WO1997009632A1 - Procedes de realisation d'un contraste de phase en imagerie par resonance magnetique et appareil associe - Google Patents

Procedes de realisation d'un contraste de phase en imagerie par resonance magnetique et appareil associe Download PDF

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WO1997009632A1
WO1997009632A1 PCT/GB1996/002172 GB9602172W WO9709632A1 WO 1997009632 A1 WO1997009632 A1 WO 1997009632A1 GB 9602172 W GB9602172 W GB 9602172W WO 9709632 A1 WO9709632 A1 WO 9709632A1
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phase
image
output signal
ofthe
information
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PCT/GB1996/002172
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English (en)
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Thomas William Tennant Redpath
Fergus Iain Mckiddie
Rosemary Carmen Dymond
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British Technology Group Limited
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Application filed by British Technology Group Limited filed Critical British Technology Group Limited
Priority to IL12354296A priority Critical patent/IL123542A/en
Priority to EP96929411A priority patent/EP0848827A1/fr
Publication of WO1997009632A1 publication Critical patent/WO1997009632A1/fr
Priority to US09/034,309 priority patent/US6150814A/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
    • G01R33/56509—Correction of image distortions, e.g. due to magnetic field inhomogeneities due to motion, displacement or flow, e.g. gradient moment nulling
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
    • G01R33/56563—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0

