Dynamic MR-Imaging with Radial Scanning, a Post-Acquisition Keyhole Approach
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Dynamic MR-Imaging with Radial Scanning, a Post-Acquisition Keyhole Approach Ralf Lethmate Laboratoire de R´esonance Magn´etique Nucl´eaire, CNRS UMR 5012, Universit´e Claude Bernard Lyon I, CPE, France Email: [email protected]
Frank T. A. W. Wajer Department of Applied Physics, Delft University of Technology, P.O. Box 5046, 2600 GA, Delft, The Netherlands Email: [email protected]
´ Yannick Cremillieux Laboratoire de R´esonance Magn´etique Nucl´eaire, CNRS UMR 5012, Universit´e Claude Bernard Lyon I, CPE, France Email: [email protected]
Dirk van Ormondt Department of Applied Physics, Delft University of Technology, P.O. Box 5046, 2600 GA, Delft, The Netherlands Email: [email protected]
Danielle Graveron-Demilly Laboratoire de R´esonance Magn´etique Nucl´eaire, CNRS UMR 5012, Universit´e Claude Bernard Lyon I, CPE, France Email: [email protected] Received 15 February 2002 A new method for 2D/3D dynamic MR-imaging with radial scanning is proposed. It exploits the inherent strong oversampling in the centre of k-space, which holds crucial temporal information of the contrast evolution. It is based on (1) a rearrangement of (novel 3D) isotropic distributions of trajectories during the scan according to the desired time resolution and (2) a post-acquisition keyhole approach. The 2D/3D dynamic images are reconstructed using 2D/3D-gridding and 2D/3D-IFFT. The scan time is not increased with respect to a conventional 2D/3D radial scan of the same image resolution, in addition one benefits from the dynamic information. An application to in vivo ventilation of rat lungs using hyperpolarized helium is demonstrated. Keywords and phrases: 2D/3D dynamic MRI, 3D isotropic radial sampling, keyhole, scan time reduction, image reconstruction, gridding.
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INTRODUCTION
Dynamic magnetic resonance imaging (MRI) is a challenging topic that opens a vast field in medical diagnosis such as contrast-enhanced MR angiography, hyperpolarized gas imaging, perfusion, interventional imaging, and functional brain imaging. Dynamic (time-resolved) images have to be acquired within a reasonable time scale and with reasonable spatial and temporal resolution. But, 3D-MRI techniques are in general very time-consuming and inadequate for recording dynamic features. In MRI, signals are measured in Fourier space, the socalled k-space [1, 2]. One commonly used approach for
improving the temporal resolution of dynamic MR imaging is the sliding window technique [3], which updates the most recently acquired region of k-space before each image reconstruction. Other techniques exist such as TRICKS [4] and Glimpse [5] which update the inner part of k-space more frequently than the outer part or have more optimum phaseencoding strategies [6]. For non-Cartesian sampling, undersampled projection reconstruction [7], the recent VIPR method [8, 9] and variable density spirals [10, 11] have also been proposed. Our method for dynamic imaging exploits the inherent strong oversampling of radial scanning in the centre of
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