Versatile cone-beam imaging apparatus and method

• US 6,944,259 B2
• Filed: 09/26/2002
• Issued: 09/13/2005
• Est. Priority Date: 09/26/2001
• Status: Expired due to Fees

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Yang, Xiaochun
• Primary Examiner(s): Bruce, David V.
• Assistant Examiner(s): Gemmell, Elizabeth
• Application Number:
  US10/256,727
• Publication Number:
  US 20030072406A1
• Time in Patent Office: 1,083 Days
• Field of Search: 378/15, 378/8, 378/4, 378/901
• US Class Current: 378/15
• CPC Class Codes: A61B 6/027 characterised by the use of a particular data acquisition trajectory, e.g. helical or spiral
• CPC Class Codes: A61B 6/032 Transmission computed tomography [CT]
• CPC Class Codes: A61B 6/4085 Cone-beams
• CPC Class Codes: G06T 11/006 Inverse problem, transformation from projection-space into object-space, e.g. transform methods, back-projection, algebraic methods
• CPC Class Codes: G06T 2211/421 Filtered back projection [FBP]
• CPC Class Codes: Y10S 378/901 Computer tomography program or processor

Abstract

A three-dimensional image of an object scanned with a plurality of cone-beam projections from a number of source positions is reconstructed using a method wherein intermediate transform functions are obtained from two-dimensional images of radiation attenuation in the scanned object. The intermediate transform functions are then filtered over the two-dimensional attenuation images using a moving-frame technique. The second-order radial derivative of the Radon transform can then be backprojected to generate an intermediate, locally-reconstructed, three-dimensional image. After repetition of this process, the plurality of intermediate, locally reconstructed, three-dimensional images are summed to obtain an ultimate, reconstructed, three-dimensional image of the object.

Claims

1. A method for generating a three-dimensional image of a scanned object from a plurality of cone-beam projections passed through the object and attenuated thereby, the method comprising:

a) positioning the source at an initial position on a predetermined scan path;

b) passing a projection of cone-beam radiation comprising a plurality of projection rays from a common focal point through an object, the cone-beam projection being attenuated by partial absorption in the object;

c) detecting radiation intensity of the attenuated cone-beam projection on an area detector;

d) obtaining a two-dimensional attenuation image of the cone-beam projection from the detected radiation intensity;

e) obtaining an intermediate transform function from the two-dimensional attenuation image on a set of planes passing through the focal point;

f) at least once, repositioning the source and repeating steps (b)-(e);

g) filtering the intermediate transform functions acquired from consecutive attenuation images at two or more source positions using a moving-frame technique to obtain the second-order radial derivative of the Radon transform;

h) backprojecting the second-order radial derivative of the Radon transform in a two-dimensional space along each projection ray to generate an intermediate, locally reconstructed, three-dimensional image with constant values assigned along each projection ray;

i) at least once, repeating steps (f)-(h); and

j) summing the plurality of intermediate, locally reconstructed, three-dimensional images obtained for the plurality of cone-beam projections to obtain an ultimate, reconstructed, three-dimensional image of the object.

2. The method of claim 1, wherein the intermediate transform function is the first-order radial derivative of the Radon transform.

3. The method of claim 2, wherein each two-dimensional attenuation image is obtained as a discrete two-dimensional data set with an area detector comprising a plurality of detector elements, and wherein the step of obtaining the first-order radial derivative of the Radon transform includes:

i) if the area detector does not coincide with the standard image plane;

a) mapping the coordinates of the detector elements to the coordinates of a set of points on a standard image plane under a projective coordinate transform; and

b) interpolating the two-dimensional data set to obtain attenuation values on a regular grid on the standard image plane;

ii) interpolating data points lying between grid nodes along a set of virtual image lines;

iii) calculating a weighted sum of attenuation along each virtual image line with weights determined by the cosine of the angle between (a) a projection ray associated to a image point and (b) a central ray, which is the shortest line from the source to the virtual image line;

iv) differentiating the weighted sum over a set of parallel image lines to obtain the first-order radial derivative of the Radon transform on a set of corresponding planes passing through the focal point; and

v) storing the resulting first-order radial derivative of the Radon transform in memory for use in a subsequent phase of reconstruction.

4. The method of claim 3, wherein the virtual image lines are rational lines.

5. The method of claim 3, wherein the regular grid has square or rectangular elements.

6. The method of claim 1, wherein the two-dimensional backprojection along a projection ray is an integral over all planes passing by that ray, wherein the unit normal vectors of these planes are confined to a great circle on the unit sphere.

7. The method of claim 1, wherein a plurality of cone-beam projections are successively passed through the object, and wherein the step of determining the second-order radial derivative of the Radon transform includes:

i) obtaining the first-order radial derivative of the Radon transform on a set of planes for the next one or more cone-beam projections;

ii) determining the global coordinates, represented by the unit normal and radial distance, of a set of corresponding planes that pass though a set of virtual image lines on the standard image plane and through the focal point;

iii) calculating the local parameters of intersection image lines between the image plane and corresponding planes in successive cone-beam projections parallel to the set of planes passing through the focal point via a coordinate transformation determined by a moving-frame basis;

iv) interpolating the first-order radial derivative of the Radon transform for the planes obtained in part (iii) from the first-order radial derivative of the Radon transform obtained in part (i); and

v) obtaining the second-order radial derivative of the Radon transform by differentiating, which can be approximated by a finite difference, the first-order radial derivative of the Radon transform over the parallel planes from consecutive cone-beam projections.

