|Pulse Sequences: Post-Cartesian|
Fast Diffusion Imaging Using Undersampled Propeller
Mathias Engström1, 2, Anders Nordell1, 3, Bo Nordell1, 3, Stefan Skare4
1Karolinska University Hospital, Stockholm, Sweden; 2Karolinska Institute, Stockholm, Sweden; 3Karolinska Institute, Sweden; 4Stanford University, Stanford, California , USA
A new DWI Propeller reconstruction is presented that dramatically reduces scan time when imaging multiple diffusion directions. By assigning diffusion directions to fewer blades than what is required to grid a full resolution k-space and using a custom gridding procedure, high resolution diffusion tensor imaging is possible in a limited scan time.
Retrospective, Reference-Less Ghosting Correction in
Jon-Fredrik Nielsen1, Krishna S. Nayak1
1University of Southern California, Los Angeles, California , USA
In PROPELLER EPI, anisotropic gradient time delays cause 2D phase errors for "oblique" blades that are not aligned with the physical gradient axes, resulting in ghosting that cannot be removed using conventional 1D correction schemes. We propose an approach to 2D phase correction that involves estimating on-axis delays from the acquired PROPELLER EPI data itself, and performing interlaced sampling reconstruction along the phase-encode direction for each blade. The proposed correction technique is performed retrospectively, and does not require reprogramming the pulse sequence or obtaining additional reference scans.
Calibration Free Distortion Correction
for Propeller EPI
Stefan Skare1, Jesper LR Andersson2, Roland Bammer1
1Stanford University, Palo Alto, California , USA; 2Oxford University, Oxford, UK
GRAPPA accelerated short-axis propeller EPI (SAP-EPI) has earlier been shown to be a promising sequence for high-resolution diffusion imaging due to its low distortion properties. In this work, we have developed an extended model of the reversed gradient polarity method to correct for the residual geometric distortions in SAP-EPI. This algorithm estimates a single distortion field from all blades simultaneously, instead of pair-wise correction of two oppositely distorted blades as previously done. This makes the field estimation process more overdetermined and allows us to perform the correction using only blades acquired over 0 to 180 degrees.
A Method for Removing Off-Resonance Artifact in
James G. Pipe1, Donglai Huo1, Eric Aboussouan1, Zhiqiang Li2
1Barrow Neurological Institute, Phoenix, Arizona , USA; 2GE Healthcare, Phoenix, Arizona , USA
Turboprop is a fast PROPELLER hybrid of FSE with EPI-like echo trains. Compared to conventional PROPELLER, it reduces SAR and increases sampling efficiency, but introduces artifacts from off-resonance. This work illustrates a method to dramatically reduce these artifacts, at the expense of narrower blade-widths.
Jie Deng1, 2, Andrew C. Larson1
1Northwestern University, Chicago, Illinois, USA
Multi-shot TSE-based PROPELLER imaging has been shown to be less sensitive to motion artifacts. However, PROPELLER imaging requires longer imaging time compared to conventional TSE due to oversampling of k-space. Also, it has been demonstrated that regional motions cannot be effectively corrected based on an entire image including both static and moving objects. In this study, we propose a targeted-PROPELLER technique employing inner-volume excitation method to limit the field-of-view (FOV) for (i) imaging a small ROI to shorten imaging time and (ii) targeting the moving objects for robust regional motion correction.
Abdominal Imaging in Free-Breathing Mice Using
Prachi Pandit1, Kevin F. King2, G Allan Johnson1
1Duke University, Durham, North Carolina, USA; 2GE Healthcare, Waukesha, Wisconsin, USA
T2-weighted and diffusion-weighted imaging is effective for tumor visualization, but it is challenging in free-breathing mice due to higher spatial resolution and faster physiologic motion. The requirements in this case are: immunity to cardiac and respiratory motion; short TE to minimize T2* decay at the higher magnetic fields required for mice; and short scan time with long TR required for T1 recovery. In this work, we use the PROPELLER sequence on a 7T scanner with high strength, rapid gradient coils to acquire artifact-free abdominal images in free-breathing mice with good T2-weighting and in-plane resolution of 97µm.
Assessment of Concomitant Gradient
Blurring in Spiral In-Vivo Scans at 1.5 T
Christopher Thomas Sica1, Craig H. Meyer1
1University of Virginia, Charlottesville, Virginia , USA
Off-resonance phase in spiral scans can lead to undesirable blurring artifacts. An important but not commonly considered source of off-resonance phase is concomitant gradients. In this abstract, we demonstrate the severity of the concomitant gradient artifact in spiral in-vivo scans at 1.5T. In-vivo data sets with varied scan plane orientations and offsets were acquired to qualitatively assess the concomitant blurring pattern, and the distance from isocenter at which blurring starts to become significant. Concomitant blurring starts to becomes an issue around 6-7.5 centimeters from isocenter in the z direction, across a multitude of scan plane orientations.
Optimization of Undersampled Variable Density Spiral
Trajectories Based on Incoherence of Spatial Aliasing
Yoon-Chul Kim1, Krishna S. Nayak1
1Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, California , USA
Variable density spiral (VDS) imaging typically samples low spatial frequencies densely and samples high spatial frequencies sparsely to achieve higher temporal or spatial resolution than uniform density spiral (UDS) imaging. Reconstructed images are prone to aliasing artifacts due to the undersampling of high spatial frequencies, but they are often considered to be acceptable. However, unlike UDS, VDS imaging requires an appropriate selection of variation in sampling density. We propose a way of optimizing VDS trajectory design based on its point spread function and demonstrate its effectiveness in phantom studies.
A Fast Method for Designing Time-Optimal Gradient
Waveforms for Arbitrary K-Space Trajectories
Michael Lustig1, Seung Jean Kim1, John Mark Pauly1
1Stanford University, Stanford, California , USA
A fast and simple algorithm for designing time-optimal waveforms is presented. The algorithm accepts a given arbitrary multi-dimensional k-space trajectory as the input and outputs the time-optimal gradient waveform that traverses k-space along that path in minimum time. The algorithm is non-iterative, and its run time is independent of the complexity of the curve, i.e. the number of switches between slew-rate limited acceleration, slew-rate limited deceleration and gradient amplitude limited regions. The key in the method is that the gradient amplitude is designed as a function of arc length along the k-space trajectory, rather than as a function of time. Several trajectory design examples are presented.
Variable Density Bunched Phase Encoding
Hisamoto Moriguchi1, Jeffrey L. Duerk2, Yutaka Imai1
1Tokai Unversity School of Medicine, Isehara, Japan; 2University Hospitals of Cleveland/Case Western Reserve University, Cleveland, Ohio, USA
Bunched Phase Encoding (BPE) has recently been proposed as a new fast data acquisition method in MRI. In BPE, zigzag k-space trajectories often need to be measured because actual trajectories often deviate from designed trajectories due to eddy currents. This may limit actual implementation of BPE. In this study, we show an improved BPE acquisition method that does not require k-space trajectory measurement. In our newly proposed method, variable density (VD) zigzag trajectories are used. VD-BPE obviates the need for cumbersome k-space trajectory measurement and achieves accurate image reconstruction. VD-BPE is quite useful and facilitates implementation of BPE in practice.