Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-04-08
2001-06-05
Oda, Christine K. (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S306000, C324S312000, C324S318000, C600S410000
Reexamination Certificate
active
06242914
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a magnetic resonance imaging method and a magnetic resonance imaging system.
2. Background Art
Magnetic resonance imaging (MRI) is a method for imaging echo signals generated from atomic nuclei such as protons in magnetic resonance by collecting positionally encoded echo signals using gradient magnetic field and by decoding using Fourier transform.
In general, it takes much time for imaging by MRI, and various proposals have been presented for reducing the imaging time.
EPI (echo planar imaging) is most representative one of high-speed MRI imaging methods. EPI is a method for collecting echo signals necessary for filling a k space as a multi-echo by switching (inverting) a gradient magnetic field very quickly in a uniform magnetostatic field generated by excitation by RF pulses.
EPI uses multi-shot switching or single-shot switching for switching of a magnetic field. Multi-shot EPI collects echo data necessary for reconstructing images over a plurality of occurrences of excitation whereas single-shot EPI collects all echo data in one step of excitation.
Since single-shot EPI enables obtaining MR images in a very short time as short as 30 to 40 ms, it is also used in functional MRI (FMRI) for catching dynamic changes of viscera, especially, images of blood flows in brains or hearts, peristalses of cardiac muscular walls and digestive tracts.
However, in case of imaging by the EPI method, read-out magnetic gradients are inverted in polarity. Therefore, when using the conventional method configured to reconstruct images by simply assembling obtained echo signals, single-shot EPI images are subject to ghost images called artifacts, and invite diagnostic errors.
Especially in FMRI which takes hundreds of images successively, an eddy current generated during imaging is liable to cause changes in intensity and phase of ghost, this is added to original images and varies the signal intensity of images together with the ghosts. This was a bar to quantitative analysis in FMRI.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a magnetic resonance imaging method and a magnetic resonance imaging system which can reduce changes in signal intensity of images with time, reduce ghost artifacts, and obtain a good image quality with maitaining same S/N ratio per time as conventional method.
According to the invention, there is provided a magnetic resonance imaging method comprising:
a process for switching (inverting) a gradient magnetic field at a high speed in a magnetostatic field applied to an object to be analyzed and obtaining magnetic resonant signals from atomic nuclei in said object to be analyzed as echo signals necessary for filling a k-space which is constructed by two axes of frequency encoding and phase encoding;
a process for converting said resonance signals into a digital form and developing them in the k-space as echo data;
a first scan process for sequentially scanning a phase encoding region corresponding to substantially a half of the k-space starting from a central portion of said k-space to an end corresponding to a Nyquist frequency, upon collecting said echo data of the k-space;
a second scan process for effecting scanning from the route scanned in said first scan process with only frequency encoding directions being opposite; and
a process for obtaining an image by classifying said echo data of the k-space by the encoding directions of the frequency encoding, reconstructing two images from the separate echo data sets obtained by the classification, and adding absolute number images of the reconstructed images.
According to the invention, there is further provided a magnetic resonance imaging system comprising:
a cylindrical hollow portion for receiving an object to be analyzed;
magnetostatic field generating means provided around said cylindrical hollow portion and generating a highly uniform magnetostatic field to be applied to said object to be analyzed;
gradient field generating mans for making a gradient to said magnetostatic field;
field inverting means for having said gradient field generating means to invert the gradient of said magnetic field;
transmitter means for applying high frequency electromagnetic waves to said object to be analyzed;
receiver means provided in said cylindrical hollow portion and detecting magnetic resonant signals from atomic nuclei in said object to be analyzed due to said high frequency electromagnetic waves and inversion of the gradient of said magnetostatic field;
k-space data developing means for obtaining k-space data by developing echo data detected by said receiver means and converted into a digital form;
data collecting means for collecting data upon collecting the echo data of the k-space by sequentially conducting first scanning of a phase encoding region corresponding to substantially a half the k-space starting from a central portion of said k-space to an end corresponding to a Nyquist frequency and by effecting second scanning the route scanned in said first scan process with an opposite frequency encoding direction; and
image forming means for making an image by obtaining an image by classifying said echo data of the k-space by the encoding directions of the frequency encoding, reconstructing two images from the separate echo data sets obtained by the classification, and averaging absolute number images of the reconstructed images.
In these method and system, two kinds of echo data in each phase encoding are obtained in response to scanning types of normal direction and opposite direction, image reconstruction can be performed using each echo data classified by scanning direction at frequency encoding. A known half-reconstruction method can be used for this image reconstruction. Each image is converted into real number by absolute number processing, and is added together for enhancing S/N ratio.
Therefore, according to this method, ghost artifacts derived from polarity inversion of a read-out magnetic gradient can be reduced since absolute number images are made for scanning directions of the frequency encoding and added together so that echo data differences caused by scanning directions are prevented from mixing as a form of phase information of the image.
Since the method and apparatus are configured to reconstruct images by collecting echo data in a half phase encoding region, positional shifts along a phase encoding direction caused by non-uniform static magnetic field are not such that opposite direction shiftings to cause overlapping of the two images, which is the case of centric view ordering. Therefore, the method and system provides another advantage that there is no deterioration in contrast of the image and space resolution.
REFERENCES:
patent: 4851779 (1989-07-01), DeMeester et al.
patent: 5652513 (1997-07-01), Liu et al.
MAGNETIC RESONANCE IN MEDICINE, Vol. 13, Feinberg et al, “Echo-Planar Imaging with Asymmetric. . .”, pp. 162-169, 1990.
PROC. OF ISMRM 4thMeeting, N.Y., Kao et al, “Nyquist Ghost Elimination in Blipped. . .”, pp.1476, 1996.
Jacobson Price Holman & Stern PLLC
Oda Christine K.
Shrivastav Brij B.
The Institute of Physical and Chemical Research
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