Method and apparatus for correcting the contrast density of a radiography image

[0001] This application claims the benefit of a priority under 35 USC 119 to French Patent Application No. 01 10924 filed Aug. 20, 2001, the entire contents of which are hereby incorporated by reference.

[0002] The invention relates to the processing of the contrast density of an image in a radiography device. More particularly, the invention is directed to interpretation or reducing the appearance of artifacts for image reconstruction.

[0003] In numerous X-ray imaging applications where the object of which images are being taken has a very low contrast, a contrast agent is injected for the purpose of improving the contrast of the object. For example, a contrast agent of this sort is used for cardiology examinations or for mammographs or for DSA (Digital Subtraction Angiography) examinations. In many cases, the visibility of the object is further improved by acquiring an image called a mask image before injecting the contrast agent. This mask image is then subtracted from the image made after the contrast agent is injected in order to obtain a final image that will be used. In this case, a logarithmic subtraction is carried out. A logarithm of the mask image and the logarithm of the image made with the contrast medium. The logarithm of the mask image is subtracted from the logarithm of the image made with the contrast medium. The resulting image gives a good idea of the distribution of the concentration of the contrast agent but has drawbacks. Because of the beam hardening of the X-ray radiation, the contrast of an image made with a given density of contrast agents depends on the amount of tissue that is superimposed. In the case of DSA examinations, which require high concentrations of contrast agents, the beam hardening of the X-ray radiation may also be due to the contrast agent itself. This drawback is problematic when trying to interpret the images resulting from the subtraction and generally leads to artifacts when using the images for image reconstruction, in particular, three-dimensional construction.

[0004] An embodiment of the invention provides a method and apparatus for correcting the contrast density of images enabling a good interpretation or reducing the appearance of artifacts. In particular, an embodiment of the present invention can be applied during subsequent calculations for three-dimensional image reconstruction.

[0005] An embodiment of the invention is directed to a method 20 (see FIG. 2) for correcting the density of an image in a radiography apparatus comprising means for providing a radiation source, such as an X-ray source; means for taking or recording or acquiring an image placed facing the source; and means for support on which an object and surroundings to be imaged are intended to be positioned. In an embodiment of the invention, the method comprises the steps of:

[0006] acquiring 22 (see FIG. 2) an image which image, will be further indicated as a mask image;

[0007] acquiring 24 (see FIG. 2) at least one image, which will further indicated as an injected image, after injecting a contrast agent into the object to be imaged; and

[0008] determining 26 (see FIG. 2) corrected images of the mask image and of the injected image from acquisition parameters of the apparatus, from the composition of the surroundings of the imaged object and from the composition of the injected contrast agent.

[0009] In an embodiment of the invention, the method for correcting the density of an image has at least one of the following characteristics:

[0010] the composition of the surroundings of the object is estimated by means of a typical composition;

[0011] during the step of determining corrected images, a radiation spectrum is determined for each of the images having to be corrected;

[0012] the radiation spectrum is determined for each of the pixels making up the image;

[0013] when determining corrected images, gray level values are determined for each of the images to be corrected;

[0014] when determining corrected images, a thickness of tissue d_tissue traversed by the radiation and a thickness of contrast medium d_contrast traversed by the radiation are determined;

[0015] d_tissue and d_contrast are approximated by polynomial functions;

[0016] all the possible values of d_tissue and of d_contrast are calculated and the value of d_contrast-d_tissue is stored in a two-dimensional table for each of the possible pairs (mask image, injected image), the corrected images of the mask image and of the injected image being determined using the corresponding value of d_constrast-d_tissue read from the two-dimensional table when acquiring an image, the acquisition parameters of the apparatus used for this image are combined with the image—during the acquisition of an image, the acquisition parameters of the device used to acquire this image are combined with the image.

[0017] In an embodiment of the invention a radiography apparatus comprising means for providing an radiation source, such as an X-ray source; means for recording an image placed facing the source; and means for support on which an object and surroundings to be X-rayed are intended to be positioned is provided, and means for implementing the method having one at least of the aforementioned characteristics.