Definitions

  • the present invention relates to methods of obtaining phase information in 5 Magnetic Resonance Imaging (MRI) and a related Apparatus.
  • MRI Magnetic Resonance Imaging
  • Magnetic Resonance Imaging is a technique for imaging a material by exciting nuclei in the material when the material is in a strong magnetic field.
  • the strong magnetic field is known as a static magnetic field.
  • Nuclei are then excited by a radio frequency magnetic field.
  • Positional information is obtained by the generation of magnetic 0 field gradients.
  • a voltage induced in a receiver coil after the gradient field is removed is measured. It is the information derived from the induced voltage which is used, with sophisticated computing systems, to produce an image which is readily recognised by the eye.
  • Magnetic Resonance Imaging is therefore a powerful medical imaging technique which generates data which contains phase as well as magnitude information, and essentially depicts the magnitude and phase ofthe transverse nuclear magnetisation within 0 an elemental unit volume called a voxel.
  • the phase ofthe data is often removed, to avoid image artefacts, which are often attributable to related motion between two or more material types being imaged.
  • MRI has parallels with optics, because light is also an electromagnetic wave having both an amplitude and a phase.
  • optics as in MRI, the phase is often discarded in imaging methods. For 5 instance a photographic emulsion is sensitive only to the intensity ofthe light.
  • the present invention has been made following recognition of an existing problem 0 and as a result of an attempt to solve that problem.
  • a problem associated with optical systems was identified by Zernike in a paper, published in July 1942 in Physica IX, No. 7 at pages 686 to 698 in which the technique of imaging a microscopic object using phase contrast was described.
  • m may have all positive and negative integral values, corresponding to the different diffracted waves on the right (positive m) and on the left (negative m) ofthe direct wave.
  • the phases ofthe different waves are compared for wave fronts passing through the origin 0.
  • a coordinate x' is defined and expressed in such units that conjugate points in P and P' have equal coordinates.
  • the interference effect of the diffracted waves in the plane P' e.g. in 0' may be found.
  • the optical paths from 0 to 0' through the objective are all equal. Therefore the relative phases ofthe different beams are the same in 0' as they were in 0, so that the resulting vibration in 0' is simply found by sunrming the complex numbers c m .
  • V ⁇ c m e -' « ' ( 4)
  • phase-contrast method transparent details of an object which differ in thickness or in refractive index appear as differences of intensity in the image.
  • An important increase of sensitivity can further be obtained by the use of an absorbing phase strip.
  • the effect of diffraction by the phase strip is then considered and practical methods to make the resulting diffraction-halo as faint as possible are discussed.
  • the strip should preferably be of circular form, with a corresponding annular diaphragm in a condenser.
  • the methods of preparing phase strips and of placing and adjusting them in the microscope are discussed.
  • a method of obtaining phase information in Magnetic Resonance Imaging (MRI) in which both magnitude and phase information are used to manipulate content in a displayed image comprising: means for applying magnetic fields to an object to be imaged; means for obtaining an output signal in such a way that the output signal contains phase information; means for applying a phase shift to said phase information in the output signal; processing means for ob ⁇ ning an image from information relating to the magnetic field signal, the output signal and the phase shifted output signal, such that both magnitude and phase information are used to manipulate the content of data representative of an image; and means for displaying said image.
  • the apparatus employs the difference between the phase ofthe output signal and the phase ofthe phase shifted signal to correct any phase errors in the output signal.
  • phase contrast in MRI is achieved by employing a phase advance or a phase retard device or method.
  • a phase advance or a phase retard device or method is employed.
  • Figure 1 is a ray diagram showing an optical analogy with lenses and an object
  • Figure 2 is a vector diagram
  • Figure 3 is a ray diagram showing spatial relationship of an image plane and k-space to illustrate the Schlieren method in optics
  • Figure 4 is a vector diagram
  • Figure 5 is a graph of a gradient switching sequence
  • Figures 6a to 61 show images of an aortic arch obtained using only magnitude of data signals without phase encoding
  • Figures 7a to 71 show images of an aortic arch obtained using only phase differences of output signals in which the phase-difference map is devoid of unwanted phase artefact;
  • Figures 8a to 81 show phase contrast images of aortic arch obtained using the present invention, the constant background vector having been chosen to highlight contrast between the aorta and the background;
  • Figure 9 shows a block diagram of a system incorporating the invention
  • Figure 10 shows a flow diagram of key steps in the method of obtaining data
  • Figure 11 shows diagrammatically an alternative method.
  • Figure 12 shows the distribution of light in the transform plane is a 2D
  • Figure 13 shows the addition of a background vector R in image space elicits contrast in the magnitude ofthe resulting vectors in two regions with similar signal magnitudes but different phases;
  • Figures 14 a) shows diagrammatically the Magnitude Fourier transform; b) shows the computed phase; c) shows the phase-contrast; d) shows a "stopped” Schlieren; and e) shows a 'phase-reversed' Schlieren cine magnetic resonance images ofthe aortic arch of a normal volunteer obtained with a delay of 150ms after the ECG R- wave.
  • one set of data can be acquired from a phantom thus the machine based artefact can be calculated and this is then used to correct the patient data.
  • Schlieren methods may be applied to the data, such that other types of imaging techniques may be used with the invention.
  • Schlieren techniques may also be introduced into MRI.
  • a practical advantage is that only approximately one half of available k-space needs to be acquired.
  • the optical analogy has been described in the above referenced letter by Zernike.
