing that was performed on the effect of magnetic resonance imaging (MRI) procedures on an implanted RF BION Microstimulator.

MR procedures MUST ONLY be performed according to the following information:

Magnetic Field Interactions.
Magnetic field interactions (translational attraction and torque) were assessed for the RF BION Microstimulator in association with exposure to a 1.5-Tesla MR system. The RF BION Microstimulator displays relatively moderate magnetic field interactions. The deflection angle measured for the RF BION Microstimulator was 58 degrees. Thus, the magnetic force is 1.6 times the gravitational force (0.42 g). The maximal magnetic torque was calculated to be equal to 3.9 times the "gravity torque", the product of the implant's length and weight. Therefore, torque can be viewed as a force equivalent to 0.98 g acting on one end of the RF BION Microstimulator with the other end fixed. Because of the internal ferrite material (mass of 32 mg), the intended in vivo use of this implant must be considered.

Notably, tissue encapsulation around the RF BION Microstimulator is a major factor that will retain this implant in situ. In vivo research demonstrated that counter-forces provided by encapsulation (i.e., fibrous tissue) that occurs 2 to 3 weeks after implantation will prevent the RF BION Microstimulator from being moved or dislodged in association with the exposure to a 1.5-Tesla MR system. Therefore, considering that the mass of the RF BION Microstimulator is relatively small (0.25 g), and this implant does not contain any sharp ends, magnetic field interactions associated with exposure to a 1.5-Tesla MR system will not move or dislodge this implant after a conservative post-op waiting period of 6 to 8 weeks.

Interactions with Time-Varying Magnetic Fields.
The potential interactions of the RF BION Microstimulator with magnetic resonance imaging (MRI)-related time-varying magnetic fields (gradient and RF) were theoretically and experimentally investigated with respect to a 1.5-Tesla/64 MHz MR system.

Effects of Radiofrequency Fields.
Based on theoretical analysis and in vitro experiments, the presence of the RF BION Microstimulator in a patient undergoing an MRI procedure at a whole-body-averaged specific absorption rate (SAR) of 2.0 W/kg for 15-min. (per pulse sequence) will not result in a substantial increase in temperature. Therefore, a patient with the RF BION Microstimulator is not at increased risk with respect to MRI-related heating of this implant under the conditions used for this assessment.

Effects of Gradient Fields.
The electric field in the body induced by the MRI pulsed gradient magnetic fields may exceed 5 V/m. Given the size and shape of the RF BION Microstimulator, this field intensity induces minimal voltage (95 mV) along the length of the implant. Importantly, concentration of the gradient currents will be no greater than that resulting from the concentration rising from bones surrounded by tissue. Therefore, no physiological impact is expected by the interaction between the RF BION Microstimulator with the gradient currents in the tissue of a patient with this implant undergoing an MR procedure at 1.5-Tesla/64 MHz. Accordingly, a patient with the RF BION Microstimulator may safely be exposed to the gradient magnetic fields (dB/dt) at levels that are currently allowed for 1.5-Tesla MR systems operating with conventional pulse sequences and standard accessories (e.g., surface RF coils).

Warning: Unconventional or non-standard MR techniques have not been assessed for the RF BION Microstimulator and, therefore, must be avoided.

MRI Procedures and Function of the RF BION Microstimulator.
The effects of MRI procedures on the functional aspects of the RF BION System were assessed. In vitro testing was performed on a phantom with nine RF BION devices placed in various orientations relative to the MR system.

MR imaging was conducted using 1.5-Tesla/64 MHz MR system (transmit/receive RF body coil) to perform 15 different MR imaging pulse sequences, as follows: (1) T1-weighted, spin echo pulse sequence, (2) T1-weighted, fast spin echo pulse sequence, (3) T2-weighted, spin echo pulse sequence, (4) T2-weighted, fast spin echo pulse sequence, (5) Gradient echo, two dimensional pulse sequence, (6) Gradient echo, three dimensional pulse sequence, (7) Fast gradient echo, two dimensional pulse sequence, (8) Fast gradient echo, three dimensional pulse sequence, (9) Fast spoiled gradient echo, two dimensional pulse, sequence, (10) Fast spoiled gradient echo, three dimensional pulse sequence, (11) T1-weighted, spin echo pulse sequence with high whole-body averaged specific absorption rate (1.1 W/kg), (12) T1-weighted, fast spin echo pulse sequence with high whole-body averaged specific absorption rate (1.1 W/kg), (13) T2-weighted, spin echo pulse sequence with high whole body averaged specific absorption rate (1.1 W/kg), (14) T2-weighted, fast spin echo pulse sequence with high whole-body averaged specific absorption rate (1.1 W/kg), and (15) Gradient echo, three dimensional pulse sequence with magnetization transfer contrast with high whole-body averaged specific absorption rate (1.1 W/kg)

The findings indicated that there was no apparent damage or alteration in the functional aspects of the RF BION Microstimulators. Thus, the RF BION Microstimulator was demonstrated to maintain full functionality after exposure to the MR imaging conditions indicated above.

Warning
The effect of using any other MR imaging pulse sequence or procedure on the RF BION Microstimulator is unknown.

Additional MRI Safety Guidelines for the RF BION Microstimulator
-DO NOT use MR systems other than 1.5 Tesla MR systems.
-Continuously monitor the patient using visual and audio means (i.e., intercom system) throughout the MR procedure.
-Instruct the patient to alert the MR system operator of any unusual sensations or problems so the MR system operator can terminate the MRI procedure, if needed.
-Provide the patient with a means to alert the MR system operator of any unusual
sensations or problems that may be experienced during the MR procedure.

Post-MRI Procedure Recommendations for the RF BION Microstimulator
After undergoing an MRI procedure, the RF BION Microstimulator should be checked to ensure that it is working properly. To do so, stimulation threshold measurements should be obtained and compared to pre-MRI procedure threshold levels.

For additional information about this product, including MR safety guidelines, please contact the Alfred E. Mann Foundation, Valencia, CA; http://www.aemf.org and Advanced Bionics Corporation, Sylmar, CA; http://www.advancedbionics.com/

For this neurostimulation system, MR healthcare professionals are advised to contact the manufacturer of the specific device to ensure that the latest safety information is obtained and carefully followed in order to ensure patient safety relative to the use of an MR procedure.

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Heetderks, WJ. RF powering of millimeter- and submillimeter-sized neural prosthetic implants. IEEE Trans Biomed Eng 1988;35:323-327.

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