Method for calculating gain correction factors in a digital imaging system

• US 6,350,985 B1
• Filed: 04/26/1999
• Issued: 02/26/2002
• Est. Priority Date: 04/26/1999
• Status: Expired due to Fees

• Current Assignee: Direct Radiography Corp. (Hologic Incorporated)
• Original Assignee: Direct Radiography Corp. (Hologic Incorporated)
• Inventors: Rodricks, Brian, Hoffberg, Michael, Williams, Cornell
• Primary Examiner(s): Ham, Seungsook
• Assistant Examiner(s): LEE, SHUN K
• Application Number:
  US09/299,316
• Time in Patent Office: 1,037 Days
• Field of Search: 250/252.1, 250/363.09, 250/339.09, 250/341.5, 250/370.08, 250/370.09
• US Class Current: 250/252.1
• CPC Class Codes: G01T 1/16 Measuring radiation intensity G01T1/29 takes precedence ; self-powered detectors G01T3/006; using an ionisation chamber filled with a liquid or solid, e.g. frozen liquid, dielectric G01T3/008
• CPC Class Codes: G06T 5/94 based on local image properties, e.g. for local contrast enhancement
• CPC Class Codes: H04N 25/67 applied to fixed-pattern noise, e.g. non-uniformity of response
• CPC Class Codes: H04N 5/325 Image enhancement, e.g. by subtraction techniques using polyenergetic X-rays

Abstract

A method for developing a set of gain correction coefficients corrected for structural noise to be used in correcting the digital values representing an image captured by a pixelated detector having a plurality of individual sensors. The gain correction coefficients are developed by first obtaining a first set gain correction coefficients using a flat exposure and adjusting the individual sensor gain output so that all sensors produce the same output value, and then applying a smoothing filter to the first set of gain correction coefficients to obtain a new set of corrected gain correction coefficients.

Claims

1. A method for developing a set of corrected digital gain correction coefficients for use in correcting digital values representing an image captured by a detector comprising a plurality of individual sensors, each sensor representing a pixel, the method comprising:

A) exposing the detector to radiation having a substantially uniform intensity distribution over all sensors to obtain an initial set of pixel values;

B) developing a first set of gain correction coefficients; and

C) applying a smoothing filter to all coefficients in said first set of gain correction coefficients to obtain a-set of corrected gain correction coefficients.

2. The method according to claim 1 further comprising the step:

D) storing said set of corrected gain correction coefficients in a Look Up Table.

3. The method according to claim 1 wherein the step of applying a smoothing filter comprises:

identifying a filter window comprising a plurality of sensors in the vicinity of a target pixel and calculating a corrected gain coefficient using only the gain coefficients within the filter window.

4. The method according to claim 1 wherein the step of applying a smoothing filter comprises:

identifying a filter window comprising a plurality of sensors in the vicinity of a target sensor and including the target sensor, each of said target sensors having one of said first set of gain correction coefficients; and

averaging the coefficient values of said first set of gain correction coefficients for said plurality of sensors of said filter window to obtain an average gain correction coefficient value and replacing said one of said first set of gain correction coefficients for said target sensor with said average gain correction coefficient value.

5. The method according to claim 1 wherein the step of applying a smoothing filter comprises:

identifiing a filter window comprising a plurality of sensors in the vicinity of a target sensor and including the target sensor, each of said target sensors having one of said first set of gain correction coefficients;

comparing the coefficient values of said first set of gain correction coefficients for said filter window, determining a highest and a lowest gain correction coefficient value and discarding said highest and said lowest of said values; and

averaging the coefficient values of said first set of gain correction coefficients for said plurality of sensors of said filter window other than the discarded values to obtain an average gain correction coefficient value and replacing said one of said first set of gain correction coefficients for said target sensor with said average gain correction coefficient value.

6. The method according to claim 1 wherein the step of applying a smoothing filter comprises:

dividing the first set of gain correction coefficients into subsets of gain correction coefficients;

for each of the subsets, identifying a filter window comprising a plurality of sensors in the vicinity of a target pixel, said filter window confined within each of said subsets, and calculating a corrected gain coefficient using only the gain coefficients within the filter window.

7. A method for correcting an image for structural noise in the output of a plurality of radiation detection sensors forming a detector, each of said sensors representing a pixel and comprising a radiation detection conversion element, the detector also comprising signal processing electronic circuitry for producing a digital output representing the radiation exposure of each of said sensors, the method comprising in the following order:

A) exposing the plurality of sensors to a substantially uniform intensity radiation;

B) obtaining a digital output pixel value from each of said plurality of sensors;

C) calculating a median value of the of all of said pixel values;

D) calculating and storing for each pixel a first gain coefficient equal to the ratio of the pixel value to the median value to form a first set of gain coefficients;

E) applying a smoothing filter to all coefficients in the first set of gain coefficients to obtain a corrected set of gain coefficients;

F) storing said corrected set of gain coefficients.

8. The method according to claim 7 wherein in step E, applying a smoothing filter comprises:

identifying a filter window comprising a plurality of sensors in the vicinity of a target sensor and including the target sensor, each of said target sensors having one of said first set of gain correction coefficients;

comparing the coefficient values of said first set of gain correction coefficients for said filter window, determining a highest and a lowest gain correction coefficient value and discarding said highest and said lowest of said values; and

averaging the coefficient values of said first set of gain correction coefficients for said plurality of sensors of said filter window other than the discarded values to obtain an average gain correction coefficient value and replacing said one of said first set of gain correction coefficients for said target sensor with said average gain correction coefficient value.

9. The method according to claim 7 wherein the step of storing the gain correction coefficients comprises storing the corrected gain correction coefficients in a look up table.

10. The method according to claim 9 further comprising, following step F, exposing the detector to imagewise modulate radiation and applying the stored corrected gain correction coefficients to correct an image data output from the detector resulting from the detector exposure to said imagewise modulated radiation.

11. An article of manufacture comprising:

a computer useable medium having computer readable program code means embodied therein causing the correction of gain correction coefficients used in radiation detector comprising a plurality of radiation sensors each sensor representing a pixel, to correct the detector output for non uniformity in the output of the sensors when exposed to substantially uniform radiation the computer readable means in said article of manufacture comprising;

A) obtaining a set of pixel values resulting from exposing the detector to radiation having a substantially uniform intensity distribution over all sensors;

B) calculating a first set of gain correction coefficients from said set of pixel values;

C) applying a smoothing filter to all coefficients in said first set of gain correction coefficients to obtain a set of corrected gain correction coefficients; and

D) storing said set of corrected gain correction coefficients in a memory.

12. A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform the method steps for correcting a plurality of gain correction coefficients used to correct non uniform sensor response in a radiation detector comprising a plurality of radiation sensors, the method comprising:

A) obtaining a set of pixel values resulting from exposing the detector to radiation having a substantially uniform intensity distribution over all sensors;

B) calculating a first set of gain correction coefficients from said set of pixel values;

C) applying a smoothing filter to all coefficients in said first set of gain correction coefficients to obtain a set of corrected gain correction coefficients; and

D) storing said set of corrected gain correction coefficients in a memory.