ISMRM 23rd Annual Meeting & Exhibition • 30 May - 05 June 2015 • Toronto, Ontario, Canada

Scientific Session • RF Field & Exogenous Agent

Tuesday 2 June 2015

Room 714 A/B

13:30 - 15:30


Thoralf Niendorf, Ph.D., T.B.A.

13:30 0374.   A rotating transmit coil and 32ch receive array for high-resolution brain imaging of DBS patients
Laleh Golestnirad1, Boris Keil1, Giorgio Bonmassar1, Azma Mareyam1, and Lawrence Leory Wald1
1Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, United States

Deep brain stimulation (DBS) is a neurosurgical procedure that involves implanting electrodes in the brain to electrical pulses to deep nuclei inside the brain for treatment of a variety of disabling movement and psychiatric disorders. We built and simulated a transmit/receive head coil, composed of a linearly polarized rotating birdcage transmitter and a close-fit 32 channel receive array. We showed that this coil has slab-like E-Null region that is thick enough at 1.5 T to encompass the whole DBS implant to significantly reduce SAR. We also showed that insertion of a 32-channel close-fit receive array has a negligible effect on the transmit field distribution and does not increase the local SAR levels at the DBS lead in 1.5 T.

13:42 0375.   Reduction of worst-case local SAR with constraints on RF shimming parameters based on principal component analysis - permission withheld
Kosuke Ito1, Yoshihisa Soutome1,2, Yukio Kaneko2, and Masahiro Takizawa1
1Hitachi Medical Corporation, Kashiwa, Chiba, Japan, 2Central Research Laboratory, Hitachi Ltd, Kokubunji, Tokyo, Japan

The inhomogeneity of B1 map and high local SAR are major problems in high field MRI. To improve the homogeneity of B1, RF shimming is used. The parameters of RF shimming affect the homogeneity of B1 and the local SAR. As the number of RF transmission channels increases, the number of possible combinations of RF shimming parameters increases, and peak local SAR may increase. In this study, we applied principal component analysis to determine constraints on RF shimming parameters. Exclusion of high local SAR combinations of RF shimming parameters without degrading the homogeneity of B1 was demonstrated by numerical simulation.

13:54 0376.   
Reconstruction of the Local SAR Deposition Based on B1+ Field Data Using CSI-EPT
Edmond Balidemaj1, Cornelis A.T. van den Berg2, Hans Crezee3, Aart Nederveen4, and Rob Remis5
1Radiotherapy, Academic Medical Center, Amsterdam, Amsterdam, Netherlands, 2Radiotherapy, UMC Utrecht, Utrecht, Netherlands, 3Radiotherapy, Academic Medical Center, Amsterdam, Netherlands, 4Radiology, Academic Medical Center, Amsterdam, Netherlands, 5Circuits and Systems Group, TU Delft, Delft, Netherlands

CSI-EPT can be used to reconstruct electric properties and the electric field based on B1+ fields. In this work we demonstrate the ability of CSI-EPT to reconstruct in vivo SAR distribution and compare it to the SAR distribution obtained with FDTD simulations.

14:06 0377.   Thermo-Acoustic Ultrasound Detection of RF Coil and Tip SAR
Greig Scott1, Maryam Etezadi-Amoli1, Pascal Stang2, Hao Nan1, Miaad Aliroteh1, Amin Arbabian1, and John Pauly1
1Electrical Engineering, Stanford University, Stanford, California, United States, 2Procyon Engineering, California, United States

Detection of RF power deposition is important in MRI SAR analysis, and in RF safety of implanted leads. We demonstrate thermo-acoustic ultrasound as a non-invasive means to detect rapid RF power deposition at MRI frequencies. Results are shown using CW modulation methods for a exposed lead and stripline RF coil.

14:18 0378.   NSsaFe study: Observational study on the incidence of nephrogenic systemic fibrosis in renal impaired patients following gadoterate meglumine administration.
Adelard I De Backer1
1Radology, General Hospital Sint-Lucas, Ghent, Oost-Vlaanderen, Belgium

Purpose: Nephrogenic Systemic Fibrosis (NSF) has been reported to occur in patients with impaired renal function after administration of gadolinium-based contrast agents (GBCAs) for Magnetic Resonance Imaging (MRI). The aim of this prospective study is to evaluate the occurrence of adverse reactions with special focus on NSF after administration of gadoterate meglumine (Dotarem®) in a large series of patients with moderate, severe and end stage renal insufficiency and after renal transplantation. Conclusion: Interim analysis of the NSsaFe study showed no cases of NSF in patients with moderate to severe renal impairment. NSsaFe study confirms the excellent safety profile of gadoterate meglumine.

