System and methods for the reduction and elimination of image artifacts in the calibration of x-ray imagers

• US 6,370,224 B1
• Filed: 06/12/2000
• Issued: 04/09/2002
• Est. Priority Date: 06/29/1998
• Status: Expired due to Term

• Current Assignee: Sofamor Danek Group, Inc. (Medtronic Plc)
• Original Assignee: Sofamor Danek Group, Inc. (Medtronic Plc)
• Inventors: Simon, David A., Smith, Kurt R.
• Primary Examiner(s): BRUCE, DAVID VERNON
• Application Number:
  US09/591,512
• Time in Patent Office: 666 Days
• Field of Search: 378/4, 378/8, 378/62, 378/98, 378/207, 378/99
• US Class Current: 378/62
• CPC Class Codes: A61B 6/5252 removing objects from field of view, e.g. removing patient table from a CT image
• CPC Class Codes: A61B 6/583 using calibration phantoms

Abstract

Image processing operations are used to improve images that include visual artifacts generated by calibration markers used in intrinsic calibration of an x-ray image. Artifacts introduced by opaque or semi-transparent calibration markers may be completely or partially removed from the image. More particularly, artifacts caused by opaque calibration markers are removed by changing the pixels corresponding to the projections of the calibration markers to blend in with pixels surrounding the calibration markers. Artifacts may also be generated with semi-transparent calibration markers. These artifacts may be eliminated from the image, while leaving intact the underlying image, by subtracting a constant offset from each marker projection.

Claims

1. A medical imaging system comprising:

an x-ray source for generating x-rays;

calibration markers positioned in a path of the x-rays;

an x-ray receiving device for receiving the generated x-rays and deriving a digital image representing objects through which the generated x-rays have passed, the digital image including representations of the calibration markers; and

a processor coupled to the x-ray receiving device for performing image processing operations on the digital image, the image processing operations modifying the visual presence of the representations of the calibration markers, based on characteristics of an image area surrounding the calibration markers.

2. The system of claim 1, wherein the x-ray receiving device further comprises an image intensifier and a charge coupled device (CCD) array for deriving the digital image from the x-rays.

3. The system of claim 1, wherein the processor modifies the visual presence of the representations of the calibration markers using a maximum likelihood estimator that maximizes a criterion function.

4. The system of claim 3, wherein the maximum likelihood estimator is an Expectation Maximizing algorithm.

5. The system of claim 1, wherein the x-ray receiving device provides a representation of calibration markers that are opaque or semi-transparent to the x-rays.

6. The system of claim 5, wherein the processor further includes means for identifying pixels in the digital image that surround the calibration markers and means for modifying the representations of the calibration markers based on the values of the pixels that surround the representations of the calibration markers.

7. The system of claim 6, wherein modifying the representation of the calibration markers further includes using an average value of the pixels that surround the representations of the calibration markers.

8. The system of claim 6, wherein modifying the representations of the calibration markers further includes separately calculating average intensity values for separate regions of the digital image surrounding the representation of the calibration markers.

9. The system of claim 6, wherein the modifying means includes an estimator using the value of the pixels surrounding the representation of the calibration markers.

10. The system of claim 6, wherein identifying the representations of the calibration markers further includes identifying the calibration markers by classifying a plurality of image pixels into image pixels corresponding to the calibration markers and image pixels corresponding to anatomy or other objects.

11. The system of claim 6, wherein identifying the pixels in the digital image includes identifying pixels that are not directly adjacent to the calibration markers.

12. The system of claim 6, wherein the processor further includes means for adding a constant offset to the modified representations of the calibration markers to enhance the visibility of the representations.

13. The system of claim 6, wherein modifying the representations of the calibration markers further includes adding an offset to the representations of the calibration markers that have been modified to enhance the visibility of the representations.

14. The system of claim 13, wherein the processor further includes means for acquiring an offset value corresponding to an intensity of one of the calibration marker representations in which the x-rays have not traversed the objects.

15. The system of claim 14, wherein the processor further includes means for subtracting the offset value from each of the calibration marker representations.

16. The system of claim 15, wherein the processor further includes means for refining the subtracted versions of the calibration marker representations by using an estimator algorithm that optimizes a criterion function.

17. A method of creating an image of a target comprising:

transmitting x-rays at the target and calibration markers;

collecting the x-rays;

producing a digital image representing the target and calibration markers; and

processing the digital image to modify the representation of the calibration markers, based on characteristics of an image area surrounding the calibration markers.

18. The method of claim 17, wherein the identifying of the representations of the calibration markers includes identifying the calibration markers by classifying a plurality of image pixels into image pixels corresponding to the calibration markers and image pixels corresponding to anatomy or other objects.

19. The method of claim 17, wherein the transmitting further includes transmitting the x-rays through semi-transparent calibration markers arranged in a predetermined pattern.

20. The method of claim 19, wherein processing includes eliminating the representations of the calibration markers by subtracting an offset from pixels in the digital image that comprise the calibration marker representations.

