Energy dependent gain correction for radiation detection

• US 4,789,930 A
• Filed: 11/15/1985
• Issued: 12/06/1988
• Est. Priority Date: 11/15/1985
• Status: Expired due to Term

• Current Assignee: Picker International, Inc. (Marconi Holdings Company Limited)
• Original Assignee: Picker International, Inc. (Marconi Holdings Company Limited)
• Inventors: Sones, Richard A., Lauro, Karen L.
• Primary Examiner(s): Smith, Jerry
• Assistant Examiner(s): Bui, Kimthanh T.
• Application Number:
  US06/798,428
• Time in Patent Office: 1,117 Days
• Field of Search: 364/414, 378/900, 378/19, 378/5, 378/99, 250/252.1, 250/255, 250/361 R
• US Class Current: 378/207
• CPC Class Codes: A61B 6/583 using calibration phantoms
• CPC Class Codes: H05G 1/26 Measuring, controlling or protecting measuring X-ray radiation G01T
• CPC Class Codes: H05G 1/60 Circuit arrangements for obtaining a series of X-ray photographs or for X-ray cinematography

Abstract

A method and apparatus for calibrating the detector gain of a dual energy digital radiography system is provided. A basis material calibration object is scanned to create low and high energy pixel data. A regression is performed on the pixel data to derive at least one high energy calibration vector and at least one low energy calibration vector. The calibration vectors are transformed into high and low energy gain functions represented by a Taylor series expansion. An examination object is scanned to create low and high energy image data. The image data is combined with the gain function to create corrected low and high energy image data.

Claims

1. A method of calibrating a dual energy digital radiography system, utilizing a radiation source, a detector array responsive to radiation of at least two different energies spaced from the source to accommodate the placement of an object in a space between the source and detector array, scanning means for effecting relative scanning motion between the detector array and an object when said object is located in said object space, said calibration method comprising the steps of;

(a) scanning a multiplicity of thicknesses of basis materials to create a matrix of low and high energy pixel data;

(b) performing a regression on said matrix of low and high energy pixel data to derive at least one low energy coefficient vector and at least one high energy coefficient vector;

(c) transforming said at least one low energy and at least one high energy coefficient vectors to low and high energy gain functions;

(d) scanning an examination object to create low and high energy image data;

(e) combining said low and high energy image data with said low and high energy gain functions to create corrected low and high energy image data.

2. A medical imaging system comprising:

(a) a radiation source;

(b) a detector array responsive to incident radiation for producing low and high energy pixel values, said detector array spaced sufficiently from the source to accommodate the interposition of an object in the space between the source and the detector array;

(c) scanning means for effecting relative scanning motion between the detector array and an object when said object is located in said space between the source and the detector array;

(d) power means for actuating the source to direct radiation through the object space and toward the detector array;

(e) circuitry coupled to the detector array for monitoring variations in the low and high energy pixel values caused by variations in the incident beam quality and for producing low and high gain correction signals, said signals being a function of the radiation beam quality and the detector array'"'"'s response thereto;

(f) correction means coupled to said detector array and said circuitry for correcting said low and high pixel data values by respectively applying said low and high correction signals to said values; and

(g) imaging circuitry coupled to the correction means for producing an image representative of corrected detector array response to incident radiation.

3. The system of claim 2 wherein said circuitry comprises:

(a) means for storing a set of calibration signals representative of variations in detector array response to variations in incident beam quality; and

(b) a function generator coupled to said detector array and said storage means for combining a set of low and high energy image signals representative of radiation transmitted through an object under examination with said calibration signal set to produce low and high gain correction signals.

4. A method of correcting the gain of a radiation detector comprising the steps of:

(a) determining a first detector response by selectively measuring detector response to a beam of radiation transmitted through a multiplicity of thicknesses of basis materials;

said radiation being in at least two energy ranges;

(b) determining a representation of the variation in detector gain in response to variations in beam quality of said radiation by performing a regression on said first detector response;

(c) determining a second detector response to radiation transmitted through an object to be imaged;

(d) determining a gain correction factor, said factor a function of said detector gain variation representation and said second detector response; and

(e) combining said second detector response with said gain correction factor.

5. The method of claim 4 wherein said gain correction factor function is represented by a Taylor series expansion.

6. The method of claim 5 wherein said Taylor series expansion is represented by the formula:

##EQU5## where A₀, A₁ and A₂ are constants of a given detector;

PIX LO, PIX HI represent low and hi energy pixel values at a given pixel location;

Ref PIX LO, Ref PIX HI represent low and high energy pixel values at a given pixel location at reference beam hardness.

7. A digital radiography system comprising:

(a) a radiation source for directing radiation along a path;

(b) a radiation detector spaced from the source to receive radiation from said source passing through a calibration standard and to produce a first set of signals indicating the detector'"'"'s response to variations in the beam quality of the incident radiation;

(c) means for storing said first set of signals;

(d) means for producing a gain correction function, said means comprising;

(i) first receiving means coupled to said storage means for receiving said first set of signals;

(ii) second receiving means coupled to the detector for receiving a second set of signals representative of radiation transmitted through an object under examination;

(iii) a function generator for combining said first signal set with said second signal set;

(e) means for combining said second set of signals with said gain correction function to produce gain corrected signals; and

(f) imaging circuitry for receiving said gain corrected signals for producing a representation of the pattern of radiation transmitted through the examination object and incident on the detector.

8. The system of claim 7, wherein said detector comprises:

a plurality of individual detector elements.

