The invention concerns the saturation management of an image of an object of high dynamics, that is, presenting too great a variation between the lowest and highest detectable gray level.
It is particularly applicable in the medical field, especially for mammograms in which the thick tissue displays on the image a high dynamics sometimes greater than that of the X-ray detector.
In the medical field, physicians generally use radiographic images of the internal organs of patients to make their diagnosis. The images are obtained by means of an acquisition chain notably including a detector. Each detector possesses a specific dynamics and can therefore register up to a certain maximum level of X-ray exposure called saturation level. If that saturation level is exceeded in an area, all the information is lost in that area. Thus, on a mammogram, the area close to the skin can be partially erased.
In the course of automatic mammography, two images are made:
a first low-dose preexposure image (saturation is thus avoided) from which certain characteristics of the breast are determined, such as its composition and its radiological thickness. That preexposure image is no longer subsequently used;
a second normal-dose exposure image from which the physician makes his diagnosis.
However, this method of operation does not make it possible to solve possible saturation problems that can occur during the exposure phase. In fact, the object to be X-rayed presents a dynamics which can be more or less appreciable in any given area and the detector of the image chain also possesses its own dynamics. Saturation occurs when the dynamics of the detector is less than that of the object to be X-rayed. Some information close to the skin can be lost.
The invention is aimed at contributing a solution to that problem and, in particular, at making it possible to obtain a digital radiographic image of a body organ whose contour is precisely detected regardless of the dynamics of the organ.
The invention proposes a method of saturation of a digital radiographic exposure image, in which there are one or more saturation areas. The image is obtained following a mammogram on an acquisition chain in automatic mode, using a preexposure image.
According to a general characteristic of the invention, a final image is constructed from the the exposure image and preexposure image.
The invention proposes using the preexposure image obtained in the course of a mammogram in automatic mode in order to recover the information lost by saturation during exposure.
The use of the preexposure image is therefore generally limited to obtaining the characteristics of the organ X-rayed the pre-exposure image is obtained from an X-ray flash at low enough dose not to involve any saturation area. The exposure image, on the other hand, is obtained from X-radiation at normal dose higher than that of the preexposure image and thus entails possible saturation areas.
The invention preferably provides for making a mask from the exposure image by taking the saturation areas into account, and then determining on the preexposure image the areas corresponding to the saturation areas of the exposure image by using the mask. The areas thus obtained are called coverage areas. The area complementing the junction of coverage areas is set at a zero gray level. A digital standardization processing of certain gray levels of both exposure and preexposure images is then carried out and the final image is determined from a combination of the two images thus processed.
According to an embodiment of the invention, the mask is made by detecting all the gray levels higher than a saturation gray level linked to the detector of the acquisition chain.
The mask is, in fact, made by keeping in memory the coordinates of the pixels thus detected. These coordinates make it possible to delimit the saturation areas and, by transferring them on the preexposure images, the pixels of the preexposure image are determined, the coordinates of which are identical to those of the pixels of the saturation areas.
It will be readily apparent to one skilled in the art that this masking operation is correctly carried out only if the two images are perfectly superposable. The two images can, if necessary, undergo a treatment of resetting relative to each other.
In general, according to one embodiment of the invention, the digital standardization processing can be carried out by enhancing the gray level of the coverage areas on the the preexposure image to the same gray level as the unsaturated areas of the the exposure image.
In fact, the pixels of the coverage areas have, on the average, a gray level much lower than that of the exposure image, for the preexposure image is made following a low-dose radiation. The object of the standardization operation is to ensure continuity between the coverage areas of the preexposure image and the unsaturated areas of the exposure image. This operation is of interest only if it is carried out homogeneously, that is, all the pixels of the coverage areas are augmented by the same quantity. This makes it possible to preserve the contrasts and, therefore, always to be able to distinguish the elements of interest.
Finally, the final image is obtained by joining the unsaturated areas of the exposure image with the enhanced coverage areas of the preexposure image.
The final image is therefore composed of the exposure image in which all the saturated areas are replaced by homologous areas of the preexposure image having undergone a digital processing. The information from the saturated areas is thus partly recovered.
According to another embodiment of the invention, the mask is limited to a saturated part of the exposure image corresponding to a peripheral region of the breast X-rayed. The information recovered then makes it possible to delimit the breast precisely.
The limitation of the mask to a small part of the image results in accelerating processing, since the number of pixels processed is less than the number of pixels processed in the case of a mask made on all the saturated areas.
The invention also concerns a system of saturation management of the exposure image. This system comprises:
an acquisition chain capable of elaborating, in addition to the exposure image containing at least one saturation area, a second preexposure image of weaker gray level;
an image processing system containing a means of detection of the pixels of the saturated area of the exposure image in order to form a mask, a means of detection of a coverage area on the second image, the coordinates of whose pixels correspond to the coordinates of the pixels of the saturated area of the exposure image, a means of enhancement of the gray levels of the coverage area to the same gray level as the unsaturated area of the exposure image and a means of addition of the enhanced coverage area of the second image with the unsaturated area of the exposure image in order to obtain the unsaturated final image; and
at least one system of display of the final image.
The digital image processing system therefore performs all the digital operations involving pixels. This system receives the images direct from the acquisition chain, but it can also obtain them from a digital data storage means on a noninstantaneous diagnosis.
The display system can consist of a video monitor and/or a printer connected to the image processing system.
According to an advantageous embodiment of the invention, the acquisition chain operates on automatic mode, for which a first low-dose flash supplies a preexposure image and a normal-dose radiation supplies an exposure image; the preexposure image therefore corresponds to the second image.
The invention is remarkable in that it uses the preexposure image on an automatic operation of the acquisition chain.
In fact, the image is ordinarily used only to determine certain parameters of the body organ X-rayed, but not at all for formation of the digital radiographic image.
Other advantages and characteristics of the invention will appear on examination of the detailed description of a nonlimitative embodiment and of the attached figures, in which: