DE19945018A1 - Method of operating digital image system of X-ray diagnostic arrangement enables calibration can be matched to current sensitivities - Google Patents

Abstract

The method involves dividing a current raw image with a sensitivity image assessed using a correction matrix to determine the image ready for reproduction on a monitor (8). The correction matrix consists of at least one X-ray image and the sensitivity image is derived as a calibration image from at least one null acquisition, whereby the correction matrix depends on the current sensitivity and the decay characteristic of preceding acquisitions.

Description

The invention relates to a method for operating a digi tal image system of an X-ray diagnostic device with an x-ray unit for generating x-ray images, with a two-dimensional one sensitive to X-rays Imager with arrayed in rows and columns th pixels for acquisition of the X-ray images and generation digital image data that the image system for processing be performed, and a monitor for the playback of the digital image system has processed x-ray images, where the digital image data of the current raw image with be standardized using a calibration image.

Flat X-ray detectors such as solid bodies detectors, imaging plate systems or X-ray image intensifiers TV systems provide digital X-ray images. The sensation The image sensors used can be affected by the irradiation depend on the history of the For example, the light turns off prey of the scintillator cesium iodide when exposed to X-rays rays to. The light output then decreases over time off again.

This effect can be particularly strong if, for example a large area detector is operated and mostly only an inner part of the sensor surface is irradiated becomes. If a full-format picture is then taken, show it the central pixels have an undesirable greater brightness.

Digital systems can have a computational sensitivity Carry out compensation, but this compensation is not in the Able to feel relatively short term, for example Sensitive Rise in minutes, decrease in sensitivity  within days, changing sensitivity changes equalize.

To carry out this arithmetical sensitivity compensation ren is "calibrated", i.e. H. there are so-called empty shots x-rays and without further ab sorber, made. This X-ray picture is essentially the information about the intensity distribution of the x-ray field and the sensitivity of the individual pixels.

In DE 195 27 148 C1 such a calibration is be wrote in which a processed calibration image for Correction of the original image is used.

With this calibration picture or "sensitivity picture" is each subsequent recording standardized. According to this standardization law All pixels have the same sensitivity.

This calibration is carried out at relatively large intervals for example carried out weeks or months and can each only capture the current sensitivities. Luminous efficacy and thus changes in sensitivity between the calibrations stances are not appropriately recorded and corrected.

A typical preprocessing step for determining the finished image F _{i, j} is the division of the current raw image A _{i, j} by the sensitivity image E _{i, j} obtained during the calibration with i, j as a matrix of size i × j:

F _{i, j} = A _{i, j} / E _{i, j} (1)

So far, this problem has been attempted through material optimization to avoid.

The invention is based on the object, a method of type mentioned in such a way that an adjustment  calibration to the current sensitivities light becomes.

The object is achieved in that for loading the finished image to be displayed on the monitor the current raw image by using a correction matrix scored sensitivity image is divided, the Cor rectification matrix from at least one x-ray image and the emp Sensitivity image as a calibration image from at least one Blank recording was obtained, with the correction matrix of the current sensitivity and decay behavior dependent recordings depends. This ensures that easily the calibration image without new calibration through the correction matrix continuously the current situation units is adjusted.

The finished image (F _{i, j} ) can be formed according to the following formula: F _{i, j} * = A _{i, j} / (E _{i, j} . K _{i, j} ), the correction matrix (K _{i, j} ) is calculated using the following formula:

K _{i, j} (t _{n + 1} ) = Π _{m = 1 to n} {1 + a (K _{i, j} (t _m )). F _{i, j, m} . exp (- (t _{n + 1} - t _m0 ) / τ)}

It has proven to be advantageous if to determine the Correction matrix X-ray image taken only with high dose which are taken into account because of them in particular of the current recordings are to be expected.

According to the invention, the image converter can be a two-dimensional one Solid state detector made of amorphous silicon or an X-ray Image intensifier with a coupled CCD image converter.

The invention is based on one in the drawing illustrated embodiment explained in more detail.

In the figure, an X-ray diagnostic device with an X-ray tube 2 provided by an X-ray generator 1 with high and heating voltage is shown as an X-ray unit in which the image system 7 of the inventive method can be used. The x-ray tube 2 generates a conical x-ray gene radiation 3 , which penetrates a patient 4 and generates 5 radiation images on a two-dimensional solid-state detector 3 sensitive to x-ray radiation. The output signal of the solid-state detector 5 , the digital image data 6 , is fed to an image system 7 . The image system 7 can have converters, image memories and processing circuits. It is connected to a monitor 8 in order to display the acquired x-ray images. Operating elements 9 are connected to the other components of the x-ray diagnostic device via a system control and communication 10 .

