Field Interactions & Safety
Tuesday 21 April 2009
Room 323ABC 16:00-18:00

Moderators:

 Penny Anne Gowland and Greig C. Scott
 
16:00  298. Influence of Strong Static Magnetic Fields on Myocardial Mechanics: Evaluation Applying Ultra-Wideband Radar
    Florian Thiel1, Mathias Hein2, Jürgen Sachs2, Ulrich Schwarz2, Tomasz Lindel1, Frank Seifert1
1Physikalisch-Technische Bundesanstalt (PTB), Berlin, Germany; 2Ilmenau University of Technology
    ECG is excessively used for triggering MR data acquisition to image the heart. It is well established that the ECG is corrupted by the magneto-hydrodynamic effect (MHD). Hence, there is increasing difficulty to use the ECG for MR-triggering especially at B0-fields beyond 1.5 T. To investigate whether the myocardial contraction is also influenced by the static magnetic field, we propose a novel method, based on an ultra-wideband radar technique (UWB radar) to monitor the global myocardial dynamics inside a MR scanner.
     
16:12 299. Effects of 7 Tesla MRI Examination on Postural Stability
    Jens Matthias Theysohn1,2, Andreas K. Bitz1, Oliver Kraff1,2, Stefan Maderwald1,2, Marcus Gerwig3, Oliver Kastrup3, Dagmar Timmann3, Franz Schmitt4, Harald H. Quick1,2, Elke Ruth Gizewski1,2, Michael Forsting1,2, Mark E. Ladd1,2, Susanne C. Ladd1,2
1Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Essen, Germany; 2Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, Germany; 3Department of Neurology, University Hospital Essen, Essen, 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. Postural stability was quantitatively measured before and after a 7 Tesla examination of the head. Twenty-five volunteers underwent a Romberg’s test. Stability with eyes closed was significantly reduced directly after the MR examination, but was fully recovered 15 minutes after examination. Physiological effects by orthostatic regulation were excluded. Stability results for tests with eyes open show no significant difference. The results show that exposition to a 7 Tesla examination causes only a temporary dysfunction or over-compensation of the vestibular system.
     
16:24 300. Numerical Evaluation of SAR Within Whole-Body Pregnant Woman Models in MRI Birdcage Coil
    Zhangwei Wang1, George X. Xu2, Victor Taracila1, Jason Jin1, Fraser J. Robb1
1GE Healthcare Coils, Aurora, OH, USA; 2Department of Biomedical Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA
    Whole body pregnant woman models at different gestational stages were adopted to investigate the SAR variation at 1.5T. The highest SAR is occurred in the mother's peripheral tissues in all pregnancy phase. And the maximum local SAR of the fetus is below IEC limitation. The results show that the local maximum SAR1g and SAR10g can be better indications as limitation factor other than the whole body average SAR. Future work will be done by using more pregnancy stages and higher resolution models.
     
16:36 301. Optimized Signal Intensity and T1r Based NMR Thermometry for Ultra-High Field RF Safety Applications
    Devashish Shrivastava1, Timo Liimatainen1, Ute Goerke1, Jeramy Kulesa1, Timothy Hanson1, Shalom Michaeli1, J. Thomas Vaughan1
1CMRR, Radiology, University of Minnesota, Minneapolis, MN, USA
    Good linear correlations were obtained between the core temperatures, and the 'optimized' signal intensities and the longitudinal magnetization relaxation rate constants in the rotating frame (i.e., T1r) in an in vivo porcine brain at 3 tesla (T). The correlations were obtained to measure RF heating induced temperatures with sub-degree Celsius accuracy in porcine brains at ultra-high fields.
     
16:48 302. Real-Time Global and Local SAR Monitoring for Parallel Transmission Systems
   

Ingmar Graesslin1, Sven Biederer2, Bjoern Annighoefer3, Hanno Homann1, Henry Stahl1, Peter Vernickel1, Ulrich Katscher1, Dennis Glaesel4, Paul Harvey5
1Philips Research Europe, Hamburg, Germany; 2University of Lübeck, Lübeck, Germany; 3TU Hamburg-Harburg, Hamburg, Germany; 4Research Establishment for Applied Science, Wachtberg, Germany; 5Philips Healthcare, Best, Netherlands
    A comprehensive RF safety concept for parallel transmission MRI systems can be realized by verifying the SAR limits before scanning and monitoring RF signals during scanning. However, using a pre-calculated safety margin over-estimates the actual SAR. This abstract extends the concept of pre-calculated safety margins with a real-time global and local SAR monitoring system. This new concept allows for a significantly increased permissible RF duty cycle and improves the detection of violations of SAR limits or any unsafe condition. The impact of patient motion on SAR is analyzed, and a scan termination in case of unsafe system operation is demonstrated.
     
