Image analysis – Applications – Biomedical applications
Reexamination Certificate
1994-11-08
2002-03-12
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
Biomedical applications
C382S274000
Reexamination Certificate
active
06356652
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of digital radiography. The invention more specifically relates to an image processing method applied for the purpose of enhancing the quality of diagnostic evaluation of a radiographic image.
BACKGROUND OF THE INVENTION
In the field of digital radiography, a wide variety of image acquisition techniques have been developed such as computerised tomography, nuclear magnetic resonance, ultrasound, detection of a radiation image by means of a CCD sensor or a video camera, radiographic film scanning etc. By means of these techniques, a digital representation of a radiographic image is obtained.
In still another technique, a radiation image, for example an image of x-rays transmitted by an object, is stored in a screen comprising a photostimulable phosphor such as one of the phosphors described in European patent publication 503 702 published on Sep. 16, 1992 and U.S. Ser. No. 07/842,603. The technique for reading out the stored radiation image consists of scanning the screen with stimulating radiation, such as laser light of the appropriate wavelength, detecting the light emitted upon stimulation and converting the emitted light into an electric representation, for example, by means of a photomultiplier and finally digitizing the signal.
The digital images obtained by one of the acquisition techniques described hereinbefore can be subjected to a wide variety of image processing techniques.
If the unprocessed original image representation. is stored, it can be subjected off-line to different types of image processing techniques as well as to processing using different values of the processing parameters as frequently as required.
The processed or unprocessed images can further be applied to a display device and/or can be reproduced by means of an image recorder such as a laser recorder or the like.
For the purpose of display and/or hard copy recording signal, values are converted into density values envisioned in the display or hard copy according to a signal-to-density mapping curve that has a predetermined shape in between a minimum and a maximum displayable or reproducible density value.
In some applications radiologists protect their subjects against unnecessary exposure to X-rays by use of X-ray opaque ‘collimation’ material. The material is placed in the path of the X-ray beam so as to shield those areas of the patient which are not regarded as diagnostically important. Besides reducing patient dosage this technique has the additional advantage of limiting the amount of scattered radiation in the image field of interest. The regions of the resulting image which comprise the shadow cast by the X-ray opaque material (shadow regions) are exposed only by scattered radiation.
The presence of the collimation shadow region however can cause a problem in the display of the radiographic image on film or on a display device. Therein, the shadow region is relatively bright, and if displayed unmodified, may impair diagnosis of subtle lesions due to dazzle, especially if the unexposed region is relatively large.
It has been proposed in European patent application 523 771 to establish the region of interest and then to convert signal values of the radiographic image into density values to be displayed either as soft copy on a display device or as hard copy on film in such a way that pixels outside the diagnostically relevant zone are visualized so that the light transmitted by said image part when the hard copy is viewed on a console screen or when it is displayed, is effectively masked off.
In accordance with one embodiment described in this patent, the electrical signals of the pixels comprised within the diagnostically not relevant image zone within the radiographic image are converted to a uniform density values comprised between 0.5 and 2.5.
By applying this method, the dazzling effect produced by the light transmitted by the irrelevant region is decreased. However, as a consequence of the application of this method also the image information that was present in the diagnostically irrelevant area is lost.
OBJECTS OF THE PRESENT INVENTION
It is an object of the present invention to provide a method of processing the radiographic image in a digital radiographic system in such a way that the dazzling effect described above does not occur and that the quality of diagnostic evaluation of a radiographic image is enhanced.
It is a further object of the invention to provide such a method wherein the information in a collimation region is not entirely lost.
It is still a further object to provide such a method for application in a system wherein a radiographic image is stored in a photostimulable phosphor screen and wherein said screen is scanned with stimulating irradiation, the light emitted upon irradiation is detected and converted into a digital signal representation.
Still further objects will become apparent from the description given hereafter.
STATEMENT OF THE INVENTION
The objects of the present invention are achieved by a method of processing an image in a radiographic imaging system wherein an electric signal representation of said image is mapped to density values for visualization as a hard or a soft copy image characterized in that the density of low density area in a diagnostically irrelevant zone in the image is enhanced and image structure in said zone is kept visible, by converting in said diagnostically irrelevant zone of the image pixels located at position (x,y) according to a conversion function g(x,y)=a.f(x,y)+(1−a).f
max
wherein f(x,y) is the signal value before conversion of a pixel located at position (x,y), a is a value between zero and one, and f
max
is equal to the maximum of values f(x,y), prior to being subjected to mapping into density values.
By applying the method according to this invention the pixel values of the radiographic image are converted in such a way that
(i) the pixels outside the diagnostically relevant zone are visualized or reproduced so that the light transmitted by said irrelevant image zone when the hard copy is viewed on a console screen or when the image is displayed, is attenuated, and
(ii) whereas the information within the diagnostically irrelevant zone is not lost.
This was obtained by processing pixels of the radiographic image differently when they were situated in the region of interest than when they were situated in the diagnostically irrelevant zone.
Pixels belonging to the irrelevant image zone where subjected to a conversion, for example being implemented in the form of a look up table, applied before signal-to-density mapping.
Pixels within the relevant image zone are converted by means of an identity transformation (i.e. they are not additionally converted) whereas pixels outside the diagnostically relevant zone are converted in accordance with a conversion function that can be represented by a straight line located above the identity transformation and that has a slope in between 0 and 1 so that pixels in the diagnostically irrelevant zone are converted to a higher average density than that of the remainder of the image and so that image information in that part of the image is not lost.
This conversion can generally be represented as: g(x,y)=af(x,y)+(1−a)f
max
, wherein g(x,y) is a pixel value after transformation, f(x,y) is the pixel value before conversion and ‘a’ is the slope of the conversion function applied to pixels outside the region of interest, f
max
is the maximum of values f(x,y). The slope ‘a’ has a value between zero and one and determines the minimum density to which pixel values in the diagnostically irrelevant zone can be converted.
In a preferred embodiment, the image is decomposed into a multi-resolution representation. That multiresolution representation is then modified for the purpose of enhancing the image quality, and the modified multi-resolution representation is finally subjected to a reconstruction process.
The additional signal conversion g(x,y) of pixels within the diagnostically irrelevant zone
Agfa-Gevaert
Hoffman, Warnick & D'Alessandro LLC
Johns Andrew W.
Merecki John A.
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