Tone-scale generation method and apparatus for digital x-ray images

• US 5,046,118 A
• Filed: 02/06/1990
• Issued: 09/03/1991
• Est. Priority Date: 02/06/1990
• Status: Expired due to Term

• Current Assignee: CareStream Health Incorporated
• Original Assignee: Eastman Kodak Company
• Inventors: Ajewole, Isaac A., Schaetzing, Ralph
• Primary Examiner(s): Boudreau, Leo H.
• Assistant Examiner(s): Fox, David
• Application Number:
  US07/475,522
• Time in Patent Office: 574 Days
• Field of Search: 358/445, 358/455, 358/461, 358/464, 358/466, 382/50, 382/51, 382/54, 364/413.22
• US Class Current: 382/169
• CPC Class Codes: G01T 1/2014 Reading out of stimulable sheets, e.g. latent image
• CPC Class Codes: G06T 2207/10116 X-ray image
• CPC Class Codes: G06T 5/40 using histogram techniques
• CPC Class Codes: G06T 5/94 based on local image properties, e.g. for local contrast enhancement

Abstract

In any digital imaging system, the digital input signals must be converted to a visible image on some output material or medium. Because the range of this output medium is usually not the same as the input image range, care must be taken in converting the digital input signals into output signals. This conversion is done via the tone-scale transformation function. The type of tone-scale transformation function used largely determines the quality of the output image. A well designed tone-scale transformation function must ensure good contrast, no clipping and the display of the minimum and maximum densities that are present in the input. This invention deals with a reliable and robust technique of generating the tone-scale transformation function for digital projection X-ray input images. The method is based on the histogram entropy of the input image.

Claims

1. A method of processing a digital diagnostic X-ray image comprising the steps of:

a. in a digital computer, generating a tone-scale transformation function by,

i forming a histogram of the digital image;

ii dividing the histogram into a region of interest and a background region by;

(a) assuming the histogram is comprised of two modes; and

(b) determining the grey level that separates the two modes by maximizing the a posteriori entropy of the image by;

(i) modeling the background grey level distribution as a uniform distribution having substantially the same entropy as the background distribution, and computing the width W of the model uniform distribution;

(ii) determining the location of a window of width W with the greatest number of pixels in the background region; and

(iii) if the window is located at the upper end of the background distribution choosing the beginning of the window as the start of the background, else compute a new window based on the average of the grey levels in the window, until the window is located at the upper end of the background distribution, and choosing the beginning of the new window as the start of the background;

iii constructing a tone-scale transformation function having a linear portion substantially over the region of interest, smoothly joined with a first portion extending from the start of the histogram to the linear portion, and a second portion extending from the linear portion to the beginning of the background region; and

b. transforming the digital image with the tone-scale transformation function.

2. The method claimed in claim 1 further comprising the steps of:

prior to transforming the digital image with the tone-scale transformation function, processing the image with an operator.

5. The method claimed in claim 1 or 3, wherein the steps of constructing a tone-scale transformation function further comprises the steps of generating the linear portion of the function by determining a slope that will cause the average density and variance of the output image in the region of interest to be equal to preselected target values.

6. The method claimed in claim 5 wherein the preselected target values are in the range 0.7-0.85 and 0.3-0.4, respectively, for the average density and variance.

7. The method claimed in claim 6, wherein the preselected target values are 0.78 and 0.35 respectively, for the average density and variance.

8. The method claimed in claim 1 or 3, wherein the first and/or second portion of the tone-scale reproduction function defines a parabola.

9. The method claimed in claim 1 or 3, wherein the first and/or second portions of the tone-scale reproduction function defines a straight line smoothly connected to the linear portion by a parabola.

