CN111097106B

CN111097106B - System and method for determining dose area product

Info

Publication number: CN111097106B
Application number: CN201811252130.9A
Authority: CN
Inventors: 孙雷; 陈鸣之; 张欣宇
Original assignee: RAYCO (SHANGHAI) MEDICAL PRODUCTS CO LTD
Current assignee: RAYCO (SHANGHAI) MEDICAL PRODUCTS CO LTD
Priority date: 2018-10-25
Filing date: 2018-10-25
Publication date: 2023-06-02
Anticipated expiration: 2038-10-25
Also published as: CN111097106A

Abstract

A method of determining a dose area product of an radiography system, comprising: defining a dose distribution function in a reference plane according to the X-ray tube type; calibrating the X-ray dose function to the radiography system under different exposure conditions; the dose area product is calculated based on the dose distribution function, the dose function, and the exposure parameters of the radiography system. The method can improve the DAP calculation accuracy under the low exposure condition and is beneficial to eliminating calculation errors caused by the non-uniformity of the X-ray field.

Description

System and method for determining dose area product

Technical Field

The present invention relates to the field of digital radiography and, more particularly, to a method of determining a dose area product.

Background

Dose area product (Dose Area product, DAP) meters are typically mounted on digital radiography systems for monitoring the radiation dose applied to a patient.

It is desirable for the skilled person to reduce or decrease the amount of X-ray radiation to which the patient is subjected during imaging, as overexposure of the patient to X-ray radiation may cause some potentially pathogenic effect. In order to determine the amount of X-ray radiation to which a patient is exposed, some systems include a device that can be used to measure the dose of X-ray radiation received by the patient.

Due to a number of factors, it is often difficult to determine the actual X-ray radiation dose to which a patient is subjected. Thus, instead of measuring the actual dose of X-rays to which the patient is subjected, another parameter, the Dose Area Product (DAP), is typically measured. DAP is a measure of the surface dose, which is obtained by multiplying the dose at a given distance by the area irradiated, and thus DAP is a measure of the X-ray radiation emitted, not the dose absorbed by the patient. DAP is considered by the practitioner to approximate the dose received by the patient.

The DAP is a useful diagnostic means, and when a patient receives a given radiography program, the DAP serves as an indication signal of the radiation dose received by the patient, so that a doctor can monitor and adjust the radiation dose while ensuring the image quality. DAP eliminates many of the uncertainties inherent in determining the actual skin dose of a patient. For example, the cumulative radiation dose at a surface is inversely proportional to the square of the distance between the surface and the X-ray source. In addition, the area being imaged is proportional to the square of the distance between the surface and the X-ray source. Thus, determining the DAP (i.e., the product of dose and area) yields a parameter that is independent of the X-ray source to imaging surface distance. The DAP provides an available figure of merit to evaluate the patient's received X-ray radiation at a distance from the emitter.

In the prior art, some radiography systems have acquired the necessary data to determine the DAP, for example, by inserting a measurement device in the vicinity of the X-ray source and exposing the measurement device to the X-ray source during imaging. Such measurement devices are typically gas-filled ionization chambers and associated electronics designed to produce dose measurements or DAP measurements. The placement of the ionization chamber near and directly exposed to the X-ray source is the only reasonable option, and therefore the ionization chamber must be larger than the entire X-ray source but not be able to shield the patient. Since the DAP is independent of distance to the X-ray emitter, the DAP measured by the instrument gives the DAP value at any location along the X-ray beam trajectory, including the location of the patient's skin surface.

However, where the ionization chamber is located (closer to the X-ray source), the X-ray beam typically includes off-axis scatter components that are not applied to the patient. In addition, the patient may generate secondary back-scattered X-rays. Additional X-ray radiation from off-axis scattered or backscattered X-rays may contribute to misleading DAP measurements. In addition, the erroneous scatter values may vary considerably and more complex with different conditions (e.g. X-ray kV settings and others).

Ionization chambers and associated instrumentation are also quite expensive. The overhead incurred by the ionization chamber is multiplied because the ionization chamber must be located in each radiography system. For various reasons, the use of an ionization chamber to determine the DAP during imaging is not a preferred approach. In addition, considerable system downtime is also required to service the ionization chamber. The ionization chamber and associated instrumentation need to be often recalibrated. In particular in the case of large systems employing mobile radiography systems rather than fixed space, various costs are worth careful consideration.

Thus, there is a need for a system that: which is capable of accurately calculating DAPs without incurring the additional overhead of additional camera system components.

