ISMRM 21st Annual Meeting & Exhibition 20-26 April 2013 Salt Lake City, Utah, USA

Correction for Eddy Currents & Off-Resonance
Wednesday 24 April 2013
Room 355 BC  16:00 - 18:00 Moderators: Michael H. Buonocore, Dan Xu

16:00 0549.   
Eddy Current Compensated IDEAL
Trevor P. Wade1,2, Curtis N. Wiens3, and Charles A. McKenzie1,3
1Medical Biophysics, University of Western Ontario, London, Ontario, Canada, 2Robarts Research Institute, University of Western Ontario, London, Ontario, Canada, 3Physics and Astronomy, University of Western Ontario, London, Ontario, Canada

In chemical shift imaging, eddy current induced phase errors in the first echo of multi-echo fat-water separation techniques result in clinically significant biases in the estimates of proton density fat fractions (PDFF). Previously, this bias has been removed by modifying the reconstruction at a cost of reduced noise performance. Instead, we propose using additional gradient pulses to induce similar eddy currents for all echoes. The proposed modification to the pulse sequence was tested on the liver of a healthy volunteer and showed reduced bias in PDFF.

16:12 0550.   
Linear Phase Shift Correction for Field Map Estimation with Bipolar Gradient Dual-Echo Sequence Using the Noise PDF
Hiroyuki Takeda1 and Boklye Kim2
1Radiology, University of Michigan, Ann Arbor, Michigan, United States, 2Radiology, University of Michigan, ann arbor, MICHIGAN, United States

We propose a field map acquisition method using a dual-echo fast field echo (DEFFE) sequence. It is advantageous that the DEFFE sequence measures an image pair with different echo times simultaneously. Consequently, there is little displacement between the images unlike the image pair measured by two separated echo sequences. However, the DEFFE introduces linear phase errors to the image pair. In this work, we present an appropriate probability density function (PDF) of nonuniform noise in the field map. Our phase correction method derived from the PDF properly removes outliers and works fine even in the presence of the wrapping effects.

16:24 0551.   Rapid Method of Mapping Eddy Current Fields
Andrew J. Wheaton1 and Wayne R. Dannels1
1Toshiba Medical Research Institute USA, Mayfield, OH, United States

The purpose of this investigation is to develop a rapid method of volumetric field mapping for the purpose of measuring linear and higher-order eddy current-induced gradient fields. The method includes a novel pulse sequence using a grid tag module to sensitize the magnetization to eddy-induced moment accumulation. Any conventional sequence can be used for detection thus enabling fast acquisition. Results are shown on a commercial scanner demonstrating eddy current maps with higher order spherical harmonic terms acquired in less than 20 seconds.

16:36 0552.   
Image Reconstruction Using the Gradient Impulse Response for Trajectory Prediction
Nadine N. Graedel1, Signe Johanna Vannesjo1, Lars Kasper1, Simon Gross1, Benjamin E. Dietrich2, Christoph Barmet1,3, and Klaas P. Pruessmann1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, 2University and ETH Zurich, Zurich, Switzerland, 3Skope Magnetic Resonance Technologies, Zurich, Switzerland

Assuming that the gradient chain behaves as a linear time-invariant system, a gradient impulse response function (GIRF) can be determined. A measured GIRF was used in this work to predict the actual k-space trajectories for EPI image reconstructions. This allowed for reduction of ghosting caused by encoding field imperfections. Some residual artifacts remain as compared to using trajectories obtained with concurrent field monitoring. The GIRF method however does provide a simple tool for artifact reduction for arbitrary k-trajectories, and depending only on a one-time calibration of the system.

16:48 0553.   
Automatic Reconstruction of Gradient Echo Imaging Sequences by Concurrent and Continuous Monitoring of Gradient and RF Waveforms
David Otto Brunner1, Benjamin E. Dietrich1, Christoph Barmet1,2, Simon Gross1, Bertram J. Wilm1, and Klaas P. Pruessmann1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Zurich, Switzerland, 2Skope Magnetic Resonance Technologies, Zurich, Switzerland

Two major shortcomings of conventional monitoring are limited image resolution (≈probe diameter/2) as well as finite acquisition and dead time due to the relaxation properties of the field probes. Therefore the monitoring approach needed to be closely entangled with the image acquisition and was bound to sequences with low repetition times. These limitations are overcome by fast re-excitation of the field probes and further recording the stray-coupled high power RF signals of the scanner. Such comprehensive information about the spin dynamics is the used for a automatic reconstruction of various gradient echo based sequences.

