ELECTRONIC POSTER - ismrm

and a relaxed fixed point iteration scheme is introduced to alter the coil windings iteratively and lower the hot spot temperature. The
new coil windings display a considerable improvement in hot spot temperature, at no cost to coil performance, when compared to
equivalent minimum power x-gradient coils. The model can be adapted easily for other geometries, thermal properties, cooling
mechanisms and non-linear constraints.
14:00 3945. A Temperature Distribution Model for Gradient Coils
Peter T. While 1 , Larry K. Forbes 1 , Stuart Crozier 2
1 School of Maths and Physics, University of Tasmania, Hobart, TAS, Australia; 2 School of Information
Technology and Electrical Engineering, University of Queensland, Brisbane, QLD, Australia
Excessive heating of gradient coils is a considerable concern. An analytic model is presented for calculating theoretically the spatial
temperature distribution for cylindrical gradient coils. The model includes Ohmic heating due to current density in resistive material,
thermal conduction through a copper layer and an epoxy former, and radial heat loss to the environment. A great number of coil
parameters can be varied, including geometry, electrical and thermal properties, and results are shown for a standard x-gradient coil
under three different types of cooling. In addition, temperature rise-times are predicted using a time-dependent solution for hot spot
temperature.
14:30 3946. Safety Considerations for a PatLoc Gradient Insert Coil for Human Head Imaging
Chris A. Cocosco 1 , Andrew J. Dewdney 2 , P Dietz 2 , M Semmler 2 , Anna M. Welz 1 , Daniel
Gallichan 1 , Hans Weber 1 , Gerrit Schultz 1 , Juergen Hennig 1 , Maxim Zaitsev 1
1 Dept of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, B.W., Germany;
2 Siemens Healthcare, Erlangen, Germany
We present the design considerations and evaluation measurements for the safety of a PatLoc (Parallel Acquisition Technique using
Localized Gradients) gradient insert coil designed for human head imaging on a 3T MRI system. This novel concept has the potential
to allow higher gradient switching rates while not exceeding the Peripheral Nerve Stimulation (PNS) limits. Based on the presented
experimental measurements and simulations, we consider imaging human volunteers with this system to be safe.
15:00 3947. First Results for Diffusion-Weighted Imaging with a 4th Channel Gradient Insert
Rebecca E. Feldman 1 , Jamu Alford 2 , Timothy Scholl 2 , Blaine A. Chronik 2
1 Medical Biophysics, University of Western Ontario, London, Ontario, Canada; 2 Physics and Astronomy,
University of Western Ontario, London, Ontario, Canada
Gradients in diffusion-weighted imaging play two distinct roles. A gradient with a linear range sufficient to cover the sample is
required for imaging. For the diffusion-weighting, the gradient is only required to be strong. A fourth gradient was inserted into the
bore for the purposes of diffusion-weighting only. The inserted gradient was pulsed twice, before and after the 180 degree RF pulse,
and the image acquisition was done using the whole body gradients. Using the insert b-values greater than 1000 s/mm2 were obtained
a time frame that would permit only a b-value of 100 using the whole-body gradients.
Hybrid Systems: MR+
Hall B Monday 14:00-16:00 Computer 53
14:00 3948. A Depth-Encoding Detector Module for an MR-Compatible PET Insert
Yibao Wu 1 , Yongfeng Yang 1 , Bo J. Peng 1 , Simon R. Cherry 1
1 Department of Biomedical Engineering, University of California, Davis, Davis, CA, United States
The next generation of MR-compatible PET insert is under development for small-animal imaging providing greater than an order of
magnitude increase in sensitivity by utilizing 20 mm thick scintillator crystal elements with excellent stopping power. A detector
module based on avalanche photodiode read out with depth-of-interaction encoding was designed and evaluated to overcome the
resolution-degrading parallax errors associated with such thick detectors. Detectors for the new PET insert were characterized in terms
of crystal identification and energy spectra. Data were acquired outside a 7T MR scanner, inside the MR scanner, with and without
sequences running. An MR phantom also was measured with the PET detector module inside the MR system. No significant
interference between the PET detector module and the MR system were observed. The design of the new PET insert based on these
detectors is presented.
14:30 3949. Design of a Dynamically-Controlled Resistive Shield for a Combined PET and
Superconducting MRI System for Small Animal Imaging
Geron Andre Bindseil 1 , Timothy J. Scholl 1 , William B. Handler 1 , Chad T. Harris 1 , Blaine
A. Chronik 1
1 Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
Combining conventional PET and MRI faces numerous technical challenges, particularly the sensitivity of photomultiplier tube-based
(PMT) PET detectors to magnetic fields. The authors describe an approach to PET/MRI in which a resistive electromagnet shield is
used to null the field at the PMTs of a conventional PET system in the vicinity of a superconducting MRI system. The electromagnetic
characteristics of the shield coil are presented. This approach benefits from allowing the use of commercially available PET systems,
which include state-of-the-art timing & energy resolution, high sensitivity, and highly optimized event processing hardware.