Disclosure of Invention

The invention aims to provide a method for accurately calculating DAP.

In order to achieve the above object, the present invention provides the following technical solutions. A method of determining a dose area product of an radiography system comprising the steps of: defining a dose distribution function in a reference plane according to the X-ray tube type; calibrating a dose function of the X-rays to the radiography system under different exposure conditions; the dose area product is calculated based on the dose distribution function, the dose function, and the exposure parameters of the radiography system.

The method for calculating the DAP provided by the invention does not generate unnecessary hardware cost, does not additionally filter X-rays, and is not easy to influence by the surrounding environment. Second, the DAP calculation accuracy under low exposure conditions is significantly improved. This DAP calculation method is advantageous in eliminating calculation errors due to non-uniformity of the X-ray field.

Drawings

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of embodiments of the invention, as illustrated in the accompanying drawings.

FIG. 1 illustrates a flow chart of a method of determining a dose area product of an radiography system in accordance with one embodiment of the present invention.

Fig. 2 shows an radiography system according to an embodiment of the invention.

Fig. 3 shows a medical diagnostic apparatus according to an embodiment of the present invention.

Detailed Description

In the following description, specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that embodiments of the invention can be practiced without these specific details. In the present invention, specific numerical references such as "first element", "second device", etc. may be made. However, a specific numerical reference should not be construed as necessarily subject to its literal order, but rather as a "first element" distinct from a "second element".

The particular details presented herein are exemplary only and the particular details may vary and yet fall within the spirit and scope of the present invention. The term "coupled" is defined as either directly connected to an element or indirectly connected to an element via another element. Furthermore, the terms "about," "substantially," and the like, as used herein for any numerical value or range, mean a suitable tolerance without affecting the practice of the present invention.

Although the embodiments are described with respect to a single combination of elements, it is to be understood that the invention includes all possible combinations of the disclosed elements. Thus, if one embodiment includes elements A, B and C, while a second embodiment includes elements B and D, the present invention should also be considered to include other remaining combinations of A, B, C or D, even if not explicitly disclosed.

The present disclosure describes a method of computing a DAP rather than using a DAP meter to determine the DAP.

The method of the present invention is applicable to all digital radiography systems having an automated beam splitter.

According to the method of the present invention, the dose function meets both high exposure conditions as well as low exposure conditions, and thus, DAP accuracy under low exposure conditions is improved, as will be described in more detail below.

According to the method of the present invention, the DAP is calculated from a dose distribution function, which may facilitate reducing or eliminating errors due to X-ray field non-uniformities.

The Dose Area Product (DAP) is a quantitative indicator for evaluating the radiation risk resulting from diagnostic X-ray examinations and interventions. Which is defined as the absorbed dose multiplied by the illuminated area in gray-centimeter squared (gy cm ² )。

In the case of permanently mounting the DAP meter to the X-ray apparatus, the DAP is considered by some to have the advantage of being easier to measure. Because as the X-ray beam emanates from the "point source" diverges, the illuminated area a increases with the square of the distance d (distance from the point source): a is ≡d ² While the radiation intensity I decreases according to the square of the distance: i.alpha.1/d ² . Thus, the product of intensity and area, and the DAP value obtained therefrom, is independent of distance from the X-ray source. For example, an X-ray field of 5cm X5 cm will produce 25mGy.cm with an entrance dose of 1mGy ² DAP value of (C). When the spot was increased to 10cm x 10cm with the same inlet dose, DAP was increased to 100mgy.cm ² This is 4 times the previous value.

In commercially available DR systems, the DAP is measured directly by a DAP meter (ionization chamber and measurement assembly) that is placed outside the X-ray beam illuminator and intercepts the entire X-ray beam.

Manufacturers of DAP meters typically calibrate them based on the absorbed dose of air. The DAP reflects not only the dose within the radiation field, but also the area of the tissue being irradiated. Thus, DAP reflects a greater overall risk of carcinogenesis than does the dose in the radiation field.

For a DAP meter, the practitioner has the following considerations:

1. DAP meters increase the cost of DR systems;

2. the DAP meter also adds additional filtering to the X-ray beam;

3. the accuracy of DAP meters is susceptible to ambient conditions (air pressure, temperature, and humidity).

Some DAP calculation methods in the prior art have the following disadvantages:

1. the dose function is calibrated only under one exposure condition, which reduces the accuracy of DAP calculation under low exposure conditions;

2. DAP calculations do not take into account the dose distribution, and thus non-uniformities in the X-ray field can introduce calculation errors.

