Image-based material decomposition

1. An imaging apparatus having a data acquisition system configured to reconstruct an image of an object and comprising a computer programmed to:

define a given projection into a first segment and a second segment different from the first segment;

acquire a first set of data in the first segment and acquire a second set of data in the second segment, wherein the first set of data is acquired at a first energy level and the second set of data is acquired at a second energy level that is different from the first energy level;

reconstruct a first image from the first set of data and a second image from the second set of data;

re-project the first image to a first set of corrected sinograms and re-project the second image to a second set of corrected sinograms;

decompose the first and second sets of corrected sinograms and determine atomic number information for each ray of each corrected sinogram, wherein the step of decomposing comprises generating an atomic number of material composition map from the decomposition;

reconstruct a first corrected image and a second corrected image from the first and the second sets of corrected sinograms; and

automatically register the first and the second corrected images into a composite image substantially free of beam hardening artifacts, wherein the composite image provides material decomposition information.

2. The apparatus of claim 1 further comprising an x-ray energy source designed to emit an x-ray beam toward an x-ray detector spaced from the x-ray source by at least an object to be imaged, and wherein the first set of data is acquired with the x-ray source biased to a first voltage and the second set of data is acquired with the x-ray source biased to a second voltage different from the first voltage.

3. The apparatus of claim 2 wherein the second voltage is considerably less than the first voltage.

4. The apparatus of claim 1 wherein the computer is further programmed to correct the first and the second images for cone beam distortions prior to registration.

5. The apparatus of claim 1 wherein the computer is further programmed to perform spectrally-sensitive beam hardening correction, with a water curve, on the first set and the second set of data to yield a first sinogram and a second sinogram, respectively.

6. The apparatus of claim 5 wherein the computer is further programmed to reconstruct the first image from the first sinogram and reconstruct the second image from the second sinogram, and display the composite image with material differentiation by color-coding image pixels corresponding to various imaged materials.

7. The apparatus of claim 1 further comprising an x-ray source and an x-ray filter, and wherein the computer is further programmed to automatically control at least one of x-ray source energization and x-ray filter filtering to generate the first energy level and the second energy level.

8. The apparatus of claim 7 wherein the x-ray source is a single anode x-ray source.

9. The apparatus of claim 1 wherein the computer is further programmed to determine material composition of the composite image based on atomic numbers of that imaged in the composite image, the determination of the material composition achieved by:

determining a first attenuation coefficient for a given pixel in the first image and a second attenuation coefficient for the given pixel in the second image;

determining a ratio between the first attenuation coefficient and the second attenuation coefficient; and

determining an atomic number for the given pixel in the composite image from the ratio.

10. The apparatus of claim 9 further comprising a computer readable storage medium having a look-up table stored thereon, and wherein the computer is further programmed to compare the determined ratio to the look-up table to determine the atomic number for the given pixel.

11. The apparatus of claim 1 wherein the first set of data includes a first sinogram and the second set of data includes a second sinogram, and wherein the computer is further programmed to:

reconstruct the first image from the first sinogram and reconstruct the second image from the second sinogram with a first beam hardening correction;

re-project the first image to a first set of corrected sinograms and re-project the second image to a second set of corrected sinograms;

decompose the first and the second sets of corrected sinograms and determine atomic number information for each ray of each corrected sinogram;

perform spectrally sensitive beam hardening correction, with the atomic number information, on the first and the second set of corrected sinograms;

reconstruct a first corrected image and a second corrected image from the spectrally sensitive beam hardening corrected sinograms;

register the first corrected image and the second corrected image to form a corrected composite image; and

display with the corrected composite image with atomic number information differentiated in the corrected composite image.

12. The apparatus of claim 1 wherein the first set of data includes a first sinogram and the second set of data includes a second sinogram, and wherein the computer is further programmed to:

determine median ρ-value of second sinogram;

set baseline thickness to the median ρ-value;

iteratively determine a first attenuation coefficient for each pixel in the first image and a second attenuation coefficient for each pixel in the second image;

determine a ratio between the first attenuation coefficient and the second attenuation coefficient for each pixel;

determine an atomic number for each pixel in the composite image from a linear attenuation coefficient curve that is defined in the x-axis by atomic number and defined in the y-axis by possible ratios.

