Method and apparatus for automatic image quality assessment

• US 6,813,374 B1
• Filed: 04/25/2001
• Issued: 11/02/2004
• Est. Priority Date: 04/25/2001
• Status: Expired due to Term

• Current Assignee: Analogic Corporation
• Original Assignee: Analogic Corporation
• Inventors: Rozas, David, Karimi, Seemeen S., Crawford, Carl R., Simanovsky, Sergey, Bechwati, Ibrahim
• Primary Examiner(s): Johns, Andrew W.
• Application Number:
  US09/842,075
• Time in Patent Office: 1,287 Days
• Field of Search: 382/128, 382/131, 378/18, 378/207, 250/252.1, 250/453.11
• US Class Current: 382/131
• CPC Class Codes: A61B 6/482 involving multiple energy imaging
• CPC Class Codes: A61B 6/583 using calibration phantoms
• CPC Class Codes: G01V 5/20 Detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects
• CPC Class Codes: G06V 20/52 Surveillance or monitoring of activities, e.g. for recognising suspicious objects recognising microscopic objects G06V20/69
• CPC Class Codes: G06V 2201/05 Recognition of patterns representing particular kinds of hidden objects, e.g. weapons, explosives, drugs

Abstract

A method of and apparatus for assessing the image quality of a CT scanner is described in which assessment can be made manually or automatically. No special image quality mode of CT scanner operation is necessary, and no precise alignment of the phantom is necessary. In general, performance of the scanner comprises: using the scanner (a) to scan a phantom in one or more of its normal modes of operation while translating said phantom along the scanner axis of rotation and (b) to produce scanned data of the phantom, and assessing the performance of the scanner from the scanned data. In accordance with another aspect, the assessment is performed by (a) using the scanner to scan a phantom in one or more of its normal modes of operation; (b) reconstructing a three-dimensional volume CT image for a region containing at least a portion of the phantom; (c) calculating properties of the CT image; and (d) using the calculated properties of the CT image to assess CT scanner performance.

Claims

1. A method of assessing performance of a computed tomography (CT) scanner including a scanner axis of rotation, comprising:

(a) using the scanner to scan a phantom in one or more of its normal modes of operation;

(b) reconstructing a three-dimensional volume CT image for a region containing at least a portion of the phantom;

(c) calculating properties of the CT image; and

(d) using the calculated properties of the CT image to assess CT scanner performance;

wherein the CT scanner includes a translator that translates the phantom substantially in the same direction as the scanner axis of rotation while operating the scanner to scan the phantom.

2. The method of claim 1, wherein calculating properties of the CT image is performed automatically.

3. The method of claim 1, wherein calculating properties of the CT image is performed manually.

4. The method of claim 1, wherein using the scanner to scan a phantom comprises acquiring CT projection data simultaneously with phantom translation.

5. The method of claim 1, wherein using the scanner to scan a phantom comprises displacing the phantom along the scanner axis of rotation at a sequence of successive fixed axial positions, and acquiring a set of CT projection data at each position.

6. The method of claim 1, wherein using the scanner to scan a phantom further comprises generally aligning the phantom with the scanner axis of rotation.

7. The method of claim 1, wherein the CT scanner has a single fixed mode of reconstructing a three-dimensional volume CT image.

8. The method of claim 1, wherein the phantom is a container with a plurality of inserts disposed therein.

9. The method of claim 8, wherein the inserts are positioned at predetermined locations within the phantom.

10. The method of claim 8, wherein portions of at least two inserts are positioned so as to overlap in the same CT image.

11. The method of claim 8, wherein portions of at least two inserts are positioned so as to overlap in the same CT axial image.

12. The method of claim 8, wherein the container is made of plastic material such that

(a) the container does not require careful handling;

(b) the number of metallic components in the container is minimized, so as to minimize CT reconstruction artifacts.

13. The method of claim 8, wherein foam is used to position and maintain the inserts separate from one another within the container.

14. The method of claim 8, wherein calculating properties of the CT image further comprises

(a) identifying a plurality of volume elements in the CT image, each volume element being associated with a density value;

(b) combining the volume elements into objects;

(c) associating the objects with phantom inserts; and

(d) calculating properties of the objects.

