The present invention relates to a method apparatus for calibrating and correction of gray levels in images. In particular, the present invention is directed to acquiring a sequence of radiographic images and correction images of an object under observation. More particularly, the present invention relates to a method and apparatus for an acquiring sequence of radiographic images and calibration and correction of images of an object under observation by subtracting from each image of the sequence the spurious variation of a gray level between the images of the same sequence. The present invention can be particularly used in the medical field, such as, for example, in mammography and in the detection of cancerous tumors.
A known radiographic apparatus is comprises a console, means for providing a beam of radiation in the direction of means for detection. The means for detecting receives the radiation after passing through an object under observation placed in the observation space arranged in the beam, between on the means for providing the beam of radiation and on the means for detection. The known apparatus also comprises means for processing enabling acquiring and processing a sequence of images of an object sent from the means for detection.
The difference in absorption of the radiation by the different parts of the object under observation enables obtaining information on the composition of the object. The image formed on the means for detection comprises different gray levels, from which information can be derived. Thus, if the object under observation is a human body part, for example, the bones will appear clearly on the image acquired by the means for detection and are distinctly separate from the part formed by the muscles.
Generally, at the time of acquisition of a sequence of radiographic images, there is always a variation in the gray levels found between the successive images of the sequence. In a succession of images acquired the means for detection measures an increase in the variation of the gray level. This phenomenon is due to a persistence or remanence of the radiographic information from one image to the other, which causes the gray level to vary between the images. The variation of the gray level in the sequence depends on the thickness and the composition of the object that is being observed. Thus, for an object observed having a first thickness, will have curve of the variation versus time different than a curve for an object having a second thickness different from the first thickness.
The variation of the gray level is due principally to the trapping of charges in the photodiodes of the means for detection. The variation can also have a number of causes. It can be a question especially of an increase in temperature of the different elements of the apparatus.
In all cases, the variation of the gray levels from one image to another perturbs the measurements acquired by the apparatus. In the case of a large remanence that is, of very considerable variations, the quality and the interpretation of the images acquired may deteriorate considerably. There may be the appearance of a “ghost” or multiple images; that is, superimposing of images acquired previously onto a new image of an object. When the remanence is weaker and does not cause the appearance of ghosts, the measures made are similarly distorted by the spurious variation of the gray level from one image to the other. The variations can be of the same order of magnitude as the dynamics in gray levels of the signal that one wants to detect.
Certain methods enable elimination of this remanence in special applications using special devices. In certain applications, it is possible to determine a law according to which the remanence diminishes over time and to subtract the remanence in the images of the sequence. Consequently, at least one black measurement is made between two acquisition instants corresponding to gray levels supplied by the means for detection in the absence of exposure by radiation. The black measurement enables determining the value of the remanence for that instant. As a result, by virtue of the knowledge of the values of remanence at a given first time the law of decrease of remanence can be deduced. From the law of decrease of remanence between two acquisitions, the value of the remanence at given second time can be deduced of the following acquired image and thus correct the images acquired in a sequence.
This method of correcting sequences of images presumes the use of devices for measuring blacks. Such devices are not always available or accessible on prior art radiographic devices. Consequently, in the majority of cases, it is practically impossible to be able to correct the variation in gray level in a sequence of successive images.