Friday, 7 may 2010

MR Safety

Room K2 10:30-12:30 Moderators: Blaine A. Chronik and Daniel J. Schaefer

10:30 774. Experimental and Theoretical Analysis of the Induced Voltage Along Implant Leads Due to Gradient Fields

Esra Abaci Turk¹, Emre Kopanoglu¹, Yigitcan Eryaman¹, Vakur Behcet Erturk¹, Ergin Atalar<sup>1

1</sup>Bilkent University, Ankara, Turkey

With the help of the simplified electric field expressions for x, y and z gradient coils, approximate voltage values to occur on the lead are derived analytically and these values are compared with the values obtained from realistic experiments. This comparison shows that, if the path of the implant lead is known, induced voltage on the lead can be determined analytically and with the obtained result the risk of the stimulation can be examined for patients with implants prior to MRI.

10:42 775. Safely Detecting Device Coupling Using Reversed RF Polarization and Pre-Spoiled EPI

William Overall¹, Pascal Stang¹, John Pauly¹, Greig Scott<sup>1

1</sup>Electrical Engineering, Stanford University, Stanford, CA, United States

The degree of coupling present in long-wire implants can be quantified by reversing the RF receiver polarization. To assess device coupling in patients with potentially dangerous implants, a four-shot projection EPI sequence may be used safely given reasonable assumptions. Image quality and reliability can be improved by adding a small pre-spoiler gradient to suppress imperfections due to electrodynamic effects.

10:54 776. Towards MRI-Safe Implanted Leads: A Comparative Evaluation of Four Designs

Paul A. Bottomley¹, William A. Edelstein¹, Ananda Kumar¹, Justin M. Allen¹, Perry Karmarkar<sup>1

1</sup>Suite B307, 1101 E 33rd Street, SurgiVision Inc, Baltimore, MD, United States

Implanted leads and devices are a contraindication for MRI, denying many patients its potential benefits. Here, the MRI safety of four passive implantable lead designs that minimize the hazards of induced currents and heating, is investigated as a function of geometry. Continuously coiled leads, leads incorporating RF traps, and single and multi-layer “billabong” leads with reversed sections wherein the current opposes the induced RF, are compared in a model phantom at 1.5T and 4W/kg exposure. In coil and trap designs factors that maximize impedance limited heating below 1-2°C, but folded lead configurations can be problematic. The billabong designs heated <1°C.

11:06 777. Controlling Induced Currents in Guidewires Using Parallel Transmit

Maryam Etezadi-Amoli¹, Pascal Stang¹, Marta G. Zanchi¹, John M. Pauly¹, Greig C. Scott¹, Adam B. Kerr<sup>1

1</sup>Electrical Engineering, Stanford University, Stanford, CA, United States

RF transmit fields during MRI can induce currents and unsafe heating in conductive structures such as guidewires and implanted device leads. In this work, we used parallel transmit to control the level of current induced in a guidewire. We found experimentally that only one transmit mode from a four-channel array induced any appreciable current in a guidewire, while the remaining three modes induced no significant current, yet still provided adequate visualization of the volume. A parallel transmit approach thus offers a safe way of imaging in the presence of implanted conductive structures.

11:18 778. MR Safety Measurements of Intracranial Fixation Devices at 7T

Jaane Rauschenberg¹, Jens Groebner¹, Armin Michael Nagel¹, Armin Biller^2,3, Wolfhard Semmler¹, Michael Bock<sup>1

1</sup>Medical Physics in Radiology, German Cancer Research Center, Heidelberg, Germany; ²Division of Radiology, German Cancer Research Center, Heidelberg, Germany; ³Neuroradiology, University Hospital, Heidelberg, Germany

So far, the widely used cranial bone fixation system CranioFix® has been evaluated to be MR-safe up to field strengths of 3T. In this work we performed ASTM measurements of the implants at 7T MRI. As the magnetic force is much less than the gravitational force, no torque could be detected, and the temperature rise was less than 1°C during 16 min the implants can be considered as MR safe for the hardware used. Further¬more, artifact width is acceptable. This result enables MR imaging studies after brain surgery to be performed at field strengths up to 7 Tesla.

11:30 779. Systemic in Vivo Radio-Frequency Heating in Porcine Models with a 12.5’’ Diameter, 8 Channel, 7 T (296 MHz) Head Coil