WO2023237443A1

WO2023237443A1 - Dose determination method

Info

Publication number: WO2023237443A1
Application number: PCT/EP2023/064840
Authority: WO
Inventors: Michael Weber; Felix Schöfer; Maximilian Bemmann
Original assignee: Dürr Dental SE
Priority date: 2022-06-07
Filing date: 2023-06-02
Publication date: 2023-12-14
Also published as: DE102022114320A1

Abstract

The invention relates to a method for determining the dose for an X-ray scan.

Description

Dose determination procedure

The invention relates to a method for determining the dose in an x-ray.

The operation of an X-ray system for use on humans is only possible within the framework of the radiation protection law applicable in Germany if a large number of regulations are adhered to. Among other things, it must be ensured that the X-ray device displays parameters for determining the exposure that the person being examined or treated received during the application. These are recorded as part of the examination or treatment. Traditionally, the parameters used in X-ray examination - the dose area product and, particularly in the dental field, the tube voltage (kV), the tube current (mA) and the exposure time (s) - are recorded in an X-ray control book together with other information about the examination or treatment. Based on the recorded parameters and information, conclusions can be drawn about the exposure of the person being treated or examined.

However, this approach is problematic in some respects for determining the exposure of the person being examined or treated. When displaying or recording the values set on the X-ray tube, errors can occur that are not immediately noticeable in the X-ray image. In addition, it is not always guaranteed that the exposure to the person calculated using the settings on the X-ray tube will occur as expected. Last but not least, with existing X-ray facilities it is not easy to archive the parameters set on the X-ray tube digitally and ideally automatically. Due to the necessary radiation protection equipment in the room in which such X-ray facilities are operated, transmission of the X-ray parameters via cable or wirelessly is difficult or can only be achieved with great effort - especially on site.

Apart from an arrangement in the C-arm, smaller flexible X-ray emitters are often used in the dental field or in the field of materials testing. In order to determine the dose area product here, you would have to use distance sensors or a tape measure The distance from the X-ray emitter to the patient, as well as the aperture cone and the image plate size must be taken into account for the correct determination of the dose.

It is an object of the present invention to provide a method for dose determination that at least partially eliminates the problems mentioned and, in particular, can be implemented subsequently without structural changes in existing X-ray devices, but in particular for flexible X-ray emitters.

This task is solved by a method according to independent claim 1. The method according to the invention for determining the dose includes raw data of a digital X-ray image that was created using an image receiver. Determining gray values that correspond to minimum X-ray absorption and/or from the raw data of a digital X-ray image that was created using an image receiver. Additional determination of gray values of one or more areas with expected X-ray absorption values; Calculating a dose based on gray values using calibration.

The method according to the invention therefore provides for an analysis of the raw data of a digital X-ray image and, using calibration, enables the dose reached to the image receiver to be calculated on the basis of this analysis.

An image receiver within the meaning of the present application can be a storage film that latently stores an X-ray image in a phosphor layer. The latent image is read out using photoluminescence using a scanner as a digital image generating device. Alternatively, the image receiver can be an electronic detector, such as a solid-state detector or another storage medium, which generates an X-ray image directly or indirectly using an intermediate device, such as a screen or a PC.

Raw data refers to the data of a digital x-ray image that is largely unprocessed immediately after the physical creation process of the image are present. In particular, no scaling or other further processing of the intensity values of the image material must have taken place.

In the present case, gray values are understood to mean the part of the raw data which, for each image pixel, contains the X-ray absorption that occurred at this image pixel as an intensity value or

represents gray value. The gray value corresponds to a measured value from the image receiver. In the case of imaging plates, this measured value is obtained when the imaging plate is read out by the output signal of the photomultiplier, which amplifies the weak photoluminescence light and outputs it as counting pulses (“counts”). These “counts” can be represented as gray values.

It is a finding of the invention that the gray values of the digital X-ray image can be understood as dose values, i.e. as a unit of measurement of the absorbed dose caused by ionizing radiation. A gray value of a single pixel therefore describes the energy absorbed per mass. It can accordingly serve as a measure for determining the dose that was deposited in the image receiver and thus ultimately also for the exposure of the person being treated or examined (patient dose).

