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SAR & Safety Hall B Tuesday 13:30-15:30 1441. Evaluation of Maximum Local SAR for Parallel Transmission (PTx) Pulses Based on Pre-Calculated Field Data Using a Selected Subset of "Virtual Observation Points" Matthias Gebhardt1, Dirk Diehl2, Elfar Adalsteinsson3, Lawrence L. Wald4, Gabriele Eichfelder5 1Siemens Healthcare, Erlangen, Germany; 2Siemens Corporate Technology, Erlangen, Germany; 3Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States; 4Martinos Center for Biomedical Imaging, Harvard University, Charlestown, MA, United States; 5Applied Mathematics II, University of Erlangen-Nuremberg, Erlangen, Germany This work addresses the complexity problem inherent in local SAR estimation for parallel transmission (pTx) and shows that a relatively small subset of carefully selected “virtual observation points” is adequate for prediction and control of maximum local SAR in pTx. We tested the proposed algorithm to detect local SAR maxima by comparison with an exhaustive search over local SAR distribution in numerical simulations of adult male and female subjects for an 8 channel whole body transmit array. The proposed method of model compression for local SAR successfully captured regions of local SAR maxima, but with dramatically reduced computation cost. 1442. Safety of 17O and 23Na MR Imaging of the Human Brain at 9.4 Tesla Ian C. Atkinson1, Rachel Sonstegaard1, Lilian Bityou2, Neil H. Pliskin2, Keith R. Thulborn1 1Center for MR Research, University of Illinois- Chicago, Chicago, IL, United States; 2Psychiatry, University of Illinois- Chicago, Chicago, IL, United States Current FDA guidelines classify MR devices operating at 8 Tesla or lower as insignificant risk. Vital sign and cognitive performance data supporting the safety of performing non-proton MR imaging of the human brain at 9.4 Tesla are presented. These data add to the growing body of results that suggest ultra-high field MR imaging can be safely performed up to 9.4 Tesla. 1443. Experimental Setup with a Whole-Body Resonator for Investigating Thresholds of Tissue Damage in Swine Model Exposed at 123 MHz – First Measurement Results Gerhard Brinker1, Christian Grosse-Siestrup2, Chie Hee Cho2, Katja Reiter2, Katherina Habekost2, Peter Wust2, Razvan Lazar1, Franz Hebrank1, Eckart Stetter1, Jacek Nadobny2 1Siemens Healthcare, Erlangen, Germany; 2Campus Virchow Klinikum, Charité, Berlin, Germany The SAR limits issued by IEC 60601-2-33 are to some extent unbalanced comparing the whole body SAR limit with the local SAR limit. In order to examine thresholds of tissue damage a measurement setup consisting by the Tx path components of a clinical 3T MR scanner is established. Initial results of RF exposure on dead swine (6.5 W/kg, 30 minutes) are presented. Temperature increments are measured at the hot spot locations both invasively in tissue as well as superficially using infrared camera. The experimental results correspond very well with results of simulation using realistic 3D-voxel model of a swine 1444. Influence of a Receive-Array Coil on Specific Absorption Rate at 3T: Simulations and Experiments with Basic Geometries Sukhoon Oh1, Yeun Chul Ryu1, Zhangwei Wang2, Fraser Robb2, Christopher M. Collins1 1PSU College of Medicine, Hershey, PA, United States; 2GE Healthcare, Aurora, OH, United States Influence of a receive array and failure of its detuning circuit on specific absorption rate (SAR) is investigated with simulations and experiments at 3T. A simple array of two copper loops was placed around an agar-gel phantom. A gap in the circuit was placed and removed to simulate perfect and defective detuning conditions, respectively. In simulations, addition of the ¡°decoupled¡± array resulted in only a 3% higher average SAR than with no array present, whereas with the ¡°defective¡± circuit, an increase of 41% was observed. Trends in the SAR distributions between the different cases are similar in experiment and calculation. 