John A. Nyenhuis

ACR Guidance Document for Safe MR Practices: 2007

E. Kanal is a consultant for, is a member of the speakers bureau of, and provides research support for Bracco Diagnostics and GE Healthcare; is a member of the speakers bureau of and provides research support for Siemens Medical Solutions; and provides research support for Berlex and Medtronic. T. Gilk is a consultant for Mednovus, Inc. J. R. Gimbel provides research support for St. Jude Medical, Medtronic, and Biotronik. J. Nyenhuis is a consultant for and provides research support to Medtronics. J. Weinreb is a consultant and member of the speakers bureau for GE Healthcare.

ACR guidance document on MR safe practices: 2013

Because there are many potential risks in the MR environment and reports of adverse incidents involving patients, equipment and personnel, the need for a guidance document on MR safe practices emerged. Initially published in 2002, the ACR MR Safe Practices Guidelines established de facto industry standards for safe and responsible practices in clinical and research MR environments. As the MR industry changes the document is reviewed, modified and updated. The most recent version will reflect these changes. J. Magn. Reson. Imaging 2013;37:501–530.

Neurostimulation systems for deep brain stimulation: In vitro evaluation of magnetic resonance imaging–related heating at 1.5 tesla

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).

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

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.

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 (ie, 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 Reillys 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. J. Magn. Reson. Imaging 2000;12:20–29.

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

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.

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

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.

Gelled versus nongelled phantom material for measurement of MRI-induced temperature increases with bioimplants

Measurements in phantoms are used to predict temperature changes that would occur in vivo for medical implants due to the radio frequency (RF) field in magnetic resonance imaging (MRI). In this study, the impact of concentration of the gelling agent in a saline-based phantom on the RF-induced temperature rise was measured using an apparatus that accurately reproduces the RF environment present in a 1.5-T whole-body MR system. The temperature was measured using fluoroptic thermometry at the electrode and other sites for a deep brain neurostimulation system. The average power deposition in the 30-kg phantom was about 1.5 W/kg. Four phantom formulations were evaluated, using different concentrations of polyacrylic acid (PAA) added to saline solution, with NaCl concentration adjusted to maintain an electrical conductivity near 0.24 S/m. The greatest temperature rises occurred at the electrode, ranging from 16.2/spl deg/C for greatest concentration of PAA to 2.9/spl deg/C for only saline solution. The temperature rise attained the maximal value for sufficient concentration of PAA. Similar behavior was observed in the temperature versus time relationship near a current-carrying resistor, immersed in gel and saline, which was used to model a localized heat source. The temperature rise for insufficient PAA concentration is reduced due to convection of phantom material. In conclusion, an appropriate gelling agent is required to accurately simulate the thermal properties of body tissues for measurements of RF-induced heating with medical implants.

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.

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°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.