[0021] With reference to the figure, a radiography apparatus 1, such as may be used with X-rays, comprises means 2 for recording or taking or acquiring radiographic images and means 3 for emission, such as an X-ray source. In this case, the means 2 for taking images may be a digital camera. The source 3 and the digital camera 2 are attached at each end of a carrying arm acting as a gantry, in this case in the form of a semicircle. The semicircular arm 7 is connected by sliding to a second arm 8. The second arm 8 itself is connected by sliding and by rotation to the stand 9 of the apparatus. The arm 8 is mainly capable of rotational movements 6 around its own axis. The semicircular arm 7 itself is capable of sliding with respect to the arm 8, so that the semicircular arm 7 makes a rotational movement 5 with respect to the center of the semicircle forming the arm 7.

[0022] In use, an object 4 to be imaged, such as a body of a patient is positioned between the source 3 and the digital camera 2, such that the object to be imaged is in a field 10 of the apparatus.

[0023] In order to be able to provide images of optimum quality of imaged objects having a low contrast during radiography, it is common practice to inject a contrast medium into the object to be imaged. This contrast agent spreads within the object to be imaged thereby making it possible to increase this contrast on the images taken by the apparatus. In order to increase the quality of the result, the operator takes an image or images before having injected the contrast agent. This image will hereinafter be indicated as a mask image. The image or images taken after injecting the contrast agent will hereinafter be indicated as an injected images.

[0024] In order to optimise the quality of the images which will be subsequently used, for example, for diagnosis by viewing or for a three-dimensional image reconstruction of the imaged object or to improve the subtraction from the corrected injected images of the corrected mask image, which makes it possible to completely isolate the imaged object from its surroundings, the apparatus will determine the radiation spectrum, i.e., of the X-rays, at each point of the image taken. To do this, the apparatus will use the acquisition parameters that it has used to take the images. Furthermore, the apparatus is acquainted with constitution or the composition of the object and with its surroundings that has been imaged. Preferably, the composition of the imaged object and/or its surroundings is generally estimated and approximated by a simple typical composition. For example, in the case of an examination targeting the abdominal region of a human body, the typical composition corresponding to water can be selected as an estimate of the surroundings of the object to be imaged in an abdominal region. Within the scope of a neurological examination targeting the brain, a typical composition of the surroundings of the object to be imaged will be bony tissue. This is because, within the scope of this examination, the majority of the attenuation comes from the bone constituting the skull. These various typical compositions can be pre-recorded directly within the apparatus allowing the rapid selection by the operator of a typical composition depending on the region of the body of a patient of which images are being taken.

[0025] Moreover, within the context of calculating the radiation spectrum for X-rays, the injected images, the apparatus uses the composition of the contrast agent that has been injected within the object to be imaged for the calculation. Generally the contrast agent used in such examinations is iodine, but the apparatus can have available a multitude of contrast agents which can be used within the context of radiography. The choice of contrast agent, and therefore its composition, is made available as above for a chosen typical composition, that is to say according to the typical composition used by the operator to carry out the injection.

[0026] Once determining the radiation spectrum of each of the images made or of each of the points or pixels of the images made (this depends on the degree of accuracy which is required during later use of the corrected images), the apparatus calculates the effective attenuation coefficient of the contrast agent or of the imaged object. The attenuation coefficient may vary throughout the image because, on the one hand, of X-ray beam hardening and because, on the other hand, of the amount of tissue which surrounds the imaged object which the X-rays have to traverse to reach the means for taking the image.