  • phase of one half of k-space can be reversed.
  • the method in MRI is to acquire approximately half of k-space and zero fill the remainder before performing any transformations. Altematively the portion of k-space may be phase-reversed before performing any transformations.
  • Applications of the present technique are in phase- velocity imaging; although any effect which produces phase-contrast could be utilised (e.g. fat-water chemical shift in a gradient-echo image).
  • the invention is described with specific reference to a preferred method of imaging flowing fluids by nuclear magnetic resonance (NMR).
  • NMR nuclear magnetic resonance
  • the method employs the phase of a voltage signal in order to obtain a measure of the magmtude and direction of flow in the fluid.
  • Live images in Figures 6, 7 and 8 are presented which demonstrate the implementation of this technique in conjunction with a spin-warp imaging method as described by Edelstein et. al. in 1980.
  • the invention may be employed in other imaging sequences which traverse k-space in a spiral trajectory or spoke trajectory and it is therefore not limited to rectilinear raster k-space trajectory types of imaging sequence.
  • phase of a signal from a stationary spin packet, excited in the presence of a static field B 0 is given by
  • a relatively strong gradient pulse of duration ⁇ 0 is applied in the desired direction, followed by an interval ⁇ -, then after a further interval, by a second pulse identical to the first pulse but of an opposite polarity.
  • a second pulse identical to the first pulse but of an opposite polarity may be applied.
  • a stationary spin experiences a net phase change of zero during duration of application of two such pulses.
  • the first of these terms is zero because the time integral of G ⁇ .(t) is zero. If rectangular pulses of magnitude G 0 are used such that
  • Two and three-dimensional Fourier Transform (2-D FT and 3-D FT) methods are the most common MR image formation techniques currently in use.
  • the Nuclear Magnetic Resonance (NMR) signal is digitally sampled using a phase sensitive detector, usually in phase and in phase quadrature.
  • the image data is related to the signal or raw data, via a two or three-dimensional discrete Fourier Transform (2D-FT or 3D-FT), as described in, for example Callaghan, 1993, Chapter 3.
  • Both S and p are complex and are a Fourier conjugate pair.
  • the space (k x ,k y ) is often referred to as k-space.
  • the variables (k ⁇ ,ky) are the time integrals ofthe magnetic field gradients (G x , G y ). In MRI, it is the magnitude of p which is most often computed for image display, and the phase is discarded, in order to avoid image artefacts.
  • Fourier Optics describes the formation of an image by a lens system using the mathematical formalism ofthe Fourier Transform as described by Hecht, in 1987, (Chapter 11).
  • a lens effectively acts as a Fourier Transformer of light scattered by an object.
  • a second lens performs the inverse transform to form the image.
  • the transform or diffraction plane For a two-dimensional object, parallel to the plane ofthe first and second lenses, the distribution of light in both the transform and image planes, can also be represented by 2D complex functions, and are related in the same way as in equations 10 and 11 above.
  • Phase contrast microscopy manipulates light in a so-called transform plane, by effectively inserting a "diffraction plate" in that plane.
  • Zernike proposed shifting the phase ofthe central area ofthe transform by 90° by means of a thin spot of transparent varnish deposited on a glass plate. Attenuation ofthe light passing through the spot was small.
  • a variety of alternatives were proposed, including an annular "phase-strip". Bennett et al. (1951, Chapter 3) describe a design of commercially available phase-plates.
  • Manipulation of optical data in the transform plane is equivalent to manipulation of Fourier MRI data in k-space. That is manipulation of a raw signal before Fourier Transforming it to form the MR image. In MRI this is done by mathematical operations on digitised raw data.
  • shifting the phase of the NMR signal in the central area of k- space in MRI is analogous to inserting a phase or diffraction plate in phase-contrast microscopy.
  • the design of phase plates for phase-contrast microscopy takes into account the physical limitation of an optical system, while application ofthe principle to MRI must take into account the physical limitations of the MR imaging technique and hardware.
  • Optical analogies are given which illustrate the theory and practice of phase-contrast imaging as applied to MRI.
  • phase information is often displayed as a computed phase image (as for example by Young et. al, 1987).
  • the phase p is calculated from the ratio ofthe real and imaginary parts ofthe complex signal (S) corresponding to each voxel.
  • S complex signal
  • phase-contrast technique effectively imports phase information into the magnitude image, thus displaying phase and magnitude contrast simultaneously.
  • phase-contrast techniques are of value in eliciting detail in samples which are virtually transparent, for example where the image is formed from transmitted light.
  • contrast may be available by virtue of differences in refractive index within a sample.
  • the differences in refractive index result in differences in optical path length as light traverses the sample.
  • there are only small differences in the intensity ofthe light across the image but substantial changes in phase.
  • MRI a number of factors can cause a change in phase ofthe magnetization within a particular voxel. Grover and Singer (in 1971) were the first to argue that magnetization moving with a velocity v, along a magnetic field gradient G, would form a spin-echo phase shifted by:
  • ⁇ is the Larmor frequency and ⁇ is the separation ofthe 90° and 180° radiofrequency pulses.
  • the same effect can be achieved by using a bipolar gradient in conjunction with a gradient echo sequence (Moran, 1982; Redpath et. al., 1984).
  • the phase-shift can be used to measure the velocity of blood flow in vivo (Nayler et. al., 1986). Differences in magnetic susceptibility between tissues effect results in a phase-shift ofthe observed signal, provided that a gradient-echo sequence is used. Young et. al. (1987) demonstrated the clinical value of this technique in cases of intracranial haemorrhage. Chemical-shift effects between fatty CH2 and water resident protons also result in a phase-difference between fatty and non-fatty tissues in gradient- echo images.
