Diffusion Acquisition
Monday 20 April 2009
Room 316BC 16:30-18:30

Moderators:

 Roland Bammer and Gareth J. Barker
 
16:30  160. Concurrent Field Monitoring Removes Distortions from In-Vivo DWI Data
    Bertram Jakob Wilm1, Christoph Barmet2, Nicola DeZanche2, Peter Boesiger2, Klaas Paul Pruessmann2
1Institute for Biomedical Engineering , University and ETH Zurich , Zurich , Switzerland; 2Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland
    In diffusion-weighted MRI, eddy current effects notoriously result in geometrical image distortions. This problem is usually approached by fine-calibration of the scanner gradient system. We present a generic way of addressing this problem by deliberately tolerating a certain degree of field deviations in terms of eddy currents, gradient delays and field drifts and rather monitor the actual magnetic field during each scan. The field evolution thereby obtained is used for distortion correction of in-vivo DWI data that was acquired in the absence of hardware eddy current compensation.
     
16:42 161. Influence of Gradient Design on the Measurement of S/V Using DWI.
    Frederik Bernd Laun1, Bram Stieltjes1
1Medical Physics in Radiology, German Cancer Research Center, Heidelberg, Baden-Württemberg, Germany
    We show, that in the slow diffusion limit, the classical Stejskal-Tanner gradient scheme is optimal to measure the surface-to-volume ratio using the time dependent diffusion constant approach. We further show data supporting the assumption that S/V can not be determined properly by just shaping the gradients, e.g. using a train of short gradients, if the slow diffusion condition is not fulfilled as has been proposed previously.
   

 
16:54 162.

Addressing a Systematic Vibration Artefact in Diffusion-Weighted MR Images
    Daniel Gallichan1, Jan Scholz1, Andreas J. Bartsch2, Timothy E. Behrens1, Matthew D. Robson3, Karla L. Miller1
1FMRIB Centre, University of Oxford, Oxford, Oxon, UK; 2Neuroradiology, University of Würzburg; 3OCMR, University of Oxford
    Diffusion-weighted imaging employs large gradient lobes which are known to cause vibration of the patient table. We identify and characterise an artefact arising from these vibrations. We suggest a method to correct affected data as well as how to choose protocol parameters to avoid the acquisition of affected data.
     
17:06 163.

b-Matrix Correction Applied to High Resolution DTI
    Murat Aksoy1, Samantha Holdsworth1, Stefan Skare1, Roland Bammer1
1Department of Radiology, Stanford University, Stanford, CA, USA
    Due to its prolonged acquisition time, the correction of motion artifacts in high resolution DTI is essential for acceptable image quality. Short-Axis PROPELLER-EPI (SAP-EPI) has been proven to be very effective in eliminating phase and motion artifacts as well as geometric distortions. However, gross patient motion has two effects on the acquired data: pixel misregistration and change in diffusion encoding direction. While the pixel misregistration can be addressed by coregistration of low-resolution images, the change in diffusion encoding direction makes it incorrect to combine different blades to get the diffusion weighted images. In this study, we addressed this issue by combining SAP-EPI with the novel non-linear tensor estimation scheme that estimates the diffusion tensors from the complex k-space data directly. The results show an increased accuracy of main eigenvector orientation compared to the conventional schemes for tensor estimation.
   

 
17:18  164. Consistent Signal for Non-CPMG Echo Trains
    James G.  Pipe1, Donglai Huo1, Zhiqiang Li2, Eric Aboussouan1
1Imaging Research, Barrow Neurological Institute, Phoenix, AZ, USA; 2MRI, GE Healthcare, Phoenix, AZ, USA
    Diffusion Weighting precludes the use of CPMG echo trains in FSE without crushing significant signal. The MLEV and LeRoux phase cycling schemes help stabilize the signal magnitude, but some residual signal dependence on the starting phase remains. This work illustrates that appropriately placed gradient pulses can remove this residual signal instability.
   