13. The apparatus of claim 1 wherein the computer is further programmed to:

determine a thickness for each pixel of the second image;

iteratively determine a first attenuation coefficient for each pixel in the first image and a second attenuation coefficient for each pixel in the second image;

determine a ratio between the first attenuation coefficient and the second attenuation coefficient for each pixel;

determine an atomic number for each pixel in the composite image from a linear attenuation coefficient curve that is defined in the x-axis by atomic number and defined in the y-axis by possible ratios, wherein the thickness determined for each pixel mitigates beam hardening artifacts for the pixel.

14. The apparatus of claim 1 configured to acquire CT data of luggage, packages, or parcels.

15. The apparatus of claim 1 configured to acquire CT data from a medical patient.

16. The apparatus of claim 1 further comprising a rotatable gantry defining a bore to receive an object for imaging, the gantry housing an x-ray source and an x-ray detector assembly that are rotated about the object during CT data acquisition, and wherein one projection of data is acquired in one gantry rotation plus twice a detector fan-angle, and wherein the first segment is defined by one-half of gantry rotation plus a fan-angle and the second segment is defined by the other one-half of gantry rotation plus a fan-angle.

17. The apparatus of claim 16 wherein the computer is further programmed to coordinate gantry rotation with linear motion of the object to effect a helical scan, and wherein periodic switching of kvp corresponds to linear motion of one-half of detector width.

18. The apparatus of claim 16 wherein the computer is further programmed to coordinate gantry rotation with linear motion of the object to effect a step-and-shoot scan, and wherein the linear motion is suspended during acquisition of one projection data corresponding to two segments and linear advancement between acquisitions.

19. A computer readable storage medium having a computer program stored thereon and representing a set of instructions that when executed by a computer causes the computer to perform image-based atomic number mapping of an object imaged in a dual-energy mode, wherein the set of instructions comprise:

defining a given projection into a first segment and a second segment different from the first segment;

acquiring a first set of data in the first segment and acquiring a second set of data in the second segment, wherein the first set of data is acquired at a first energy level and the second set of data is acquired at a second energy level that is different from the first energy level;

reconstructing a first image from the first set of data and a second image from the second set of data;

re-projecting the first image to a first set of corrected sinograms and re-projecting the second image to a second set of corrected sinograms;

decomposing the first and second sets of corrected sinograms and determining atomic number information for each ray of each corrected sinogram, wherein the step of decomposing comprises generating an atomic number of material composition map from the decomposition;

reconstructing a first corrected image and a second corrected image from the first and the second sets of corrected sinograms; and

automatically registering the first and the second corrected images into a composite image substantially free of beam hardening artifacts, wherein the composite image provides material decomposition information.

20. The computer readable storage medium of claim 19 wherein the computer is further caused to:

automatically register a high kvp image and a low kvp image to form a composite image;

associate an atomic number of reach pixel of the composite image; and

color-code the composite image to differentiate imaged materials from one another based on atomic number.

21. The computer readable storage medium of claim 20 wherein the computer is further caused to iteratively determine the atomic number for each pixel of the composite image based on a linear coefficient curve.

22. A method of CT imaging comprising the steps of:

determining a thickness for each pixel;

determining a first attenuation coefficient for a given pixel in a first image;

determining a second attenuation coefficient, different from the first attenuation coefficient, for the given pixel in a second image, different from the first image;

determining a linear coefficient ratio between the first attenuation coefficient and the second attenuation coefficient;

determining an atomic make-up for the given pixel from the linear coefficient ratio, wherein the thickness determined for each pixel mitigates beam hardening artifacts for the pixel; and

displaying a registered image comprised of the first and the second images with the given pixel in the registered image displayed in a conspicuous manner to indicate the atomic make-up of that imaged in the given pixel.

23. The method of claim 22 wherein the first image is acquired at a first energy level and the second image is acquired at a second energy level, different from the first energy level.

24. The method of claim 23 further comprising the step of alternating an x-ray source between a high kvp and a low kvp to acquire the first and the second images, respectively.

25. The method of claim 22 wherein the registered image is substantially free of beam hardening artifacts.

26. The method of claim 22 wherein the atomic make-up of that imaged in the given pixel is indicated by at least one of color-coding or shading.