MR Safety
Friday 7 May 2010
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 Atalar1
1Bilkent 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 Scott1
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 - not available
Paul A. Bottomley1, William A. Edelstein1, Ananda Kumar1, Justin M. Allen1, Perry Karmarkar1
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. Kerr1
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 Biller2,3, Wolfhard Semmler1, Michael Bock1
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
Devashish Shrivastava1, Timothy Hanson, Jeramy Kulesa, Jinfeng Tian2, Gregor Adriany, John Thomas Vaughan

1CMRR, Radiology, University of Minnesota, Minneapolis, MN, United States; 2Univeristy of Minnesota, Minneapolis, MN, United States

In vivo radio-frequency (RF) heating was measured due to a 7T head coil in four anesthetized porcine models (N = 4). Temperatures were measured using fluoroptic probes in the scalp; 5 mm, 10 mm, 15 mm, and 20 mm in the brain; and rectum. Continuous wave, 296 MHz, RF power was delivered for ~3 hours using the head coil. The whole head average SAR was maintained close to 3 W/kg. Systemic, uniform heating up to ~1.85 °C was produced. No RF heating induced adverse thermo-physiologic temperature response was detected as measured by the difference in post-RF and pre-RF temperature slopes.

11:42 780

An Automated Method for Subject Specific Global SAR Prediction in Parallel Transmission
Leeor Alon1,2, Cem Murat Deniz1,2, Riccardo Lattanzi1, Graham Wiggins1, Ryan Brown1, Daniel K. Sodickson1,2, Yudong Zhu

1Center for Biomedical Imaging, Department of Radiology, NYU School of Medicine, NYU School of Medicine, New York, NY, United States; 2Sackler Institute of Graduate Biomedical Sciences, NYU School of Medicine, New York, NY, United States

Current SAR measurement schemes are missing the capability to track and manage SAR under in-vivo conditions. Existing hardware schemes monitor forward and reflective power in real time only, but offer no prediction capability and tend to considerably overestimate SAR by assuming complete constructive interference of electric fields. In this study, we present, and demonstrate in vivo, a rapid and simple calibration method for the accurate prediction of subject specific global power deposition on an 8-channel transmit 7T MR system. This global SAR prediction capability is scalable to parallel transmit systems with any number of transmit channels.

11:54 781

Real Time RF Monitoring in a 7T Parallel Transmit System
Borjan Aleksandar Gagoski1, Rene Gumbrecht1,2, Michael Hamm3, Kawin Setsompop4,5, Boris Keil4,5, Joonsung Lee1, Khaldoun Makhoul4,5, Azma Mareyam4, Kyoko Fujimoto4, Thomas Witzel4,6, Ulrich Fontius7, Josef Pfeuffer3, Elfar Adalsteinsson1,6, Lawrence L. Wald4,6

1Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States; 2Department of Physics, Friedrich-Alexander-University Erlangen, Erlangen, Germany; 3Siemens Healthcare, Charlestown, MA, United States; 4A.A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, United States; 5Harvard Medical School, Boston, MA, United States; 6Harvard-MIT Division of Health Sciences and Technology, MIT, Cambridge, MA, United States; 7Siemens Healthcare, Erlangen, Germany

Current challenges to high-field applications of parallel RF transmission (pTx) in vivo include the monitoring and management of local SAR. We developed and tested real-time RF monitoring system for MAGNETOM 7T (Siemens Healthcare, Erlangen, Germany) with an 8-channel prototype pTx system that limits local SAR based on numerical simulation of E fields and power deposition in a segmented head model, and tracks and compares RF waveforms on each channel to the expected digital pulse waveform and shuts down the scan in the event of a mismatch due to spurious sources of pTx RF errors.

12:06 782.

Effects of a High Static Magnetic Field on (Higher) Cognitive Functions
Jöran Lepsien1, Karsten Müller1, D. Yves von Cramon1,2, Harald E. Möller1
Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany; 2Max Planck Institute for Neurological Research, Cologne, Germany

The possibility of exposure to high static magnetic fields altering cognitive performance in human volunteers was tested in a strictly controlled fashion. 24 participants conducted 6 different well-established paradigms covering a variety of cognitive processes. Sessions took place inside a 3T magnet with the main magnetic field being switched on and off. The analysis of reaction time and accuracy revealed no significant effect of the magnetic field in any of the 6 tasks related to the static field. The results indicate that exposure to a 3T field does not alter performance in cognitive tasks.

12:18 783

Effects of 7 Tesla MRI on Postural Stability with and Without RF, Gradient Switching, or B0 Exposure
Jens M. Theysohn1,2, Oliver Kraff1,2, Stefan Maderwald1,2, Marcus Gerwig3, Dagmar Timmann3, Franz Schmitt4, Lena Schaefer1,2, Sebastian Blex1,2, Elke R. Gizewski1,2, Michael Forsting1,2, Mark E. Ladd1,2, Susanne C. Ladd1,2, Andreas K. Bitz1,2
Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, NRW, Germany; 2Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Essen, NRW, Germany; 3Department of Neurology, University Hospital Essen, Essen, NRW, Germany; 4Siemens Healthcare, Erlangen, Germany

Ultra-high-field MRI (7 Tesla and above) generates more temporary side-effects compared to 1.5T and 3T, e.g. dizziness. In this study, postural stability was quantitatively measured before and after exposure to magnetic and electromagnetic fields of a 7 Tesla MR system. Forty-nine volunteers underwent Romberg’s tests. Stability shortly after MRI exposure was significantly reduced; when no RF was applied, the effect showed a similar trend but did not achieve significance. The results show that exposure to 7 Tesla causes only a temporary dysfunction of the vestibular system which does not appear to be related to the RF field.



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