Method for imaging fat plaque with nuclear magnetic resonance tomography

In a method for producing an image of fat plaque employing nuclear magnetic resonance tomography, a radio-frequency excitation pulse is emitted with wavelet coding in the presence of a first gradient, refocusing pulses are emitted which are selective to the spectral frequency of fat, and the resulting nuclear magnetic resonance signals are read out in the presence of a further magnetic field gradient. An image of fat plaque with enhanced spatial resolution is thereby produced.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention is directed to a method employing nuclear magnetic
 resonance tomography for producing an image of fat plaque in an
 examination subject.
 2. Description of the Prior Art
 The presentation of calcification by computed tomography is usually
 employed as a non-invasive modality for diagnosing a constriction or a
 closure of arteries. This type of diagnosis is particularly important with
 respect to coronary arteries since a cardiac infarction can result from
 the constriction. The presentation of the calcification, however, is a
 relatively unreliable criterion for identifying the presence of
 constriction. First, the deposit of calcium need not necessarily indicate
 a constriction; second, a constriction can be present before a deposit of
 calcium occurs. The method hitherto applied is therefore unsatisfactory
 with respect to the sensitivity as well as with respect to the
 specificity.
 The deposit of lipids (fats) in the vessels is considered a more reliable
 indicator for a constriction or a closure of arteries. Nuclear magnetic
 resonance tomography fundamentally allows fat deposits to be
 non-invasively displayed. A problem, however, is to display an adequate
 spatial resolution. Large coronary vessels have a diameter of
 approximately 3 through 5 mm. The fat plaque occupies 10 through 20% of
 the vessel diameter in an early phase and can amount to 70% later. An
 illustration of fat plaque in the coronary arteries and an evaluation of
 the degree of stenosis therefore presumes a spatial resolution in the
 sub-millimeter range. The spatial resolution also is degraded by the
 movement of the heart.
 The presentation of coronary arteries in a MR image is known in general. In
 order to keep the measuring time optimally short, for example, the
 especially fast EPI (Echo Planar Imaging) technique is applied as
 proposed, among others, in the reference P. Mansfield, "Multiplanar Image
 Formation Using NMR Spin Echos", Journal of Physics C, 10 (1977). Since
 the originally proposed "single shot" EPI method wherein the k-space is
 sampled after one excitation makes extreme demands on the magnetic field
 gradients, a segmented EPI method is also often employed. After an
 excitation, only a part of the k-space is sampled, i.e. the entire
 measurement for the data of a tomogram comprises a number of excitations.
 Methods are also known wherein essentially only fat is portrayed in the
 acquired image. These methods can be classified into methods having
 spectrally-selective excitation or saturation, and phase-difference
 methods. For example, the article by J. Pauly et al., Echo Planar Spin
 Echo and Inversion Pulses, MRM 29, pages 776-782 (1993), presents a
 possibility of designing excitation pulses which, under the influence of a
 gradient, are both spatially selective (i.e., for example, excite only one
 slice of an examination subject) and spectrally selective, so that, for
 example, only fat protons are excited. In the saturation method, the water
 protons are saturated in a preparation phase and the fat protons are
 subsequently excited, so that only the latter have a signal-producing
 effect.
 The phase-difference method known, for example, from W. Thomas Dixon,
 Simple Proton Spectroscopic Imaging, Radiology 1984, 153, pages 189-194,
 makes use of the fact that the Larmor frequencies of fat and water protons
 are somewhat different, and thus the phase of the corresponding transverse
 magnetization diverges. By forming the difference at suitable points in
 time, the fat signal can be separated from the water signal.
 The FFT (Fast Fourier Transform) method applied in a standard way in
 nuclear magnetic resonance tomography has the property that the entire
 field of view (FOV) is acquired with constant resolution per direction.
 U.S. Pat. No. 5,687,725 discloses a method wherein wavelet coding is used
 as an alternative to the FFT method. Individual image regions within an
 observation window can thereby be presented with higher resolution.
