MR safety in patients with implanted deep brain stimulation systems (DBS).

INTRODUCTION: While it is desirable to perform MRI examinations in patients with deep brain stimulators (DBS), a major safety concern exists regarding the potential for excessive heating secondary to magnetically induced electrical currents. This study was designed to determine the safety of MRI and DBS. METHODS: Standard configurations of DBS systems were tested. In vitro testing was performed using a 1.5-Tesla MR system, a gel-filled phantom, and the body and head RF coils with varying levels of RF energy (SAR). A fluoroptic thermometry system was used to record temperatures. RESULTS: Using the 1.5-T MRI and body RF transmit coil, the temperature changes ranged from 2.5 to 25.3 degrees C. Using the 1.5-T MRI and head RF transmit coil, the temperature changes ranged from 2.3 to 7.1 degrees C. CONCLUSIONS: Excessive heating does occur with certain MR imaging conditions. Under certain conditions determined in this study, patients with DBS may safely undergo anatomical MR imaging. In the future, standardized testing and more comprehensive studies will be needed to ensure the MR safety of neurostimulation systems.

Review of patient safety in time-varying gradient fields.

In magnetic resonance, time-varying gradient magnetic fields (dB/dt) may stimulate nerves or muscles by inducing electric fields in patients. Models predicted mean peripheral nerve and cardiac stimulation thresholds. For gradient ramp durations of less than a few milliseconds, mean peripheral nerve stimulation is a safe indicator of high dB/dt. At sufficient amplitudes, peripheral nerve stimulation is perceptible (i.e., tingling or tapping sensations). Magnetic fields from simultaneous gradient axes combine almost as a vector sum to produce stimulation. Patients may become uncomfortable at amplitudes 50%-100% above perception thresholds. In dogs, respiratory stimulation has been induced at about 300% of mean peripheral nerve thresholds. Cardiac stimulation has been induced in dogs by small gradient coils at thresholds near Reilly's predictions. Cardiac stimulation required nearly 80 times the energy needed to produce nerve stimulation in dogs. Nerve and cardiac stimulation thresholds for dogs were unaffected by 1.5-T magnetic fields.

Physiologic effects of intense MR imaging gradient fields.

The strength duration relationship for peripheral nerve stimulation by MR imaging pulsed gradient magnetic fields was measured in 84 human subjects. The data were fitted to the hyperbolic strength-duration relationship: dB/dt=b(1 + c/d), where b is rheobase, c is chronaxie, and d is duration, and dB/dt is reported as the maximal value on the axis of the bore. For sensation threshold, average (b,c) (15 T/s, 0.37 ms) for the y-gradient and (26 T/s, 0.38 ms) for the z-gradient coil. The dB/dt intensity to induce a sensation which the subject described as uncomfortable was about 50% above the sensation threshold. Experiments with dogs showed that the cardiac stimulation by pulsed magnetic gradient fields is exceedingly unlikely.

Evaluation of MRI compatibility of the modified nucleus multichannel auditory brainstem and cochlear implants.

Magnetic resonance imaging (MRI) has been contraindicated for users of cochlear implants because of the internal magnet and other possible interactions. Users of the Nucleus Mini-22 Cochlear Implant (CI) or the experimental Multichannel Auditory Brainstem Implant (ABI) may have other disorders that are best diagnosed by MRI. The CI and ABI were modified by replacing the internal magnet and integrated circuit lid with nonmagnetic material. Tests were conducted in a 1.5-T MRI machine. Safety tests for force, heating, induced current, unintentional implant output, and implant damage were conducted by using various phantom models. Image distortion was evaluated in two subjects with implants. The maximum force measured was 2,818 dynes. There was < 0.1 degree C temperature increase in the vicinity of the implant. The maximum induced charge was > or = 667 times less than the minimum charge for auditory stimulation. There was no unintentional output during MRI scans and no change in implant function after 10 repeated scans. Image distortion consisted primarily of darkening and was worst in the axial plane, where it extended 1-2 cm medially and inferiorly from the receiver/stimulator. Compatibility-test results were acceptable, with a large margin of safety. Image distortion is limited to darkening in the immediate vicinity of the implant.

Closed-chest cardiac stimulation with a pulsed magnetic field.

Magnetic stimulators, used medically, generate intense rapidly changing magnetic fields, capable of stimulating nerves. Advanced magnetic resonance imaging systems employ stronger and more rapidly changing gradient fields than those used previously. The risk of provoking cardiac arrhythmias by these new devices is of concern. In the paper, the threshold for cardiac stimulation by an externally-applied magnetic field is determined for 11 anaesthetised dogs. Two coplanar coils provide the pulsed magnetic field. An average energy of approximately 12 kJ is required to achieve closed-chest magnetically induced ectopic beats in the 17-26 kg dogs. The mean peak induced electric field for threshold stimulation is 213 V m-1 for a 571 microseconds damped sine wave pulse. Accounting for waveform efficacy and extrapolating to long-duration pulses, a threshold induced electric field strength of approximately 30 V m-1 for the rectangular pulse is predicted. It is now possible to establish the margin of safety for devices that use pulsed magnetic fields and to design therapeutic devices employing magnetic fields to stimulate the heart.

Guidelines for energy-efficient coils: coils designed for magnetic stimulation of the heart.

Magnetic stimulation of the heart requires high magnetic field energy and results in considerable Joulean dissipation. Energetically efficient and mechanically robust coils were designed for magnetic stimulation of the canine heart. Circular coils with rectangular cross section oriented either parallel or perpendicular to the skin were employed. A reasonable compromise between coil size and energy efficiency is achieved when the outer radius is twice the target depth. Nearly optimal efficiency is obtained by coils with annular width and height of 60% and 20% of the radius, respectively. A coplanar pair of coils parallel to the skin surface requires less energy and provides a more localized stimulus than a single coil. Placing a single coil perpendicular to the skin provides stimulus localization comparable to that of a coplanar pair; however, a single coil requires approximately 4 times the energy. The large coil currents required for cardiac stimulation exert Lorentz forces on the conductors that may exceed their tensile strength, emphasizing the need for an adequate support structure. Coils fabricated along the preceding guidelines were used to stimulate dog hearts repeatedly, requiring magnetic fields with energy greater than 12,000 J, without coil failure. Although the principles for coil design described herein were applied to a cardiac stimulator, they can also be applied to coils for other tissues, resulting in less coil heating, better stimulus localization and less power consumption.