Method and apparatus for generation of a digital x-ray image of an examination subject

In a method and apparatus for generation of a digital x-ray image of an examination subject with an x-ray tube and a digital x-ray detector, a preliminary total dose required for correct exposure is determined using prior knowledge and a complete pre-image of the examination subject is acquired with a partial dose that is smaller than the preliminary total dose. A remaining dose is determined from the pre-image by image evaluation. A complete following image is acquired with this remaining dose, and the final x-ray image is assembled from the pre-image and the following image.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a method and an apparatus for generation of a digital x-ray image of an examination subject.

2. Description of the Prior Art

In the acquisition of an x-ray image of an examination subject, for example in mammography, it must be ensured that the acquisition parameters (in particular the applied dose) are correctly set in order to ensure an image quality suitable for diagnostic evaluation. In order to limit the radiation exposure of the irradiated region of the examination subject to the diagnostically-required minimum, it is sought to already correctly set the acquisition parameters in the first image acquisition. For this purpose, an arrangement known as an exposure automatic (AEC, automatic exposure control) is used.

In the generation of an analog x-ray image with an x-ray film, for example in analog mammography, a number of solid-state detectors are arranged behind the x-ray film (in the direction of propagation of the x-rays). These detectors measure the intensity of the x-rays transmitted through the x-ray film, and the output signals of which are used to control the acquisition parameters (for example exposure time, operating voltage of the x-ray tube, x-ray current, anode filter combination).

Due to the higher absorption of the x-ray detectors used in digital x-ray imaging, such an exposure control ensuing during the image acquisition is, however, not possible. In x-ray apparatuses with digital x-ray detectors the exposure control ensues by making an exposure known as a pre-shot with a low dose in a first step. The dose radiated in the pre-shot is so low that detector and quantum noise influence the measurement signal respectively acquired by the individual detectors to a significant degree. The signals of a number of individual detectors are therefore combined (binning, undersampling) into respective measurement values, such that only a few hundred measurement values have to be evaluated instead of multiple tens of thousands (orders of magnitude more). These measurement values are used for determination of the correct exposure time or total dose. The data acquired in the pre-shot, however cannot be used for the diagnostic x-ray image (i.e., an image of sufficient quality to allow a competent diagnosis to be made there from) due to the low dose, and thus represent a small but nevertheless undesirable additional dose exposure.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method for generation of a digital x-ray image of an examination subject in which the disadvantages cited above are avoided. An object of the invention is to also provide an apparatus for implementation of such a method.

With regard to the method, the above object is achieved by determining using prior knowledge, a preliminary total dose required for correct exposure and a complete pre-image of the examination subject is acquired with a partial dose that is smaller than the preliminary total dose. A remaining dose is determined from the complete pre-image by evaluation thereof, and a complete following image is acquired with this remaining dose. The final x-ray image (used for diagnosis) is then assembled from the pre-image and the following image.

With this method it is ensured that the entire radiation exposure for the x-ray image can be utilized. Since a complete pre-image is acquired, meaning that a pre-image is generated having a spatial resolution corresponding to that of the final x-ray image, by the signals of all individual detectors being acquired separately from one another, this pre-image can be diagnostically utilized and can be assembled with the following image into an overall image.

The term “dose” as used herein is equivalent to the product of x-ray current and time (mAs), since the acquisition parameters that do not concern this current-time product but rather concern the other operating conditions of the x-ray tube are identical for pre-image and following image.

The image evaluation for determination of the remaining dose ensues with the methods as are known in the automatic exposure control (explained in the preceding) in digital x-ray diagnostics. There is a desired or target value of an image parameter for this exposure control, for example an average intensity or brightness value (pixel value) or a signal-to-noise ratio, in an image region (region of interest) evaluated for the exposure control. The required remaining dose then can be determined from the value of the image parameter measured in the pre-image and the desired value by means of a correlation between the dose and the value of the image parameter that is known from previous measurement and tabulation.

