Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-11-23
2001-12-18
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06331777
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of forming a magnetic resonance image of an object which is arranged in a steady magnetic field, which method involves repeated execution of an image pulse sequence and includes the following steps:
signals are measured along a first number of lines in the k-space by application of gradients, the method also including the following steps:
2. Description of the Related Art
MR image. The invention also relates to a device for carrying out such a method.
In the context of the present patent application a gradient is to be understood to mean a magnetic field gradient of the steady magnetic field.
The method of the kind set forth is known from the article “The Use of Intelligent Re-acquisition to Reduce Scan Time in MRI degraded by Motion”, Q. Nguyen et al., proceedings SMRM 1998, page 134. The known method can be used for the in vivo imaging of diffusion phenomena in tissue of a human or animal body to be examined, for example a part of the brain. Diffusion-weighted magnetic resonance (MR) images may be of assistance for making diagnoses of diseases, for example brain infarcts, and for characterizing brain tumors. In order to enable the imaging of diffusion phenomena, the known method also includes a magnetization preparation pulse sequence which includes a gradient pair and a refocusing RF pulse in order to realize an amplitude modulation in the MR signals received, said amplitude modulation being dependent on diffusion of material in the part to be selected in the brain of the body to be examined. The imaging pulse sequence succeeding the magnetization preparation pulse sequence yields navigator MR signals and MR signals wherefrom the MR image of the part of the tissue is reconstructed. MR signals are measured along the number of lines in the k-space as a result of the generation of RF pulses and the application of magnetic field gradients. Repeating the imaging pulse sequence ensures that the imaging MR signals are measured along a number of lines in the k-space which is large enough to ensure that diagnostically useful MR images become available after reconstruction. The applied gradients serve for slice selection, phase encoding and frequency encoding of the MR signals. After all imaging MR signals have been measured, the MR image of the part of the brain to be examined is reconstructed from the measured imaging MR signals while utilizing, for example a two-dimensional Fourier transformation. Subsequently, an imaging quality of the reconstructed MR image is determined. According to the known method an imaging quality of this kind is determined from a ratio of a mean value of intensities of pixels within a boundary of an imaged part of the object to a mean value of intensities of pixels within a boundary of a ghost image of the imaged part of the object. In the context of the present patent application a ghost image is to be understood to mean an image of the imaged part which has been shifted relative to the image and has a lower intensity. If the imaging quality is too low, according to the known method the MR image is corrected by replacing at least one of the measured imaging MR signals by an MR signal to be newly measured for said imaging MR signal, a magnitude of the associated navigator MR signal being smaller than or equal to the magnitudes of the navigator MR signals associated with the other measured imaging MR signals. The imaging MR signal to be measured anew is measured along the same trajectory in the k-space as that along which the imaging MR signal to be replaced has been measured. Subsequently, a new MR image is reconstructed again from the imaging MR signals and the imaging quality of this new MR image is determined again. Subsequently, said steps are repeated until the imaging quality of the MR image is satisfactory.
It is a drawback of the known method that the imaging quality of the MR image is determined after the final reconstruction. It is a further drawback that the imaging quality cannot be unambiguously defined, because the boundary of the imaged part of the object and the boundary of its ghost image can be determined from the reconstructed MR image only afterwards.
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 whereby the imaging quality of the MR image is defined in advance. To this end, the method according to the invention is characterized in that it includes a step in which, prior to the reconstruction of the MR image, a quality criterion of the measured navigator MR signal is compared with a predetermined limit value, followed, if the quality criterion of the measured navigator MR signal does not meet the predetermined limit value, by the step for measuring the at least one of the imaging MR signals again in order to correct the MR image, the latter step being executed by executing the imaging pulse sequence again, the navigator MR signal and the imaging MR signals then being measured again along the first number of lines in the k-space.
The invention is based on the recognition of the fact that by imposing the quality criterion on the measured navigator MR signal it is ensured that the measured imaging MR signals, wherefrom ultimately the MR image is reconstructed, a priori satisfy predetermined requirements and that the MR image always has a predetermined quality after reconstruction. As opposed to the known method, therefore, a quality criterion is not determined afterwards from the reconstructed MR image. When, for example a magnitude of the navigator MR signal is chosen as the quality criterion of the navigator MR signal, the magnitude of the navigator MR signal of an object in motion will be small relative to the magnitude of a navigator MR signal of a stationary object and the imaging MR signals associated with the navigator MR signal will have to be measured again. In that case the MR image will contain fewer motion artefacts. It is a further advantage that the number of successive reconstructions is reduced to a single reconstruction; this offers a saving in time and also reduces the overall acquisition time during which the body to be examined must remain in a magnetic resonance imaging device. A further advantage resides in the fact that, after completion of the measurements, there are also present navigator MR signals which satisfy the predetermined quality criterion and hence all navigator MR signals can be used for a phase correction of the imaging MR signals.
A special version of the method according to the invention is characterized in that the quality criterion is dependent on the magnitude of the measured navigator MR signal. The magnitude of the navigator MR signal can thus be quickly determined. Preferably, the measured navigator MR signal is corrected for eddy-currents. Such eddy-currents are caused by the switching of gradient fields. If no steps are taken, these eddy-currents may deteriorate the MR navigator signal which is acquired from the center of k-space. Notably, compensation for slowly decaying eddy-currents, e.g. having a life time of about 10 ms or more, yields accurate results.
A further version of the method according to the invention is characterized in that the quality criterion is dependent on a modulus-weighted phase of the measured navigator MR signal. The modulus-weighted phase is a weighted mean value of the phases of successive samples of the navigator MR signal weighted by the modulus of the corresponding samples of the navigator MR signal.
A further version of the method according to the invention is characterized in that a corrected MR image is determined by phase correction of the measured MR signals, the phase correction of a sample of the measured MR signal being dependent on the phases of corresponding samples of the navigator MR signal associated with the measured MR signa
Van Den Brink Johan S.
Van Muiswinkel Arianne M. C.
Arana Louis
U.S. Philips Corporation
Vodopia John F.
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