According to an embodiment of the present invention, there is provided a method of determining a DAP for an X-ray radiography system, as shown in FIG. 1, comprising the steps of S10-S12-S14:

step S10, defining a dose distribution function in a reference plane according to the X-ray tube type.

Step S12, calibrating the dose function of the X-rays to the radiography system under different exposure conditions.

Step S14, calculating a dose area product based on the dose distribution function, the dose function and the exposure parameters of the radiography system.

In summary, a dose distribution function and a dose function are first established with respect to exposure parameters. The dose distribution function is predetermined for a particular X-ray tube type. The dose function will be calibrated to the particular radiography system under high exposure and low exposure conditions. In clinical applications, DAP is calculated by applying exposure parameters to a dose distribution function and a dose function.

The invention can at least realize the following technical effects: (1) The DAP can be calculated by software without unnecessary hardware cost, additional filtering, and the DAP calculation is not susceptible to the surrounding environment. (2) The dose functions are adapted under high exposure and low exposure conditions, respectively, so that the DAP accuracy under low exposure conditions is significantly better than in the prior art. (3) The DAP is calculated based on a dose distribution function, which eliminates errors due to non-uniformity of the X-ray field.

The following is a detailed description of embodiments of the invention made with reference to the accompanying drawings. Wherein like reference numerals identify like structural elements in each of the several figures.

Fig. 2 shows an radiography system, which can be used for DAP calculation, comprising an X-ray tube, an X-ray generator, an automatic beam splitter, and a control unit. An X-ray tube is coupled to the generator as an X-ray generating device for exposing the irradiated object to X-rays. The X-ray generator acts as a high voltage generating means for providing the high voltage required by the X-ray tube, which is connected to and controlled by the control unit. The automatic beam splitter is also connected to the control unit and can be controlled remotely by it. The control unit is typically a computer or processor on which DAP calculation software or instructions can be run.

The system calculates the DAP from the exposure parameters. The specific calculation process is as follows.

Since the DAP is independent of distance from the X-ray source, the system calculates the DAP on a freely selected reference plane that is substantially perpendicular to the central X-ray beam. As an example, the reference plane is 100cm from the X-ray source (the position of the X-ray tube focus).

DAP is calculated based on the following equation (1):

DAP(kV,mAs,area)＝k(area)·dose(kV,mAs)·area (1)

where k (area) is a factor related to the dose distribution of the irradiated area in the reference plane, area is the area of the irradiated area, and dose (kV, mAs) is the dose function at a freely chosen reference point in the reference plane. As an example, the reference point is the intersection of the reference plane with the central ray beam.

k (area) is a unitless factor calculated according to the following equation (2):

wherein, the dose _average Is the average dose of the irradiated region, dose _ref Is the dose at the reference point, dose (w, l) is the dose distribution at the coordinates (w, l) in the reference plane, (w) ₀ ,l ₀ ) Is the coordinates of the reference point. The dose distribution function depends on the type of tube and is almost independent of the exposure parameters.

The dose distribution function is then a predefined function, depending on the type of tube installed in the system. As an example, the dose distribution function may be defined from gray values of an X-ray image, which is taken by a digital Flat Panel Detector (FPD). The definition here is because the gray values in the X-ray image are proportional to the dose at the location. For a given X-ray tube type, in the laboratory, under appropriate conditions (e.g. no overexposure, 70kv,3.2 mas), an image of the largest irradiated region in the reference plane is taken.

After determining the coordinates and the reference point positions, the gray values of the image may be converted into a gray distribution function, which may in turn be used as a dose distribution function in the reference plane. Equation (2) above may be rewritten as equation (3):

wherein, grey _average Is the average gray scale of the irradiated region, grey _ref Is the gray at the reference point, grey (w, l) is the gray distribution at the coordinates (w, l) in the reference plane, (w) ₀ ,l ₀ ) Is the coordinates of the reference point.

The dose function dose (kV, mAs) is a parameter that is the product (mAs) of the tube voltage kV, the tube current mA, and the exposure time s. The dose function is pre-calibrated to the specific radiography system under high exposure and low exposure conditions. Typically, a radiation dosimeter is used to calibrate the dose function. As an example, the calibration procedure comprises the following steps:

1) Adjusting the geometric alignment of the camera system;

2) Placing a dosimeter in the center of a reference plane;

3) The irradiated area is limited to, for example, an area of 25 x 25 cm;

4) Selecting a focus and an appropriate filter type;

5) Performing a plurality of exposures, for example, 10 times;

6) Inputting the readings of the radiation dosimeter into a control unit to fit a dose function;

7) Repeating steps 4) -6) until all the focuses and filters are selected.