The method according to the invention provides for analysis of the raw data in two ways. According to a first alternative, the gray values of the raw data are analyzed and those gray values are determined which correspond to a minimum X-ray absorption in the examined structure and, conversely, a maximum absorption in the image receiver. Depending on the type of definition, it can be the lightest or the darkest gray values. In the present case, it is assumed that the gray value decreases as the X-ray absorption increases, i.e. the lightest gray values have experienced the least X-ray absorption in the structure being examined. For example, displaying it in a histogram can be helpful in this analysis.

Alternatively or in addition to this, the raw data is analyzed with regard to the image content, so that one or more areas with expected X-ray absorption are determined. These can, for example, be the areas already mentioned with minimal X-ray absorption. In the x-ray image, for example, these are areas where Only thin soft tissue has been irradiated or areas in which no significant X-ray absorption has evidently taken place. Additionally or alternatively, those areas can also be analyzed that are subject to significant X-ray absorption, but for which the X-ray absorption characteristics are known. For example, artificial structures used in dentistry, which are used to hold the image receiver and are clearly visible in the x-ray image, can be useful for this purpose due to their known nature and dimensions.

After determining the relevant gray values either by analyzing all gray values with regard to their frequency in order to identify those that correspond to minimal X-ray absorption or/and by analyzing the image content in order to determine gray values with expected X-ray absorption, a calibration can be applied to these identified gray values in order to calculate the x-ray dose that occurred to the patient. The calibration assigns a corresponding dose to each gray value that appears in the X-ray image. The calibration takes into account, among other things, the image receiver's conversion characteristics of X-ray absorption into an intensity value or gray value.

In this context, it is preferred if the gray values of one or more areas with expected X-ray absorption are determined on the basis of the gray values which correspond to a minimal X-ray absorption. If, in a first step, those gray values are identified for which minimal x-ray absorption can be seen in the raw data during the x-ray process, these can be identified in the second step as areas in the digital x-ray image. This allows for subsequent corrections if necessary and thus offers greater accuracy when calculating the dose.

It can advantageously be provided that the determination of the gray values of one or more areas with expected X-ray absorption takes into account the homogeneity of the raw data within the respective area. The aim is to ensure that the homogeneity of the gray values is as high as possible in the areas that are to be used to determine the dose. Conversely, a high variation in the gray values indicates a low homogeneity, indicating a range that is less suitable for calculating the dose. However, if the gray values of the raw data are sufficiently homogeneous, all gray values of the specific areas can be taken into account when calculating the dose. An average value is advantageously formed across all gray values of the areas. Alternatively or additionally, median or other statistical parameters such as variance, standard deviation, skewness, excess, etc. can be used as a measure for evaluation.

The calibration advantageously includes calibrating the X-ray image generation chain. In particular, a factory-provided calibration can also be used.

In a preferred embodiment, the X-ray image generation chain includes an X-ray source, the image receiver and a digital image generation device for generating the raw data of the digital X-ray image. As already mentioned, the image receiver can be an imaging plate or a solid-state sensor. In the case of an imaging plate, the digital image generating device is, for example, the scanner, which reads the latent X-ray image from the imaging plate. Electronic detectors such as solid-state detectors can, for example, be the evaluation electronics.

A calibration phantom is preferably used for calibration. The calibration phantom can preferably be designed in such a way that it has suitable absorption materials with correspondingly suitable material thicknesses for the maximum X-ray absorption or the expected X-ray absorption.

In a preferred embodiment, the calibration includes a calibration factor and/or a calibration characteristic curve. If there is a linear relationship between the X-ray absorption and the gray value, the calibration characteristic curve is a straight line. If the image receiver outputs a gray value of 0 in the absence of X-ray absorption, the calibration characteristic curve can be represented as a calibration factor. In a preferred embodiment, the calibration factor and/or the calibration characteristic curve take into account a resolution of the X-ray image receiver, a sensitivity of the X-ray image receiver and/or the nature of the calibration phantom. After calibration, a change in the X-ray image generation chain can be reflected in the calibration characteristic curve or the calibration factor without having to be calibrated again. For example, a change in the resolution or sensitivity of the digital image creation device can be taken into account in the calibration factor.

