Digital radiography with image brightness and contrast normalization

• US 4,868,651 A
• Filed: 05/17/1988
• Issued: 09/19/1989
• Est. Priority Date: 05/17/1988
• Status: Expired due to Term

• Current Assignee: Varian Medical Systems Incorporated (Siemens AG)
• Original Assignee: S&S Inficon Incorporated
• Inventors: Elderbroom, Fred, Chou, Tzeyang J., Ma, Marshall, Flatau, Ralph
• Primary Examiner(s): Church, Craig E.
• Assistant Examiner(s): Freeman, John C.
• Application Number:
  US07/195,023
• Time in Patent Office: 490 Days
• Field of Search: 378/99, 358/111, 358/282, 358/80, 358/257
• US Class Current: 378/98.7
• CPC Class Codes: G06T 2207/10016 Video; Image sequence
• CPC Class Codes: G06T 2207/10121 Fluoroscopy
• CPC Class Codes: G06T 2207/30004 Biomedical image processing
• CPC Class Codes: G06T 5/40 using histogram techniques
• CPC Class Codes: G06T 5/90 Dynamic range modification of images or parts thereof

Abstract

A process for enhancing a still video image can correct digital fluoroscopy images which are under- or overexposed and have their image brightness values concentrated to one end of the gray scale. A pixel histogram is formed, but is clipped at a low value. Then, a cumulative histogram is formed from the clipped histogram. The cumulative histogram, which has a generally linear slope, is normalized and is used for mapping the brightness of luminance values of the original image. The new values form an optimal video image with the gray scale values spread over the entire range. The image shows details not only in the middle gray areas, but in the light and dark areas as well.

Claims

1. A process of enhancing a still video image to achieve ideal contrast and brightness, comprising dividing the image into a tableau of pixels, each with a respective luminance value along a gray scale from a predetermined minimum to a predetermined maximum;

digitizing the pixel luminance values to respective digital pixel values in a range from a predetermined minimum value to a predetermined maximum digital value;

forming a histogram distribution of said digital pixel values by storing, for each possible digital pixel value from said predetermined minimum digital value to said predetermined maximum digital value, a number of pixels having said digital pixel value up to a predetermined finite low threshold number, but storing said threshold number if the number of pixels having said digital pixel value exceeds the threshold number;

forming a cumulative histogram from said histogram distribution by storing for each said possible digital pixel value the sum of the stored numbers of pixels having values of said histogram distribution less than or equal to said digital pixel value;

normalizing said cumulative histogram by adjusting said stored sums by an appropriate factor such that the adjusted stored sums range between zero and a number equal to said maximum digital value;

assigning new digital pixel values for said pixels, wherein said new digital pixel values are that adjusted stored sums for the corresponding digital pixel values of the normalized cumulative histogram; and

reproducing a visible image from said new digital pixel luminance values, the reproduced image having an optimal luminance distribution over the entire gray scale, with high contrast and superior detail in bright and dark regions of the reproduced image.

2. The process of claim 1 wherein said gray scale consists of digital pixel values form zero to 255.

3. The process of claim 1 wherein said threshold number is selected such that the resulting correspondence between said new digital pixel values and said digital pixel values of said normalized cumulative histogram is approximately linear.

4. The process of claim 1 wherein said still video image is formed by energizing a low-energy x-ray device which radiates low-energy x-rays through a human or veterinary patient onto a fluoroscopic image forming device which produces an image;

and forming a video image by directing the image from said fluoroscopic image forming device to a high-resolution video camera.

5. Digital fluoroscopy apparatus that comprises:

an x-ray tube;

an imager responsive to radiation from said x-ray tube to produce a visible image;

a video camera directed at the visible image which produces a still video image comprising a tableau of pixels each having a luminance value along a gray scale between zero and a maximum value;

a patient-positioning stage for positioning a human or veterinary patient between the x-ray tube and the imager;

an imaging computer device which converts the pixel luminance values to respective digital pixel values between zero and a maximum digital pixel value, and corrects and digital pixel values to correspond to a corrected image having corrected digital pixel values distributed over said gray scale; and

means for reproducing a visible image from said corrected digital values having high contrast and superior detail in bright and dark areas;

wherein said imaging computer device comprises means for forming a histogram distribution of said digital pixel values, including means for storing, for each possible digital pixel value from zero to said maximum digital value, a number of pixels having said digital pixel value up to a predetermined low finite number of pixels, but storing said low finite number if the number of pixels having said digital pixel value exceeds the low finite number;

means forming a cumulative histogram from said histogram distribution including means for storing, for each said possible digital pixel value of said histogram distribution the sum of the stored numbers of pixels having values less than or equal to said digital pixel value;

means for normalizing said cumulative histogram by adjusting said stored sums by an appropriate factor such that the adjusted stored sums range between zero and a number substantially equal to said maximum digital value; and

means for assigning new digital pixel values for said pixels, wherein said new digital pixel values are the adjusted stored sums for the corresponding digital pixel values of the normalized cumulative histogram.