According to some embodiments of the invention, the high exposure conditions are: the voltages of the ray tube are 40, 70, 95, 120 and 150kV, the current of the ray tube is 200mA, and the exposure time is 0.05s; the low exposure conditions were: the tube voltages were 40, 70, 95, 120, 150kV, the tube currents were 200mA, and the exposure time was 0.005s.

According to some embodiments of the invention, the dose function is linear with the square of the voltage and linear with the product of the current and the exposure time. The dose function is determined by fitting the measured data, as follows.

Step one, fitting dose (kV) under high exposure and low exposure conditions respectively.

Specifically, the dose (kV) is fitted by using a weighted least squares method. The weighting coefficient w is obtained by the following equation (4):

wherein, the dose _measured (kV) is the reading of the radiation dosimeter. The relative accuracy at low voltage kV can be improved by the weighting coefficients. At the end of step one, two dose functions can be obtained, as shown in equation (5):

wherein, the dose _high (kV) represents the dose function under high exposure conditions, dose _low (kV) represents the dose function at low exposure conditions. a, a _high 、b _high 、c _high A is a calculation factor obtained by a least square method under high exposure condition _low 、b _low 、c _low Is a calculation factor under low exposure conditions.

And step two, determining the dose (kV, mAs).

Specifically, instead of using one point, two points are used to determine the dose (mAs). This is because, although the dose (mAs) is a linear function, the intercept of the linear function is not trivial at low mAs. By using two points for linear operation, the dose function is finally written as:

wherein mAs _high And mAs _low mAs values at high exposure and low exposure conditions, respectively.

It should be noted that the dose function needs to be calibrated to a specific focus type and filter type. The correct dose function determined from a given focus and filter type should be used in DAP calculations to improve the accuracy of the calculations.

In equation (1) above there is also a parameter area, which is the area of the irradiated area in the reference plane. As an example, the parameter area is calculated from the aperture size of the beam splitter. For each exposure, the aperture size of the beam splitter can be read from the automatic beam splitter. Further, the parameter area is calculated according to the following equation (7):

where r is the distance from the X-ray source to the reference plane, e.g. 100cm, area _aperture Is the aperture size of the beam splitter, r _aperture Is the distance from the X-ray source to the aperture, which is fixed and known.

In general, the process of computing the DAP can be performed in three steps:

step A, defining a dose distribution function for a given ray tube type;

step B, calibrating a dose function to a specific radiography system;

step C, for each exposure, calculating the DAP according to equations (1), (3), (6) and (7) above.

According to some embodiments of the invention, step a is performed once for each tube type. Step B is performed periodically for a particular radiography system. Step C is performed on the control unit (or other processing unit) for each exposure.

It should be understood that although the above embodiment illustrates 3 steps, after reading the description of the present invention, those skilled in the art will be able to combine, omit, simply modify or perform the steps described above in other suitable order without affecting the technical effect of the present invention and all fall within the scope of the present invention.

By adopting the method provided by the invention, the dose function is fitted under the conditions of high exposure and low exposure respectively, so that the DAP precision under the condition of low exposure is improved; meanwhile, DAP is calculated based on a dose distribution function, which eliminates errors due to non-uniformity of the X-ray field.

Fig. 3 shows a medical diagnostic apparatus that can be used for DAP calculation, allowing a physician to monitor and adjust radiation dose while maintaining image quality. Which comprises an X-ray generating device, a beam splitter and a control unit (not shown in the figures). The control unit is respectively coupled with the X-ray generating device and the beam splitter, so that the X-ray generating device can be controlled to expose under different exposure conditions, and the beam splitter can be controlled to adjust the size (area) of the X-ray radiation area. The control unit may be integrated with the X-ray generating device or separate from the X-ray generating device and the beam splitter as a separate unit. The X-ray generating device may include an X-ray generator and an X-ray tube, the X-ray generator supplying high voltage to the X-ray tube to generate X-rays.