17:00 0554.   
Correction of Dynamic B0 Field Changes in MRSI of the Prostate at 7T Using an Internal Field Probe
Catalina S. Arteaga de Castro1, Vincent Oltman Boer1, Mariska P. Luttje1, Marco van Vulpen1, Peter R. Luijten1, Uulke A. van der Heide2, and Dennis W.J. Klomp1
1Imaging Division, University Medical Center Utrecht, Utrecht, Utrecht, Netherlands, 2Radiotherapy, The Netherlands Cancer Institute, Amsterdam, Amsterdam, Netherlands

Physiological and patient dependent susceptibility effects take place during the MR sessions, particularly in the human body at high field strengths. Without prior knowledge of where the effects originate, these B0 field dynamic variations can be monitored and corrected as demonstrated by using an internal field probe for MRSI of the human prostate. Correcting MRSI data with the simultaneously acquired field probe data, line width sharpens, artifacts levels are reduced and peaks are better resolved compared to the non-corrected data.

17:12 0555.   Sliding Interleaved Cylinder (SLINCY) Imaging with Dynamic Center Frequency Adjustment
Kie Tae Kwon1, Adam B. Kerr1, Bob S. Hu2,3, and Dwight G. Nishimura1
1Electrical Engineering, Stanford University, Stanford, CA, United States, 2Palo Alto Medical Foundation, Palo Alto, CA, United States, 3HeartVista Inc., Los Altos, CA, United States

A sliding interleaved cylinder (SLINCY) acquisition uses a 3D concentric cylinders trajectory to reduce venetian blind artifacts. Previously, we employed the SLINCY acquisition for a non-contrast-enhanced magnetization-prepared 3D SSFP sequence to improve artery-vein contrast in the lower extremities. One challenge for this approach is to cover a large FOV without SSFP banding artifacts to yield a uniform arterial signal. In this work, we exploited the thin-slab-scan nature of SLINCY to dynamically adjust the center frequencies of slabs for banding artifact reduction using a separately acquired 2D field map. In vivo studies on healthy volunteers demonstrated the feasibility of this approach.

17:24 0556.   An Improved Maxwell Term Compensation Method for FSE Images
Weiwei Zhang1, Yongchuan Lai1, and Xiaocheng Wei1
1GE Healthcare, Beijing, Beijing, China

We proposed an improved method for Maxwell compensation which compensates slice gradient (Gx) and readout gradient (Gy) mutually. This method has 3 benefits compared to traditional methods: (1) Special attention is paid on minimizing the ESP prolonging. (2) The gradients are derated as much as possible without prolonging the ESP, which helps to reduce the cross axis Maxwell term and eddy current. (3) Mutual compensation makes the Maxwell compensation for FSE with flow compensation more effective.

17:36 0557.   
Fast Imaging of Metallic Implants by Data Subsampling
Bragi Sveinsson1, Pauline W. Worters2, Garry E. Gold1, and Brian Andrew Hargreaves1
1Stanford University, Stanford, CA, United States, 2GE Healthcare, Menlo Park, CA, United States

Slice Encoding for Metal Artifact Correction (SEMAC) corrects for magnetic field variations by applying view-angle tilting in the readout direction to avoid in-plane distortion, and phase encoding in the slice direction to resolve distorted slice profiles. Phase encoding in the y and z directions can lead to long scan times. We demonstrate a way to reduce the scan time by 50% by subsampling ky-kz space in a checkerboard fashion. The resulting aliased copies do not interfere with the desired image because of the shape of the slice profile and can be removed during post-processing. We demonstrate the feasibility of this approach on a phantom and for in vivo scans, in combination with partial Fourier and parallel imaging.

17:48 0558.   
Accelerating Sequences in the Presence of Metal by Exploiting the Spatial Distribution of Off-Resonance
Matthew R. Smith1, Nathan S. Artz1, Kevin M. Koch2, and Scott B. Reeder1,3
1Radiology, University of Wisconsin, Madison, WI, United States, 2Applied Sciences Laboratory, General Electric, Waukesha, WI, United States, 3Medical Physics, University of Wisconsin, Madison, WI, United States

To successfully image near metallic implants, multiple independent acquisitions are often required at different RF offsets and combined. This dramatically increases scan time. However, each frequency bin contains unique spatial information that is analogous to the spatial sensitivity of multi-channel receiver coils. We demonstrate that this behavior can be exploited to accelerate k-space encoding using a reconstruction similar to parallel imaging. This work demonstrates feasibility of acceleration using a femoral stem of a titanium hip implant in a phantom. Independent acquisition of each off-resonance bin provides a distinct advantage to optimize acceleration by enabling off-resonance dependent reduction factors.