Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-10-10
2002-10-15
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000, C324S309000, C324S318000, C324S322000
Reexamination Certificate
active
06466016
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a phase distribution measurement method and apparatus, phase correction method and apparatus, and magnetic resonance imaging method and apparatus, and more specifically, to a method and apparatus for measuring the phase distribution in an image obtained by magnetic resonance imaging, to a method and apparatus for correcting the phase of pixel data based on the measured phase distribution, and to a magnetic resonance imaging method and apparatus which obtains water and lipid images based on the phase-corrected pixel data.
In a magnetic resonance imaging apparatus, a stationary magnetic field is formed in a space which houses an imaging object, a gradient magnetic field and a high frequency magnetic field are formed in the stationary magnetic field space, and an image is formed (reconstituted) based on a magnetic resonance signal generated by the spin of the imaging object. Due to a chemical shift, the magnetic resonance signal of lipids has a different frequency from the magnetic resonance signal of water, so water and lipids can be imaged separately using a phase difference based on the frequency difference.
The phase of the magnetic resonance signal is affected by the non-uniformity of the stationary magnetic field intensity, so in order to image water and lipids separately without being affected by the magnetic field non-uniformity, a phase distribution, i.e. phase map, representing the stationary magnetic field non-uniformity, is calculated, and phase correction of the image is performed based thereon.
The phase map is obtained by calculating the phase of image data given by complex numbers for each pixel. To obtain a precise phase map, noise is first removed by processing the image with a lowpass filter.
FIGS.
1
(A) and
1
(B) show the concept of the phase map for a typical one-dimensional image. The origin of the phase map is taken as the center of the stationary magnetic field. The phase at the origin is 0. This diagram is a phase map when the stationary magnetic field intensity has a linear gradient, and the phase varies linearly according to the distance from the origin. When the phase exceeds +&pgr;, it returns to the −&pgr; side, and when it exceeds −&pgr;, it returns to the +&pgr; side, as shown by (a) in the figure. In other words, phase wrap around takes place.
Therefore, in parts where there is wraparound, a correction for wrap around, i.e. unwrapping, is performed to obtain a phase map without wraparound, as shown in FIG.
1
(
b
). The presence or absence of wraparound is detected by whether or not the absolute value of the phase difference of the image data for adjacent pixels is 2&pgr;, and for pixels where wraparound is detected, 2&pgr; is added to the phase with a reverse sign to that of the phase difference.
When lowpass filtering was performed on the image as a preliminary step to calculating the phase map, the pixel data was locally disturbed in some places resulting from change in the magnetization of the imaging object, phase variation due to lipids, ghosting due to blood circulation or body movement, or high noise. In these places, it appeared as if there were a wraparound phase state, but a precise phase map could not be obtained by performing unwrapping based thereon.
Further, if the magnetic field nonuniformity is nonlinear, the phase map comprises high order components. Lowpass filtering must be set so that these high order components are not removed, but this lowpass filtering setting is not easy to perform.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a phase distribution measuring method and apparatus which can calculate a precise phase map even when there is local phase disturbance, to provide a phase correction method and device using this calculated phase map, and to produce a magnetic resonance imaging method and apparatus which implements this phase correction.
It is a further object of this invention to provide a phase correction method and apparatus which permits an easy phase correction including high frequency components, and to provide a magnetic resonance imaging method and apparatus which performs such a phase correction.
The invention from one aspect for resolving the aforesaid problems is a phase distribution measuring method, comprising the steps of lowpass filtering an image obtained by magnetic resonance imaging, detecting pixel positions for which the ratio of a value after lowpass filtering to a value before lowpass filtering does not exceed a predetermined ratio by comparing pixel data for each corresponding pixel for an image before lowpass filtering and an image after lowpass filtering, calculating a phase distribution based on the image before lowpass filtering or the image after lowpass filtering excluding pixel data at the detected pixel positions, and compensating by estimating the phase at said excluded pixel positions in the calculated phase distribution from the phase at neighboring pixel positions.
In the invention from this aspect, pixel data for each corresponding pixel are compared for an image before and after lowpass filtering, pixel positions where there is a phase disturbance are detected, a phase map is calculated excluding pixel data at these pixel positions, and compensation is then made by estimating the phase at the excluded pixel positions from the phase at neighboring pixel positions so as to obtain a phase map without anomalies.
The invention from another aspect for resolving the above problems is a phase distribution measuring apparatus, comprising a filtering means which lowpass filters an image obtained by magnetic resonance imaging, a pixel position detecting means which detects pixel positions for which the ratio of a value after lowpass filtering to a value before lowpass filtering does not exceed a predetermined ratio by comparing pixel data for each corresponding pixel for an image before lowpass filtering and an image after lowpass filtering, a phase distribution calculating means which calculates a phase distribution based on the image before lowpass filtering or the image after lowpass filtering excluding pixel data at the detected pixel positions, and phase compensating means which compensates by estimating the phase at the excluded pixel positions in the calculated phase distribution from the phase at neighboring pixel positions.
In the invention from this aspect, pixel data for each corresponding pixel are compared for an image before and after lowpass filtering by the pixel position detecting means to detect pixel positions where there is a phase disturbance, a phase map is calculated excluding pixel data at these pixel positions by the phase distribution calculating means, and compensation is then made by the phase compensating means which estimates the phase at the excluded pixel positions from the phase at neighboring pixel positions so as to obtain a phase map without anomalies.
The invention from another aspect for resolving the above problems is a phase correction method, comprising the steps of lowpass filtering an image obtained by magnetic resonance imaging, detecting pixel positions for which the ratio of a value after lowpass filtering to a value before lowpass filtering does not exceed a predetermined ratio by comparing pixel data for each corresponding pixel for an image before lowpass filtering and an image after lowpass filtering, calculating a phase distribution based on the image before lowpass filtering or the image after lowpass filtering excluding pixel data at the detected pixel positions, compensating by estimating the phase at the excluded pixel positions in the calculated phase distribution from the phase at neighboring pixel positions, and performing phase correction of the image using the compensated phase distribution.
In the invention from this aspect, pixel data for each corresponding pixel are compared for an image before and after lowpass filtering, pixel positions where there is a phase disturbance are detected, a phase map is calculated excluding pixel
GE Medical Systems Global Technology Company LLC
Lefkowitz Edward
Moonray Kojima
Vargas D.
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