14:30 0379.   
A Vectorized Formalism for Efficient SAR Computation in Parallel Transmission
Mihir Pendse1 and Brian Rutt1
1Radiology, Stanford University, Stanford, CA, United States

We formulate an expression for a normalized SAR cost function that encapsulates all local/global SAR and average power terms into a single expression that is evaluated to determine whether a given RF pulse sequence abides by all time-averaged safety/hardware limits. A vectorized oracle for evaluating this function and its derivatives is described and implemented. With the use of a GPU, the computation time of the oracle is shown to be approximately constant even for large number of local SAR voxels and subpulses. The vectorized oracle allows for direct SAR-aware pulse optimization without the need for SAR matrix formation and compression.

14:42 0380.   Correlation of psSAR and tissue specific temperature for 7T pTx head coils - a large scale simulation study
Frank Seifert1, Gerd Weidemann1, and Bernd Ittermann1
1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany

Currently, psSAR10g is the basic safety measure for transmit coil arrays. But there is little knowledge about the correlation of psSAR10g with local tissue temperature which determines the risk of tissue damage according to the thermal dose concept. In this study we performed thermal simulations for a large set of steering conditions, i.e. amplitudes and phases of the driving voltages of a 7T 8-channel head coil array, in order to determine the maximum steady state temperatures of different tissue types. A fast implementation of Pennes Bioheat Equation on a GPU was developed which allows to perform >1000 simulation runs in a reasonable time.

14:54 0381.   Prospective assessment of transient dyspnea and arterial oxygen saturation after injection of gadoxetic acid in a large patient cohort
Utaroh Motosugi1,2, Peter Bannas1,3, Candice A. Bookwalter1, and Scott B. Reeder1,4
1Radiology, University of Wisconsin, Madison, WI, United States, 2Radiology, University of Yamanashi, Yamanashi, Japan, 3Radiology, University Hospital Hamburg-Eppendorf, Humburg, Germany, 4Medical Physics, University of Wisconsin, Madison, WI, United States

Self-limited shortness of breath (transient dyspnea) after the injection of gadoxetic acid is a newly recognized adverse effect. This phenomenon is clinically relevant because breath-hold failure can leads to image degradation in dynamic contrast-enhanced liver MRI, particularly in the arterial phase. However, it is not clear whether the image degradation is only associated with transient dyspnea. In this study we prospectively evaluated the association between subjective transient dyspnea, oxygen saturation, breath-hold fidelity and image degradation.

15:06 0382.   Variations in peak local SAR due to coupling – comparison between various pTx array simulation methods
Shubham Gupta1, R Allen Waggoner1, Keiji Tanaka1, and Kang Cheng1,2
1Lab. for Cognitive Brain Mapping, RIKEN Brain Science Institute, Wako, Saitama, Japan, 2RRC, RIKEN Brain Science Institute, Wako, Saitama, Japan

This study compared three methods of Parallel Transmission array simulation, namely, A) “Ideally decoupled” method, B) “Coupled” method and C) “Field superposition” method using S-parameters and calculated peak 10gm average SAR (pk-10gmAvgSAR) for 5000 random amplitude and phase combinations in two adult human models for a 4-channel pTx array coil. The results suggest that compared to “Coupled” method, both “Ideally decoupled” and “Field superposition” methods underestimate the pk-10gmAvgSAR for about 80% of times, hence it is crucial to include coupling in pTx array simulations. Since “field superposition” method can both under- and over-estimate the pk-10gmAvgSAR, it is better to perform simulations again with optimum capacitor values being incorporated into the simulation model.

15:18 0383.   B1-based SAR determination for local RF transmit coils
Ulrich Katscher1, Marina Braun2, Christian Findeklee1, Christoph Leussler1, Ingmar Graesslin1, Peter Vernickel1, and Michael Morlock2
1Philips Research Europe, Hamburg, Germany, 2University of Technology, Hamburg, Germany

For safety management, local SAR is usually estimated via models, but could alternatively be estimated from individually measured B1-maps. A key issue of this method is that the longitudinal component of B1, Bz, cannot be measured via MRI, and thus, has to be estimated. Satisfactory results were reported assuming Bz = 0 for birdcage-type coils, which is less valid for the upcoming type for local surface RF coils, particularly near transversely oriented conductors. This study investigates different ways for Bz modelling and the resulting impact on local SAR, based on phantom / realistic patient model simulations at 3 T.