Image analysis – Applications – Biomedical applications
Reexamination Certificate
1998-12-30
2001-03-13
Johns, Andrew W. (Department: 2721)
Image analysis
Applications
Biomedical applications
Reexamination Certificate
active
06201890
ABSTRACT:
FIELD OF THE INVENTION
This disclosure concerns an invention relating generally to radiographic imaging, and more specifically to conversion of digital radiographic images to images printed on film-like material which are simulative of analog radiographic images.
BACKGROUND OF THE INVENTION
The classic radiographic or “X-ray” image is obtained by situating the object to be imaged between an X-ray emitter and an X-ray detector made of photographic film. Emitted X-rays pass through the object to expose the film, with the degree of film exposure at the various points on the film largely being determined by the attenuation of the object along the path of the X-rays.
It is proposed to utilize solid-state digital X-ray detectors, e.g., an array of photodiodes, in place of film detectors. After the X-ray exposure is terminated, the charges generated on the various points of the detector are read and processed to generate a digital image of the object in electronic form, rather than an analog image on photographic film. Digital imaging is advantageous because the image can later be electronically transmitted to other locations, subjected to diagnostic algorithms to determine properties of the object, and so on.
However, digital images present problems when printed for analysis by radiologists. Because the characteristics of the digital detectors are significantly different from those of film, the images look quite different from analog film images, even when printed on transparent film. This is due to the differing exposure response curves of digital and film detectors. As an example, the digital image data generated by a detector may be linearly proportional to the received radiation (or nearly so), whereas film has a non-linear response to radiation. As a result, the contrast in digital images is not as great as that with radiographic film. To avoid error, radiologists analyzing digital images must keep these differences between analog and digital X-ray images prominently in mind when making such analyses. Therefore, there has been a need for a means of “translating” digital images into analog-simulative digital images which mimic the results of standard prior filmed images, and which may be printed on transparent film so that they resemble filmed radiographic images. This would allow the use of light boxes and other tools commonly in use for analysis of analog filmed images.
SUMMARY OF THE INVENTION
The invention, which is defined by the claims set out at the end of this disclosure, is directed to a method for converting a digital image to an analog-simulative film-like digital image. Input pixel values from the original digital image are obtained, wherein these input values (designated X) have a dynamic range extending from X
min
to X
max
. The dynamic range is divided into an integer number of intervals N which is equal to at least 2. In each interval, the input value X for each pixel is converted into an analog-simulative film-like output value Y=&Sgr;(A
i
X
pi
+B
i
), where &Sgr; signifies a summation wherein i ranges from 1 to M, A
i
, pi, and B
i
are real numbers, and each interval generally has different values of A
i
, pi, and B
i
. This can be reexpressed as
Y
=
∑
i
=
1
M
A
i
X
p
i
+
B
i
=
A
1
X
p1
+
B
1
+
A
2
X
p2
+
B
2
+
…
+
A
i
X
pi
+
B
i
+
…
+
A
M
X
pM
+
B
M
=
A
1
X
p1
+
A
2
X
p2
+
…
+
A
i
X
pi
+
…
+
A
M
X
pM
+
B
wherein this conversion is applied to each of the N intervals, and wherein each interval generally has different values of A
i
, pi, and B
i
.
In the most preferred embodiment of the invention, it has been found sufficient to have M=1. In this case, the foregoing expression simplifies to
Y=AX
p
+B
wherein this conversion is applied to each of the N intervals (again, with each interval usually having different values of A, p, and B). It has also been found suitable to set N=3, thereby defining three intervals representing low, medium, and high radiation/light response intervals wherein the low radiation/light interval is defined at input values X<X
1
, the high light/radiation interval is defined at X>X
2
(with X
2
>X
1
), and the medium light/radiation interval is defined therebetween. The values of X
1
and X
2
are chosen such that the low and high light/radiation intervals cover some desired portion of the low and high ends of the input dynamic range, e.g., the lower and upper 30% of the input dynamic range. In the high, medium, and low light intervals, the parameters A, p, and B preferably have the following behavior.
First, p will generally be greater than 1 for intervals on the low end of the dynamic range and less than 1 for intervals on the high end of the dynamic range, with p decreasing in each successive interval after the first. Further, where one or more medium radiation/light response intervals are defined, p will generally be approximately 1 in these intervals so as to provide an output response which is approximately linear with respect to the input. However, these medium radiation/light response intervals will generally have A greater than 1 so as to amplify the input, thereby providing greater contrast in the medium interval.
Second, A will generally increase in each successive interval after the first, and will generally be less than 1 in the first interval to better suppress quantum noise at the low end of the dynamic range.
Finally, B will be chosen so as to provide continuity between intervals, and to also provide a desired level of optical density (i.e., lightness/darkness) at chosen points along the output dynamic range.
After the output pixel values Y are determined for each pixel in the digital input image, these are compiled to generate an output analog-simulative film-like digital image. This may be displayed on a screen, printed on translucent film-like media for use in the same manner as an analog (filmed) image, or otherwise used in a desired manner.
While the preferred embodiment of the invention utilizes M=1 and N=3, it is noted that higher values of M and N can be used to more accurately simulate the nonlinear response of film. However, since the conversion is preferably automated in software or hardware form, higher M and N will generally lead to increased processing requirements and longer processing times. As a result, in view of the present state of the art, M=1 and N=3 are regarded as providing a suitable exchange between accuracy and processing time.
Further advantages, features, and objects of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings.
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patent: 5220357 (1993-06-01), Kubo et al.
patent: 5371562 (1994-12-01), Hahm et al.
patent: 5483259 (1996-01-01), Sachs
patent: 5522067 (1996-05-01), Swire
patent: 5691961 (1997-11-01), Paranjpe
patent: 5786902 (1998-07-01), Morton
patent: 5927191 (1999-07-01), Wheatley, Jr. et al.
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patent: 5987270 (1999-11-01), Hulan et al.
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Hill James Martin
Kump Kenneth Scott
Shi Shuanghe
Chaskin Jay L.
Fieschko, Esq. Craig A.
General Electric Company
Johns Andrew W.
Nakhjavan Shervin
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