9. An imaging system comprising:

(a) a radiation source;

(b) an array of detector elements facing the source each element being responsive to incident radiation to produce electrical signals indicative of said radiation;

(c) circuitry coupled to said detector elements and responsive to said electrical signals for determining the detectors response to variations in the spectral distribution of the incident radiation;

(d) a function generator coupled to said circuitry for producing a detector gain correction signal, said signal being a function of the detectors response to variations in the spectral distribution of the incident radiation;

(e) correction circuitry coupled to said detector elements and to said function generator for generating gain corrected electrical signals by combining the electrical signals and the gain correction signal; and

(f) imaging circuitry coupled to said correction circuitry and responsive to said gain corrected electrical signals for producing a representation of a pattern of said incident radiation.

10. The imaging system of claim 9 wherein the gain corrected electrical signals are generated substantially simultaneously to the production of said incident radiation pattern representation.

11. A dual energy digital x-ray imaging system comprising;

(a) a source of x-radiation;

(b) a dual energy detector array spaced from the source to accommodate an object therebetween, said detector array comprising a first element layer preferentially responsive to low energy x-radiation for producing low energy pixel data and a second element layer preferentially responsive to high energy x-radiation for producing high energy pixel data;

(c) logger means for generating the logarithm of said low energy pixel data and said high energy pixel data;

(d) means for storing first, second and third high energy coefficients and first, second and third low energy coefficients;

(e) sum of products calculator means for producing;

(i) a high energy gain correction signal derived from the sum of the product of the first high energy coefficient and the logarithm of the low energy pixel data plus the product of the second high energy coefficient and the logarithm of the high energy pixel data plus the third high energy coefficient; and

(ii) a low energy gain correction signal derived from the sum of the product of the first low energy coefficient and the logarithm of the low energy pixel data plus the product of the second low energy coefficient and the logarithm of the high energy pixel data plus the third low energy coefficient;

(f) multiplier means for producing;

(i) corrected high energy pixel data by multiplying the high energy pixel data by the high energy gain correction signal; and

(ii) corrected low energy pixel data by multiplying the low energy pixel data by the low energy gain correction signal; and

(g) display means for displaying an image representative of at least one of said corrected low energy pixel data and said high corrected energy pixel data.

12. The imaging system of claim 11 wherein said first, second and third, low and high energy coefficients are constants of the first and second element layers, respectively.

13. The imaging system of claim 12 wherein said constants of the first and second element layers are determined through a basis calibration.

14. A dual energy digital x-ray imaging system comprising;

(a) means for producing at least a high energy radiation spectrum and a low energy radiation spectrum;

(b) a radiation detector responsive to said radiation spectra for producing low energy pixel data and high energy pixel data;

(c) logger means for generating the logarithm of said low energy pixel data and said high energy pixel data;

(d) means for storing first, second and third high energy coefficients and first, second and third low energy coefficients;

(e) sum of products calculator means for producing;

(i) a high energy gain correction signal derived from the sum of the product of the first high energy coefficient and the logarithm of the low energy pixel data;

the product of the second high energy coefficient and the logarithm of the high energy pixel data; and

the third high energy coefficient; and

(ii) a low energy gain correction signal derived from the sum of the product of the first low energy coefficient and the logarithm of the low energy pixel data;

the product of the second low energy coefficient and the logarithm of the high energy pixel data; and

the third low energy coefficient;

(f) multiplier means for producing;

(i) corrected high energy pixel data by multiplying the high energy pixel data by the high energy gain correction signal; and

(ii) corrected low energy pixel data by multiplying the low energy pixel data by the low energy gain correction signal; and

(g) display means for displaying an image representative of at least one of said corrected low energy pixel data and said corrected high energy pixel data.

15. The imaging system of claim 14 wherein said first, second and third, low and high energy coefficients are constants of the radiation detector.

16. The imaging system of claim 15 wherein said constants of the radiation detector are determined through a basis calibration.

17. The system of claim 14 wherein said means of producing the radiation spectra comprises an x-ray tube operating at multiple energy levels.

18. The system of claim 14 wherein said detector is spaced from the radiation producing means to accommodate an object therebetween.

19. A method of calibrating a dual energy radiation imaging system utilizing a radiation source for directing radiation along a beam path, a radiation detector positioned in the beam path and spaced from the source to accommodate the placement of an object therebetween, said detector responsive to radiation incident thereon and capable of producing high energy and low energy pixel signals representative of said incident radiation, said method comprising the steps of:

(a) placing a multiplicity of thicknesses of basis materials in the space between the source and detector;

(b) directing radiation through said basis materials;

(c) monitoring the high energy and low energy pixel signals produced by said detector in response to radiation passing through said basis materials;

(d) transforming said high energy and low energy pixel signals monitored in step (c) into high energy and low energy transfer functions;

(e) replacing said basis materials with an object to be examined;

(f) directing radiation through the object to be examined;

(g) monitoring the high energy and low energy pixel signals produced by said detector in response to radiation passing through said object to be examined;

(h) combining in a predetermined way said high energy low energy pixel signals monitored in step (g) with the high energy transfer functions to produce a high energy correction factor;

(i) combining in a predetermined way said high energy and low energy pixel signals monitored in step (g) with the low energy transfer function to produce a low energy correction factor;

(j) combining the high energy pixel value monitored in step (g) with the high energy correction factor to produce corrected high energy pixel data; and

(k) combining the low energy pixel value monitored in step (g) with the low energy correction factor to produce corrected low energy pixel data.

20. The method of claim 19 wherein the step of transforming comprises performing a regression on said high energy and low energy pixel signals monitored in step (c).

21. The method of claim 19 wherein the steps of combining in a predetermined way comprise a Taylor series expansion.