21. The method of claim 17, wherein the transmitting further includes transmitting the x-rays at calibration markers that are opaque or semi-transparent to the x-rays.

22. The method of claim 21, wherein the processing includes identifying pixels in the digital image that surround the calibration markers and modifying the representations of the calibration markers based on the values of the pixels that surround the representations of the calibration markers.

23. The method of claim 22, wherein the identifying further includes identifying pixels that are not directly adjacent to the calibration markers.

24. The method of claim 22, wherein the modifying of the representation of the calibration markers further includes using an average value of the pixels that surround the representation of the calibration markers.

25. The method of claim 22, further comprising adding an offset to the modified representations of the calibration markers to enhance the visibility of the representations.

26. The method of claim 22, wherein modifying of the representation of the calibration markers includes separately calculating average intensity values for separate regions of the digital image surrounding the representation of the calibration markers.

27. The method of claim 22, wherein the modifying of the representation of the calibration markers further includes using an estimator utilizing the value of the pixels surrounding the representation of the calibration markers.

28. A computer readable medium containing computer instructions for causing a processor to perform processing comprising:

storing a digital image representing anatomy of a patient, the digital image including representations of calibration markers that at least partially occlude portions of the patient anatomy; and

performing image processing operations on the digital image to modify the representations of the calibration markers, based on characteristics of an image area surrounding the calibration markers.

29. The computer readable medium of claim 28, wherein the instructions further include instructions for eliminating the representations of the calibration markers by subtracting an offset from pixels in the digital image that comprise the calibration marker representations.

30. The computer readable medium of claim 28, wherein the instructions further include instructions for modifying the visual presence of the representations of the calibration markers using a maximum likelihood estimator that maximizes a criterion function.

31. The computer readable medium of claim 30 wherein the maximum likelihood estimator is an Expectation Maximizing algorithm.

32. The computer readable medium of claim 28, wherein the instructions further include instructions for identifying pixels in the digital image that surround the calibration markers and modifying the representations of the calibration markers based on the values of the pixels that surround the representations of the calibration markers.

33. The computer readable medium of claim 32, wherein the instructions further include instructions for identifying pixels in the digital image that are not directly adjacent to the calibration markers.

34. The computer readable medium of claim 32, wherein the instructions for modifying further include using an average value of the pixels that surround the representation of the calibration markers.

35. The computer readable medium of claim 32, wherein the instructions further include separately calculating average intensity values for separate regions of the digital image surrounding the representation of the calibration markers.

36. The computer readable medium of claim 32, wherein the instructions further include using an estimator to calculate the value of the pixels surrounding the representation of the calibration markers.

37. The computer readable medium of claim 32, wherein the instructions include identifying the calibration markers by classifying a plurality of image pixels into image pixels corresponding to the calibration markers and image pixels corresponding to anatomy or other objects.

38. The computer readable medium of claim 32, further including instructions for adding an offset to the modified representations of the calibration markers to enhance the visibility of the representations.

39. A computer system comprising:

a first computer memory storing a digital image representing anatomy of a patient, the digital image including representations of calibration markers that at least partially occlude portions of the patient anatomy;

a second memory storing instruction for performing image processing operations on the digital image to modify the representations of the calibration markers based on characteristics of an image area surrounding the calibration markers; and

a processor coupled to the first and second memory for executing the instructions stored in the second memory.

40. The computer system of claim 39, wherein the processor modifies the visual presence of the representations of the calibration markers using a maximum likelihood estimator that maximizes a criterion function.

41. The computer system of claim 40, wherein the maximum likelihood estimator is an Expectation Maximizing algorithm.

42. The computer system of claim 39, wherein the processor further includes means for identifying pixels in the digital image that surround the calibration markers and means for modifying the representations of the calibration markers based on the values of the pixels that surround the representations of the calibration markers.

43. The computer system of claim 42, wherein the processor further includes means for adding a constant offset to the modified representations of the calibration markers to enhance the visibility of the representations.

44. The computer system of claim 42, wherein the processor further includes means for modifying the representations of the calibration markers by identifying pixels in the digital image that are not directly adjacent to the calibration markers.

45. The computer system of claim 42, wherein the processor further includes means for modifying the representations of the calibration markers by using an average value of the pixels that surround the representation of the calibration markers.

46. The computer system of claim 42, wherein the processor further includes means for modifying the representations of the calibration markers by separately calculating average intensity values for separate regions of the digital image surrounding the representation of the calibration markers.

47. The computer system of claim 42, wherein the processor further includes means for modifying the representations of the calibration markers using an estimator utilizing the value of the pixels surrounding the representation of the calibration markers.

48. The computer system of claim 42, wherein the processor further includes means for modifying the representations of the calibration markers by classifying a plurality of image pixels into image pixels corresponding to the calibration markers and image pixels corresponding to anatomy or other objects.