Method for automatic detection of region of interest for digital x-ray detectors using a filtered histogram

• US 6,118,892 A
• Filed: 11/19/1998
• Issued: 09/12/2000
• Est. Priority Date: 11/19/1998
• Status: Expired due to Term

• Current Assignee: Direct Radiography Corp. (Hologic Incorporated)
• Original Assignee: Direct Radiography Corp. (Hologic Incorporated)
• Inventors: Williams, Cornell L.
• Primary Examiner(s): MEHTA, BHAVESH M
• Application Number:
  US09/196,391
• Time in Patent Office: 663 Days
• Field of Search: 382/128, 382/132, 382/168, 382/170, 382/171, 382/274, 382/172, 378/98, 378/98.2, 378/98.5, 378/98.12, 250/581, 250/583, 250/584, 250/587
• US Class Current: 382/132
• CPC Class Codes: G06T 2207/10116 X-ray image
• CPC Class Codes: G06T 2207/20021 Dividing image into blocks, subimages or windows
• CPC Class Codes: G06T 2207/20132 Image cropping
• CPC Class Codes: G06T 2207/30061 Lung
• CPC Class Codes: G06T 7/0012 Biomedical image inspection
• CPC Class Codes: G06T 7/11 Region-based segmentation

Abstract

A method for automatically forming a filtered subset of digital values from a first set of digital values corresponding to a plurality of picture elements representing a radiographic exposure of a subject for use in displaying a radiograph of said subject, the method comprising: (1) generating a first histogram of the first set of digital values; (2) generating an integral curve representing an integral of the first histogram; (3) identifying an inflection point and corresponding digital value V_peak1 on the first histogram integral where the integral curve first changes slope in a direction decreasing from the highest digital value; (4) calculating a parameter V_test =V_peak1 -AR wherein AR represents an anatomical range of the subject; (5) spatially dividing the first set of digital values into multiple block subsets of digital values, each block subset comprising digital values from a neighborhood of pixels consisting of a predetermined number of picture elements; (6) constructing a subset histogram representing the frequency of occurrence of digital values within each block subset; (7) determining a low point of each block subset, BV_low, corresponding to a predetermined low point of the subset histogram; (8) comparing each BV_low to V_test ; and (9) forming the filtered subset of digital values from the digital values of all the block subsets having BV_low greater than V_test.

Claims

1. A method for automatically forming a filtered subset of digital values from a first set of digital values corresponding to a plurality of picture elements representing a radiographic exposure of a subject for use in displaying a radiograph of said subject, the method comprising:

(1) generating a first histogram representing a frequency of occurrence of digital values within the first set of digital values;

(2) generating an integral curve representing an integral of said first histogram;

(3) obtaining a digital value V_peak1, corresponding to an inflection point on the first histogram integral curve where the integral curve first changes slope starting from a point of highest digital value and traversing along the integral curve in the direction of decreasing digital values;

(4) calculating a parameter V_test =V_peak1 -AR wherein AR is a parameter that represents an anatomical range of said subject;

(5) spatially dividing the first set of digital values into multiple block subsets of digital values, each block subset comprising digital values from a neighborhood of pixels consisting of a predetermined number of picture elements;

(6) constructing a block subset histogram representing a frequency of occurrence of digital values within each block subset;

(7) determining on each block subset a digital value BV_low, corresponding to a predetermined low point of the block subset histogram;

(8) comparing each BV_low to V_test ; and

(9) forming said filtered subset of digital values from the digital values of all the block subsets having BV_low greater than V_test.

2. The method according to claim 1 wherein the step of determining a low point on each block subset of digital values includes obtaining a block subset histogram integral curve for each of said block subset histograms and setting BV_low in a range of between 1% to 10% of the integral curve.

3. The method according to claim 1 wherein in calculating V_test there is applied to the set of digital values an exam-specific algorithm.

4. The method according to claim 1 wherein in calculating V_test there is applied to the set of digital values a preselected one of a plurality of exam specific algorithms.

5. The method according to claim 1 wherein in calculating BV_low there is applied to the subset of digital values an exam specific algorithm.

6. The method according to claim 1 further comprising obtaining a digital value V_edge, corresponding to an edge point on said first histogram where the frequency of occurrence first drops to a predetermined percentage of a preceding peak frequency of occurrence when traversing along the histogram starting from V_low in a direction of increasing exposure, wherein AR=[100+{(V_max -V_edge)-ARC_ref)}], where ARC_ref is an experimentally derived constant.