15. The method of claim 14, wherein combining the volume elements into objects further comprises using region growing.

16. The method of claim 14, wherein combining the volume elements into objects further comprises using connected components labeling (CCL).

17. The method of claim 14, wherein the CT scanner includes an in-plane, and calculating properties of the objects further comprises measuring the in-plane resolution of the CT scanner.

18. The method of claim 17, wherein measuring the in-plane resolution of the CT scanner comprises:

(a) mounting a comb insert inside the phantom so that it is scanned generally perpendicular to the scanner axis of rotation, said insert including a plurality of fin groupings;

(b) calculating CT number variation within at least one of the fin groupings;

(c) comparing the CT number variation to a reference value so as to determine the response of the CT scanner to the spatial frequency represented by the at least one fin grouping.

19. The method of claim 14, wherein calculating properties of the objects further comprises measuring the axial resolution of the CT scanner.

20. The method of claim 19, wherein measuring the axial resolution of the CT scanner comprises:

(a) mounting a comb insert inside the phantom so that it is scanned generally parallel to the scanner axis of rotation, the insert including a plurality of fin groupings;

(b) calculating CT number variation within at least one of the fin groupings;

(c) comparing the CT number variation to a reference value so as to determine the response of the CT scanner to the spatial frequency represented by the at least one fin grouping.

21. The method of claim 14, wherein calculating properties of the objects further comprises associating a characteristic CT number with an insert associated with each object.

22. The method of claim 21 wherein associating the characteristic CT number of each insert is the mean CT number of all the volume elements comprising the insert.

23. The method of claim 21, wherein assigning a characteristic CT number with an insert comprises:

(a) identifying a region of interest within the corresponding object;

(b) calculating the mean CT number of all the volume elements within the region of interest.

24. The method of claim 21, further comprising comparing the differences in said characteristic CT numbers of at least two inserts to a reference value so as to assess scanner CT number uniformity.

25. The method of claim 24, wherein at least two inserts are placed at different locations within the CT scanner field of view.

26. The method of claim 24, wherein at least two inserts are placed at different locations along the scanner axis of rotation.

27. The method of claim 21, further comprising comparing the ratios of the characteristic CT numbers of at least two inserts to a reference value so as to assess the scanner CT number linearity.

28. The method of claim 27, wherein at least two inserts are made of different materials.

29. The method of claim 27, wherein at least two inserts are of different size.

30. The method of claim 14, wherein calculating properties of the objects further comprises measuring image noise.

31. The method of claim 30, wherein measuring image noise comprises:

(a) identifying a region of interest within an object;

(b) calculating standard deviation of CT numbers within said region of interest;

(c) comparing the standard deviation to a reference value so as to assess the image noise of the CT scanner.

32. The method of claim 30, further including translating the phantom with the translator when scanning the phantom, wherein the insert associated with at least one object has a length greater than the distance traveled by the phantom in one rotation of the gantry of the CT scanner, and measuring the image noise comprises:

(a) selecting an axial slice of the at least one object;

(b) identifying a region of interest within the at least one object;

(c) subtracting the selected axial slice from an axial slice located a predetermined number of slices away from the selected slice, thereby creating a difference image;

(d) calculating standard deviation of CT numbers within said region of interest in the difference image;

(e) comparing the standard deviation to a reference value so as to assess the image noise of the CT scanner.

33. The method of claim 14, further comprising selecting a plurality of fiducial marks in the CT image of the phantom.

34. The method of claim 33, wherein said fiducial marks are selected from the plurality of volume elements associated with the container.

35. The method of claim 33, wherein the translator is a conveyor, and the phantom is supported on the conveyor as the phantom is translated through the scanner, the method further comprising:

calculating relative coordinates of the fiducial marks so as to determine;

(a) the position;

(b) the slope; and

(c) the translational speed of the conveyor.

36. The method of claim 8, wherein the calculation of the properties of the CT image is automatically done on a separate automatic diagnostics computer configured to perform automatic diagnostics.

