Esra Turk, Abstract Co-Author: Nothing to Disclose

Kadir Emre Bugdayci, Abstract Co-Author: Nothing to Disclose

Yusuf Ziya Ider, Abstract Co-Author: Nothing to Disclose

Ergin Atalar, Presenter: Nothing to Disclose

The purpose of this study is to analyze the risk of undesired nerve stimulation for patients with active medical implants, via ex-vivo frog experiments.

In a previous study we simplified expressions for field distributions inside the uniform region of the gradient field. If the position of the implant is known, the voltage on the lead can be computed approximately utilizing these expressions. In this study, experiments are conducted to observe to what extent these induced voltages can stimulate a nerve. In the experiments, a frog leg and a wire are fixed inside a phantom filled with Ringer’s solution. One tip of the wire is soldered to a piece of copper plate (emulates the pulse generator) and the other tip, which is left uninsulated (emulates the electrode), is kept touching the nerve. The position of the wire in the phantom is determined by MR images. Utilizing these position data, voltage values are computed. 20 experiments are conducted on a Siemens TimTrio MR machine. Each gradient is set to 166 T/m/s slew rate and 5 ms duration with a 120 microsecond ramp up and down time. Stimulation threshold voltage for the frog sciatic nerve is determined by an external signal generator with an independent experiment.

Measured and calculated induced voltages on the lead matched within a 40 % error. For example, when using the y-gradient, a 15 cm lead positioned at a distance of 8.6 cm from the isocenter, theoretically computed voltage value is larger than the frog nerve stimulation threshold, and stimulation is observed as expected. Induced voltage is proportional with the distance from the isocenter and increases with the square of the lead length. In the presentation, frog nerve stimulation in a MR scanner because of the induced current on a pacemaker lead will be shown as a video demonstration.

In a clinical scanner, risk of nerve stimulation for patients with implants has been demonstrated with ex-vivo frog nerve experiments.

Frog nerve experiments show that in patients with low stimulation threshold, unwanted cardiac stimulation can be observed.

Turk, E, Bugdayci, K, Ider, Y, Atalar, E, Demonstration of Nerve Stimulation Due to Induced Current on an Implant Lead during MRI.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11009462.html