In order to minimize detector and quantum noise in the pre-image and in the following image, i.e. to ensure that the pre-image and the following image each can be diagnostically utilized, it is advantageous for the partial dose to be between 40% and 60% of the preliminary total dose. In other words, the first exposure ensues with a partial dose that does not in fact correspond to the actual necessary total dose or the predicted total dose, but is in the same order of magnitude and thus is significantly higher than the dose used in the pre-shot in the prior art for exposure control.

The prior knowledge necessary for the determination of the preliminary total dose encompasses experimental values (properties, for example tissue composition, bone ratio and thickness of the examination subject) stored, for example, in a look-up table for the respective application case. In mammography it has proven to be particularly simple and advantageous when a dose is selected as the preliminary total dose that is approximately 80 to 90% of the known (from the prior knowledge) average total dose of a 2 cm-thick compressed breast.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown inFIG. 1, an apparatus for generation of a digital x-ray image B (in the shown example a mammography apparatus) has an x-ray tube2for generation of x-rays3that penetrate an examination subject4. In the example the examination subject4is a female breast that is held between a compression plate6and a patient positioning table8. The x-rays4penetrating the examination subject4, the compression plate6and the patient positioning table8are acquired by a large-surface digital x-ray detector10that is composed of a number of individual detectors12arranged in a matrix-like array. A scattered-ray grid can be arranged in front of the x-ray detector10for suppression of scatter radiation.

The control of the x-ray tube2as well as of its operating parameters ensues by control signals S that are generated by a control and evaluation device14. The acquisition parameters required for generation of a complete pre-image V can be set by the user using input and display elements, represented in the example as a keyboard16and a monitor18. This pre-image V is acquired with a partial dose that is determined using a total dose determined by the user.

An automatic evaluation of the pre-image V acquired with the partial dose then ensues in the control and evaluation device14with the method as is known for automatic exposure control in order to determine the remaining dose required for generation of the x-ray image. Given otherwise unchanged acquisition parameters, a complete following image F is then acquired with this remaining dose. The complete pre-image V and the complete following image F are then combined into a final x-ray image B in the control and evaluation device14. The actual total dose required for the final x-ray image B is the stored units of partial dose and remaining dose and normally does not coincide with the previously-predicted preliminary total dose.

The selection of the acquisition parameters and of the preliminary total dose for the pre-image V ensues by the personnel implementing the image acquisition using prior knowledge (for example look-up tables) in which are listed, for example, tissue composition, size and further subject parameters relating to the examination subject4as well as operating parameters of the x-ray tube2and the total dose normally required for these parameters or a first exposure time required for generation of the pre-image V. The preliminary total dose is a rough estimate. However, since the pre-image V is acquired with a significantly lower partial dose, namely approximately between 40% and 60% of this estimated or predicted preliminary total dose, an over-exposure and an unnecessary dose exposure associated with this are precluded in practice.

In mammography, i.e. in the generation of an x-ray image B of the female breast, the partial dose can also be selected with the aid of tables in that approximately 80 to 90% of the known (from the prior knowledge) average total dose for the thinnest breast occurring in practice (i.e. for a compressed breast thickness of approximately 2 cm) is selected by default as the preliminary total dose. Given an average breast this then leads to a partial dose of approximately 50% of the second total dose, and given a breast with the highest compression thickness this then leads to a partial dose of approximately 10 to 20% of the second total dose.

In the flow chart ofFIG. 2, a partial dose DVor partial exposure time is entered manually in a first step. The control signals SVassociated with this partial dose DVare then generated in the control and evaluation device for the x-ray tube and a first exposure is conducted in a first time window20. The measurement values acquired by the individual detectors are read out at the end of the time window20and assembled into the complete pre-image V as well as into a data set AEC that is suitable for the automatic exposure control. A calculation of the actual required, correct total dose or, respectively, the remaining dose DRor the remaining exposure time from this data set AEC ensues in a next step and the associated control signals SRfor the x-ray tube are generated. A new exposure is subsequently conducted in a second time window22. The individual detectors are read out and combined into a complete following image F, whereby the pre-image V and the following image F are subsequently added and assembled into the x-ray image B in this manner.