Fig. 3 shows a distance r1 from the X-ray source to the beam splitter aperture, and r2 from the X-ray source to the reference plane. By combining the aperture size of the beam splitter and the distances r1 and r2, one skilled in the art can calculate the parameters in equation (7) and thus calculate the area of the irradiated region of the reference plane. Further, in combination with the defined dose distribution function for a given tube type and the dose function of the radiography system, the control unit can directly calculate the DAP to help the healthcare staff obtain diagnostic data about the patient while controlling the X-ray radiation dose such that it does not cause harm to the patient.

According to some embodiments of the present invention, a computer-readable storage medium is provided having stored thereon a set of machine-executable instructions which, when executed by a processor, will implement the method of determining a DAP provided by the above embodiments.

According to some embodiments of the present invention, there is further provided a computer control apparatus including a memory and a processor, wherein the memory has stored thereon a computer program, and the processor is capable of implementing the method for determining a DAP provided in the above embodiments when executing the computer program.

The above description is only for the preferred embodiments of the invention and is not intended to limit the scope of the invention. Numerous variations and modifications can be made by those skilled in the art without departing from the spirit of the invention and the appended claims.

Claims

1. A method of determining a dose area product of an radiography system comprising the steps of:

a. defining a dose distribution function in a reference plane according to the X-ray tube type;

b. calibrating a dose function of X-rays to the radiography system under different exposure conditions; and

c. calculating the dose area product based on the dose distribution function, the dose function, and an exposure parameter of the radiography system, wherein the exposure parameter comprises an area of the irradiated region in the reference plane and the dose area product DAP is determined according to the following formula:

DAP(kV，mAs，ctrea)＝k(area)·dose(kV，mAs)·area

where k (area) is a factor related to the dose distribution of the irradiated area in the reference plane, area is the area of the irradiated area, and dose (kV, mAs) is the dose function at the reference point in the reference plane.

2. The method of claim 1, wherein the dose function is linear with the square of the voltage of the X-ray tube and linear with the product of the current and exposure time of the X-ray tube.

3. The method according to claim 1, wherein the dose distribution function is defined in accordance with a gray scale distribution function of the X-ray image.

4. The method of claim 1, wherein the area of the irradiated region is calculated from an aperture size of a beam splitter in the radiography system and a distance from an X-ray source to an aperture.

5. The method of claim 4, wherein the dose distribution function is determined according to the formula:

where k (area) is a factor related to the dose distribution of the irradiated region in the reference plane, grey _average Is the average gray scale of the irradiated region, grey _ref Is the gray at the reference point, grey (w, l) is the gray distribution at the coordinates (w, l) in the reference plane, (w) ₀ ，l ₀ ) Is the coordinates of the reference point.

6. The method of claim 5, wherein the dose function is determined according to the formula:

wherein, the dose function dose (kV, mAs) takes the product (mAs) of the voltage kV of the ray tube, the current mA of the ray tube and the exposure time s as parameters, and the dose _high (kV) represents the dose function under high exposure conditionsCount, dose _low (kV) represents the dose function under low exposure conditions, mAs _high And mAs _low mAS values under high exposure and low exposure conditions, respectively.

7. An radiography system comprising:

an X-ray generating device for emitting X-rays to expose an irradiated object;

a beam splitter for adjusting an area of the irradiated region in the reference plane; and

a control unit coupled with the X-ray generating device and the beam splitter, respectively, for controlling the X-ray generating device to expose under different exposure conditions and controlling the beam splitter to adjust the area of the irradiated region;

wherein the control unit calculates a dose area product based on a dose function of the X-rays, a dose distribution function in a reference plane, and an exposure parameter of the radiography system, wherein the exposure parameter comprises an area of the irradiated region in the reference plane and the dose area product DAP is determined according to the following formula:

DAP(kV，mAs，ctrea)＝k(area)·dose(kV，mAs)·area

8. The system of claim 7, wherein the system periodically calibrates the dose function.

9. The system of claim 7, wherein the control unit reads an aperture size from the beam splitter for calculating an area of the irradiated region.

10. The system of claim 7, wherein the control unit calculates the dose area product for each exposure.

11. The system according to claim 7, wherein the X-ray generating device comprises an X-ray tube and an X-ray generator, the control unit defining the dose distribution function according to the type of the X-ray tube.

12. A computer readable storage medium having stored thereon machine executable instructions which, when executed by a processor, will implement the method of any of the preceding claims 1-6.

13. A computer control device comprising a memory and a processor, wherein the memory has stored thereon a computer program, which when executed by the processor implements the method of any of claims 1-6.

14. A medical diagnostic apparatus comprising the radiography system according to any one of claims 7-11.