Image analysis – Applications – Biomedical applications
Reexamination Certificate
1994-11-08
2001-06-19
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
Biomedical applications
C382S308000
Reexamination Certificate
active
06249596
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The present invention is in the field of digital radiography. The invention more specifically relates to a method and an apparatus for displaying a radiographic image and for locating saturated pixels in the displayed image.
2. Background of the Invention
In the field of digital radiography, a wide variety of image acquisition techniques have been developed such as computerized 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 16.09.92 and U.S. Ser. 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.
Prior to display or hard copy recording the image is commonly subjected to a window/levelling operation. A subrange within the entire range of acquired signal values is defined. Within this range signal values are mapped to density values between said minimum and maximum density values and outside this range signal values are mapped onto the minimum density value if they are smaller than the minimum value of said subrange and onto the maximum density value if they are larger than the maximum value in said subrange.
The determination of this subrange, i.e. its position and its width relative to the entire range of acquired signal values, can be predefined for example as a specific characteristic of a particular examination type. Alternatively the width and position of the range, also called the window and the level, can be determined under visual control by applying the acquired image signal to a monitor for example part of a workstation, and by changing brightness and contrast of said monitor until the display provides an image that satisfies the operator.
Contrast and brightness controls provide a shift of the signal-to-density conversion curve relative to the acquired signal range. Contrast and brightness will be adjusted so that the display provides a visual image on which image detail in the diagnostically relevant area is discernable.
It is advantageous that the shape of the signal-to-density mapping curve in between the minimum and maximum density values referred to hereinbefore is similar to the shape of the sensitometric curve of a radiographic film because the radiologist is acquainted with the characteristics, more specifically with the sensitometry of the conventional radiographic film.
For display purposes, it is furthermore advantageous to incorporate into the signal-to-density transformation a compensation for non-linear behavior of the display device.
Because of the digitization process at the input of a display device commonly having a dynamic range that is smaller than the range of acquired pixel values, more than one original pixel value is mapped onto a single input value for the display device. This input value of the display device is then later converted into a single density value so that different original pixel values are finally mapped into a single density value.
Such pixels originating from different original pixel values and being mapped onto a single density value in the display are referred to as “saturated pixels”.
In a saturated pixel area no differences between density values originating from pixels with different pixel values can be discerned with the result that valuable information might get lost.
In addition, the saturated pixels may not be recognized as being “saturated” given the limited dynamic range of the state of the art display devices. Under these circumstances discrimination on the screen of a saturated image area relative to non-saturated image area or pixels is difficult.
When evaluating a radiographic image on a display, the radiologist may not be aware of the fact that relevant pixels have become saturated and that in fact the effective density in these pixels is different from the density seen on the displayed image.
Moreover, when making a hard copy of an image (using the above mapping curve) wherein saturated areas are present on a hard-copy reproduction medium such as a photographic film, the saturated areas become visible due to the larger dynamic range of the hard copy material relative to that of the display device. Saturated areas are rather disturbing during image interpretation.
Objects of the Present Invention
It is an object of the present invention to provide a method of determining the location of pixels that are saturated in a system wherein a radiographic image is acquired in the form of a digital signal representation and wherein the radiographic image is visualized on a display device.
It is a further object to display an image in such a way that pixels which are important for the diagnostical evaluation process of a radiographic image are not saturated.
It is a further object to provide such a method for application on a display monitor of a workstation.
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.
Further objects will become apparent from the description hereinafter.
Statement of the Invention
The objects of the present invention are achieved by a method of determining the location of pixels in a radiographic image that are saturated when digital pixel values of said image are transformed into density values in accordance with a first signal-to-density transformation comprising the steps of
-converting the digital pixel values of said radiographic image into density values in accordance with a second signal-to-density transformation that is substantially identical to said first transformation except that it comprises at least one discontinuity in a pixel value corresponding with a pixel of said image that would be saturated when being converted in accordance with said first transformation, and
-applying pixel values converted according to said second transformation to a display device.
In this context by the term “saturated pixels” is meant pixels originating from different pixel values that are converted onto a single input value for the display device. These different pixel values are then further converted into a single density value.
By evaluation of the signal-to-density mapping transformation the pixel values can be found that would result in saturated pixels when being converted in accordance with that transformation. However, the location of the saturated pixels in the image cannot be fo
Buytaert Tom
Van de Velde Stefan
AGFA-GEVAERT
Johns Andrew W.
Schmeiser Olsen & Watts
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