17:00 303. Safety and Initial Results with a Dedicated Human 7T Eye Coil
    Peter A. Wassenaar1, Kathryn Richdale2, Katharine Teal Bluestein1, John Christoforidis3, Titus Lanz4, Michael Vincent Knopp1, Petra Schmalbrock1
1Radiology, The Ohio State University, Columbus, OH, USA; 2Optometry, The Ohio State University, Columbus, OH, USA; 3Ophthalmology, The Ohio State University, Columbus, OH, USA; 4Rapid MR International, LLC, Columbus, OH, USA
   

Ultra-high field MRI allows higher resolution for examination of the fine structures of the eye. This work focuses on the safety testing and implementation of a custom surface coil at 7T. A transmit head coil was used with a 4cm receive coil. Safety was insured through both active and passive decoupling. Testing indicated good homogeneity of the imaging region and signal drop off as expected for the coil diameter. Animal testing demonstrated that tissue heating did not exceed 0.3°C, even with high RF power. The use of a custom coil yielded high quality images of the human eye in vivo.
     
17:12 304. RF Safety Assessment of a 7T Head Coil Using Thermal Modeling with Discrete Vessels
   

Astrid L.H.M.W. van Lier1, Cornelis A.T. van den Berg1, Dennis W.J. Klomp1, Bas W. Raaymakers1, Jan J.W. Lagendijk1
1Radiology, Radiotherapy, Nuclear Medicine, University Medical Center Utrecht, Utrecht, Netherlands
    The RF safety assessment of a commercially available 7T head coil using thermal simulations with a discrete vessels model is shown. This model incorporates the heating of blood in the arteries and veins as it traverses through a heated volume, which is an addition to Pennes’ bio heat transfer equation. Comparing the maximum SAR with the maximum temperature rise within the head it was concluded that the restrictions based on volume integrated SAR calculations are over-conservative, which results in the sub-optimal imaging.
   

 
17:24 305. Reduction of Implant RF Heating by Modification of Electric Field Distribution
    Yigitcan Eryaman1, Haydar Celik1, Burak Akin1, Ergin Atalar1
1Electrical Engineering, Bilkent University, Ankara, Turkey
    In this work, the optimum EM field that reduces the RF heating of implant leads is calculated. The resulting field also satisfies a given transmit target sensitivity constraint in an axial plane. Among infinite number of solutions satisfying these constraints, the one that minimizes the average SAR is presented. The field distribution which satisfies the electric field and the target transmit sensitivity constraints is realized by using a linearly polarized birdcage coil. Finally, heating experiments of resonant length wires is conducted by using linearly polarized and quadrature coils.
     
17:36 306. Detecting Unsafe Device Coupling Using Reversed RF Polarization
    William Overall1, Pascal Stang1, John Pauly1, Greig Scott1
1Electrical Engineering, Stanford University, Stanford, CA, USA
    Patients with long-wire implants are ineligible for MR scans due to the risk of RF burns arising from RF fields coupling to their device. We propose the use of a low-power prescan to quantify the level of coupling and determine the burn risk for a particular patient in a particular scanner. This is accomplished through use of reversed RF polarization imaging, which produces images with signal proportional to RF coupling. Simulations and experimental data suggest that this technique can be reliably used to determine burn risk in a prescan setting.
     
17:48 307. Characterization of the Relationship Between MR-Induced Distal Tip Heating in Cardiac Pacing Leads and the Electrical Performance of Novel Filtered Tip Assemblies
    Robert S. Johnson1, Holly Moschiano1, Robert Stevenson1, Scott Brainard2, Sam Ye2, Joseph E. Spaulding1, Warren Dabney1
1Cardiac & Neurology, Greatbatch, Inc., Clarence, NY, USA; 2Cardiac & Neurology, Greatbatch, Inc., Plymouth, MN, USA
    The impedance of inductor-capacitor band stop filters were characterized with impedance spectroscopy. If was found the impedance of the device was controllable with proper selection of the inductor and capacitor values. The filters were incorporated in implantable leads and the temperature rise of the distal end was measured during a MRI scan with an RF-intensive imaging protocol. The minimum heating was 9°C with the highest impedance filter and the maximum heating was 51°C with a control lead. Systematic changes in the lead configuration and location reduced the heating to near instrumentation noise level.