 
17:30 165. Echo-Planar Diffusion-Tensor Imaging of Inner Field-Of-Views in the Human Brain and Spinal Cord Using 2D-Selective RF Excitations
    Jürgen Finsterbusch1,2
1Dept. of Systems Neuroscience, University Medical Center Hamburg-Eppendorf, Hamburg, Germany; 2Neuroimage Nord, Hamburg-Kiel-Lübeck, Germany
    Echo-planar imaging suffers from geometric distortions caused by magnetic field inhomogeneities in particular at high static magnetic fields. Because these artifacts depend on the field-of-view in the phase-encoding direction, several techniques to acquire inner field-of-views without aliasing have been presented, one of them involves 2D-selective RF excitations. In this work, the feasibility of this approach for diffusion-tensor imaging of inner field-of-views in the human brain and cervical spinal cord at in-plane resolution of up to 0.5x0.5 mm2 is demonstrated. Major fibres in the cerebellum can be identified as well as the reduced anisotropy of gray matter in the spinal cord.
   

 
17:42 166. High-Resolution DWI Outside the CNS Using Reduced-FOV Single-Shot EPI
    Emine Ulku Saritas1, Ajit Shankaranarayanan2, Eric T. Han2, Joelle K. Barral1, Jin Hyung Lee1, Dwight George Nishimura1
1Department of Electrical Engineering, Stanford University, Stanford, CA, USA; 2Applied Science Laboratory, GE Healthcare, Menlo Park, CA, USA
    DWI has recently been recognized as a potential clinical tool for the diagnosis, assessment and treatment monitoring of cancer outside the central nervous system (CNS). Even though single-shot EPI (ss-EPI) is the preferred method for these applications, its resolution is limited. Recently, a reduced FOV method using a 2D echo-planar RF (2D-EPRF) excitation pulse has been proposed for high-resolution ss-EPI DWI. In this work, we present the investigated improvements on this method, particularly optimization of the 2D-EPRF pulse to allow its use in various parts of the body. Specifically, we apply the improved method to in vivo prostate, breast and larynx DWI to demonstrate the high-resolution DWI capability of the reduced-FOV ss-EPI method, with improved coverage in the slice direction.
   

 
17:54 167. Steady-State Diffusion-Weighted Imaging with Trajectory Using Radially Batched Internal Navigator Echoes (TURBINE)
    Jennifer Andrea McNab1, Daniel Gallichan1, Matthew D. Robson2, Karla L. Miller1
1Clinical Neurology, Oxford University, Oxford, UK; 2Cardiology, Oxford University Centre for Clinical Magnetic Resonance Research, Oxford, UK
    2D segmented DWI with motion correction can improve image quality and in-plane resolution, relative to conventional single-shot EPI. The problems associated with thin slice-selection, however, render a 3D segmented pulse sequence the only way to achieve very small isotropic voxels. One challenge with 3D segmented DWI is the time-prohibitive nature of acquiring a 3D navigator echo along with each k-space segment. Here we present a novel approach to this problem with a fully 3D pulse sequence called steady-state DWI with Trajectory Using Radially Batched Internal Navigator Echoes (TURBINE).
   

 
18:06 168.

Steady-State Free Precession (SSFP) Diffusion Imaging Using 3D Rotating Spirals (3DRS)
    Jian Zhang1,2, Chunlei Liu2, Michael Moseley2
1Department of Electrical Engineering, Stanford University, Stanford, CA, USA; 2Department of Radiology, Stanford University, Stanford, CA, USA
    A new 3D diffusion imaging technique has been presented by using Steady-State Free Precession (SSFP) DWI and 3D rotation spirals (3DRS). The novel acquisition scheme offers very high SNR efficiency and low sensitivity to motion artifacts. In addition, the 0th order phase errors can be extracted and corrected with 3DRS. Experimental results have shown that high quality DWI and DTI whole brain volumes can be rapidly acquired with high SNR.
   

 
18:18 169. Optimized EPI-DTI and TSE-DTI at 3 T and 7 T in the Brain
    Eric Edward Sigmund1, David Gutman2, Mariana Lazar1, Jens H. Jensen3, Joseph A. Helpern1
1Radiology, New York University, New York, NY, USA; 2School of Medicine, New York University, New York, NY, USA; 3Radiology, New York University, New  York, NY, USA
   

Diffusion tensor imaging (DTI) and its higher order variants are powerful tools for elucidating brain tissue microstructure, but their processing schemes demand high SNR. The benefits of high field MRI (7 T) can be harnessed to amplify DTI processing or resolution, with the right strategy. Two single-shot sequences, echo-planar (EPI) and turbo spin echo (TSE) are often used for diffusion, but their migration to 7 T is ongoing and nontrivial. We present a validation study successfully achieving high quality DTI brain data from both sequences at both 3 T and 7 T through a combination of parallel imaging and post-processing.