Method of and system for computing effective atomic number images in multi-energy computed tomography

1. A method of computing Z images from the CT projection data acquired by scanning a set of objects using at least two x-ray spectra, wherein said projection data includes a set of low energy projections and a set of high energy projections, comprising:

a. reconstructing a CT image from one of the two sets of projections, which are not processed by any decomposition methods;

b. obtaining the photoelectric projections from the high energy projections and low energy projections without decomposition to Compton projections;

c. reconstructing a photoelectric image from the photoelectric projections; and

d. computing the Z image from the photoelectric image and the CT image, without using any Compton images.

2. The method of claim 1 , wherein reconstructing said CT image includes reconstructing said CT image from the set of low energy projections.

3. The method of claim 1 , wherein reconstructing said CT image includes reconstructing said CT image from the set of high energy projections.

4. The method of claim 1 , wherein reconstructing said CT image from one of the two sets of projections includes using nonlinear beam hardening correction.

5. The method of claim 1 , wherein computing the Z image from the photoelectric image and the CT image is a function of at least two parameters K and n, wherein K is a factor for scaling the Z image, and n is an exponent of the ratio of the photoelectric image to the CT image.

6. The method of claim 1 , wherein obtaining the photoelectric projections from the high energy projections and low energy projections includes using a two-dimensional look-up table to generate the photoelectric projections.

7. The method of claim 5 , further including obtaining values for said parameters K and n through a calibration procedure comprising:

a. scanning at least two different materials with known effective atomic numbers;

b. reconstructing CT images;

c. obtaining the photoelectric projections;

d. reconstructing photoelectric images from the photoelectric projections;

e. generating the Z images with k=1 and n=1;

f. obtaining mean Z values for each of the two materials; and

g. calculating K and n based on the mean Z values and the known effective atomic numbers of the two materials.

8. A system for computing Z images from the projection data acquired by scanning a set of objects using at least two x-ray spectra, wherein said projection data includes a set of low energy projections and a set of high energy projections, comprising:

a. a reconstruction subsystem constructed and arranged so as to reconstruct CT images from one of the two sets of projections, which are not processed by any decomposition methods;

b. a dual energy decomposition subsystem constructed and arranged so as to obtain the photoelectric projections from low energy projections and high energy projections without decomposition to Compton projections;

c. a reconstruction subsystem constructed and arranged so as to reconstruct photoelectric images from the photoelectric projections; and

d. a Z image calculator constructed and arranged so as to compute the Z images from the CT images and the photoelectric images, without using any Compton images.

9. The system of claim 8 , wherein the reconstruction subsystem is constructed and arranged so that said CT image can be reconstructed from the set of low energy projections.

10. The system of claim 8 , wherein the reconstruction subsystem is constructed and arranged so that said CT image can be reconstructed from the set of high energy projections.

11. The system of claim 8 , wherein said reconstruction subsystem for reconstructing CT images from one of the two sets of projections comprises a module for providing nonlinear beam hardening correction.

12. The system of claim 8 , wherein said Z image calculator is constructed and arranged so as to compute the Z images from the photoelectric image and the CT image is a function of at least two parameters K and n, wherein K is a factor for scaling the Z image, and n is an exponent of the ratio of the photoelectric image to the CT image.

13. The system of claim 8 , wherein the dual energy decomposition subsystem includes a two-dimensional look-up table configured and arranged so as to generate the photoelectric projections.

14. The system of claim 12 , wherein said system further includes a calibration module constructed and arranged so as to obtain parameters K and n for the Z image calculator.