1445. Effects of Simplifying Rf Coil 3-D EM Simulation Models on Power Balance and SAR Mikhail Kozlov1, Robert Turner 1Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Sachsen, Germany We have investigated the effect on power balance and SAR of simplifying rf coil 3-D EM simulation models, using co-simulation of the RF circuit and 3-D EM fields. We find that sufficiently accurate SAR and power balance estimation cannot be achieved for actual coils using simplified coil models and any renormalization approach. Simulations must converge by mesh refinement not only for B1+ values, but also for radiated and load absorbed powers, and must include simulation of the scanner gradient shield. The effect on SAR of distance to absorbing boundaries and gradient shield depends strongly on RF coil design. Thus coils and their environment need to be specified as accurately as possible, much better than current common practice. If simulation fails to give the correct power balance it is pointless to calculate SAR. In their current implementations, the Ansoft HFSS frequency domain solver provides much more reliable data, and much faster, than the CST time domain solver. 1446. Patch Antenna in Comparison to and in Combination with a Volume Coil for Excitation at 7T: Whole-Brain B1 Shimming and Consequent SAR Chien-ping Kao1, Zhipeng Cao1, Sukhoon Oh1, Yeun Chul Ryu1, Christopher M. Collins1 1PSU College of Medicine, Hershey, PA, United States Here we compare the homogeneity in whole-brain excitation that can be achieved with two adjustable transmit channels used to drive a patch antenna and to drive a body array with elements combined as to form two orthogonal mode 1 field patterns. Optimal whole-brain homogeneity achievable with B1 shimming and the resulting SAR is examined for the two approaches separately and combined. Combining the patch and the body array results in appreciable better achievable whole-brain homogeneity with significantly lower average and maximum local SAR than possible with either the patch antenna or body coil alone. 1447. Comprehensive Numerical Study of 7T Transmit-Only, Receive-Only Array Coils Mikhail Kozlov1, Robert Turner1 1Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Sachsen, Germany We present a reliable and fast workflow for comprehensive joint numerical study of 7T transmit-only, receive-only RF coils as complete devices. The flexibility of the workflow allows inclusion of almost all the RF and DC components used for tune/de-tune/match/decoupling. The S-parameter data and B1+ and B1- profiles obtained for a family of loop-based 8-channel TX-only, 8-channel RX-only coils allow estimation of the required impedance of de-tuning circuits. 1448. Fast SAR Estimation Via a Hybrid Approach Shumin Wang1, Jeff H. Duyn1 1LFMI/NINDS/NIH, Bethesda, Center Dr. , United States Estimation of local specific absorption rate (SAR) is a major challenge in high-field (>3.0 Tesla) multi-channel transmission systems. Conventional Finite-Difference Time-Domain (FDTD) method is often found inefficient to give subject-specific results. Recent studies sped up the process by applying Ampere’s law to obtain electrical field distributions via measured transverse B1 fields. The drawback is that only the longitudinal electrical fields can be calculated, which may yield large discrepancy. We present a fast numerical approach by calculating all three components of magnetic field by an integral-equation method and obtain the electric fields by Ampere’s law subsequently. Combined with fast subject modeling, this method may provide a viable approach for subject-specific SAR estimations. 1449. Parallelized Algorithm for the Computation of N-Gram Specific Absorption Rate (SAR) on a Graphics Processor Lohith Kini1, Matthias Gebhardt2, Khaldoun Makhoul3, Lawrence L. Wald3,4, Elfar Adalsteinsson1,4 1EECS, Massachusetts Institute of Technology, Cambridge, MA, United States; 2Siemens Medical Solutions, Erlangen, Germany; 3MGH, Harvard Medical School, A. A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States; 4Harvard-MIT Division of Health Sciences and Technology, MIT, Cambridge, MA, United States Specific Absorption Rate (SAR) is a major concern in parallel transmission. Calculating the average N-gram SAR arising at each spatial location is a computationally intractable problem. We propose and demonstrate improvements to a recent algorithm, the fast region-growth algorithm, and exploit its parallelizable computation structure by implementing it on a commercial graphics card. This algorithm is up to 4-7 times faster in runtime regardless of model resolution and number of RF pulse time samples while maintaining accuracy with the fast region-growth algorithm. 1450. Fast GPU FDTD Calculations: Towards Online SAR and B1+ Assessment and Control Davi Correia1, Astrid van Lier2, Martijn de Greef1, Henny Petra Kok1, Johannes Crezee1, Cornelis Antonius Theodorus van den Berg2 1Academic Medical Center, Amsterdam, Noord Holland, Netherlands; 2University Medical Center, Utrecht, Netherlands We present a novel implementation of the FDTD method on a graphics card and how the possibility of simulating SAR during an MRI procedure. It is now possible to simulate the electromagnetics fields distribution in minutes or even in seconds, depending on the resolution. 