X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-11-12
2001-11-27
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S008000, C378S901000
Reexamination Certificate
active
06324240
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to computer assisted tomography and quantitative computed tomography (both known as CT imaging). More particularly, it relates to a method for processing CT imaging data that corrects for beam hardening errors.
BACKGROUND OF THE INVENTION
CT imaging is extensively used for medical imaging and the imaging of objects. In CT imaging, X-rays are projected through the object being imaged, and these X-rays are detected by arrays of detectors. The X-rays are projected through the object in many different directions. The combination of X-ray trajectories through the object provides data from which the internal structure of the object can be determined. Contrast in CT images is provided by variations in X-ray attenuation within the object. No other contrast parameters are available. Therefore, accurate measurements of X-ray attenuation are required for high quality CT images.
Quantitative computed tomography (QCT) is a technique that allows for quantitative measurements of physical properties related to X-ray attenuation. QCT has been used for in-vivo quantitative measurements of bone density, for example. The uses of QCT include assessment of spinal trabecular bone, evaluation of drug therapy in the treatment of osteoporosis, screening for osteoporosis, fracture risk assessment and many others. Although QCT is now an established tool for bone densitometry, there exist major issues affecting the accuracy and precision of QCT measurements.
QCT measurements and CT images are affected by beam hardening error in X-ray attenuation measurements. Beam hardening error is caused by the energy-dependence of X-ray transmission within an object being imaged. In any material, low-energy X-rays are attenuated more strongly than high-energy X-rays. Therefore, as a polychromatic X-ray beam passes through an object, the proportion of high energy X-rays in the beam increases, and attenuation decreases. Long path lengths through an object therefore appear to have an excessively small attenuation. When an image is computed, the center of an object appears to have a lower attenuation than the outer regions of the object. In this way, beam hardening error produces inaccurate measurements in QCT. Correction of beam hardening errors has been an active area of research since 1975. Some popular current correction techniques for beam hardening require strict assumptions about the X-ray attenuation characteristics of the materials within the object. The two most commonly used techniques are the water and bone corrections which assume that the materials in the scan field are either water-equivalent or dense bone-equivalent in X-ray attenuation characteristics. For more information regarding these techniques, reference can be made to “A Method for Correcting Bone Induced Artifacts in Computed Tomography Scanners” by P. M. Joseph and R. D. Spital in the
Journal of Computer Assisted Tomography
vol. 2, pp. 481-487, 1978 and “Post-Reconstruction Method for Beam Hardening in Computed Tomography” by O. Nalcioglu and R. Y. Lou in
Physics in Medicine and Biology
, vol 24, pp.330-340, 1979.
In another beam hardening correction method, calibration tubes having a known transmission characteristic are used. However, this method is often not effective for two reasons: 1) different regions in the scan field experience different degrees of beam hardening, and 2) calibration tubes cannot capture the beam hardening characteristics in vicinity of a patients bone because the calibration tube must be placed outside the body.
There exists a need in the art of CT imaging and QCT for an improved method of beam hardening correction. An improved beam hardening correction technique will provide QCT measurements and CT images with improved accuracy.
OBJECTS AND ADVANTAGES OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide an improved method for beam hardening correction that:
1) accurately corrects beam hardening errors;
2) does not require knowledge of the attenuation characteristics of the X-ray detectors used in the CT imager;
3) does not require calibration tubes;
4) can be used with objects comprising many different materials.
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
SUMMARY OF THE INVENTION
The present invention provides a method for beam hardening correction in CT imaging data. The present method includes reiterative calculations that converge on accurate measurements of X-ray attenuation. In a first variation of the present invention, the following information is required:
1) The attenuation spectra for materials within the object being imaged. Each voxel is assumed to contain at most two materials.
2) The output spectra of the X-ray source.
3) The output data from the X-ray detectors.
An initial estimation is made of volume fraction of the two materials in each voxel. Then, a reiterative calculation is performed that converges upon the true volume fraction for each material in each voxel.
In a second variation of the present invention, the following information is required:
1) Two basis attenuation spectra. The attenuation spectra do not need to correspond to real materials.
2) Output spectra from the X-ray source at two different settings (e.g. two different X-ray tube voltages).
3) Output data from the X-ray detectors at the two X-ray source settings.
An initial estimation is made of the relative weighting of the two basis attenuation spectra based on a linear combination of the basis spectra. Then, a reiterative calculation is performed that converges upon the relative weightings of the basis attenuation spectra that produce the observed measurements. A separate reiterative calculation is performed for each voxel. In this way, an accurate measurement of the attenuation spectrum for each voxel is provided.
REFERENCES:
patent: 6018562 (2000-01-01), Wilson
Joseph, P. et al., amethod for simultaneous correction of spectrum hardening artifacts in CT images containing both bone and iodine, Med. Phys., 24(10), pp. 1629-1634, Oct. 1997.
Chase, R., et al.,An improved image algorithm for CT scanners, Med. Phys., 5(6), pp. 497-499, Nov./Dec. 1978.
Alvarez, R. et al.,Energy-selective reconstructions in X-ray computerized tomography, Phys. Med. Biol., 21(5), pp. 733-744, 1976.
Duerinckx, A. et al.,Polychromatic streak artifacts in computed tomography images, J. Comp. Assis. Tomography, vol. 2, pp. 481-487, Sep. 1978.
Joseph, P. et al.,A method for correcting bone induces artifacts in computed tomography Scanners, J. Comp. Assis. Tomography, vol. 2, pp. 100-108, Jan. 1978.
Kijewski, P. et al.,Correction for beam hardening in computed tomography, Med. Phys., 5(3), pp. 209-214, May/Jun. 1978.
McDavid, W. et al.,Correction for spectral artifacts in cross-sectional reconstruction from X-rays, Med. Phys. 4(1), pp. 54-57, Jan./Feb. 1977.
Nalcioglu, O. et al.,Post-reconstruction method for beam hardening in computerised Tomography, Phys. Med. Biol., 24(2), pp. 330-340, 1979.
Napel Sandy
Whalen Robert T.
Yan Chye Hwang
Bruce David V.
Lumen Intellectual Property Services
The Board of Trustees of the Leland Stanford Junior University
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