Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-11-12
2001-08-14
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S306000
Reexamination Certificate
active
06275036
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and device for imaging diffusion parameters of a body by means of magnetic resonance (MR).
2. Description of Related Art
In the context of the present application an MG (Meibohm, Gill) component is to be understood to mean a component of a transverse magnetization in the direction of a first axis of a rotating reference system, the magnetization being rotated about said first axis by the refocusing RF pulses. A method of the described kind is known form the article “Phase Insensitive Preparation of Single-Shot RARE: Application to diffusion Imaging in Humans”, by D. C. Alsop, published in Magnetic Resonance in Medicine No. 38, pp. 527-533, 1997.
The known method is used for the in vivo imaging of diffusion phenomena in tissue of a human or animal body to be examined, for example the brain. The magnetization preparation pulse sequence of the known method includes, for example a gradient pair and a refocusing RF pulse for applying amplitude modulation which is dependent on diffusion of material in a part to be selected of the tissue of the body to be examined. An imaging pulse sequence which succeeds the magnetization preparation pulse sequence images the selected part of the tissue. The magnetic field gradients provide slice selection, phase encoding and frequency encoding of the MR signals, respectively. In order to reduce phase sensitivity in the successive MR signals, which sensitivity may be due to, for example motion of the body or motion of the tissue in the body during the magnetization preparation pulse sequence, a first crusher gradient is applied and the additional RF pulse is generated, respectively. The crusher gradient is a magnetic field gradient which causes a rotation of spins in such a manner that the magnetization due to the spins within a small area is distributed across the transverse plane relative to the steady magnetic field. The further first and second crusher gradients refocus and defocus the MG component. Subsequently, position-dependent MR signals are measured. A diffusion image of the selected part of the brain of the body to be examined is reconstructed from the measured position-dependent MR signals by means of two-dimensional transformation.
It is a drawback of the known method that artefacts occur in the image; for example, ghost images occur in the actual image of the part of the tissue. A ghost image is a second or subsequent image of the part of the tissue which has been shifted relative to the first image and has a reduced intensity.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method which mitigates the artefacts in the reconstructed image. To this end, the method according to the invention is characterized in that it utilizes an adjustment pulse sequence which includes the following steps: generating a preparatory excitation RF pulse, applying a preparatory magnetization preparation pulse sequence, generating a preparatory additional RF pulse in order to select an MG component, generating preparatory refocusing RF pulses, applying first and second preparatory crusher magnetic field gradients in order to measure preparatory magnetic resonance signals in the adjustment pulse sequence, and adjusting the additional RF pulse in the imaging pulse sequence in dependence on a parameter which is determined from the preparatory MR signals measured in the adjustment pulse sequence. The invention is based on the recognition of the fact that deviations of the additional RF pulse relative to the desired RF pulse cause an additional modulation in the successive MR signals. The deviations may arise, for example due to amplitude and phase distortion in an RF chain for generating the RF pulses or due to eddy current effects caused by the application of the magnetic field gradients. Measurement of this modulation in the adjustment pulse sequence from successive preparatory MR signals and formation of a correction for the additional RF pulse in the imaging pulse sequence therefrom enable a reduction of the modulation in the MR signals wherefrom the diffusion image is reconstructed. This method also offers the advantage that it can be executed without requiring the intervention of an operator. The deviations which may occur concern the flip angle, being the angle wherethrough the magnetization is rotated, and deviations of the phase of the RF pulse relative to a phase of the refocusing RF pulses. It has been found that amplitude deviations are much more sensitive to a deviation of the additional RF pulse than to a similar deviation of the refocusing RF pulses, the refocusing RF pulses having a flip angle of approximately 180 degrees. The use of an adjustment pulse sequence is known per se from European patent application EP-A-577188. According to the method disclosed in the cited patent application an adjustment pulse sequence which is substantially identical to the spin echo imaging pulse sequence used is applied prior to a multiple spin echo imaging pulse sequence. The imaging pulse sequence includes an excitation RF pulse and a number of refocusing RF pulses. In the adjustment pulse sequence preparatory MR signals are measured and on the basis of the measured preparatory MR signals there is determined a correction whereby the phase of the excitation RF pulse of the imaging pulse sequence is adjusted so as to satisfy the CPMG condition.
According to a special version of the method according to the invention amplitude modulation of the position-dependent MR signals can thus be counteracted. The nominal flip angle of the additional RF pulse amounts to 90 degrees. Due to deviations of a nominal flip angle, the non-MG component is not completely rotated in the z direction and a residual non-MG component remains, giving rise to the amplitude modulation in successive MR signals. The correction of the nominal flip angle of the additional RF pulse can be determined by measurement of the amplitude of successive preparatory MR signals in the adjustment pulse sequence.
It has been found that amplitude deviations are much more sensitive to a deviation of the additional RF pulse than to a similar deviation of the refocusing RF pulses, the refocusing RF pulses having a flip angle of approximately 180 degrees.
According to another version of the method according to the invention, in order to satisfy the CPMG (Carr, Purcel, Meibohm, Gill) condition, in this pulse the nominal phase difference between the phase of the additional RF pulse and the phase of the refocusing RF pulses is chosen to be zero. The phase difference relative to the nominal phase difference produces a phase modulation in the phase of the successive MR signals. The phase correction for the phase of the additional RF pulse in the imaging pulse sequence is determined from the measured phase difference between two successive preparatory MR signals in the adjustment pulse sequence. The phase difference between successive MR signals, measured by means of an imaging pulse sequence, is thus constant. The phase differences of MR signals measured in successive imaging pulse sequences, however, may vary.
In a further version of the method according to the invention, one of the position-dependent MR signals is used as a navigator MR signal for relating the position-dependent MR signals measured in successive imaging pulse sequences to one another. The phase of successive imaging pulse sequences can be related to one another by measurement of a navigator signal.
The invention also relates to a device for carrying out such a method. A device of this kind comprises means for sustaining a steady magnetic field, means for applying gradients, means for generating RF pulses applied to the object to be examined in the steady magnetic field, a control unit for controlling the means for applying gradients and
Jenniskens Hans G.
Van Den Brink Johan S.
Van Yperen Gerrit H.
Arana Louis
U.S. Philips Corporation
Vodopia John F.
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