X-ray or gamma ray systems or devices – Specific application – Absorption
Reexamination Certificate
2000-03-15
2002-04-16
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Absorption
C378S004000
Reexamination Certificate
active
06373918
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for automatically detecting the contours of structures having a high X-ray absorption in an X-ray image. The invention also relates to an X-ray apparatus for carrying out such a method.
2. Description of Related Art
In radiological practice there are a variety of structures with a high X-ray absorption whose contours must be detected so as to enable optimum processing of the X-ray image:
1. The contours of the shutters whereby the X-ray beam striking a patient in the examination zone is restricted. Exact knowledge of the orientation in space of the shutter contours in an X-ray image can be used in various ways:
a) The dynamic range of a monitor or a so-called hardcopy unit can be automatically adapted to the image section enclosed by the shutter contours in such a manner that the contrasts are optimally reproduced (so-called auto ranging).
b) Upon output of an X-ray image, the region covered by the shutters can be reproduced in black or in color. If this step is not taken, these parts of the image would be very bright and could dazzle the observer.
c) The images can be rotated and, in the case of oblique projection, corrected so that the shutter contours extend horizontally and vertically in the image reproduced. Moreover, a plurality of images can be arranged adjacent to one another in such a manner that the surface area available on a monitor or a hardcopy is optimally used.
d) The image processing methods to be applied to the X-ray image can be limited to the section defined by the shutter contours. The amount of calculation work required for image processing can thus be significantly reduced.
2. The contours of implants in an X-ray image, for example of an artificial hip, Such implants have an X-ray absorption which is greater than that of bones but less than that of the X-ray diaphragm. The presence of such an implant in an X-ray image changes the mean gray scale value (image value) and other statistical properties of a gray scale histogram of the X-ray image. Consequently, it may happen that the automatic setting, derived from the X-ray image, of a display unit for displaying the X-ray image is not optimum. Improvements in this respect would be possible if the regions of the X-ray image in which the implant is reproduced were excluded from the histogram analysis on which the contrast setting of the display apparatus is based.
U.S. Pat. No. 5,651,042 already discloses a method for the automatic detection of shutter contours in an X-ray image wherein contour point candidates (i.e. pixels which could be situated on the contour of the shutter) are determined from the spatial variation of the image values associated with the pixels of the X-ray image (or a source image derived therefrom). The contour point candidates can be derived from a gradient image (or a high-pass image), derived from the source image, as those points for which the gradient exhibits a maximum. Using so-called linear regression, straight line candidates are calculated from such contour point candidates, which lines are arranged so that the contour point candidates lie thereon or in the immediate vicinity thereof. The line candidates may (but need not) be coincident with the shutter contours. Generally speaking, more line candidates appear as there are contours of the shutter. From these line candidates those lines are selected (4 at the most) which correspond best to one of 14 archetypes of shutter contours stored in a library. It is a prerequisite for this method that the shutter contours extend in parallel or at right angles to one another, and that the area enclosed by the shutter contours is centrally situated.
These conditions are not always satisfied in radiological practice. Even when the X-rays are confined by means of pairs of shutters extending parallel and perpendicularly to one another, the shutter contours in the X-ray image will no longer extend parallel or perpendicular to one another in the case of oblique projection. It may also occur that a part of the examination zone in which the X-ray beam is incident is covered by a lead apron or the like, the boundary of which extends parallel or perpendicular to the diaphragm edges in the X-ray image only in rare cases. The X-ray image is more likely to contain a contour which no longer coincides with a rectangle.
Moreover, digital X-ray image converters which convert the X-ray image into electric signals that can be digitized may have comparatively large dimensions (for example, 43 cm×43 cm). If, for example, only a hand or a finger is imaged by means of such an X-ray image converter, the X-ray image defined by the shutters usually will not be situated at the center but near the edge.
The same limitation holds for the method which is known from U.S. Pat. No. 5,081,680 and wherein points exhibiting a maximum gradient and situated on various straight lines through the center are determined. If more than two of such points are present on a straight line, as it will always be the case in a normal X-ray image with skeleton structures, the additional points must be excluded in a separate ranking process. The remaining points are considered to be points situated on the shutter contour, and a number of straight lines, forming a polygon, is derived from such shutter contour points by means of a Hough transformation; these lines are to represent the shutter contour.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for the automatic detection of the contours of structures having a high X-ray absorption which can be reliably performed and is not restricted to given contour shapes. This object is achieved according to the invention by taking the following steps: determining a number of closed paths which serve as contour candidates in an X-ray image or an image derived therefrom, selecting, in dependence on the contrasts along the closed paths, the contour as a closed path from the number of closed paths.
Thus, according to the invention a number of closed paths is derived in the image, one of said closed paths representing the contour searched. According to the invention no individual segments of the closed paths are analyzed and rejected or confirmed, and paths having a given shape are not selected either. The underlying consideration is that along the contour particularly strong contrasts arise between the image values inside and outside the image region defined thereby. The closed path along which the strongest contrasts occur is thus selected as the contour.
A method which is particularly suitable for determining a shutter contour includes the steps of determining contour point candidates from the spatial variation of the image values associated with the pixels of the X-ray image or a source image derived therefrom, determining line candidates from the contour point candidates in such a manner that a row of contour point candidates is situated on each line candidate or immediately adjacent to such a candidate, forming closed paths as shutter contour candidates composed of segments of different line candidates, and selecting a closed path as the shutter contour in dependence on the contrasts along the closed paths. Among the contour point candidates initially derived from the source image there are a large number in clinical practice which are not situated on a shutter contour. In this version however, no attempts are made to exclude such contour point candidates from further processing from the very start. They rather serve to determine line candidates on which, or in the immediate vicinity of which, the contour point candidates are situated. Such line candidates may form, at least partly, a part of the shutter contour; however, they may also be associated with an anatomical contour in the X-ray image.
Once more no attemp
Blaffert Thomas
Buzug Thorsten
Dippel Sabine
Stahl Martin
Wiemker Rafael
Bruce David V.
U.S. Philips Corporation
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