37. The method of claim 36, wherein said automatic diagnostics is configured for explosive detection.

38. The method of claim 36, wherein said automatic diagnostics is configured for medical scanning.

39. The method of claim 36, wherein at least one insert is to test a property of the automatic diagnostics.

40. The method of claim 36, wherein the plurality of inserts includes at least one simulated sheet explosive.

41. The method of claim 36, wherein the plurality of inserts includes at least one simulated bulk explosive.

42. The method of claim 8, wherein each insert is characterized by a predetermined shape and density.

43. The method of claim 8, wherein the plurality of inserts includes:

(a) one or more comb inserts;

(b) a diagonal rod;

(c) a Nylon cylinder;

(d) a PVC cylinder; and

(e) a Teflon cylinder.

44. The method of claim 8, further comprising inferring the identity of the phantom.

45. The method of claim 44, wherein inferring the identity of the phantom comprises modifying at least one insert in a manner unique to each phantom.

46. The method of claim 45, wherein modifying at least one insert comprises making one or more indentations in at least one insert.

47. The method of claim 44, wherein CT performance is assessed in a manner unique to each phantom.

48. The method of claim 8, wherein calculating properties of the CT image further comprises

(a) identifying a plurality of volume elements in the CT image, each volume element being associated with a density value;

(b) combining the volume elements into objects;

(c) associating the objects with phantom inserts; and

(d) calculating properties of the objects.

49. The method of claim 1, further comprising:

(a) computing the distance between the center of the phantom and the scanner axis of rotation;

(b) comparing the distance to a predetermined reference value;

(c) declaring the CT image of the phantom unsuitable for assessing performance of the CT scanner if the distance exceeds the predetermined reference value.

50. The method of claim 1, further comprising:

(a) computing the angle between the phantom and the scanner axis of rotation;

(b) comparing the angle to a predetermined reference value;

(c) declaring the CT image of the phantom unsuitable for assessing performance of the CT scanner if the angle exceeds the predetermined reference value.

51. The method of claim 1, wherein the phantom is a container with a plurality of inserts, and further including using at least one insert to test communication between the CT scanner and an automatic diagnostics computer.

52. The method of claim 51, wherein using at least one insert to test communication between the CT scanner and the automatic diagnostics computer comprises:

(a) identifying the volume elements in the CT image corresponding to the at least one. insert;

(b) establishing contiguity of any of the identified volume elements;

(c) declaring a communication error if the identified volume elements are not contiguous.

53. The method of claim 51, wherein the at least one insert is a diagonal rod.

54. A method of assessing performance of a computed tomography (CT) scanner comprising:

using the scanner (a) to scan a phantom in one or more of its normal modes of operation while translating said phantom along the scanner axis of rotation and (b) to produce scanned data of the phantom, and assessing the performance of the scanner from the scanned data.

55. The method according to claim 54, wherein using the scanner to scan a phantom and to produce scanned data comprises reconstructing a three-dimensional volume CT image for a region containing at least a portion of the phantom and calculating the properties of the CT image, and assessing the performance of the scanner comprises assessing the performance of the scanner from assessing the predetermined properties of the CT image.

56. A computed tomography (CT) scanner including a scanner axis of rotation including apparatus for assessing the performance of the scanner, comprising:

a translator for translating a phantom through the scanner substantially in the same direction as the scanner axis of rotation while operating the scanner in one or more of its normal modes of operation; and

a processor constructed and arranged so as to (a) reconstruct a three-dimensional volume CT image for a region containing at least a portion of the phantom scanned by the scanner while operating the scanner in one or more of its normal modes of operation;

(b) provide information for calculating properties of the CT image; and

(c) use of the calculated properties of the CT image to assess CT scanner performance.

57. The scanner of claim 56, wherein the processor calculates properties of the CT image automatically.

58. The scanner of claim 56, wherein the properties of the CT image is calculated manually.

59. The scanner of claim 56, wherein the scanner scans the phantom so as to acquire CT projection data simultaneously with phantom translation.

60. The scanner of claim 56, wherein the translator displaces the phantom along the scanner axis of rotation at a sequence of successive fixed axial positions, and a set of CT projection data is acquired at each position.

