Gradient & Shim Engineering
Friday 24 April 2009
Room 314 10:30-12:30

Moderators:

 Labros S. Petropoulos and Brian K. Rutt
 
10:30 772. The Equivalent Magnetization Current Method Applied to the Design of Gradient Coils for MRI
    Hector Sanchez1, Michael Poole1, Feng Liu1, Stuart Crozier1
1School of Information Technology & Electrical Engineering, The University of Queensland, Brisbane, QLD, Australia
    This paper presents an approach for designing gradient coils that is independent of the shape of the current-carrying surface. The approach employs the equivalency between a uniformly magnetized volume and a surface current density. A linear variation of the magnetization in each boundary element was assumed which is equivalent to a uniform current density. A suite of electromagnetic properties can be parameterised in terms of a thin, piecewise, linearly-magnetised shell; the magnetic flux density, stored energy, power, torque, force and eddy-current induced magnetic flux density were all considered. A QP optimization algorithm was employed to find the magnetisation distribution that satisfies the constraints of the electromagnetic design problem.
     
10:42 773. Curved Planar Gradient Coil Design Using the Boundary Element Method
    Chad Harris1, Blaine Alexander Chronik1
1Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
    In the present study, we have implemented a boundary element method to design and compare the performance of a single axis (y-axis) curved surface gradient coil with varying degree of curvature from planar up to a half cylinder. A curved surface gradient would be of use in the context of providing a specific fourth gradient channel exclusively for very high performance diffusion weighted imaging in a specified volume of tissue such as the breast, prostate, or posterior regions of the brain. It has also been shown experimentally that planar gradient designs offer significantly improved peripheral nerve stimulation properties as compared to traditional whole-body gradient designs.
     
10:54 774. Cool Gradient Coils Designed with Adaptive Regularisation
    Michael Poole1, Hector Sanchez Lopez1, Adnan Trakic1, Stuart Crozier1
1ITEE, University of Queensland, Brisbane, QLD, Australia
    Joule heating in gradient coils can raise their temperature and cause coil deformation, patient discomfort, passive shim heating and in some rare cases can result in failure of the gradient coil. A boundary element method with adaptive regularisation is presented to design coils with reduced maximum current density. Simulations indicate significantly reduced "hot spots" in gradient coils designed with the adaptive regularisation method. The method can also be used to design coils with higher efficiency for a given buildable wire separation and is particularly effective when space for the coil set is limited.
     
11:06 775. Design of a Cylindrical Passive Shim Insert for Human Brain Imaging at High Field
    Mohan Jayatilake1,2, Judd Storrs1,3, Jing-Huei Lee1,3
1Center for Imaging Research, University of Cincinnati, Cincinnati, OH, USA; 2Physics, University of Cincinnati, Cincinnati, OH, USA; 3Biomedical Engineering, University of Cincinnati, Cincinnati, OH, USA
    Magnetic susceptibility variation leads to B0 field inhomogeneity and causes artifacts including line broadening, signal dropout and image distortions. We present a novel method to choose the magnetic susceptibility and dimensions of shim elements in the design of a cylindrical shim insert and validate our technique using simulation.
     
11:18 776. A Transverse Gradient Detection Coil for Dynamic Pre-Emphasis
    Karl Edler1, David I. Hoult2
1National Research Council Institute for Biodiagnostics, Winnipeg, Manitoba, Canada; 2National Research Council Institute for Biodiagnostics, Canada
    It has been shown previously that the effects of eddy currents on a switched z-gradient Gz can be annulled using negative feedback control instead of standard pre-emphasis. Here the method is extended to the more difficult case of transverse gradients. A Gx detection coil was designed with induced voltage proportional to dGx/dt but without response to other field components such as x3, x5, etc. Incorporation of the coil in a feedback loop allowed excellent compensation of eddy currents without a priori knowledge of their effects. Annulment of Bo and higher order transients is amenable to the same approach.
     