  • phase is to be used to quantify a parameter, for example velocity of a following blood bolus
  • two measurements are often required. For instance, one measurement is made with a bipolar velocity phase-encoding magnetic field gradient pulse, and one is made without the pulse.
  • the or each velocity pulse can be inverted to give an equal and opposite phase shift.
  • the phase maps are then subtracted from each other to give the phase change due to the velocity encoding pulses alone, thus removing artefactual phase changes arising from other sources, such as poor static magnetic field homogeneity.
  • the complex difference technique subtracts complex signals A and B directly, as shown in Figure 4a. Thus some magnitude information is preserved for moving nuclear spins.
  • the MRI phase-contrast method proposed, and also optical phase contrast methods are based on the following principle.
  • a and B corresponding to two different positions in image space, and representing either the electromagnetic field (in optics) or transverse magnetisation (in MRI). Both A and B are of approximately equal magnitude, so that magnitude images will show little contrast.
  • Figure 4b and 4c shows how the addition of a third constant background or reference constant R can elicit contrast in a magnitude image.
  • the reference R can be added to the complex image data after obtaining the
  • the operation can be performed in k-space by addition of a complex constant R to the dc or k(0,0) point.
  • phase-contrast can be achieved by the addition of the complex reference constant R to every pixel value. This is straightforward to implement provided the image is free of artefact.
  • the implementation ofthe technique in k-space has the advantage of computational speed, in the only one point ofthe raw data need be modified.
  • the image is then formed by taking the FT of the raw data. Since this step has to be done anyway, the excess computation required over that needed to form a conventional magnitude image, is minimal.
  • the central data point ofthe complex raw data array may not be the effective dc point, for a number of reasons. This results in an artefactual phase- gradient across the image. This effect will dominate phase-contrast effects made visible by the addition ofthe reference constant R, so that the subtle shifts due to susceptibility and velocity which are of value, will not be seen. This point is discussed in detail below for a gradient-echo image.
  • the effect ofthe invariant term E is to add a constant phase offset across the field of view.
  • the effect ofthe second term, the term in x is to shift the position ofthe spin- echo. Referring to Figure 5, this can be understood by imagining the effect of an offset to the gradient waveform, of an amount equal to the term in x. Suppose that the offset is positive, then the size of the dephasing area G j) will be diminished, and the size of the rephasing gradient increased, so that the echo occurs at an earlier time. That is the effective TE is reduced. This results in a shift of the gradient echo signal along the frequency- encoding direction in k-space.
  • phase-contrast technique therefore requires either the removal the phase-gradient in image space, or an implementation which can operate in the presence of phase-gradients.
  • Phase-correction methods for image space data are available. Ahn and Cho (1987) described a first-order technique (i.e. phase-gradients are corrected, but not second-order derivatives) which calculates an image auto-correlation function to estimate the gradients present in the image.
  • a technique employing second and higher order corrections is also available, but is not generally applicable to gradient-echo images with both fat and water protons in the field of view (Bernstein et. al., 1989).
  • phase-gradient in ⁇ p matches the existing average artefactual phase-gradient in
  • the images shown in Figures 6, 7 and 8 show that the technique works, and that it shows promise in displaying data more readily than by jointly viewing magnitude and phase-difference images.
  • the images show from i.e. a to 1 respectively, the heart and aortic arch in 50ms steps after ECG R-wave detection. Flow is seen in the aorta in the Figure 6c after the aortic valve opens. At the end ofthe cycle at Figure 6, aortic flow has ceased.
  • Magnetic Resonance Imaging (MRI) apparatus 10 together with electronic control apparatus 12 and a host computer 14, is capable of producing magnetic field gradients and applying radio frequency (rf) pulses.
  • Induced voltage signals are captured by RF receive and converted to digital data by an analog-to-digital converter 18 (ADC) and data is then sent to processing system 20.
  • ADC analog-to-digital converter
  • the second image data set is obtained shortly after the first image data set.
  • the two data sets undergo mathematical processing which incorporates a Fast Fourier Transform. Phase shifting is then effected digitally on one ofthe two image data sets.
  • a converted image is obtained using the second image data set.
  • Phase contrast techniques as described above, are performed on the data sets in image or k-space.
  • a phase contrast image is then obtained and an image is displayed.
  • a radiographer is able to enhance an image at this stage using conventional image enhancement techniques or a digital image may be stored electronically.
  • an image is considered to be unsatisfactory the radiographer may intervene in the imaging process and recommence the cycle, as indicated by decision box in Figure 10.
  • An alternative method is now described with reference to Figure 11.
  • An image data set with phase contrast information is obtained.
  • Phase contrast enhancement is performed on the data sets in k-space as described above.
  • a phase contrast image is obtained on a monitor.
  • the process parameters may be modified or, as is illustrated by the dotted line, a fresh set of data may be obtained.
  • the contrast detail is poor the phase and/or magnitude ofthe background vector may be modified. Such modifications are carried out by a skilled radiographer.