[0027] These assumptions permit calculation of the X-ray spectrum of each of the images produced, so as to write the equations relating the X-ray spectrum to the gray level values for each the images produced:

[0028] $\hspace{1em}\{\begin{array}{c}{I}_{\mathrm{mask}}={\int }_0^{E_{\max }}\eta \left(E\right)·N\left(E\right)·e^{-{\mu }_{\mathrm{tissue}}\left(E\right)·d_{\mathrm{tissue}}}·dE\\ I_{\mathrm{injected}}={\int }_0^{E_{\max }}\eta \left(E\right)·N\left(E\right)·e^{-{\mu }_{\mathrm{tissue}}\left(E\right)·d_{\mathrm{tissue}}}·e^{-{\mu }_{\mathrm{contrast}}\left(E\right)·d_{\mathrm{contrast}}}·dE\end{array}$
In which:

[0029] I_mask represents the gray level values in the mask image;

[0030] I_injected represents the gray level values of the injected image in question;

[0031] η(E) represents the conversion factor of the digital camera sensor;

[0032] (E) represents the number of photons having energy E (this actually represents the X-ray spectrum);

[0033] d_tissue represents the thickness of the tissue traversed by the incident X-ray beam;

[0034] d_constrast represents the thickness of the contrast medium traversed by the incident X-ray beam;

[0035] μ_tissue represents the estimated typical composition of the surrounding tissues and/or of the X-rayed object; and

[0036] μ_contrast represents the composition of the contrast agent used to produce the injected images.

[0037] On the basis of these two equations, the two unknowns d_tissue and d_contrast, can easily be calculated by solving the system formed by the two aforementioned equations. Knowledge of d_tissue and d_constrast provides the corrected images of the mask image and of the injected image.

[0038] Knowledge of d_tissue and d_contrast provides the following advantages:

[0039] the same amount of contrast agent creates the same contrast on an image, independently of the structure and of the amount of these structures surrounding the object to be imaged;

[0040] when the corrected mask image is subtracted from the corrected injected image, the contrast of the vessels is not degraded when it is behind the bone, for example;

[0041] when carrying out the three-dimensional image reconstruction from the corrected images, this three-dimensional image reconstruction has considerably fewer artifacts due to the X-ray radiation beam hardening.

[0042] Solving the aforementioned equation system provides a first function to obtain d_tissue and a second function to obtain d_contrast. These two functions are implemented within the apparatus to calculate the corrected images. To do this, it is possible to use a polynomial approximation which is simple to program.

[0043] Another implementation or embodiment of the method comprises calculating a two-dimensional table containing all the possible combinations of I_mask and I_injected using the X-ray spectrum calculated on the basis of the acquisition parameters of a given image. For a given spectrum, the two-dimensional table can be constructed as follows:

[0044] for a given value of I_mask (from all the possible values which are finite in number), d_tissue is calculated as described above, by using the first equation of the above system of equations;

[0045] for a given value of I_injected (among all the possible values which are finite in number), d_constrast is calculated as described above, by using the second equation of the above system of equations, and this for each of the values of d_tissue calculated at the previous step; and

[0046] the value d_constrast-d_tissue corresponding to the images I_mask and I_injected in question are stored in the two-dimensional table, at the location (I_mask, I_injected).

[0047] In this case, the apparatus that has just taken an image, in order to determine the corrected image thereof, would be placed at the intersection (I_mask, I_injected) of a two-dimensional table corresponding to the I_mask of the mask effectively taken and to the I_injected of the injected image effectively taken.

[0048] By using this method for correcting images in an radiography apparatus, the corrected images obtained are independent of the composition and of the amount of tissue surrounding the imaged object. Moreover, the corrected images obtained are quantitative and can be compared firstly between different examinations and secondly between different viewing angles are also independent of the acquisition parameters used between each of the views or of the examinations. During subtraction of the mask image and of the injected image or of the three-dimensional reconstruction on the basis of the corrected images, the result has considerably fewer artifacts introducing errors that can be damaging to this type of calculation.

[0049] Various modifications in the structure and/or the steps and/or the function or equivalents thereof of the disclosed embodiments may be made by one skilled in the art without departing from the scope and extent of the invention as recited in the claims.