 None of these methods supplies a satisfactory presentation of fat plaque.
 SUMMARY OF THE INVENTION
 It is therefore an object of the invention to provide a method for the
 presentation of fat plaque with nuclear magnetic resonance tomography that
 satisfies diagnostic requirements.
 The above object Is achieved In accordance with the principles of the
 present invention in a method for producing an image of fat plaque using
 nuclear magnetic resonance tomography, wherein a radio-frequency
 excitation pulse is emitted with wavelet coding in the presence of a first
 magnetic field gradient, refocusing pulses are emitted which are selective
 to the spectral frequency of fat, and the resulting nuclear magnetic
 resonance signal is read out in the presence of a further magnetic field
 gradient.
 In the inventive method a fat image is obtained wherein an adequate
 resolution can be achieved on the basis of wavelet coding in the region of
 the arteries to be portrayed, so that fat plaque can be well-presented.
 The direction of enhanced resolution can be oriented by wavelet coding in a
 direction perpendicular to a vessel wall so that the evaluation of the
 degree of stenosis is enabled.
 The aforementioned inventive method can be embodied in an overall
 examination procedure wherein an overview or planning exposure is first
 obtained, and a fat image is produced with "normal" resolution. Locations
 which are likely to contain fat plaque are then identified from the normal
 resolution image, and a fat image with enhanced resolution is then
 produced in the identified regions, in accordance with the above-described
 inventive method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 According to the flowchart of FIG. 1, an overview exposure of the heart is
 first produced in the form of a conventional MR image. With the assistance
 of this overview image, only the arteries to be examined, for example the
 coronary arteries, are spatially identified. Fundamentally, any
 conventional pulse sequence is suitable for this overview image. The EPI
 sequence already mentioned has the advantage that it is especially fast.
 In the second step, a fat image is produced with "normal" resolution. As
 used herein normal resolution is typically a spatial resolution of 1-2 mm.
 Fat plaque which may be present in the arteries can be identified at this
 resolution, but a reliable evaluation of the degree of stenosis, and thus
 the threat of a vessel constriction is not yet possible. Fundamentally all
 methods for the acquisition of fat images cited in the introduction to the
 specification are suitable for this step, i.e. both methods with
 frequency-selective excitation or, respectively, saturation as well as
 phase-difference methods. These first and second steps can ensue in one
 measurement wherein the MR signals for fat and water image are
 simultaneously acquired.
 The fat plaque identified in the second step is presented with enhanced
 resolution in a third step. A new method that is based on wavelet coding
 is applied for this purpose. FIGS. 2 through 5 show an exemplary
 embodiment of a corresponding pulse sequence.
 FIGS. 2 through 5 show a pulse sequence for the third step of the method
 i.e. for the acquisition of a fat image with enhanced topical resolution.
 A wavelet coding as was already fundamentally described in an article by
 J. Weaver et al., Magnetic Resonance in Medicine 24, 275-287 (1992) is
 thereby applied in one direction. The application of a wavelet coding was
 disclosed in U.S. Pat. No. 5,687,725, but for motion tracking of
 interventional instruments. The aforementioned publications are referenced
 with respect to the fundamentals of wavelet coding.
 Given the exemplary pulse sequence according to FIGS. 2 through 5, a
 radio-frequency pulse RF1 is first emitted under the influence of a
 gradient G.sub.y. In combination with the gradient G.sub.y, the frequency
 spectrum of the radio-frequency pulse RF1 determines dilatation and
 translation of the wavelet function. A stripe profile perpendicular to the
 direction of the gradient G.sub.y can thereby be intentionally selected,
 whereby this stripe profile is placed such that it contains the vessel to
 be observed in greater detail. An enhanced resolution is achieved in the
 y-direction within the stripe profile. The envelope of this
 radio-frequency pulse RF1 and the stripe profile required here are a
 Fourier transform pair for small flip angles of the radio-frequency pulse
 RF1. The dilatation a and the intensity of the gradient G.sub.y behave
 proportionally relative to one another. Given an intensification of the
 gradient Gy, a is therefore enlarged and the stripe width is therefore
 reduced. The required translation b can be achieved by shifting the center
 frequency of the radio-frequency pulse RF1 or by an offset of the gradient
 G.sub.y. Subsequently, the gradient G.sub.y is inverted in order to cancel
 the dephasing caused by the positive sub-pulse.