A sensitivity image E _{i, j} determined in a calibration process at an earlier point in time is stored in an image memory 11 .

A first processing circuit 12 of the image system 7 acts as a preprocessing step for determining the finished image F _{i, j} by dividing the current raw image A _{i, j} with the product of the sensitivity image E _{i, j} obtained during the calibration with a new one for each image newly calculated correction matrix K _{i, j} with i, j as matrix of size i × j:

F _{i, j} * = A _{i, j} / (E _{i, j} . K _{i, j} ) (2)

This correction matrix K _{i, j} is calculated by a second processing circuit 13 . To determine K _{i, j} , in principle, every image irradiated with X-rays is evaluated. In an approximation, however, it is sufficient to only consider images that have been taken with a relatively high dose.

Since the selected amplification factors are known, the image F _{i, j contains} approximately the information about the locally applied dose, which is only falsified by the instantaneous change in sensitivity to be corrected.

In the second processing circuit 13 of the image system 7 , the images F _{i, j} are evaluated, stored as necessary and using an algorithm which describes the change in the sensitivity change over time, the respective current correction image K _{i, j is} calculated.

The following equation is an example of such an algorithm:

or advertised

K _{i, j} (t _{n + 1} ) = Π _{m = 1 to n} {1 + a (K _{i, j} (t _m )). F _{i, j, m} . exp (- (t _{n + 1} - t _m0 ) / τ)}

for all pixels i, j with
n - Number of recordings made or evaluated so far, ie the "history" of the sensor
n + 1 - number of the current recording
t _{n + 1} - time of current recording (n + 1)
t _m0 - time of taking the _mth image
a (K (t _m )) - factor which depends on the sensitivity at the time t _m and
τ - time constant.

In the first processing circuit 11 of the image system 7 , the correction matrix K _{i, j} calculated in this way is then multiplied by the stored sensitivity image (E _{i, j} ) obtained during the calibration. The possibly temporarily stored current raw image A _{i, j} is then divided by this product so that the finished image (F _{i, j} ) is obtained, which can then be displayed on the monitor 8, if necessary, after further corrections.

In the method according to the invention, knowledge of the application, the material properties and the systems used are combined and taken into account in the imaging by the correction matrix K _{i, j} .

Claims (6)

1. A method for operating a digital image system ( 7 ) of an x-ray diagnostic device with an x-ray unit ( 1 , 2 ) for generating x-ray images, with a two-dimensional image converter ( 5 ) sensitive to x-ray radiation ( 3 ) with points arranged in matrix form in rows and columns Detection of the x-ray images and generation of digital image data ( 6 ), which are fed to the image system ( 7 ) for processing, and a monitor ( 8 ) for displaying the x-ray images processed by the digital image system ( 7 ), in which the digital image data ( 6 ) of the current raw image (A _{i, j} ) are standardized with a calibration image, characterized in that for determining the finished image (F _{i, j} ) to be displayed on the monitor ( 8 ) _, the current raw image (A _{i, j} ) by the divided with a correction matrix (K _{i, j} ) evaluated sensitivity image (E _{i, j} ), the correction matrix (K _{i, j} ) from at least one X-ray n image and the sensitivity image (E _{i, j} ) was obtained as a calibration image from at least one blank recording, the correction matrix (K _{i, j} ) depending on the current sensitivity and the decay behavior of previous recordings. 2. The method according to claim 1, characterized in that the finished image (F _{i, j} ) is formed according to the following formula:
F _{i, j} * = A _{i, j} / (E _{i, j} . K _{i, j} ). 3. The method according to claim 1 or 2, characterized in that for determining the correction matrix (K _{i, j} ) X-ray images taken only with a high dose are taken into account. 4. X-ray diagnostic device according to one of claims 1 to 3, characterized in that the correction matrix (K _{i, j} ) is calculated according to the following formula:
K _{i, j} (t _{n + 1} ) = Π _{m = 1 to n} {1 + a (K _{i, j} (t _m )). F _{i, j, m} . exp (- (t _{n + 1} - t _m0 ) / τ)} 5. X-ray diagnostic device according to one of claims 1 to 4, characterized in that the image converter ( 5 ) is a two-dimensional solid-state detector ( 5 ) made of amorphous silicon. 6. X-ray diagnostic device according to one of claims 1 to 5, characterized in that the image converter ( 5 ) is an X-ray image intensifier with a coupled CCD image converter.