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

Scientific Session Gradient Field Engineering & Monitoring
 

Friday 5 June 2015

Room 716 A/B

08:00 - 10:00

Moderators:

Klass P. Pruessmann, Ph.D., Brian K. Rutt, Ph.D.

08:00 1013.   
Field Monitoring During High-Power Transmission Pulses: A Digital Noise Cancelling Approach
David O. Brunner1, Benjamin E. Dietrich1, Simon Gross1, Thomas Schmid1, Christoph Barmet1,2, and Klaas P. Pruessmann1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, 2Skope Magnetic Resonance Technologies LLC, Zurich, Switzerland

Field monitoring by NMR based field probes can accurately record the dynamic behavior of the B0 and gradient fields in an MRI scanner. During high-power transmission pulses the acquisition of the field probe signals was prevented by receiver saturation by large signals coupled from the transmitter. Although the dynamic range problem was recently solved, noise emitted by the power amplifier at the fluorine frequency decreased the SNR if no appropriately filtered by the scanner. Here we present an approach of digitally cancelling the noise from the power amplifier omitting the high-power filters in the scanner system.

08:12 1014.   Spiral imaging trajectory mapping using high density 25-channel field probe array
Ying-Hua Chu1, Yi-Cheng Hsu1, and Fa-Hsuan Lin1
1Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan

We developed a 2D 25-channel probe array in order to monitor the k-space trajectory and to use the calibrated k-space trajectory to improve the reconstruction of a spiral scan. Images reconstructed using magnetic field estimates up to the 1st order polynomial show the most prominent improvement in homogeneity and anatomical details. Further information from the 2nd and 3rd order field distribution estimates only improved the reconstruction marginally.

08:24 1015.   Placement of field probes for stabilization of breathing-induced B0-fluctuations in the brain
Mads Andersen1,2, Kristoffer H. Madsen1, Lars G. Hanson1,2, Vincent Boer3, Tijl van der Velden3, Dennis Klomp3, Joep Wezel4, Matthias J. van Osch4, Andrew G. Webb4, and Maarten J. Versluis4,5
1Danish Research Centre for Magnetic Resonance, Copenhagen University Hospital, Hvidovre, Denmark, 2Biomedical Engineering Group, DTU Elektro, Technical University of Denmark, Kgs. Lyngby, Denmark, 3Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands, 4C.J. Gorter center, Department of Radiology, Leiden University Medical Center, Leiden, Netherlands, 5Philips Healthcare, Best, Netherlands

B0-fluctuations induced by breathing and body motion lead to artifacts at 7T. A promising solution is to monitor the B0-fluctuations during the scan using external field probes, and update the shims in real-time. The probes must be placed carefully. Here, we provide a simulation of breathing-induced B0-fluctuations and use this simulated field to test different sets of probe positions. We also formulate two optimization problems to guide placement of the field probes. Based on the simulations we recommend two sets of positions and do not recommend full 3rd order shimming based on only 16 field probe measurements.

08:36 1016.   
Continuous 3rd-order field monitoring: Design and application for single-shot shim characterization - permission withheld
Benjamin E. Dietrich1, David O. Brunner1, S. Johanna Vannesjo1, Yolanda Duerst1, Bertram J. Wilm1, and Klaas P. Pruessmann1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

A recently proposed method based on time interleaved acquisition of sets of fast relaxing NMR probes allows for continuous, sequence independent dynamic magnetic field monitoring. As compared to field monitoring relying on single coherence probe acquisitions, this approach removes the limitations in terms of coverable k-space range, acquisition duration and duty cycle. In this work a new 32 channel field camera designed specifically for continuous higher order field monitoring is introduced. The field camera is then used to measure shim impulse response functions within a single-shot of just a few seconds.

08:48 1017.   
Movement Monitoring for MRI via Measurement of Changes in the Gradient Induced EMF in Coil Arrays
E. H. Bhuiyan1, M. E. H. Chowdhury1, P. M. Glover1, and R. Bowtell1
1SPMIC, School of Physics and Astronomy, University of Nottingham, Nottingham, United Kingdom

Here we describe an initial evaluation of a new approach for monitoring head motion, which involves measuring the pattern of voltage amplitudes induced in an array of small coils by the time-varying magnetic field gradients of the MR scanner. We have constructed a rig carrying five small coils, which can undergo controlled translations (in x, y and z) and rotations (about the x- and y-axes) inside the scanner, and have measured the changes in the voltages induced in these coils by time-varying x-,y- and z-gradients as the rig position changes, comparing the results with simulations.