1451. Local SAR in High Pass Birdcage and TEM Body Coils for Human Body Models at 3T Desmond Teck Beng Yeo1, Zhangwei Wang2, Wolfgang Loew3, Mika Vogel3, Ileana Hancu1 1GE Global Research, Niskayuna, NY, United States; 2GE Healthcare Coils, Aurora, OH, United States; 3GE Global Research, Munich, Germany The high pass birdcage body coil is commonly used in MRI for homogeneous excitation while the transverse electromagnetic volume coil is increasing used in high-field parallel transmit systems. In this work, the numerically computed normalized local SAR of four human body models, placed at three clinically relevant landmark positions, were compared for a 16-rung high-pass birdcage coil and a TEM body coil at 3T. Results show that while high local SAR may be predicted under certain conditions, any comparative generalizations of local SAR between these coils are untenable unless validated with a diverse set of human models at key landmark positions. 1452. Evaluation of E-Field Distributions in Parallel Transmit Systems by Time-Domain Optical Electric-Field Sensors Frank Seifert1, Tomasz Dawid Lindel1, Peter Ullmann2 1Physikalisch-Technische Bundesanstalt (PTB), Abbestr. 2-12, D-10587 Berlin, Germany; 2Bruker BioSpin MRI GmbH, Ettlingen, Germany Ensuring RF safety is crucial for parallel transmit MRI. During parallel transmission the E-field distributions inside and outside the body exhibit a complex time evolution which cannot be covered by calorimetric SAR measurements. We introduce an optical electric field sensor (OEFS) to map non-stationary E-fields during parallel transmit MRI. We tested a customized OEFS system for certain static phase settings of a 4-channel transmit array. Our measurements confirmed a pathologic single hot-spot SAR-distribution as expected from FDTD simulations. In conclusion, the featured OEFS is an appropriate tool to identify possible safety issues of parallel transmit technologies. 1453. Flip Angle and SAR Maps Induced by a Head Displacement in Parallel Transmission and with Cartesian Feedback Nicolas Boulant1, Martijn Cloos1, Alexis Amadon1, Guillaume Ferrand2, Michel Luong2, Christopher Wiggins1, Denis Le Bihan1, David Ian Hoult3 1NeuroSpin, CEA, Gif sur Yvette, France; 2Irfu, CEA, Gif sur Yvette, France; 3Institute for Biodiagnostics, National Research Council, Winnipeg, MB, Canada Despite the considerable variations one can observe in peak local SAR among all possible driving configurations of a Tx-array, the question of how robust both SAR and flip angle distributions are with respect to small perturbations remains. A head displacement for instance not only changes the solutions of Maxwell’s equations but also the tuning of the coils. Now Cartesian feedback has been proposed as a solution to the latter problem. We therefore present the results of simulations showing the effects of small rotations of the head upon the flip angle and SAR with and without Cartesian feedback being applied. 1454. Real Time RF Power Prediction of Parallel Transmission RF Pulse Design at 7T Cem Murat Deniz1,2, Leeor Alon1,2, Ryan Brown1, Hans-Peter Fautz3, Daniel K. Sodickson1, Yudong Zhu1 1Center for Biomedical Imaging, Department of Radiology, NYU School of Medicine, New York, NY, United States; 2Sackler Institute of Graduate Biomedical Sciences, NYU School of Medicine, New York, NY, United States; 3Siemens Medical Solutions, Erlangen, Germany Prediction of parallel transmission RF pulse power deposition into a patient is one of the principal challenges at ultra-high field magnetic strength. We introduced real time calibration system to predict the net power consequences of the parallel transmission RF pulse design before employing designed RF pulses into scanner. High accuracy was achieved in the power deposition estimation of designed RF pulses with respect to actual net power measurements. 