It is particularly preferred if the calculated dose corresponds to the exposure of a person being examined or treated. This means that the requirements of the Radiation Protection Ordinance can be met without having to create complex data transmission options from an X-ray machine.

The task is also solved by a method for calibrating an X-ray image receiver. The procedure includes the steps:

irradiating an x-ray image receiver with a known dose; Reading out the raw data from the X-ray image receiver and creating a digital X-ray image; Assigning the gray values of the X-ray image to the dose. This means that the associated dose can be taken from the X-ray image with the calibration.

Advantageously, in one embodiment, irradiating the X-ray image receiver includes irradiating a calibration phantom. This enables a particularly precise determination of the calibration.

Assigning the gray values preferably includes determining a homogeneous area of the x-ray image.

The invention can advantageously be implemented as a computer program or computer program product, in particular on a scanner or an image output unit. Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. Show in these:

Figure 1A shows a schematically illustrated measurement setup for an embodiment of a calibration method;

Figures 1 B-1 E Diagrams explaining the calibration process; and

Figures 2A-2D diagrams to explain an embodiment of a dose determination method.

Figure 1 illustrates in a highly schematic representation a measurement setup for calibrating an X-ray image generation chain. The X-ray image generation chain includes an X-ray emitter 10, an X-ray image receiver 12 and a digital image generation device 14. In the embodiment shown in FIG. 1A, a storage plate 13 is provided as the image receiver and a so-called scanner 15 is provided as the digital image generation device. Alternatively, for example, the image receiver 12 could also be an electronic detector/sensor, for example solid-state based, with corresponding evaluation electronics as a digital image generation device 14.

For the calibration process, the X-ray beam 16 emanating from the X-ray emitter 10 is limited by means of an aperture 18 so that it completely covers the X-ray image receiver 12. A calibration exposure is then carried out. Before, during or after the calibration exposure, the dose actually delivered by the X-ray emitter 10 can be measured by means of a dose measuring device 20, which is irradiated instead of the X-ray image receiver 12 or simultaneously with it. At the same time or alternatively, the dose emitted by the X-ray emitter 10 can be determined using the X-ray parameters set on the X-ray emitter 10, namely the X-ray voltage, the X-ray current and the exposure time.

Furthermore, when determining the calibration, it can be advantageous if an absorber is in the beam path between the X-ray emitter 10 and the X-ray image receiver 12 22, for example in the form of a calibration phantom 23, is arranged. Depending on its nature, the absorber 22 can have locally different absorption coefficients and thus effect recording with different but defined doses in the X-ray image receiver 12. In addition, the absorption through the absorber 22 can be adapted to the real recording conditions and thus bring about the calibration close to the later operating point. This enables an exact calibration and thus also a subsequent exact calculation of the dose at the image receiver 12.

Advantageously, an insert for a dose measuring device can be provided in the calibration phantom 23, which can carry out a dose measurement during the irradiation.

Particularly preferably, the calibration can take place at the factory or subsequently for products that have already been delivered.

Carrying out a calibration is described below with reference to FIG. 1E.

Based on extensive measurements, it was determined that the two technologies, imaging plate and electronic detector, show a linear relationship between the dose delivered to an image point and the corresponding gray value displayed in the raw data of the X-ray image. It would also be possible to create a characteristic curve for a non-linear relationship. For this, several calibration points would have to be created.

Assuming that a gray value of zero is displayed in the case of no X-ray absorption in the examined structure, the calibration or the calibration characteristic curve can be represented as a calibration factor.

The calibration factor is as follows:

_{Ap =} PosiSKal

Gray value _Kai This calibration factor can be adapted to different circumstances of the X-ray process or the generation of the X-ray image.

The exemplary embodiment shown in FIG. 1 A uses imaging plate technology to create the x-ray image, in which the image latently stored in the imaging plate 13 is read out using the scanner 15. The imaging plate 13 can be read out on the scanner 15 with various parameters that can be included in the calibration factor. If one or more of the parameters are changed, the following formulas can be used individually or in combination to adjust the calibration factor accordingly to the changed parameters: r „ „ P Cor — 1 + 0.125

The size LP _Kor is a correction factor that takes into account the reading of imaging plates with different resolutions (line pairs). LP _Kor represents a correction factor which, at usual resolutions of 5-50, particularly preferably 10-40, particularly preferably 20-30 line pairs per millimeter, takes into account the fact that when reading out the imaging plate with a higher resolution, there is a lower light yield at the same dose results.

