METHOD AND APPARATUS FOR CALIBRATION AND CORRECTION OF GRAY LEVELS IN IMAGES
CROSS-REFERENCE TO RELATED
APPLICATIONS
[0001] This application is a continuation of U.S. application Ser. No. 10/775,912 filed Feb. 10, 2004 which claims priority under 35 USC 119(a)-(d) to French Patent Application No. 03 01797 filed Feb. 14, 2003, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 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, suchas, forexample, in mammography and in the detection of cancerous tumors.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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. [0007] 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.
[0008] 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.
[0009] 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.
BRIEF DESCRIPTION OF THE INVENTION
[0010] An embodiment of the invention provides a method and an apparatus for correction of gray levels in images. An embodiment of the invention is directed to correcting the remanence in a sequence of radiographic images. The method and the apparatus are able to eliminate the utilization of black measurement devices.
[0011] An embodiment of the invention provides a method and an apparatus for calibration a device capable of acquiring a sequence of radiographic images. Calibration is done so as to be able to correct the effects of the variation of gray level in a sequence of radiographic images of an object under observation.
[0012] An embodiment of the invention provides a method and an apparatus for calibration and correction of radiographic images applicable to all repeatable variation phenomena; that is, variation phenomena that repeat from one measurement of an acquisition sequence to the next when maintaining identical acquisition phenomena from one measurement of an acquisition sequence to the next. [0013] An embodiment of the invention provides a method for calibrating an apparatus capable of acquiring a sequence of radiographic images and correcting images of an object under observation comprising: for each image of a sequence acquired by the apparatus and for a given frequency of acquisition of the sequence, the apparatus is calibrated by determining the value of the variation of a mean of gray levels in at least one zone of interest of the current image of at least one calibration device, the variation being determined relative to the mean gray level of the first image of the sequence in each zone of interest;the determination of the variation is reiterated for a series of images sequences acquired using calibration devices resulting in first images of mean gray levels different from one sequence to another: each image of an image sequence of the object under observation is corrected, comprising zones of observation having different gray levels by subtracting from the current image the variation of one gray level relative to the first image of the object, the subtraction being a function of the gray level considered from each zone of observation.
[0014] The invention similarly relates to an apparatus capable of acquiring a sequence of radiographic images implementing a method according to an embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The embodiments of the invention will better understood from the following description, which is purely illustrative and non-limiting, and which should be read with reference to the drawings annexed hereto, wherein: [0016] FIG. 1 schematically represents a known apparatus for acquiring a sequence of radiographic images; [0017] FIG. 2 schematically represents a known development of remanence as a function of time in a sequence of radiographic images;
[0018] FIG. 3 schematically represents a known correction method for remanence according to the state of technology using a black measurement;
[0019] FIG. 4 schematically represents a method of calibration and correction of sequences of image according to an embodiment of the invention, wherein the calibration is done prior to acquisition of the sequence of images of the object under observation;
[0020] FIG. 5 schematically represents a spatially adaptive embodiment of the method according to FIG. 4; [0021] FIG. 6 schematically represents a method according to an embodiment of the invention, wherein the calibration is done at the same time that the acquisition of the sequence of images of the object to be observed;
[0022] FIG. 7 schematically represents a spatially adaptive embodiment of the method according to FIG. 6; [0023] FIG. 8 schematically represents a longitudinal section of a calibration device used in an embodiment of the invention;
[0024] FIG. 9 schematically represents the different steps of an embodiment of the invention wherein calibration data of at least two successive sequences are combined; and [0025] FIG. 10 schematically represents the different steps of an embodiment of the invention wherein calibration data of at least two sequences having different calibration devices are combined.
DETAILED DESCRIPTION OF THE INVENTION
[0026] An example of a known radiographic apparatus is represented schematically in FIG. 1. In Figure, an apparatus 1 comprises a console 5 (having a substantially vertical extension) and means for providing a radiographic beam emitter 4 facing in the direction of a plate 7 comprising means for detection 2. The means for detection 2 may comprise a pho
todiode matrix. The means for detection 2 collects the radiation, for example, X-rays, after passing through an object under observation placed in the observation space 6 arranged in the beam 4, between on the one hand the emitter 3 and on the other hand the detector 2 of the apparatus 1. The extensions of the emitter 3 and the detector 2 can be, for example, horizontal and perpendicular to the console 5 but any observation direction is possible, especially due to the fact of the possible rotation of the assembly formed by the emitter 3 and the detector 2 about a substantially horizontal axis of extension. The apparatus 1 may also comprise means for processing 8 enabling acquiring and processing a sequence of images of an object sent from the detector 2.
[0027] As noted above, the difference in absorption of the radiation by the different parts of the obj ect under observation enables obtaining information on the composition of the object. In fact, an image formed on the detector 2 comprises different gray levels, from which information can be derived. Thus, if the object under observation is a humanbody part, for example, the bones will appear clearly on the image acquired by the detector and are distinctly separate from the part formed by the muscles.
[0028] As noted above, 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. Thus, FIG. 2 shows that in a succession of images acquired at times t1; t2, t3, for example, the detector measures an increase in the variation L 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. In the graph of FIG. 2 the variation L of the gray level of the image running at time t1 is represented relative to a first image acquired at time 0. FIG. 2 similarly shows that the variation L 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, a curve while for an object having a second thickness different from the first thickness, a curve |2 different from the first. [0029] As noted above, the variation L of the gray level is due principally to the trapping of charges in the photodiodes of the detector. The variation L can also have a number of causes. It can be a question especially of an increase in temperature of the different elements of the device. [0030] As noted above, in all cases, the variation of the gray levels from one image to another perturbs the measurements acquired by the device. In fact, 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. In fact, 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.
[0031] As noted above, in fact, in certain applications, it is possible to determine the law according to which the remanence diminishes over time and to subtract said remanence in the images of the sequence. FIG. 3 shows that between two acquisitions of images done at times tx and t2, the remanence diminishes according to an exponential distribution that can
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