1455. Development of an Anatomical Accurate Porcine Head Model to Study Radiofrequency Heating Due to MRI Devashish Shrivastava1, J Thomas Vaughan 1CMRR, Radiology, University of Minnesota, Minneapolis, MN, United States An anatomically accurate porcine head model was developed. The model was developed to help validate the temperature predictions of bioheat equations against direct in vivo fluoroptic measurements and predict non-uniform brain RF heating in swine and humans wearing/not-wearing implantable, conductive medical devices for a variety of field strengths, coil configurations, and head loading positions. The head model was developed by obtaining high resolution images of a porcine head using a Siemens 3T Trio (1.02 mm X 1.02 mm X 1.00 mm, Sequence type: T1MPRAGE) and manually segmenting the brain, cerebral spinal fluid, bone, cartilage, muscle, and air-cavity using MIMICS. 1456. An MRI Simulator for Effects of Realistic Field Distributions and Pulse Sequences, Including SAR and Noise Correlation for Array Coils Zhipeng Cao1, Christopher T. Sica1, Sukhoon Oh1, John McGarrity1, Timothy Horan1, Bu Sik Park1, Christopher M. Collins1 1PSU College of Medicine, Hershey, PA, United States We present a Bloch-based MRI simulator that considers desired sample, field distributions (B0, B1, E1, Gx, Gy, Gz) and pulse sequence for determining effects of field distributions on images (including noise correlation) and SAR. The software is provided with a few basic libraries for simulation of head and body in 8-element transceiver arrays at 3T and 7T and a GUI for designing and executing some simple pulse sequences, but structure of input files is intentionally simple so users can generate their own sample, field distributions, and sequence files. Many fundamental capabilities are demonstrated. 1457. MRI Acoustic Noise Can Harm Research and Companion Animals Amanda M. Lauer1, Abdelmonem M. El-Sharkawy2, Dara L. Kraitchman2, William A. Edelstein2 1Otolaryngology-HNS, Johns Hopkins School of Medicine, Baltimore, MD, United States; 2Radiology/MRI Division, Johns Hopkins School of Medicine, Baltimore, MD, United States Vertebrate animal MRI is an important part of medical research, and veterinary MRI imaging of companion animals is increasing. Human subjects are generally provided with hearing protection against the loud, potentially damaging acoustic noise produced by MRI scanners; this is generally not done for animal MRI subjects. Hearing damage can interfere with research functions for research animals or quality of life for companion animals. We compare typical MRI noise levels to animal hearing thresholds and conclude that MRI exposes many animals to levels of noise and duration that would exceed NIOSH limits for human exposure. 1458. Subjective Acceptance of Ultra-High-Field MR Imaging at 7T in 573 Volunteers Christina Heilmaier1,2, Jens M. Theysohn1,2, Stefan Maderwald1,2, Oliver Kraff1,2, Lale Umutlu1,2, Mark E. Ladd1,2, Susanne C. Ladd1,2 1Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, NRW, Germany; 2Erwin L. Hahn-Institute for Magnetic Resonance Imaging, Essen, NRW, Germany Subjective acceptance of ultra-high-field MRI has not been evaluated in larger study groups yet. For this purpose, 573 volunteers underwent a 7T examination and were afterwards asked about sensations and side effects. Analysis revealed an overall high subjective acceptance of 7T examinations with mainly non-specific factors such as unpleasant room temperature, little contact to the staff or noise being criticized. Compared to 1.5T volunteers described considerably more often nausea or a metallic taste on 7T; however, the average degree of these effects was very low. Volunteers lying ”head first” expressed more complaints than those lying “feet first”. 1459. Dynamic Slew Rate Pulse (DSRP) for PNS Alleviation Yongchuan Lai1 1GE healthcare, Beijing, China In this study, a new gradient pulse, Dynamic Slew Rate Pulse (DSRP), is designed to reduce peripheral nerve stimulation (PNS). Compared with traditional trapezoid pulse, DSRP¡¯s pulse width is much smaller when dB/dt limitation is dominant. 1460. Exposure Measurements on MR-Workers at the Stray Field of 1.5T, 3T, and 7T MR Systems Jens Groebner1, Reiner Umathum2, Michael Bock2, Wolfhard Semmler2, Jaane Rauschenberg2 1Medical Physics in Radiology , German Cancer Research Center, Heidelberg, Germany; 2Medical Physics in Radiology, German Cancer Research Center, Heidelberg, Germany Due to the insufficient experimental data on static magnetic field exposure and new ICNIRP limitations an exposure measurement instrument was developed. With the new probe actual B0 and time varying magnetic fields due to movement in the stray field can be detected simultaneously. Therefore, personal exposure of 10 healthcare workers and MR physicists during normal operating procedures could be determined. Peak exposure limits are exceeded during all procedures. 