CN111097106A - System and method for determining dose-area product - Google Patents
System and method for determining dose-area product Download PDFInfo
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- CN111097106A CN111097106A CN201811252130.9A CN201811252130A CN111097106A CN 111097106 A CN111097106 A CN 111097106A CN 201811252130 A CN201811252130 A CN 201811252130A CN 111097106 A CN111097106 A CN 111097106A
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
- A61N5/00—Radiation therapy
- A61N5/10—X-ray therapy; Gamma-ray therapy; Particle-irradiation therapy
- A61N5/103—Treatment planning systems
- A61N5/1031—Treatment planning systems using a specific method of dose optimization
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
- G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
- G01T1/16—Measuring radiation intensity
- G01T1/161—Applications in the field of nuclear medicine, e.g. in vivo counting
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 an X-ray tube type; calibrating an 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 the calculation error caused by the non-uniformity of the X-ray field.
Description
Technical Field
The present invention relates to the field of digital radiography, and more particularly, to a method for determining a dose-area product.
Background
Dose Area Product (DAP) meters are typically mounted on digital radiography systems for monitoring the radiation Dose delivered to a patient.
Reducing or diminishing the amount of X-ray radiation to which a patient is exposed during imaging is desirable by the skilled person, since overexposure of a patient to X-ray radiation may cause certain potentially pathogenic effects. 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 X-ray radiation dose to which the patient is actually subjected. Therefore, instead of measuring the actual dose of X-rays to which the patient is exposed, another parameter, the Dose Area Product (DAP), is typically measured. The DAP is a measure of the surface dose, which is determined by multiplying the dose at a given distance by the area irradiated, and is thus a measure of the emitted X-ray radiation, not the dose absorbed by the patient. DAP is considered by the practitioner to approximate the dose received by the patient.
DAP is a useful diagnostic tool as an indicator of the radiation dose received by a patient when the patient is subjected to a given radiographic procedure, allowing the physician to monitor and adjust the radiation dose while ensuring image quality. DAP eliminates many of the uncertainties inherent in determining the actual skin dose for a patient. For example, the radiation dose accumulated at a surface is inversely proportional to the square of the distance between the surface and the X-ray source. In addition, the area 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 X-ray radiation received by the patient at a distance from the transmitter.
In the prior art, some radiographic 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. Placing the ionization chamber in close proximity to and directly exposed to the X-ray source is the only reasonable option, and thus, the ionization chamber must be larger than the entire X-ray source but not block the patient. Since the DAP is independent of the distance to the X-ray emitter, the instrumentation of the DAP gives DAP values anywhere along the X-ray beam trajectory, including the position 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 backscatter 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 greatly and more complicatedly depending on 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 placed in each radiography system. For various reasons, using an ionization chamber to determine the DAP during imaging is not the preferred approach. In addition, servicing the ionization chamber also requires significant system downtime. Ionization chambers and associated instrumentation need to be recalibrated often. Various costs are of concern, particularly in the case of mobile radiography systems, rather than large systems of fixed space.
Thus, there is a need for a system that: which is capable of accurately calculating the DAP without incurring the overhead of additional camera system components.
Disclosure of Invention
The invention aims to provide a method for accurately calculating a 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 X-ray radiography system, comprising the steps of: defining a dose distribution function in a reference plane according to an 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 does not generate unnecessary hardware cost, does not filter X rays additionally, and is not easily influenced by the surrounding environment. Secondly, the DAP calculation accuracy under low exposure conditions is significantly improved. This DAP calculation method is advantageous for 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 shows a schematic flow diagram of a method of determining a dose area product of an radiography system according to an embodiment of the invention.
Fig. 2 shows an X-ray radiography system according to an embodiment of the present invention.
Fig. 3 illustrates 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 may be practiced without these specific details. In the present invention, specific numerical references such as "first element", "second device", and the like may be made. However, specific numerical references should not be construed as necessarily subject to their literal order, but rather construed as "first element" as opposed to "second element".
The specific details set forth herein are merely exemplary and may be varied while remaining within the spirit and scope of the invention. The term "coupled" is defined to mean either directly connected to a component or indirectly connected to the component via another component. Further, as used herein, the terms "about" and "substantially" for any numerical value or range indicate that the deviation is properly tolerated without affecting the performance of the invention.
Although 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 invention should also be considered to include A, B, C or the other remaining combinations of D, even if not explicitly disclosed.
The present disclosure describes a method of calculating 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 automatic beam splitter.
According to the method of the present invention, the dose function conforms to high exposure conditions as well as low exposure conditions and, therefore, DAP accuracy under low exposure conditions is improved, as will be described in more detail below.
In accordance with the method of the present invention, the DAP is computed according to a dose distribution function, which may facilitate reducing or eliminating errors due to X-ray field inhomogeneity.