7. The method according to claim 6 wherein ARC_ref represents a standard value for V_max -V_edge for a predetermined exam, type, said standard value derived from a representative distribution of histograms from a representative clinical database.

8. The method according to claim 1 further comprising displaying on a display medium an image representing the radiograph by:

I) storing in a memory a first normalized lookup table (LUT) representing a density transfer function for the display medium;

II) storing in said memory a second LUT containing a plurality of experimentally determined constants characteristic of the type of image to be displayed;

III) forming a filtered histogram representing a frequency of occurrence of digital values within said filtered subset of digital values and generating an integral curve representing an integral of said filtered histogram;

obtaining a digital value V_low corresponding to a low point on the filtered histogram integral curve at a predetermined low percentage of the filtered histogram integral;

obtaining a digital value V_edge corresponding to an edge point on the filtered histogram where the filtered histogram frequency of occurrence first drops to a predetermined percentage of a preceding peak frequency of occurrence when traversing along the filtered histogram starting from V_low in the direction of increasing exposure;

obtaining a digital value V_peak2, corresponding to a point on the filtered histogram integral curve where the filtered histogram integral curve first changes slope when traversing along the filtered histogram integral curve starting from the point of highest exposure in the direction of decreasing exposure; and

calculating a starting value V_min and an ending value V_max using a exam-specific equation containing said experimentally determined constants and one or more of the values V_low, V_peak2 and V_edge ;

IV) replacing all digital values in the filtered subset which are lower than V_min with V_min, and all digital values which are higher than V_max, by V_max ; and

V) normalizing the digital values in the filtered subset to a new set of values representing the difference V_max -V_min and using the normalized digital values and the first LUT to display the image.

9. A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform the method steps for automatically forming a filtered subset of digital values from a set of digital values corresponding to a plurality of picture elements representing a radiographic exposure of a subject for use in displaying a radiograph of said subject, said method steps comprising:

(1) generating a first histogram of the set of digital values;

(2) generating an integral curve representing an integral of said first histogram;

(3) identifying an inflection point on the first histogram integral where the integral curve first changes slope when traversing along the integral curve starting from a point of highest digital value in the direction of decreasing digital values and obtaining a digital value V_peak1 for the inflection point;

(4) calculating a parameter V_test =V_peak1 -AR wherein AR is a constant representing an anatomical range of said subject;

(5) spatially dividing the set of digital values into multiple block subsets of digital values, each block subset comprising digital values from a neighborhood of pixels consisting of a predetermined number of picture elements;

(6) constructing a subset histogram representing the frequency of occurrence of digital values within each block subset;

(7) determining a low point on each block subset, BV_low, corresponding to a predetermined low point of the block subset histogram;

(8) comparing each BV_low to V_test ; and

(9) forming said filtered subset of digital values from the digital values of all the block subsets having BV_low greater than V_test.

10. The program storage device according to claim 9, wherein the method further comprises displaying on a display medium an image representing the radiograph by:

I) storing in a memory a first normalized lookup table (LUT) representing a density transfer function for the display medium;

II) storing in said memory a second LUT containing a plurality of experimentally determined constants characteristic of the type of image to be displayed;

III) forming a filtered histogram representing a frequency of occurrence of digital values within said filtered subset of digital values and generating an integral curve representing an integral of said filtered histogram;

obtaining a digital value V_low corresponding to a low point on the filtered histogram integral curve at a predetermined low percentage of the filtered histogram integral;

obtaining a digital value V_edge corresponding to an edge point on the filtered histogram where the filtered histogram frequency of occurrence first drops to a predetermined percentage of a preceding peak frequency of occurrence when traversing along the filtered histogram starting from V_low in the direction of increasing exposure;

obtaining a digital value V_peak2, corresponding to a point on the filtered histogram integral curve where the filtered histogram integral curve first changes slope when traversing along the filtered histogram integral curve starting from the point of highest exposure in the direction of decreasing exposure; and

calculating a starting value V_min and an ending value V_max using a exam-specific equation containing said experimentally determined constants and one or more of the values V_low, V_peak2 and V_edge ;

IV) replacing all digital values in the filtered subset which are lower than V_min with V_min, and all digital values which are higher than V_max, by V_max ; and

V) normalizing the digital values in the filtered subset to a new set of values representing the difference V_max -V_min and using the normalized digital values and the first LUT to display the image.

11. The program storage device according to claim 9, wherein said machine is a component in a digital radiography system.

12. The program storage device according to claim 10, wherein said machine is a component in a digital radiography system.