1461. Physical Simulation Study of Active Noise Control Up to 5 KHz Rudd W. Bernie1, Li Mingfeng1, Teik C. Lim1, Jing-Huei Lee2 1Mechanical Engineering, University of Cincinnati; 2Biomedical Engineering, University of Cincinnati, Cincinnati, OH, United States We demonstrate that active noise control (ANC) is feasible to treat MRI noises at frequencies greater than 2 KHz. Several MRI scan noise and gradient signals were recorded and replayed in a sound quality chamber to simulate the MRI environment. A dummy “wore” headphones containing piezoceramic speakers with condenser microphones installed inside and outside the earpiece to measure the environmental sound in the immediate patient vicinity. During the simulation study, the sound pressure level (SPL) was measured, both with and without the ANC. Results presented show the ANC system attained significant SPL reduction at all frequencies up to 5 kHz. 1462. Dental MRI: Compatibility of Dental Materials Olga Tymofiyeva1, Sven Vaegler1, Kurt Rottner2, Julian Boldt2, Peter Christian Proff3, Ernst-Juergen Richter2, Peter Michael Jakob1 1Dept. of Experimental Physics 5, University of Wuerzburg, Wuerzburg, Bavaria, Germany; 2Dept. of Prosthodontics, Dental School, University of Wuerzburg, Wuerzburg, Germany; 3Dept. of Orthodontics, Dental School, University of Regensburg, Regensburg, Germany Recently, new approaches for application of MRI in various branches of dentistry have been proposed. Dental materials present in the subject’s mouth pose a major concern for dental applications of MRI. Partly contradictory results have been reported regarding the severity of image artifacts caused by different dental materials, usually without consideration of dental applications of MRI. This paper provides classification of standard dental materials from the standpoint of dental MRI, and can serve as a guideline in future dental MRI research. 1463. De-Fibrillation in an MRI Environment Gene Payne1,2, Sathya Vijayakumar1,2, Eugene Kholmovski1,2, Jayne Davis3, Josh Blauer, 2,4, Chris Gloschat, 2,4, Kimberly Lilbok5, Rob MacLeod, 3,4, Dennis Parker1,2, Nassir F. Marrouche5 1UCAIR, Department of Radiology, University of Utah, Salt Lake City, UT, United States; 2CARMA Center, University of Utah, Salt Lake City, UT, United States; 3CVRTI, University of Utah, Salt Lake City, UT, United States; 4Dept. of Biomedical Engineering, University of Utah, Salt Lake City, UT, United States; 5Dept. of Cardiology, University of Utah, Salt Lake City, UT, United States While imaging subjects in an MRI scanner, it may sometimes be necessary to de-fibrillate in order to restore sinus rhythm or resuscitate. Normal de-fibrillator equipment is incompatible with the scanner's magnetic field. Presented is a procedure for de-fibrillating inside the MRI environment. With this procedure, the defibrillator unit was located outside the MRI environment, with select non-ferrous equipment inside and on the subject. This procedure was employed on four separate occasions with an animal subject on the scanner table, and was found to be successful. No problems or safety concerns were observed. 1464. A Simple Cost-Efficient Magneto Alert Sensor (MALSE) Against Static Magnetic Fields Tobias Frauenrath1, Wolfgang Renz2, Thoralf Niendorf1,3 1Berlin Ultrahigh Field Facility, Max-Delbrueck Center for Molecular Medicine, Berlin, Germany; 2Siemens Medical Solutions, Erlangen, Germany; 3Experimental and Clinical Research Center (ECRC), Charité Campus Buch, Humboldt-University, Berlin, Germany While MR is a diagnostic imaging tool which saves lives, magnetic forces of fringe magnetic field components of MR systems on ferromagnetic components can impose a severe occupational health and safety hazard. With the advent of ultrahigh field MR Systems – including passively shielded magnet versions – this risk - commonly known as missile effect /1/ - is pronounced. Numerous accidents have been reported ranging from mechanical damage to patient death. These casualties are probably most widely known through television documentaries and printed media but still present the tip of the iceberg of safety violations (/2/ /3/ /4/). Various policies have been implemented around the world to safeguard healthcare workers, volunteers and patients with the ultimate goal of avoiding unforeseen disasters and injuries. Yüklə 4,25 Mb. Dostları ilə paylaş:
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