11:30 777. Dynamic Bo Shimming at 7 Tesla
    Saikat Sengupta1, Brian E. Welch1,2, Yansong Zhao2, David Foxall2, Piotr Starewicz3, Adam Anderson1, Malcolm Avison1, John Gore1
1VUIIS, Vanderbilt University, Nashville, TN, USA; 2Philips Medical Systems, Cleveland, OH, USA; 3Resonance Research, Inc., Billerica, MA, USA
    Dynamic Shimming (DS) is a technique for obtaining optimal Bo field homogeneity over a volume by updating the shim coil currents for every slice (or in general, location) in a multislice acquisition in real time. DS can produce better field homogeneity within each slice than global volume shimming methods and hence lower susceptibility artifacts. We have implemented DS on a human 7T system. Considerable improvements in Bo homogeneity and image distortions compared to global shimming have been shown. The use of actively shielded Z2 shim coil is also shown to be necessary for maintaining image quality with dynamic shimming.
     
11:42 778. Towards Dynamic Shimming in a 31cm Bore 9.4T System: Analysis of Shim-Shim Inductive Interactions
    Dustin Wesley Haw1, Blaine Alexander Chronik1
1Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
    There are two types of challenges surrounding dynamic shimming: one is shim-shim interactions, and the other is interactions between the shim coils and the rest of the system. We report on the severity of shim-shim interactions for all axes up to 2nd order in tesseral and 3rd order in zonal. The mutual inductances between all shim axes were calculated based on realistic discrete wire patterns. Mutual inductive interactions between realistic shim axes are dominated by only a few coil combinations. Furthermore, it is possible to redesign these axes to reduce this interaction to manageable levels.
     
11:54 779. Improving the Efficiency of Digitally Controlled Switching Gradient Amplifiers for Driving Different Gradient Insert Coils
    Francisco Manuel Martinez-Santiesteban1, Jian-xiong Wang2, Brian K. Rutt1
1Robarts Research Institute, University of Western Ontario, London, Ontario, Canada; 2Applied Science Laboratory, GE Healthcare, London, Ontario, Canada
    A method of tuning Switching Gradients Amplifiers to drive different gradient insert coils is presented. A systematic search of the global minimum of RMS(V-Drive) improved the efficiency of the amplifiers whereas the error of the gradient fields was reduced by minimizing RMS(I-Error). The tuning method allowed the use of the same gradient amplifiers for ten different coils, with inductances in the range of 200 to 1300 μH and estimated cut-off frequencies between 10 and 125 KHz. We achieved an average improvement of 55% for RMS(V-Drive) and 47% for RMS(I-Error) with respect to the values obtained for default parameter settings.
     
12:06 780. A Third-Order Field Camera with Microsecond Resolution for MR System Diagnostics
    Christoph Barmet1, Bertram J. Wilm1, Matteo Pavan1, Klaas P. Pruessmann1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland
    A magnetic field-camera, based on 16 NMR probes, with microsecond temporal resolution is presented. It allows for simultaneous dynamic magnetic field measurements of all field components up to 3rd spatial order. Contrary to conventional field-cameras, it captures the full BW of the gradient fields. Three situations were studied on a 3T scanner: the response to abrupt gradient changes, drifts and eddy currents during an fMRI scan and the subsequent magnet ‘cooling’. This field-camera is a valuable tool for MR systems engineering and diagnostics: the assessment of gradient coils and amplifiers, fast pre-emphasis calibration, testing and tuning of higher-order shim systems.
     
12:18 781. Continuous Magnetic Field Mapping with Pulsed 1H NMR Probes
    Pekka Sipilä1,2, James Tropp3, Sebastian Greding1, Gerhard Wachutka2, Florian Wiesinger1
1GE Global Research, Munich, Bavaria, Germany; 2Institute for Physics of Electrotechnology, Munich University of Technology, Munich, Germany; 3Global Applied Sciences Laboratory, GE Healthcare, CA, USA
    Description of apparatus to improve image quality during MRI-scan by measuring the magnetic fields with pulsed NMR probes. Probes are excited multiple times during each TR for maximum SNR, and, thus, the NMR samples within are not required to be susceptibility matched anymore.