09:00 1018.   Total Current Reduced Design for Brain B0 Shim Coil using Singular Value Decomposition - permission withheld
Kohjiro Iwasawa1, Yosuke Otake1, and Hisaaki Ochi1
1Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo, Japan

Local B0 shim coils placed close to the head have been shown to greatly reduce the B0 inhomogeneity of the brain. However, the total current increases according to the number of channels which were conventionally 32 or more. Here, we demonstrate a total current reduced brain shim coil design using singular value decomposition (SVD). An 18ch shim coil was designed using SVD to mitigate insignificant currents running in opposite directions. Simulated B0 shimming comparison showed that the designed shim coil can greatly reduce the total current compared to conventional local shim coils while maintaining the B0 shimming performance.

09:12 1019.   Development of a Dedicated Asymmetric Head-only Gradient Coil for High-Performance Brain Imaging with a High PNS Threshold
Jean-Baptiste Mathieu1, Seung-Kyun Lee1, Dominic Graziani1, Jian Lin2, Eric Budesheim1, Joseph E. Piel1, Naveen Thiagarajan1, Christopher J. Hardy1, John F. Schenck1, Ek Tsoon Tan1, Eric Fiveland1, Keith Park1, Yihe Hua2, Matt A. Bernstein3, John Huston III3, Yunhong Shu3, and Thomas K.-F. Foo1
1GE Global Research, Niskayuna, NY, United States, 2GE Global Research, China Technology Center, Shanghai, China, 3Mayo Clinic, Rochester, MN, United States

A high-performance, highly-accessible head-only gradient coil with an asymmetrically located, 26 cm field of view was built and tested. The coil was force- and torque-balanced, and can dissipate 25 kW of power on all axes with a standard 3T scanner systems cabinet. The coil was tested inside a conventional, 70-cm bore, whole-body 3T magnet for image quality and patient comfort assessment. Initial results showed high-quality brain images with all three gradients at 80 mT/m and 500 T/m/s without significant peripheral nerve stimulation.

09:24 1020.   Lorentz Damping and the Field Dependence of Gradient Coil Vibroacoustics
Simone Angela Winkler1, Trevor P Wade2, Andrew Alejski2, Charles McKenzie2, and Brian K Rutt1
1Dept. of Radiology, Stanford University, Stanford, CA, United States, 2Robarts Research Institute, The University of Western Ontario, London, Ontario, Canada

We present a new comprehensive modeling approach for understanding acoustic noise in MR gradient coils that accounts for previously neglected but essential Lorentz damping in vibration analysis. We used this approach to predict the dependence of acoustic noise and vibration levels on main field strength. We found that for main field strengths of 3/7/10.5 T, the spectrally-averaged sound pressure levels were 91.2/97.5/100.8 dB and 92.1/89.8/90.5 dB, for the no damping and Lorentz damping cases, respectively. Inclusion of Lorentz damping suggests that gradient acoustics and vibration / mechanical stress may be much more manageable at ultra high fields than previously thought.

09:36 1021.   Thermal Characterization of an All Hollow Copper Insertable Head Gradient Coil
Trevor Paul Wade1,2, Andrew Alejski1, Janos Bartha1, Dina Tsarapkina2, Brian K. Rutt3, and Charles A. McKenzie2
1Robarts Research Institute, Western University, London, Ontario, Canada, 2Medical Biophysics, Western University, London, Ontario, Canada, 3Radiology, Stanford University, Stanford, CA, United States

An insertable head gradient coil, with a target strength of 120 mT/m, was designed and built using hollow copper tubing for all axes to maximize its thermal capability. To establish a safe threshold with respect to temperature, internal and surface temperatures were monitored using thermocouples and a thermal camera while the coil was powered with up to 150amps DC and simultaneously cooled with 15C water at 8.6 L/min. The maximum observed surface temperature rise was 10C, the response roughly similar for each axis. Assuming a safe limit of 30C rise, the coil should be able to operate at 49mT/m RMS.

09:48 1022.   Shielded Matrix Gradient Coil
Sebastian Littin1, Feng Jia1, Stefan Kroboth1, Kelvin Layton1, Huijun Yu1, and Maxim Zaitsev1
1Medical Physics, University Medical Center Freiburg, Freiburg, Germany

A wide variety of imaging strategies relying on nonlinear gradient fields such as parallel image acquisition, reduced field of view acquisition or curved slice imaging have been presented recently. All these techniques require a certain type of nonlinear gradient field. Here we present a design of a shielded matrix gradient coil and show field maps of prototype elements. Greater flexibility of the achievable gradient field shapes might be beneficial for all of the above mentioned methods.