Method of normalising a digital signal representation of an image

• US 7,321,674 B2
• Filed: 01/30/2003
• Issued: 01/22/2008
• Est. Priority Date: 02/22/2002
• Status: Active Grant

• Current Assignee: Agfa Nv (Agfa-Gevaert N.V.)
• Original Assignee: AGFA Healthcare NV (Agfa-Gevaert N.V.)
• Inventors: Vuylsteke, Pieter
• Primary Examiner(s): Do; Anh Hong
• Application Number:
  US10/354,268
• Publication Number:
  US 20030161517A1
• Time in Patent Office: 1,818 Days
• Field of Search: 382/128, 382/251, 382/215, 382/278, 382/131, 382/132, 375/240.03, 375/240.02, 375/240.17, 709/247, 710/68, 370/521, 708/203, 358/3.19, 378/98.11
• US Class Current: 382/128
• CPC Class Codes: G06T 2207/10081 Computed x-ray tomography [CT]
• CPC Class Codes: G06T 2207/10116 X-ray image
• CPC Class Codes: G06T 2207/20016 Hierarchical, coarse-to-fine, multiscale or multiresolution image processing; Pyramid transform
• CPC Class Codes: G06T 2207/20064 Wavelet transform [DWT]
• CPC Class Codes: G06T 5/30 Erosion or dilatation, e.g. thinning
• CPC Class Codes: G06T 5/40 using histogram techniques
• CPC Class Codes: G06T 5/92 based on global image properties

Abstract

A digital signal representation of an image is normalized by applying a normalization parameter to all pixels of the digital signal representation of the image or to the pixels of a multi-scale representation of the image. The normalization parameter can be derived from the digital signal representation itself or from a multi-scale representation of the image. Different criteria for deriving the normalization parameter are disclosed.

Claims

1. A method of a computer program stored in a memory or on a disk for operation in a computed radiography system for generating, processing and storing radiographic images for medical diagnosis, the method comprising the steps of:

rendering a digital representation of one of said radiographic images;

decomposing said digital representation into a multi-scale representation comprising at least two detail images representing detail at successive scales in which pixels of the at least two detail images represent an amount of variation of pixel values of the digital representation at the scales of the detail images;

deriving a normalization parameter from said multi-scale image representation; and

applying said normalization parameter to all pixels of said multi-scale representation.

2. The method according to claim 1 wherein said normalization parameter is a combination of parameters

3. The method according to claim 2 wherein said normalization parameter is a product of

4. The method according to claim 1 wherein the at least two detail images are derived from only a single digital representation that was rendered from only a single radiographic image.

5. A method of a computer program stored in a memory or on a disk for operation in a computed radiography system for generating, processing and storing radiographic images for medical diagnosis, the method comprising the steps of:

rendering a digital representation of one of said radiographic images;

decomposing said digital representation into a multi-scale representation comprising at least two detail images representing detail at successive scales in which pixels of the at least two detail images represent an amount of variation of pixel values of the digital representation at the scales of the detail images;

deriving a normalization parameter from said multi-scale image representation; and

applying said normalization parameter to said digital signal representation.

6. The method according to claim 5 wherein said normalization parameter is a combination of parameters

7. The method according to claim 6 wherein said normalization parameter is a product of

8. The method according to claim 5 wherein the at least two detail images are derived from only a single digital representation that was rendered from only a single radiographic image.

9. A method of a computer program stored in a memory or on a disk for operation in a computed radiography system for generating, processing and storing radiographic images for medical diagnosis, the method comprising the steps of:

rendering a digital representation of a radiographic image;

deriving a normalization parameter from said digital signal representation;

decomposing said digital signal representation into a multi-scale representation comprising at least two detail images representing detail at successive scales in which pixels of the at least two detail images represent an amount of variation of pixel values of the digital representation at the scales of the detail images; and

applying said normalization parameter to pixels of said multi-scale representation.

10. The method according to claim 9 wherein said normalization parameter is a combination of parameters

11. The method according to claim 10 wherein said normalization parameter is a product of

12. The method according to claim 9 wherein the at least two detail images are derived from only a single digital representation that was rendered from only a single radiographic image.