The dose-area product (DAP) is a quantitative indicator for assessing radiation risk resulting from diagnostic X-ray examinations and interventional procedures. Which is defined as the absorbed dose multiplied by the irradiated area in gray-scale-centimeters squared (gy2)。
In the case of permanently mounting the DAP meter to the X-ray device, DAP is considered by some to have the advantage of being easier to measure. Because the area a illuminated increases with the square of the distance d (distance from the point source) with the divergence of the X-ray beam emanating from the "point source": a-2Whereas the radiation intensity I decreases according to the square of the distance: i-2. Thus, the product of intensity and area, and the resulting DAP value, is independent of distance from the X-ray source. For example, an X-ray field of 5cm by 5cm will yield 25mGy cm with an inlet dose of 1mGy2The DAP value of (c). When the field increased to 10cm by 10cm with the same inlet dose, the DAP increased to 100mgy cm2This 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), which is placed outside the X-ray beam illuminator and intercepts the entire X-ray beam.
Manufacturers of DAP meters typically calibrate them for the amount of absorbed dose to air. DAP reflects not only the dose within the radiation field, but also the area of tissue illuminated. Thus, DAP is more reflective of the overall risk of carcinogenesis than is the dose in the field of radiation.
For a DAP meter, the practitioner has the following considerations:
1. DAP meters increase the cost of DR systems;
2. the DAP gauge also adds additional filtering to the X-ray beam;
3. the accuracy of the DAP meter 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 at one exposure condition, which reduces the accuracy of the DAP calculation at low exposure conditions;
2. the DAP calculation does not take into account the dose distribution, and thus non-uniformity of 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 of a radiography system, as shown in fig. 1, including the following steps S10-S12-S14:
step S10, a dose distribution function in the reference plane is defined according to the X-ray tube type.
Step S12, calibrating a dose function of 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 with respect to exposure parameters are first established. The dose distribution function is predetermined for a specific X-ray tube type. The dose function will be calibrated to the particular radiographic system under high and low exposure conditions. In clinical applications, the DAP is calculated by applying exposure parameters to the dose distribution function and the dose function.
The invention can realize at least the following technical effects: (1) the DAP can be computed by software without incurring unnecessary hardware costs, additional filtering, and the DAP computation is not susceptible to the surrounding environment. (2) The dose function is adapted at high and low exposure conditions, respectively, so that the DAP accuracy at 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 X-ray field inhomogeneities.
The following is a detailed description of embodiments of the present invention made with reference to the accompanying drawings. Wherein like reference numerals identify like structural elements in each of the several views.
Fig. 2 shows an radiography system that can be used for DAP calculations, the system comprising an X-ray tube, an X-ray generator, an automatic beam light, and a control unit. The X-ray tube and the generator are coupled to form an X-ray generator, which performs X-ray exposure on an irradiated object. The X-ray generator serves as a high voltage generating device for supplying the high voltage required by the X-ray tube, and is connected to and controlled by the control unit. The automatic beam bunching device is also connected to and remotely controllable by the control unit. The control unit is typically a computer or processor on which the DAP calculation software or instructions can run.
The system calculates the DAP from the exposure parameters. The specific calculation procedure 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 (position of the X-ray tube focus).
The DAP is calculated based on equation (1) below:
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 a 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 and the central ray beam.
k (area) is a unitless factor, calculated as equation (2) below:
therein, doseaverageIs the average dose of the irradiated area, doserefIs the dose at the reference point, dose (w, l) is the dose distribution at the coordinates (w, l) in the reference plane, (w0,l0) Are the coordinates of the reference point. The dose distribution function depends on the tube type 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 the gray values of an X-ray image taken by a digital Flat Panel Detector (FPD). This is defined here because the grey 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, 3.2mAs), an image of the maximum irradiated area in the reference plane is taken.
After determining the coordinates and the reference point positions, the grey values of the image may be converted into a grey distribution function, which may in turn be used as a dose distribution function in the reference plane. Equation (2) above can be rewritten as equation (3):
wherein, greyaverageIs the mean gray of the irradiated area, grayrefIs the gray level at the reference point, grey (w, l) is the gray level distribution at the coordinates (w, l) in the reference plane, (w0,l0) Are the coordinates of the reference point.
The dose function dose (kV, mAs) is a parameter which 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 radiographic system under high and low exposure conditions. Typically, a radiation dosimeter may be 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 the radiation dosimeter in the center of the reference plane;
3) the irradiated area is limited to, for example, an area of 25 × 25 cm;
4) selecting a focal point and an appropriate filter type;
5) multiple exposures are performed, for example, 10 times;
6) inputting the radiation dosimeter readings into a control unit to fit a dose function;
7) repeating the steps 4) -6) for a plurality of times until all the focuses and filters are respectively selected.
According to some embodiments of the invention, the high exposure conditions are: the voltage of the tube is 40, 70, 95, 120 and 150kV, the current of the tube is 200mA, and the exposure time is 0.05 s; the low exposure conditions were: the tube voltage was 40, 70, 95, 120, 150kV, the tube current was 200mA, and the exposure time was 0.005 s.