8. The method of claim 1, wherein the step of backprojecting the second-order radial derivative includes:

for each projection ray associated with a node on the image plane, calculating a weighted sum of the second-order radial derivative of the Radon transform over a set of planes passing through the projection ray with weights determined by the number of intersections of each plane with the scan path and by the angular interval between two adjacent planes, wherein the weighted sum is constantly-assigned to all points along the projection ray; and

resampling and interpolating the projection rays so that each node on a three-dimensional Cartesian grid representing the object is positioned on one of the rays and is assigned a value to produce a three-dimensional, locally reconstructed image.

9. The method of claim 1, wherein the source is displaced along a complete scan path relative to the object, wherein the scan path intersects with almost all planes passing by the object and the scan path is everywhere differentiable, with exception allowed for a zero measure set, and the cone-beam is generated and passed through the object at a plurality of locations on the complete scan path.

10. The method of claim 9, wherein the scan path is a biquadratic curve.

11. The method of claim 1, wherein the radiation is X-ray radiation.

12. The method of claim 1, wherein the cone-beam projections are detected with an area detector on an opposite side of the object from the radiation source.

13. The method of claim 1, wherein the area detector has a vertical axis parallel with the rotational axis and a horizontal axis parallel with the projection of the tangent line of the scan path at a current source position onto a horizontal plane.

14. The method of claim 1, wherein the detector is placed orthogonal to a line connecting the source to the center of the object, and the horizontal axis of the detector is aligned with the projection of the tangent of the scan path at a current source position onto a surface of the detector.

15. The method of claim 1, wherein the moving frame technique involves the generation of a set of orthonormal bases with the origin of each orthonormal basis positioned at a focal point such that each time the source is repositioned to generate a new focal point and the two-dimensional attenuation image is obtained, the orthonormal basis has a new origin, wherein the moving frame technique enables the second-order radial derivative of the Radon transforms to be evaluated by filtering over a set of parallel planes from consecutive cone-beam projections.

16. The method of claim 15, wherein the moving frame technique enables location of the parallel planes from a set of coordinate transforms.

17. A computer system for generating a reconstruction of cone-beam radiation attenuation in an object, the computer system comprising:

means for obtaining an intermediate transform function from a signal representing the amount of radiation transmitted through the object;

means for calculating the second-order radial derivative of the Radon transform by filtering the intermediate transform function over a pair of or multiple consecutive two-dimensional cone-beam measurement using a moving-frame technique;

means for backprojecting the second-order radial derivative of the Radon transform to generate an intermediate, locally reconstructed, three-dimensional image with constant values assigned along each projection ray; and

an accumulation buffer to sum the plurality of intermediate, locally reconstructed, three-dimensional images obtained for the cone-beam projections to obtain an ultimate, reconstructed, three-dimensional image of the object.

18. A computer-readable storage medium storing computer-executable software for generating a reconstruction of cone-beam radiation attenuation in an object, the software comprising:

code for obtaining an intermediate transform function from a signal representing the amount of radiation transmitted through the object;

code for calculating the second-order radial derivative of a Radon transform by filtering over the intermediate transform function using a moving-frame technique;

code for backprojecting the second-order radial derivative of the Radon transform to generate an intermediate, partially reconstructed, three-dimensional image with constant values assigned along each projection ray; and

code for summing the plurality of intermediate, locally reconstructed, three-dimensional images obtained for the cone-beam projections to obtain an ultimate, reconstructed, three-dimensional image of the object.

19. A cone-beam tomography apparatus comprising a computer-readable storage medium storing computer-executable software for generating a reconstruction of cone-beam radiation attenuation in an object, the software comprising:

code for obtaining an intermediate transform function from a signal representing the amount of radiation transmitted through the object;

code for determining the second-order radial derivative of the Radon transform by filtering over the intermediate transform function using a moving-frame technique;

code for backprojecting the second-order radial derivative of the Radon transform to generate an intermediate, locally reconstructed, three-dimensional image with constant values assigned along each projection ray; and

code for summing the plurality of intermediate, locally reconstructed, three-dimensional images obtained for the cone-beam projections to obtain an ultimate, reconstructed, three-dimensional image of the object.

20. The cone-beam tomography apparatus of claim 19, further comprising a radiation source, a radiation detector and a support for an object to be scanned by radiation from the radiation source.

21. The cone-beam tomography apparatus of claim 20, wherein the software further comprises code for displacing the source and detector relative to the support to provide a biquadratic scan path for radiation transmitted from the source, through an object positioned by the support, and to the detector.

22. A cone-beam tomography apparatus for generating a reconstruction of cone-beam radiation attenuation in an object, the apparatus comprising

means for obtaining an intermediate transform function from a signal representing the amount of radiation transmitted through the object;

means for determining the second-order radial derivative of the Radon transform by filtering over the intermediate transform function using a moving-frame technique;

means for backprojecting the second-order radial derivative of the Radon transform to generate an intermediate, locally reconstructed, three-dimensional image with constant values assigned along each projection ray; and

means for summing the plurality of intermediate, locally reconstructed, three-dimensional images obtained for the cone-beam projections to obtain an ultimate, reconstructed, three-dimensional image of the object.