David O. Brunner¹, Benjamin E. Dietrich¹, Simon Gross¹, Thomas Schmid¹, Christoph Barmet^1,2, and Klaas P. Pruessmann^1
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.

Ying-Hua Chu¹, Yi-Cheng Hsu¹, and Fa-Hsuan Lin^1
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.

Mads Andersen^1,2, Kristoffer H. Madsen¹, Lars G. Hanson^1,2, Vincent Boer³, Tijl van der Velden³, Dennis Klomp³, Joep Wezel⁴, Matthias J. van Osch⁴, Andrew G. Webb⁴, and Maarten J. Versluis^4,5
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.

Benjamin E. Dietrich¹, David O. Brunner¹, S. Johanna Vannesjo¹, Yolanda Duerst¹, Bertram J. Wilm¹, and Klaas P. Pruessmann^1
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.

E. H. Bhuiyan¹, M. E. H. Chowdhury¹, P. M. Glover¹, and R. Bowtell^1
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.

Kohjiro Iwasawa¹, Yosuke Otake¹, and Hisaaki Ochi^1
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.

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. Foo^1
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.

Simone Angela Winkler¹, Trevor P Wade², Andrew Alejski², Charles McKenzie², and Brian K Rutt^1
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.

Trevor Paul Wade^1,2, Andrew Alejski¹, Janos Bartha¹, Dina Tsarapkina², Brian K. Rutt³, and Charles A. McKenzie^2
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 15°C water at 8.6 L/min. The maximum observed surface temperature rise was 10°C, the response roughly similar for each axis. Assuming a safe limit of 30°C rise, the coil should be able to operate at 49mT/m RMS.

Sebastian Littin¹, Feng Jia¹, Stefan Kroboth¹, Kelvin Layton¹, Huijun Yu¹, and Maxim Zaitsev^1
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.