61. The scanner of claim 56, wherein the scanner scans the phantom while the phantom is generally aligned with the scanner axis of rotation.

62. The scanner of claim 56, wherein scanner includes a single fixed mode of reconstructing a three-dimensional volume CT image.

63. The scanner of claim 56, wherein the phantom is a container with a plurality of inserts disposed therein.

64. The scanner of claim 63, wherein the inserts are positioned at predetermined locations within the phantom.

65. The scanner of claim 63, wherein portions of at least two inserts are positioned so as to overlap in the same CT image.

66. The scanner of claim 63, wherein portions of at least two inserts are positioned so as to overlap in the same CT axial image.

67. The scanner of claim 63, wherein the container is made of plastic material such that

(a) the container does not require careful handling;

(b) the number of metallic components in the container is minimized, so as to minimize CT reconstruction artifacts.

68. The scanner of claim 63, wherein foam is used to position and maintain the inserts separate from one another within the container.

69. The scanner of claim 63, wherein the processor further:

(a) identifies a plurality of volume elements in the CT image, each volume element being associated with a density value;

(b) combines the volume elements into objects;

(c) associates the objects with phantom inserts; and

(d) calculates properties of the objects.

70. The scanner of claim 69, wherein the processor combines the volume elements using region growing.

71. The scanner of claim 69, wherein the processor combines the volume elements into objects using connected components labeling (CCL).

72. The scanner of claim 69, wherein the CT scanner includes an in-plane, and the processor calculates properties of the objects by measuring the in-plane resolution of the CT scanner.

73. The scanner of claim 72, wherein the processor measures the in-plane resolution of the CT scanner by:

(a) mounting a comb insert inside the phantom so that it is scanned generally perpendicular to the scanner axis of rotation, said insert including a plurality of fin groupings;

(b) calculating CT number variation within at least one of the fin groupings;

(c) comparing the CT number variation to a reference value so as to determine the response of the CT scanner to the spatial frequency represented by the at least one fin grouping.

74. The scanner of claim 69, wherein the processor measures the axial resolution of the CT scanner.

75. The scanner of claim 74, wherein the axial resolution of the CT scanner is measured by:

(a) mounting a comb insert inside the phantom so that it is scanned generally parallel to the scanner axis of rotation, the insert including a plurality of fin groupings;

(b) calculating CT number variation within at least one of the fin groupings;

(c) comparing the CT number variation to a reference value so as to determine the response of the CT scanner to the spatial frequency represented by the at least one fin grouping.

76. The scanner of claim 69, wherein the processor associates a characteristic CT number with an insert associated with each object.

77. The scanner of claim 76, wherein the characteristic CT number associated with each insert is the mean CT number of all the volume elements comprising the insert.

78. The scanner of claim 76, wherein the processor assigns a characteristic CT number with an insert by:

(a) identifying a region of interest within the corresponding object; and

(b) calculating the mean CT number of all the volume elements within the region of interest.

79. The scanner of claim 76, wherein the processor compares the differences in the characteristic CT numbers of at least two inserts to a reference value so as to assess scanner CT number uniformity.

80. The scanner of claim 79, wherein at least two inserts are placed at different locations within the CT scanner field of view.

81. The scanner of claim 79, wherein at least two inserts are placed at different locations along the scanner axis of rotation.

82. The scanner of claim 76, wherein the processor compares the ratios of the characteristic CT numbers of at least two inserts to a reference value so as to assess the scanner CT number linearity.

83. The scanner of claim 82, wherein at least two inserts are made of different materials.

84. The scanner of claim 82, wherein at least two inserts are of different size.

85. The scanner of claim 69, the processor measures image noise.

86. The scanner of claim 85, the processor measures image noise by:

(a) identifying a region of interest within an object;

(b) calculating standard deviation of CT numbers within the region of interest;

(c) comparing the standard deviation to a reference value so as to assess the image noise of the CT scanner.