The HV _Kor factor takes into account the fact that when reading out the latent image in the imaging plate, one has to work with very small amounts of light and therefore photomultipliers are used. These are operated with a voltage of HV. Changing this voltage has a major impact on the resulting gray value of the X-ray image in the raw data. The exponent X represents the typical design of the photomultiplier that is used in the scanner 15 of the exemplary embodiment. The exponent can vary according to the design of the scanner 15, which was marked by X. A typical value for If a phantom is used during calibration, the influence of the calibration phantom can be represented using a phantom factor PF. Overall, this results in the following calibration factor for a single image:

KF _BUd = AP * LP _Kor * HV _Kor * PF

This calibration factor KF _B iid can be stored in the header data of an X-ray image and enables the dose to be determined accordingly.

Figures 1B-1D illustrate the calibration process. Figure 1 B shows an X-ray image 30 of an exposed imaging plate 13 obtained with a scanner 15, whereby no calibration phantom 23 was used. The numbers along the X and Y axes shown in Figures 1B and 1C represent a coordinate system for identifying individual pixels.

The X-ray image 30 shown in FIG. 1 B shows the essentially unprocessed raw data of the X-ray image as gray values. A homogeneous area 32 is determined in the x-ray image 30, in which the gray values shown are subject to slight fluctuations. The selected homogeneous area 32 is shown enlarged as FIG. 1C.

Figure 1D illustrates the gray value distribution of the homogeneous area 32 as a histogram. The gray value of a pixel is plotted on the x-axis, i.e. the abscissa. The ordinate, i.e. the Y-axis, shows the associated number of pixels, here normalized to a distribution with the value 1.

As can be seen from Figure 1D, the gray values show a Gaussian distribution. Accordingly, the mean value of the Gaussian distribution is determined. The dose measured when the X-ray image 30 was recorded can then be assigned to this.

Figure 1 E illustrates an alternative embodiment in which a calibration phantom 23 was used when taking the x-ray image 34, as shown in Figure 1. The calibration phantom 23 can simultaneously serve as an acceptance phantom during the initial acceptance of an X-ray device and shows various structures that are used in the Fluoroscopy have different absorption coefficients. Accordingly, three areas 36, 38, 40 are shown in FIG. 1 E, where different material thicknesses of the acceptance or calibration phantom 23 are present and which can therefore be used for calibration.

Figures 2A-2D illustrate a method for determining the dose of a digital x-ray image 50. Figure 2A illustrates the x-ray image 50 in its raw data directly after its creation. The black borders that appear when reading out an imaging plate have already been cut off using masking. Before further processing and analysis of the image 50, Gaussian filtering can be carried out. This can reduce image noise and is equivalent to smoothing with a low-pass filter. This means that the algorithms for determining the dose-relevant pixels are more robust.

FIG. 2B shows a histogram of the gray values of the X-ray image 50 shown in FIG. 2A. For determining the dose, it is of interest to identify the gray values with the lowest X-ray absorption in the image receiver. It can be assumed that they reached the image receiver largely unweakened and therefore provide a good measure for the dose calculation.

For example, a certain percentage of the histogram can be selected, for example the 10% of the gray values with the highest gray value. Alternatively, peak detection and analysis or another selection of pixels with the desired gray value can also be carried out. This is shown as field 52 in Figure 2B. After determining the areas for the dose-relevant gray values, the determined area mask is applied again to the raw data image in order to be able to use the pixel values of the raw data for dose determination.

In principle, the dose can be calculated simply from the selection of these pixels. In order to improve the calculation result, a further analysis of the dose-relevant pixels can be carried out, namely in the pictorial representation. This is shown in Figure 2C. With The brightest 10% of the pixels selected in the step described with reference to FIG. 2B are shown in FIG. 2C as contiguous areas 54, 56.