3. A method of processing a digital diagnostic X-ray image comprising the steps of:

a. in a digital computer, generating a tone-scale transformation function by,

i forming a histogram of the digital image;

ii dividing the histogram into a region of interest and a background region by;

(a) modeling the region of interest of the histogram as a uniform distribution having substantially the same entropy as the region of interest and computing the width w of the model uniform distribution;

(b) determining the location of a window of width w with the greatest number of pixels in the region of interest; and

(c) if the window is located at the low end of the region of interest, choosing the window as the region of interest, else compute a new window based on the average of the grey levels in the window, until the window is located at the low end of the region of interest, and choosing the new window as the region of interest;

iii constructing a tone-scale transformation function having a linear portion substantially over the region of interest, smoothly joined with a first portion extending from the start of the histogram to the linear portion, and a second portion extending from the linear portion to the beginning of the background region; and

b. transforming the digital image with the tone-scale transformation function.

4. Apparatus for processing digital diagnostic X-ray images, comprising:

a. means for generating a tone-scale transformation function having;

i means for generating a histogram of the digital image;

ii means for dividing the histogram into a region of interest, and a background region by determining the grey level that separates two modes of the histogram by maximizing the a posteriori entropy of the image, said dividing means including,

(a) means for modeling the region of interest of the histogram as a uniform distribution having substantially the same entropy as the region of interest and computing the width w of the model uniform distribution;

(b) means for determining the location of a window of width w with the greatest number of pixels in the region of interest; and

(c) means for choosing the window as the region of interest if the window is located at the low end of the region of interest, else computing a new window based on the average of the grey levels in the window until the window is located at the low end of the region of interest, and choosing the new window as the region of interest;

iii means for generating a linear portion of the tone scale transformation function substantially over the histogram region of interest, and for generating a first portion smoothly joining the linear portion, and extending from the beginning of the histogram, and a second portion extending from the linear portion to the background region of the histogram;

b. means for generating a look-up table representing the tone-scale transformation function; and

c. means for processing the digital image by addressing the look-up table.

10. Apparatus for processing digital diagnostic X-ray images, comprising:

a. means for generating a tone-scale transformation function having;

i means for generating a histogram of the digital image;

ii means for dividing the histogram into a region of interest, and a background region by determining the grey level that separates two modes of the histogram by maximizing the a posteriori entropy of the image said dividing means including;

(a) means for modeling the background grey level distribution as a uniform distribution having substantially the same entropy as the background distribution, and computing the width W of the model uniform distribution;

(b) means for determining the location of a window of width W with the greatest number of pixels in the background region; and

(c) means for choosing the beginning of the window as the start of the background, if the window is located at the upper end of the background distribution else computing a new window based on the average of the grey levels in the window, until the window is located at the upper end of the background distribution, and choosing the beginning of the new window as the start of the background;

iii means for generating a linear portion of the tone scale transformation function substantially over the histogram region of interest, and for generating a first portion smoothly joining the linear portion, and extending from the beginning of the histogram, and a second portion extending from the linear portion to the background region of the histogram;

b. means for generating a look-up table representing the tone-scale transformation function; and

c. means for processing the digital image by addressing the look-up table.

11. The apparatus claimed in claim 10, further comprising means for processing the image with an operator prior to transforming the digital image with the tone-scale transformation function.

12. The apparatus claimed in claim 10 or 4, wherein the means for constructing a tone-scale transformation function further comprises means for generating the linear portion of the function by determining a slope that will cause the average density and variance of the output image in the region of interest to be equal to preselected target values.

13. The apparatus claimed in claim 12 wherein the preselected target values are in the range 0.7-0.85 and 0.3-0.4, respectively, for the average density and variance.

14. The apparatus claimed in claim 12, wherein the preselected target values are 0.78 and 0.35 respectively, for the average density and vaiance.

15. The apparatus claimed in claim 10 or 4, wherein the first and/or second portion of the tone-scale reproduction function defines a parabola.

16. The apparatus claimed in claim 10 or 4, wherein the first and/or second portions of the tone-scale reproduction function defines straight line connected to the linear portion by a parabola.