X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-06-17
2001-05-22
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000
Reexamination Certificate
active
06236705
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to image reconstruction in computer tomography and more particularly relates to a method for tracking organ motion and for removing motion artifacts.
BACKGROUND OF THE INVENTION
In computer tomography, 2-D or 3-D image reconstruction is performed using projection data acquired over a period of time in a scan comprised of a series of projections. Each projection is a snapshot of a patient's organs from a different angle, or perspective, and a scan typically includes hundreds of projections. Prior art methods used to reconstruct images from such data presume the patient and his organs are motionless during the entire scan such that a same fixed object is the subject of all acquired projections. Organ motion such as cardiac motion, blood flow, lung respiration or a patient's restlessness during an acquisition process produces artifacts that appear as a blurring effect in the reconstructed image. Such blurring effects substantially complicate diagnosis or may even lead to inaccurate diagnosis putting the patient's health at risk. Furthermore, repeating a scan in case of a complicated diagnosis due to blurring effects exposes the patient unnecessarily to radiation such as X-rays.
Speeding up data acquisition to reduce the blurring effects of organ motion is not possible with current x-ray tube technology. Therefore, signal processing algorithms accounting for organ motion have to be applied in the image reconstruction process.
Several techniques have been proposed to reduce the effects of organ motion. Srinivas, C. and Costa, M. H. M. in “Motion-compensated CT image reconstruction”, Proceedings of the IEEE Ultrasonics Symposium, 1, pp. 849-853, 1994, teach motion compensation using a linear model assuming translation and rotation. In U.S. Pat. No. 5,323,007 issued Jun. 21, 1994, Wernick et al. disclose a method for motion compensation using two projections of an object taken from different locations at different time instances. Organ motion is then measured from known image elements and an image is then corrected by solving a set of linear equations. Other techniques model organ motion as a periodic sequence and take projections at a particular point of the motion cycle or to correct image data using motion trajectories obtained from Fourier harmonics as disclosed in U.S. Pat. No. 5,615,677 to Pelc et al. issued Apr. 1, 1997. However, organ motion is too complex for these methods to substantially reduce the blurring effects and makes the prior art methods useful only in a very limited number of cases. In “Tomographic Reconstruction Of Time Varying Object From Linear Time-Sequential Sampled Projections”, Proceedings of the IEEE, 0-7803-1775-0/94, pp. 309-312, 1994, Chiu, Y. H. and Yau, S. F. teach a method for compensating for organ motion by iteratively suppressing motion effects from the projections. This method reduces assumed spectral characteristics of the motion artifacts. The method depends on knowledge of at least some properties of the organ motion and requires a substantial number of iterations to converge, thereby requiring a large amount of computing time. In U.S. Pat. No. 5,671,263 issued Sep. 23, 1997, Ching-Ming discloses another spectral method for motion compensation. A high frequency signal of the organ motion is obtained using a high pass filter. The high frequency signal is then subtracted from the projection signal to remove motion artifacts. Unfortunately, removing high frequency components from the projection signal removes small size spatial structures from the image, as well.
In U.S. Pat. No. 5,806,521 issued Sep. 15, 1998, Morimoto et al. disclose a method for motion compensation based on correlating overlapping converted image data in an ultrasound imaging apparatus. In successive image frames a majority of information results from a same geometry. Due to this redundant information the data of two successive image frames is highly correlated. Organ motion between the acquisition of two frames will result in a shift of the correlation peak of the two frames with respect to each other corresponding to the amount of relative motion. Unfortunately, correlation of the two successive image frames taken from different spatial locations results only in a minor reduction of motion artifacts and, furthermore, may lead to cancellation of image details.
It is an object of the invention to provide a method for tracking organ motion and removing motion artifacts, which overcomes the aforementioned problems and substantially reduces motion artifacts in images of a large variety of CT scans.
It is further an object of the invention to provide a method for tracking organ motion and removing motion artifacts for implementation in current CT systems.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided, a method of tracking motion present during computer tomography scan data acquisition of an object, the method comprising the steps of:
capturing a plurality of image pairs, each image within an image pair captured from substantially a same perspective view of the object;
comparing each image within an image pair of the image pairs to another image of the same image pair to extract differences; and,
determining motion using the extracted differences.
In accordance with another aspect of the invention, there is provided, a method of removing motion artifacts in image data of computer tomography scans of an object comprising the steps of:
providing to a processor image data of the object, the image data containing motion artifacts due to motion of the object;
providing to the processor data indicative of the motion of the object; and,
using the processor, removing the motion artifacts from the image data using adaptive interference canceling.
In accordance with yet another aspect of the invention, there is provided, a method of tracking motion of components of an object present during computer tomography scan data acquisition of the object, the method comprising the steps of:
rotating intermittently a scanner comprising a source and a receiving sensor array in angular step increments around the object such that the object is irradiated from successive locations at successive time instances;
acquiring projection measurement data at each time instance using the receiving sensor array; and,
providing to a processor the projection measurement data for processing, the processing comprising the steps of:
determining successive sensor time series segments, each segment being associated with a full rotation of the scanner and a time difference &Dgr;t to a previous segment;
determining an amplitude difference between successive time series segments; and,
determining data indicative of organ movement by processing the amplitude differences using a filter back-projection algorithm.
In accordance with the invention, there is further provided, a computer tomography system for acquiring projection image data of an object and for tracking motion of the object during the data acquisition comprising:
a source disposed at a first position for emitting energy in order to irradiate the object; a receiving sensor array for capturing image pairs, wherein each image within an image pair is captured from substantially a same perspective view of the object;
means for irradiating the object such that the emitted energy appears to originate from a second other position;
a mechanism for moving the source to the second other position; and,
a processor for comparing the image pairs to extract differences and for determining motion using the extracted differences.
In accordance with another embodiment of the invention, there is provided, a method of tracking phases of a motion cycle of an object present during computer tomography scan data acquisition of the object, the method comprising the steps of:
a) providing to a processor sensor time series, the sensor time series being indicative of projection image data of the object;
b) using the processor, processing the sensor time series to determine spatial overlap co
Dhanantwari Amar C.
Stergiopoulos Stergios
Bruce David V.
Her Majesty the Queen in right of Canada as represented by the
Larson & Taylor PLC
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