Additional logical security precautions can be taken here. For example, it can be checked that a minimum number of pixels is used for evaluation. Alternatively or additionally, the boundary of the relevant areas can be eroded, i.e. relocated to the interior of the area so as not to use edge areas, for example in a tooth structure or in dense tissue, for the calculation.

After this step, the number of dose-relevant pixels and the associated gray values are determined. This is shown in Figure 2D. Now the edge clipping (masking) and Gaussian filtering can be canceled again in order to use the actual raw data for calculating the dose. The mean value is now calculated according to the following formula and multiplied by the calibration factor to calculate the dose of the X-ray image:

The method for determining the dose is based on the assumption that in real patient images there are image components in which the incident radiation is only weakened by soft tissue - in the dental area, for example the cheeks. Experiments have shown that image areas in real images that only show thin cheek tissue without jaw bones, teeth, implants, bite plates or other highly absorbent or thick materials are particularly suitable for dose evaluation because they are homogeneous areas in the image receiver show. Through suitable calibration, the soft tissue factor can be recalculated accordingly and dose values can be calculated that are approximately equivalent to unshielded radiation.

While in the calibration method of Figures 1A-1D a central area was selected for the calibration, in a further development of the invention the image could The receiver must be calibrated specifically across the entire receiver area in order to compensate for local differences in sensitivity. The calibration factors stored for the respective dose-relevant ranges could then be used when determining the dose. When determining the dose, care should be taken to ensure that there is no saturation at the image receiver when the radiation is absorbed in the image receiver.

Claims

PATENT CLAIMS

1. Method for determining the dose, comprising:

Determining, from the raw data of a digital X-ray image that was created using an image receiver, gray values that correspond to a maximum X-ray absorption and/or

Determining, from the raw data of a digital X-ray image created using an image receiver, gray values of one or more areas of expected X-ray absorption;

Calculating a dose based on gray values using calibration.

2. The method according to claim 1, wherein the gray values of one or more areas with expected X-ray absorption are determined based on the gray values which correspond to a minimum X-ray absorption.

3. The method according to claim 1 -2, wherein determining the gray values of one or more areas with expected X-ray absorption takes into account the homogeneity of the raw data within the respective area.

4. The method according to claim 3, wherein all gray values of the specific areas are taken into account when calculating the dose.

5. The method according to any one of claims 1-3, wherein the calibration comprises calibrating the X-ray image generation chain.

6. The method according to claim 5, wherein the X-ray image generation chain comprises an X-ray source, the X-ray image receiver and a digital image generation device for generating the raw data of the digital X-ray image.

7. The method according to any one of claims 1-6, wherein a calibration phantom is used for the calibration.

8. The method according to any one of claims 1 -7, wherein the calibration comprises a calibration factor and/or a calibration characteristic curve.

9. The method according to claim 8, wherein the calibration factor and/or the calibration characteristic curve takes into account a resolution of the X-ray image receiver, a sensitivity of the X-ray image receiver and/or the nature of the calibration phantom.

10. The method according to any one of claims 1-9, wherein the calculated dose corresponds to the exposure of an examined or treated person.

11. Procedure for calibrating an X-ray image receiver, with the steps:

irradiating an x-ray image receiver with a known dose;

Reading out the raw data from the X-ray image receiver and creating a digital X-ray image;

Assigning the gray values of the X-ray image to the dose.

12. The method according to claim 11, wherein irradiating the X-ray image receiver comprises irradiating a calibration phantom.

13. The method according to claim 11 or 12, wherein assigning the gray values includes determining a homogeneous area of the x-ray image.

14. Computer program product, comprising commands which, when the program is executed by a computer, cause it to carry out the method according to one of claims 1-10.

15. Computer program product comprising instructions which, when the program is executed by a computer, cause it to carry out the steps:

Assigning the gray values of the X-ray image to a dose.

16. Computer program product according to claim 15, wherein assigning the gray values includes in particular determining a homogeneous area of the x-ray image.

17. Scanner, comprising means for carrying out the method according to one of claims 1-10.

18. Scanner, comprising means for executing the computer program product according to one of claims 15-16.