  • phase-contrast microscopy is a method of visualising structure in objects which are practically transparent, where, as a result, little information is available in the intensity ofthe transmitted light. Differences in refractive indices within the object, and hence in the optical path lengths ofthe transmitted light, result in phase contrast in the final image. Conventional optical techniques discard this information and are sensitive only to the intensity of the light forming the image. Phase-contrast and Schlieren methods are techniques which exploit these phase differences to generate contrast. In magnetic resonance imaging, as mentioned above the signal phase is also often discarded when displaying data, despite the fact that it may contain information on blood flow velocity, chemical shift and tissue magnetic susceptibility. The implementation of the magnetic resonance imaging analogues of phase-contrast and Schlieren optical techniques and present images of blood flow in the human aorta are described in detail below, as an illustrative example.
  • Figure 12 illustrates the optical principle ofthe phase-contrast technique.
  • Solid rays 100 show undiffracted monochromatic light transmitted by object 102 focused in the centre of the transform of diffraction plane 104.
  • Broken rays 106 show light diffracted by the object 102 being focused at a point offset from the centre.
  • a first lens 108 can be considered to yield a two-dimensional Fourier transform of light from the object 102 in the transform plane, while a second lens 110 carries out a second Fourier transform to give the image.
  • This is equivalent to Abbe's theory of image formation which states that: "plane monochromatic wavefronts are diffracted by the object and yield a Fraunhofer diffraction pattem in the focal plane ofthe first lens".
  • Points in the transform plane can be viewed as point sources of Huygen's wavelets so that a second diffraction pattern is created at an image plane 112.
  • the image is formed by a two stage diffraction process.
  • Phase-contrast methods phase-shift, and may attenuate, the undiffracted or zeroth order beam by using a phase-plate in the transform plane. (Zernike originally accomplished this by means of a small central spot of varnish on a glass plate).
  • Schlieren methods the phase plate is replaced by a knife-edge which completely attenuates or stops one-half of the light in the transform plane.
  • MRI magnetic resonance imaging
  • the electrical signals observed at a receiving coil are effectively Fourier transforms of an object under investigation. Signals are sampled in-phase and quadrature so that raw data is effectively complex, having both magmtude and phase.
  • the raw data is often referred to as k-space data since, for a two- dimensional image, the nuclear spin distribution is given by
  • k x and k y are the respective time integrals ofthe frequency (x) and phase encoding (y) gradients at the time of signal (S) observation and
  • k-space data in MRI is the analogue ofthe distribution ofthe amplitude and phase of light in the optical transform plane of Figure 12.
  • MR images are often formed from the magnitude of the Fourier transform, so that phase information is discarded.
  • the MRI Schlieren method is implemented by setting one-half of the k-space data to zero before transformation, equivalent to a complete stop in optics. Altematively, the phase of the complex signals may be changed by ⁇ , effectively obtaining phase reversal.
  • Figure 13 illustrates how the addition of a background vector R gives rise to a difference in their resultant magnitudes, thus giving rise to contrast in the magnitude MR image.
  • R should be chosen to maximise the contrast, thus:
  • This limit is chosen to exceed the velocities encountered in vivo, to avoid velocity aliasing.
  • the sign ofthe phase shift is determined by the direction of motion.
  • the slice thickness is 10mm, the flip angle 30°, the matrix 256x256 with 3 signal averages.
  • phase-contrast and Schlieren methods cannot be applied directly to the velocity encoded set as the phase information is contaminated with errors caused by static field inhomogeneity.
  • the phase shift ⁇ between the two sets of information (magnitude and phase), corresponding to velocity encoding alone, is calculated on a pixel by pixel basis, and a corrected image set formed by calculating
  • can be either the encoded or un-encoded image set.
  • Fourier transformation then yields a corrected k-space data set for the Schlieren method to be applied.
  • the phase- contrast method can be implemented in either k-space or image space (see equation 26).
  • the un-encoded set can be acquired at a lower spatial resolution, as the phase contamination induced by static field inhomogeneities is only slow varying in space. In this case the encoded image data would be used to calculate
  • phase information in MRI is viewed by displaying magnitude images
  • An altemative approach to visualise blood flow in MRI is to acquire two data sets, one with positive velocity encoding p + , the other with negative velocity encoding p., and to display the magnitude of their complex difference
  • This has the disadvantage of suppressing stationary tissue so that the relative position of blood vessels to the surrounding anatomy is lost.
  • Figures 14a and 14b show respectively magnitude and computed phase-difference images for the 4th cine frame ofthe corrected image set, acquired 150ms after the R-wave, at the onset of systole.
  • the random phase ofthe noise in low signal areas is distracting, but may be removed by magnitude masking.
  • Figure 14c shows the phase contrast image with the background vector chosen to maximise contrast between blood flowing in the ascending aorta, and slower moving blood in the left ventricle. It is interesting to note the lack of contrast between the aortic wall and the top ofthe arch where there is no component of velocity in the head-to-foot direction, and hence no velocity-encoded phase shift. Good wall-blood contrast is seen elsewhere, especially in the ascending aorta.
  • Figures 14d and 14e show the Schlieren images obtained by respectively i) setting the k y ⁇ 0 half of the k- space data to zero and ii) 'phase-reversal' of the same portion of the raw data.
  • the y direction is anterior-posterior and is the spin-warp phase-encode direction.
  • the effect of the Schlieren processing is to emphasise phase boundaries at right angles to y.
  • Rotation of the cut-off edge in k-space rotates the directionality of the phase-boundary enhancement effect.
  • the stopped Schlieren image emphasises only the inner surface ofthe curve ofthe aortic arch, while the 'phase-reversed' image emphasises both inner and outer surfaces.
  • Phase-contrast and Schlieren MRI offer methods of integrating phase and magnitude information into a single MR image, with the potential of emphasising structural and functional detail which may not be readily visible by conventional MR imaging techniques.