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 measurement data as follows.
Step one, fitting dose (kV) under the conditions of high exposure and low exposure respectively.
Specifically, dose (kv) is fitted by using a weighted least squares method. The weighting coefficient w is obtained by the following equation (4):
therein, dosemeasured(kV) is the radiation dosimeter reading. The relative accuracy in the case of low voltages kV can be improved by means of the weighting coefficients. At the end of step one, two dose functions can be obtained, as shown in equation (5):
wherein，dosehigh(kV) dose function under high exposure conditions, doselow(kV) represents the dose function under low exposure conditions. a ishigh、bhigh、chighCalculated factor, a, determined for the least-squares method under high-exposure conditionslow、blow、clowIs a calculation factor under low exposure conditions.
And step two, determining dose (kV, mAs).
Specifically, instead of using one point, two points are used to determine dose (mas). This is because although 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 mAshighAnd mAslowThe mAs values under high 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 the DAP calculation to improve the calculation accuracy.
Also in equation (1) above is a parameter area, which is the area of the irradiated region 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, areaapertureIs the aperture size of the beam bunching device, rapertureIs the distance from the X-ray source to the aperture, which is fixed and known.
In general, the process of computing a DAP can proceed in three steps:
step A, defining a dose distribution function aiming at a given type of a ray tube;
step B, calibrating the dose function to a specific radiography system;
step C, for each exposure, the DAP is calculated 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 describes 3 steps, after reading the description of the present invention, the skilled person will be able to combine, omit, simply modify or perform the above steps in other suitable orders without affecting the technical effect of the present invention, and all that falls 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, the DAP is calculated based on a dose distribution function, which eliminates errors due to non-uniformity of the X-ray field.
Fig. 3 illustrates a medical diagnostic apparatus that can be used for DAP calculations, 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 figure). The control unit is coupled with the X-ray generating device and the beam light device respectively, so that the X-ray generating device can be controlled to expose under different exposure conditions, and the beam light device can be controlled to adjust the size (area) of an X-ray radiation area. The control unit may be integral with the X-ray generation device or separate from the X-ray generation device and 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 a high voltage to the X-ray tube to generate X-rays.
The distance r1 shown in FIG. 3 represents the distance from the X-ray source to the beam splitter aperture and r2 represents the distance from the X-ray source to the reference plane. In combination with 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) to determine the area of the irradiated area of the reference plane. Further, in combination with the dose distribution function defined for a given tube type and the dose function of the radiography system, the control unit can directly solve the DAP to help the medical staff to 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 invention, a computer-readable storage medium is provided having stored thereon a collection of machine-executable instructions that, when executed by a processor, will implement the method of determining a DAP provided by the above-described embodiments.
There is also provided, according to some embodiments of the present invention, a computer control apparatus including a memory and a processor, wherein the memory has stored thereon a computer program, and the processor, when executing the computer program, is capable of implementing the method of determining a DAP provided by the above-described embodiments.
The above description is only for the preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Various modifications may be made by those skilled in the art without departing from the spirit of the invention and the appended claims.
Claims (15)
1. A method of determining a dose area product of an X-ray radiography system, comprising the steps of:
a. defining a dose distribution function in a reference plane according to an 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.
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 of the X-ray tube and the exposure time.
3. The method according to claim 1, characterized in that the dose distribution function is defined in terms of a grey scale distribution function of an X-ray image.
4. The method of claim 1, wherein the exposure parameters include an area of an irradiated region in the reference plane.
5. The method of claim 4, wherein the area of the irradiated region is calculated based on the size of an aperture of a beam splitter in the radiography system and the distance from an X-ray source to the aperture.
6. An radiography system, comprising:
an X-ray generating device for emitting X-rays to expose the irradiated object;
a beam splitter for adjusting the area of the irradiated area in the reference plane; and
a control unit coupled to the X-ray generation device and the beam splitter, respectively, for controlling the X-ray generation 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 exposure parameters of the radiography system.
7. The system of claim 6, wherein the system periodically calibrates the dose function.
8. The system of claim 6, wherein the exposure parameters include an area of an irradiated region in the reference plane, the control unit reading an aperture size from the beam laser for calculating the area of the irradiated region.
9. The system of claim 6, wherein the control unit calculates the dose area product for each exposure.
10. The system according to claim 6, 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 a type of the X-ray tube.
11. The system of any of claims 6-10, wherein the system does not include a DAP meter.
12. The system of any one of claims 6-10, wherein the system comprises a mobile radiography system.
13. A computer readable storage medium having stored thereon machine executable instructions which when executed by a processor will implement the method of any of claims 1-5.
14. A computer-controlled apparatus 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 1-5.
15. A medical diagnostic apparatus comprising the radiography system according to any one of claims 6-12.
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