To reduce noise in velocity images of magnetic resonance (MR) phase-contrast measurements, the authors implemented and evaluated a pulse sequence that enables automatic optimization of the velocity-encoding parameter V(enc) for individual heart phases in pulsatile flow on the basis of a rapid prescan. This sequence was prospectively evaluated by comparing velocity-to-noise ratios with those from a standard MR flow scan obtained in the carotid artery in eight volunteers. This sequence was shown to improve velocity-to-noise ratios by a factor of 2.0-6.0 in all but the systolic heart phase and was determined to be an effective technique for reducing noise in phase-contrast velocity measurements.
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"For phase-contrast blood velocity mapping scans, some a priori knowledge about expected peak velocities is necessary in order to set velocity encoding correctly. Techniques have been described whereby acquisition of an extra segment allows automatic correction for aliased velocities to eliminate this restriction [25, 26]. With scan durations of the presented acquisitions of less than 30 min combined, they can all be acquired in a single scanning session. "
[Show abstract][Hide abstract]ABSTRACT: Current standards in magnetic resonance imaging of congenital heart disease are based mostly on anisotropic protocols to image both morphology and function. Operator-dependent acquisition planning is typically needed to obtain the desired images. We propose to instead use operator-independent, three-dimensional, isotropic imaging protocols to acquire both morphology and function (cine and flow) of the entire heart in a few standardized acquisitions. Subsequently, due to the isotropic property of the data, any desired imaging plane can be "imaged" offline by interactive planar reformatting and used for qualitative and quantitative diagnostic evaluation.
Morphological data was acquired in patients using 3D steady state free precession (SSFP) protocols, and functional data in volunteers using multislice 2D or 3D cine SSFP as well as 3D, three-component phase-contrast velocity mapping with EPI readouts. Tools to integrate morphological and functional offline image evaluation based on interactive planar reformatting, volume rendering, and corresponding quantification tools were implemented and discussed.
We successfully acquired and integrated morphology and flow and demonstrated potential clinical applications.
User independent acquisitions of morphological and functional isotropic 3D datasets with real-time, interactive planar reformatting, volume rendering, and integration of morphology and function, have the potential to replace conventional, user dependent, anisotropic 2D imaging in patients with cardiac malformations.
Full-text · Article · Apr 2005 · The International Journal of Cardiovascular Imaging
[Show abstract][Hide abstract]ABSTRACT: Magnetic resonance velocimetry (MRV) is a non-invasive technique capable of measuring the three-component mean velocity field
in complex three-dimensional geometries with either steady or periodic boundary conditions. The technique is based on the
phenomenon of nuclear magnetic resonance (NMR) and works in conventional magnetic resonance imaging (MRI) magnets used for
clinical imaging. Velocities can be measured along single lines, in planes, or in full 3D volumes with sub-millimeter resolution.
No optical access or flow markers are required so measurements can be obtained in clear or opaque MR compatible flow models
and fluids. Because of its versatility and the widespread availability of MRI scanners, MRV is seeing increasing application
in both biological and engineering flows. MRV measurements typically image the hydrogen protons in liquid flows due to the
relatively high intrinsic signal-to-noise ratio (SNR). Nonetheless, lower SNR applications such as fluorine gas flows are
beginning to appear in the literature. MRV can be used in laminar and turbulent flows, single and multiphase flows, and even
non-isothermal flows. In addition to measuring mean velocity, MRI techniques can measure turbulent velocities, diffusion coefficients
and tensors, and temperature. This review surveys recent developments in MRI measurement techniques primarily in turbulent
liquid and gas flows. A general description of MRV provides background for a discussion of its accuracy and limitations. Techniques
for decreasing scan time such as parallel imaging and partial k-space sampling are discussed. MRV applications are reviewed in the areas of physiology, biology, and engineering. Included
are measurements of arterial blood flow and gas flow in human lungs. Featured engineering applications include the scanning
of turbulent flows in complex geometries for CFD validation, the rapid iterative design of complex internal flow passages,
velocity and phase composition measurements in multiphase flows, and the scanning of flows through porous media. Temperature
measurements using MR thermometry are discussed. Finally, post-processing methods are covered to demonstrate the utility of
MRV data for calculating relative pressure fields and wall shear stresses.
Preview · Article · Dec 2007 · Experiments in Fluids
[Show abstract][Hide abstract]ABSTRACT: To describe a portable, easily assembled phantom with well-defined bore geometry together with a series of tests that will form the basis of a standardized quality assurance protocol in a multicenter trial of flow measurement by the MR phase mapping technique.
The phantom consists of silicone polymer layers containing parallel straight and stenosed flow channels in one layer and a U-bend in a second layer, separated by hermetically sealed agarose slabs. The phantom is constructed by casting low melting-point metal in an aluminum mold precisely milled to the desired geometry, and then using the low melting-point metal core as a negative around which the silicone is allowed to set. By melting out the metal, the flow channels are established. The milled aluminum mold is reusable, ensuring faithful reproduction of the flow geometry for all phantoms thus produced. The agarose layers provide additional loading and static background signal for background correction. With the use of the described phantom, one can evaluate flow measurement accuracy and repeatability, as well as the influence of several imaging geometry factors: slice offset, in-plane position, and slice-flow obliquity.
The new phantom is compact and portable, and is well suited for reassembly. We were able to demonstrate its facility in a battery of tests of interest in evaluating MR flow measurements.
The phantom is a robust standardized test object for use in a multicenter trial. Such a trial, to investigate the performance of MR flow measurement using the phantom and the tests we describe, has been initiated.
No preview · Article · May 2005 · Journal of Magnetic Resonance Imaging