RF Safety at High Field: SAR
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Wednesday May 11th
Room 520B-F |
16:00 - 18:00 |
Moderators: |
Blaine Chronik and Christopher Collins |
16:00 |
488. |
Introduction
Christopher M. Collins |
16:12 |
489. |
Validation and Comparison
of Patient-Specific SAR Models
Hanno Homann1, Peter Börnert2, Kay
Nehrke2, Holger Eggers2, Olaf
Dössel1, and Ingmar Graesslin2
1Institute of Biomedical Engineering,
Karlsruhe Institute of Technology, Karlsruhe, Germany, 2Philips
Research Europe, Hamburg, Germany
Estimation of the specific absorption rate (SAR) is
typically performed by numerical simulations using
generic body models. However, this represents the local
SAR in the individual patient only to a limited extent.
This study evaluates a recently proposed approach for
generating patient-specific body models based on
whole-body water-fat-separated MR data. A comparison of
measured and simulated B1-fields showed qualitative and
quantitative agreement. Local SAR values vary
significantly between patients. However, similar
locations of SAR hotspots were observed. These results
increase confidence in the validity of simulated SAR
values.
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16:24 |
490. |
Assessment of RF Safety of
Transmit Coils at 7 Tesla by Experimental and Numerical
Procedures
Andreas Klaus Bitz1,2, Oliver Kraff1,2,
Stephan Orzada1,2, Stefan Maderwald1,2,
Irina Brote1,2, Sören Johst1,2,
and Mark E. Ladd1,2
1Erwin L. Hahn Institute for Magnetic
Resonance Imaging, Essen, Germany, 2Department
of Diagnostic and Interventional Radiology and
Neuroradiology, University Hospital Essen, Essen,
Germany
To guarantee safe operation of RF transmit coils during
MR examinations, the maximum permitted input power at
which SAR limits are not exceeded must be determined. In
particular, SAR estimation in high-field MR demands RF
simulations with detailed numerical models including
transmit coil and heterogeneous human bodies. For
validation of the numerical results, an iterative
procedure for RF safety testing of transmit coils is
proposed which incorporates validation by field
measurements in well-defined, canonical test
configurations. The proposed test procedure has been
applied for safety tests of local and volume coils at 7
Tesla. Results are presented for an 8-channel head and
an 8-channel body coil.
|
16:36 |
491. |
Do constraints on |B1+|
also constrain |E| and SAR in high field MR?
Leeor Alon1,2, Cem Murat Deniz1,2,
Daniel K Sodickson1,2, and Yudong Zhu1,2
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
Since the local specific absorption rate (SAR) is
difficult to measure in vivo, one approach to ensure
patient safety utilizes simulation software such as
xFDTD to compare simulated and measured |B1+| fields,
under the assumption that a high correlation between the
two implies that the simulated local electric fields
also align with the unmeasured experimental local
electric fields. The numerical simulation results in
this abstract indicate, however, that substantial
variations in electric field and SAR are indeed possible
with minimally-varying |B1+| in the setting of distinct
tissue properties at 7T field strength.
|
16:48 |
492. |
SAR Analysis of Parallel
Transmission in Cardiac Imaging at 7T
Xiaoping Wu1, Sebastian Schmitter1,
J. Tian1, J. T. Vaughan1, Kamil
Ugurbil1, and P-F. Van de Moortele1
1CMRR, Radiology, University of Minnesota,
Minneapolis, MN, United States
Recently, there has been an increasing interest in body
imaging at 7T. One critical issue to be addressed in
those applications is the severe transmit B1 (B1+)
inhomogeneity present at such high magnetic field.
Parallel transmission (pTX) shown to be able to reduce
B1+ inhomogeneity holds great potential for body
imaging. However, pTX at 7T involves complicated
behavior of specific absorption rate (SAR) as was shown
in the human brain. Here, we investigated SAR behavior
of pTX when using 3D spoke RF pulses for homogenizing
B1+ in cardiac imaging at 7T.