 Although a spatial encoding of the excited signals is obtained with this
 type of excitation, the excitation is not spectrally selective with
 respect to fat and water, i.e. protons in fat as well as in water are
 excited. The spectral sensitivity is obtained by subsequent inversion
 pulses RF2 in combination with a gradient G.sub.z of alternating polarity.
 Such a pulse sequence--as explained in the aforementioned article by J.
 Pauly et al.--can be fashioned such that a spin inversion selectively
 ensues spectrally as well as spatially. In the present case, only the fat
 protons are refocused in a slice lying perpendicular to the gradient
 G.sub.z, so that a nuclear magnetic resonance signal S is obtained only
 from these protons in a following readout phase. This signal is readout
 under the influence of a readout gradient G.sub.x and is thus
 frequency-coded in the x-direction.
 With the described pulse sequence, a nuclear magnetic resonance signal of
 the fat protons thus is obtained that has a wavelet coding with respect to
 the y-direction and a frequency coding with respect to the x-direction. A
 slice selection is present in the y-direction. Using known reconstruction
 methods, a fat image can be obtained from a number of such signals that
 exhibits an enhanced resolution in the y-direction because of the
 above-described, advantageous properties of the wavelet coding within a
 stripe profile selected in the wavelet coding. Since this stripe profile
 is placed such that it covers the vessel to be observed, the desired fat
 image for the vessel is acquired with enhanced spatial resolution.
 For illustration, a coronary artery 1 with fat plaque deposit 2 is shown in
 FIG. 6. The vessel constriction by the fat plaque must be identified for
 evaluating the degree of stenosis. It is thus particularly the resolution
 in a direction perpendicular to the coronary artery 1 that is of
 significance. The wavelet coding is therefore implemented perpendicularly
 to the coronary artery 1, whereas the spatial resolution ensues in the
 artery direction by frequency coding during the readout phase. The
 selected slice lies parallel to the coronary artery 1. Transferred to the
 pulse sequence according to FIGS. 2 through 5, this means that the
 x-direction, i.e. the direction of the gradient G.sub.x, lies in the
 direction of the coronary artery 1 and the y-direction and the z-direction
 are perpendicular thereto. The region of enhanced spatial resolution is
 designated 3 in FIG. 6.
 In order to enable an exact spatial allocation of the fat plaque identified
 with enhanced spatial resolution with the wavelet coding relative to the
 anatomy of the patient, the conventional image according to the first
 step, the fat image according to the second step and the fat image with
 enhanced spatial resolution according to the third step can be
 superimposed with precise spatial allocation.
 A poorer signal-to-noise ratio is unavoidable with wavelet coding compared
 to that with conventional phase coding. As needed, however, a number of
 signals can be averaged in order to achieve an adequate signal-to-noise
 ratio. The overall image acquisition can generally not ensue within one
 heartbeat. Typically, therefore, the image acquisition will be made over a
 number of heartbeats and the data acquisition will be triggered with the
 heartbeat, for example derived from the ECG. One measurement or a series
 of measurements are respectively triggered in the same phase of a heart
 cycle. Another possibility would be to "gate" the data acquisition, i.e.
 to evaluate the respective data that were acquired in the same heart phase
 a continuously proceeding pulse sequence. The motion due to respiration
 can, for example, be largely eliminated by placing the patient in a prone
 position, however, an additional synchronization of the measurement with
 respiratory motion is also possible.
 Although modifications and changes may be suggested by those skilled in the
 art, it is the intention of the inventor to embody within the patent
 warranted hereon all changes and modifications as reasonably and properly
 come within the scope of his contribution to the art.