13. A method of a computer program stored in a memory or on a disk for operation in a computed radiography system for generating, processing and storing radiographic images for medical diagnosis, the method comprising the steps of:

rendering a digital representation of one of said radiographic images;

decomposing said digital representation into a multi-scale representation comprising at least two detail images representing detail at successive scales;

deriving a normalization parameter from said multi-scale image representation, wherein the step of deriving said normalization parameter comprises the steps of;

deriving a representative grey value rl from a histogram of grey values of pixels of said detail images;

mapping said representative grey value onto a constant reference level Tr; and

determining said normalization parameter as Tr/rl; and

applying said normalization parameter to all pixels of said multi-scale representation.

14. The method according to claim 13 wherein the step of deriving said representative grey value is further determined by the steps of:

calculating a restricted grey value histogram of pixels of said image having a local contrast-to-noise ratio between predetermined margins; and

determining said representative grey value as median of said restricted grey value histogram.

15. The method according to claim 13 wherein said normalization parameter is a ratio of a predefined target noise level value and a value representing an estimation of the noise level.

16. The method according to claim 13 wherein said normalization parameter is a ratio of a predefined target contrast level value and a value representing an original contrast level.

17. The method according to claim 16 wherein said original contrast level is determined by computing a histogram of local standard deviation at a predefined scale of said multi-scale representation and by calculating a median value of local standard deviations in the histogram.

18. A method of a computer program stored in a memory or on a disk for operation in a computed radiography system for generating, processing and storing radiographic images for medical diagnosis, the method comprising the steps of:

rendering a digital representation of one of said radiographic images;

decomposing said digital representation into a multi-scale representation comprising at least two detail images representing detail at successive scales;

deriving a normalization parameter from said multi-scale image representation, wherein the step of deriving said normalization parameter is obtained by the steps of;

deriving a representative grey value rl from a histogram of grey values of pixels of said image;

mapping said representative grey value onto a constant reference level Tr; and

determining said normalization parameter as Tr/rl; and

applying said normalization parameter to said digital signal representation.

19. The method according to claim 18 wherein the step of deriving said representative grey value is further determined by the steps of:

calculating a restricted grey value histogram of pixels of said image having a local contrast-to-noise ratio between predetermined margins; and

determining said representative grey value as median of said restricted grey value histogram.

20. The method according to claim 18 wherein said normalization parameter is a ratio of a predefined target noise level value and a value representing an estimation of the noise level.

21. The method according to claim 18 wherein said normalization parameter is a ratio of a predefined target contrast level value and a value representing an original contrast level.

22. The method according to claim 21 wherein said original contrast level is determined by computing a histogram of local standard deviation at a predefined scale of said multi-scale representation and by calculating a median value of local standard deviations in the histogram.

23. A method of a computer program stored in a memory or on a disk for operation in a computed radiography system for generating, processing and storing radiographic images for medical diagnosis, the method comprising the steps of:

rendering a digital representation of a radiographic image;

deriving a normalization parameter from said digital signal representation, wherein the step of deriving said normalization parameter is obtained by the steps of;

deriving a representative grey value rl from a histogram of grey values of pixels of said radiographic image;

mapping said representative grey value onto a constant reference level Tr; and

determining said normalization parameter as Tr/rlU;

decomposing said digital signal representation into a multi-scale representation comprising at least two detail images representing detail at successive scales; and

applying said normalization parameter to pixels of said multi-scale representation.

24. The method according to claim 23 wherein said representative grey value is further determined by the steps of:

calculating a restricted grey value histogram of pixels of said detail images having a local contrast-to-noise ratio between predetermined margins; and

determining said representative grey value as median of said restricted grey value histogram.

25. The method according to claim 23 wherein said normalization parameter is a ratio of a predefined target noise level value and a value representing an estimation of the noise level.

26. The method according to claim 23 wherein said normalization parameter is a ratio of a predefined target contrast level value and a value representing an original contrast level.

27. The method according to claim 26 wherein said original contrast level is determined by computing a histogram of local standard deviation at a predefined scale of said multi-scale representation and by calculating a median value of local standard deviations in the histogram.