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Abstract

L'invention concerne un procédé permettant d'obtenir une information de phase en imagerie par résonance magnétique (RMN) et d'employer cette information pour améliorer l'image obtenue. Dans le passé, les appareils de RMN obtenaient des données contenant des informations de phase et de magnitude. L'information de phase a souvent été rejetée afin d'éviter des artefacts d'image, généralement dus au mouvement. La présente invention fait appel à une technique permettant d'utiliser cette information de phase pour améliorer la qualité de l'image, selon un procédé comparable à celui utilisé en microscopie optique.
PCT/GB1996/002172 1995-09-04 1996-09-04 Procedes de realisation d'un contraste de phase en imagerie par resonance magnetique et appareil associe WO1997009632A1 (fr)

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IL12354296A IL123542A (en) 1995-09-04 1996-09-04 Apparatus and methods of obtaining phase information in magnetic resonance imaging
EP96929411A EP0848827A1 (fr) 1995-09-04 1996-09-04 Procedes de realisation d'un contraste de phase en imagerie par resonance magnetique et appareil associe
US09/034,309 US6150814A (en) 1995-09-04 1998-03-03 Methods of achieving phase contrast in magnetic resonance imaging and a related apparatus

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GBGB9600296.9A GB9600296D0 (en) 1995-09-04 1996-01-08 Methods of obtaining phase information in magnetic resonance imaging and related apparatus

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2006094354A1 (fr) * 2005-03-10 2006-09-14 The University Of Queensland Bobine a agencement de phase pour une irm
CN100409022C (zh) * 2003-12-29 2008-08-06 中国科学院电工研究所 一种阻抗成像方法及装置

Citations (4)

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EP0280310A2 (fr) * 1987-02-27 1988-08-31 Hitachi, Ltd. Méthode et appareil d'imagerie par résonance magnétique
EP0371477A2 (fr) * 1988-11-30 1990-06-06 Hitachi, Ltd. Procédé d'imagerie par résonance magnétique d'un objet en mouvement
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CN100409022C (zh) * 2003-12-29 2008-08-06 中国科学院电工研究所 一种阻抗成像方法及装置
WO2006094354A1 (fr) * 2005-03-10 2006-09-14 The University Of Queensland Bobine a agencement de phase pour une irm
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IL123542A (en) 2001-07-24
IL123542A0 (en) 1998-10-30
EP0848827A1 (fr) 1998-06-24

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