87. The scanner of claim 85, wherein the insert associated with at least one object has a length greater than the distance traveled by the phantom in one rotation of the gantry of the CT scanner, and the processor measures the image noise by:

(a) selecting an axial slice of the at least one object;

(b) identifying a region of interest within the at least one object;

(c) subtracting the selected axial slice from an axial slice located a predetermined number of slices away from the selected slice, thereby creating a difference image;

(d) calculating standard deviation of CT numbers within said region of interest in the difference image; and

(e) comparing the standard deviation to a reference value so as to assess the image noise of the CT scanner.

88. The scanner of claim 69, further comprising a plurality of fiducial marks selected in the CT image of the phantom.

89. The scanner of claim 88, wherein the fiducial marks are selected from the plurality of volume elements associated with the container.

90. The scanner of claim 88, wherein the translator is a conveyor, and the phantom is supported on the conveyor as the phantom is translated through the scanner, where in the processor:

calculates relative coordinates of the fiducial marks so as to determine;

(a) the position;

(b) the slope; and

(c) the translational speed of the conveyor.

91. The scanner of claim 63, further including a separate automatic diagnostics computer configured to calculate properties of the CT image automatically done on the automatic diagnostics computer.

92. The scanner of claim 91, wherein said automatic diagnostics computer is configured for explosive detection.

93. The scanner of claim 91, wherein said automatic diagnostics computer is configured for medical scanning.

94. The scanner of claim 91, wherein at least one insert is to test a property of the automatic diagnostics.

95. The scanner of claim 91, wherein the plurality of inserts includes at least one simulated sheet explosive.

96. The scanner of claim 91, wherein the plurality of inserts includes at least one simulated bulk explosive.

97. The scanner of claim 63, wherein each insert is characterized by a predetermined shape and density.

98. The scanner of claim 63, wherein the plurality of inserts includes:

(a) one or more comb inserts;

(b) a diagonal rod;

(c) a Nylon cylinder;

(d) a PVC cylinder; and

(e) a Teflon cylinder.

99. The scanner of claim 63, wherein the processor infers the identity of the phantom.

100. The scanner of claim 99, wherein at least one insert in a manner is modified to be unique within the phantom so as inferring the identity of the phantom.

101. The scanner of claim 100, the one insert is modified by making one or more indentations in at least one insert.

102. The scanner of claim 99, wherein CT performance is assessed in a manner unique to each phantom.

103. The scanner of claim 63, wherein the processor:

(a) identifies a plurality of volume elements in the CT image, each volume element being associated with a density value;

(b) combines the volume elements into objects;

(c) associates the objects with phantom inserts; and

(d) calculates properties of the objects.

104. The scanner of claim 56, wherein the processor:

(a) computes the distance between the center of the phantom and the scanner axis of rotation;

(b) compares the distance to a predetermined reference value;

(c) declares the CT image of the phantom unsuitable for assessing performance of the CT scanner if the distance exceeds the predetermined reference value.

105. The scanner of claim 56, wherein the processor:

(a) computes the angle between the phantom and the scanner axis of rotation;

(b) compares the angle to a predetermined reference value; and

(c) declares the CT image of the phantom unsuitable for assessing performance of the CT scanner if the angle exceeds the predetermined reference value.

106. The scanner of claim 56, wherein the phantom is a container with a plurality of inserts, wherein at least one insert is configured to test communication between the CT scanner and an automatic diagnostics computer.

107. The scanner of claim 106, wherein the processor tests communication between the CT scanner and the automatic diagnostics computer by:

(a) identifying the volume elements in the CT image corresponding to the at least one insert;

(b) establishing contiguity of any of the identified volume elements; and

(c) declaring a communication error if the identified volume elements are not contiguous.

108. The scanner of claim 106, wherein the at least one insert is a diagonal rod.

109. A CT scanner configured to assess its own performance, the scanner comprising:

a translator for conveying a phantom through the scanner along the scanner axis of rotation while operating the scanner in one or more of its normal modes of operation, and

a processor for receiving scanned data of the phantom during a scan of the phantom, and assessing the performance of the scanner from the scanned data.

110. The CT scanner according to claim 109, wherein the processor is configured to reconstruct a three-dimensional volume CT image for a region containing at least a portion of the phantom and calculate the properties of the CT image, and the processor assesses the performance of the scanner from assessing the predetermined properties of the CT image.