|
17:00 |
493. |
Quality assessment of
B1-based local SAR estimation as a function of position
within a parallel transmit coil at 3T
Stefanie Buchenau1, Martin Haas1,
Juergen Hennig1, and Maxim Zaitsev1
1Department of Radiology, Medical Physics,
University Medical Center Freiburg, Freiburg, Germany
Published methods for estimating specific absorption
rate via post-processing of measured transmit fields are
restricted to transmission and receive configurations in
quadrature or RF shimming mode. Focusing on the
challenge of estimating unknown magnetic field
components within the present study an extension of the
existing method is demonstrated to make it applicable to
arbitrarily excited multi-transmit arrays. Based on
simulations the quality of this extended method is
investigated as a function of z position within a
particular transmit array designed for parallel
transmission experiments at 3T.
|
17:12 |
494. |
Single element SAR
measurements in a multi-transmit system
Ulrich Katscher1, Christian Findeklee1,
and Tobias Voigt1
1Philips Research Europe, Hamburg, Germany
The additional degrees of freedom in parallel
transmission hamper straight-forward SAR estimations as
applied for single channel transmit systems. Recently, a
method was presented estimating SAR from B1 maps, which,
however, was restricted to quadrature volume coils due
to difficulties distinguishing phase contributions from
RF transmission and reception. This study presents a
method separating these two phase contributions, and
thus, enables SAR estimation not only for quadrature
volume coils, but also for single elements of a transmit
array. The good, quantitative agreement between
simulated and experimental phantom results found in this
study underlines the feasibility of the proposed method.
|
17:24 |
495. |
Generalized model
compression method for peak local SAR estimation
Joonsung Lee1, Matthias Gebhardt2,
Lawrence L Wald3,4, and Elfar Adalsteinsson1,4
1Electrical Engineering and Computer Science,
Massachusetts Institute of Technology, Cambridge, MA,
United States, 2Siemens
Healthcare, Erlangen, Germany, 3A.
A. Martinos Center for Biomedical Imaging, Department of
Radiology, Massachusetts General Hospital, Charlestown,
MA, United States, 4Harvard-MIT
Division of Health Sciences and, Massachusetts Institute
of Technology, Cambridge, MA, United States
Parallel transmit (pTx) applications in MRI are limited
by local SAR constraints. The peak local SAR can be
estimated by monitoring so-called virtual observation
points (VOPs) instead of searching exhaustively over all
voxels in a 3D model. The VOPs can be pre-computed once
for a given model and array configuration and applied in
subsequent computation to efficiently estimate peak
local SAR due to a given pTx RF pulse. We present a
generalization of the original model compression method
by VOPs, whereby we maintain the same accuracy in peak
local SAR estimates, but with a reduced number of VOPs.
|
17:36 |
496. |
A Multi-Channel, High
Dynamic Range, Real Time RF Power Deposition Monitor -permission
withheld
AbdELMonem M. El-Sharkawy1, Di Qian1,2,
Paul A. Bottomley1,2, and William A.
Edelstein1
1Russell H. Morgan Department of Radiology
and Radiological Science, Johns Hopkins University,
Baltimore, Maryland, United States, 2Electrical
and Computer Enginnering, Johns Hopkins University,
Baltimore, Maryland, United States
Accurate and independent real-time measurement of RF
power deposition is essential for MRI safety. We
designed and built a six channel MR compatible,
real-time power profiling system suitable for MRI up to
400MHz. The bench-calibrated system is highly linear
over a 90dB dynamic range and does not interfere with
scanner operation. RF power deposition was measured in
eleven volunteers in a 3T scanner during whole-body MRI.
The results showed considerable differences between the
true power delivered and that estimated by the scanner
supporting the view that scanner specific absorption
rates do not accurately reflect true individual RF
exposure.
|
17:48 |
497. |
Total Proton Resonance
Frequency Shift Coefficient in the Porcine Brain to Image
Radiofrequency Heating in Ultra-high Field MRI
Devashish Shrivastava1, Ute Goerke1,
Shalom Michaeli1, Jingeng Tian1,
Lance DelaBarre1, and John T Vaughan1
1University of Minnesota, Minneapolis, MN,
United States
Total proton resonance frequency (PRF) shift coefficient
was measured in the porcine brain to image
radiofrequency heating with sub-degree C accuracy in
ultra-high field MRI. The total PRF shift coefficient
accounted for the effects of the change in molecular
screening, volume susceptibility, and conductivity on
the local magnetic field with temperature. The total PRF
shift coefficient was measured as -0.02 ppm/0C and was
significantly different from the traditionally used
molecular shielding based PRF shift coefficient of -0.01
ppm/0C.
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