阅读说明：本技术 一种实现ct精确校直的方法 (Method for realizing CT accurate straightening ) 是由 晏雄伟 邢占峰 晓鹤 吴艳荣 司君涛 李万锋 相会财 于 2020-10-20 设计创作，主要内容包括：本发明涉及一种实现CT精确校直的方法,具体说是一种实现CT的球管焦点x-轴位置以及iso通道精确校直的迭代计算方法,在CT校直过程中,根据扫描得到的投影数据,结合探测器的每一个通道具体的光束的方向的角度信息,通过迭代的方法,精确计算出iso通道的位置以及球管焦点在x-轴的位置坐标,进而确定球管焦点位置的调整量,然后对球管焦点进行机械调整,如此反复,使得iso通道达到期望位置,最终实现对CT系统的精确校直。本发明能够计算和实现球管焦点的位置以及iso通道的精确校直。(The invention relates to a method for realizing CT accurate alignment, in particular to an iterative calculation method for realizing CT ball tube focus x-axis position and iso channel accurate alignment. The invention can calculate and realize the position of the bulb focus and the accurate alignment of the iso channel.)

1. A method for realizing CT accurate alignment is characterized in that: which comprises the following steps:

1) scanning the slender metal pin to obtain scanned projection data;

2) assuming that the tube focus is at the desired position;

3) calculating the corresponding iso channel position according to the geometric layout of the CT;

4) according to the geometric layout of the CT, calculating the angle information of the direction of the incident beam of each channel of the detector, namely the included angle between the beam and the y-axis;

5) calculating the gravity center value of the metal projection values of all channels of each view relative to the light beam angle of each channel by using the scanned projection data, wherein the value represents the angle of the light beam direction in which the projection gravity center is positioned;

6) averaging the projection gravity center values under all views to obtain the angle of the light beam direction corresponding to the iso channel；

7) Calculating a new position f of the x-axis of the focal point of the bulb;

8) repeating the steps 3) to 7) until a specified iteration number is reached, or the error of the x-coordinate of the focus of the bulb obtained by two adjacent calculations is less than a specified value, or the error of the value of the iso channel obtained by two adjacent calculations is less than a specified value;

9) calculating the adjustment amount d of the focal position according to the calculated x-coordinate of the focal point of the bulb tube, and mechanically adjusting the focal position of the bulb tube;

10) repeating 1) to 9) until the error between the value of the iso channel obtained by scanning and calculation and the expected value of the design is less than a specified value, and finishing the accurate alignment of the CT.

2. The method for realizing accurate CT alignment according to claim 1, wherein: the iteration number specified in the step 8) is 30, the error of the x-coordinate of the focus of the bulb tube obtained by two adjacent times of calculation is less than 1.0e-4 mm, and the error of the value of the iso channel obtained by two adjacent times of calculation is less than 1.0e-3 channels.

3. The method for realizing accurate CT alignment according to claim 1, wherein: and in the step 10), accurate alignment of the CT is completed until the error between the value of the iso channel obtained by scanning and calculation and the designed expected value is less than 1.0e-3 channels.

4. The method for realizing accurate CT alignment according to claim 1, wherein:

the iso channel and corresponding focal position are calculated according to a conventional method and set as the initial focal position to speed up the iterative calculation process.

5. The method for realizing accurate CT alignment according to claim 1, wherein: and setting the focus position of the bulb tube before mechanical adjustment as an initial focus position to accelerate the iterative calculation process.

6. The method for realizing accurate CT alignment according to claim 1, wherein: the average value of the focal position of the bulb before mechanical adjustment and the expected position is set as the initial focal position to accelerate the iterative calculation process.

Technical Field

The invention relates to a method for realizing CT accurate alignment, in particular to an iterative calculation method for realizing CT accurate alignment of a bulb tube focus x-axis position and an iso channel.

Background

The alignment of CT systems is an important step in the design and manufacturing process of CT systems. During CT straightening, an operator obtains scanning projection data by mechanically adjusting the position of a focus of a bulb tube on an x-axis and scanning an eccentrically placed elongated metal pin with a circular cross section, then obtains the position of an iso channel and the position of the focus by calculation, and repeats the steps until the error between the calculated value of the iso channel and the designed expected value is smaller than a specified value.

When an iso channel is calculated in the traditional method, the channel where the center of gravity of each view metal pin is located based on the number of channels is calculated first, and then averaging is carried out. However, for modern CT with the following characteristics, the conventional calculation method has a large error due to the non-uniform angular distribution of the beam directions of the channels:

1) the device has the function of flying focus scanning, and the focus position deviates from the circular point position of the circular arc of the detector.

2) In some large-caliber CT, the focal point position is about 40-60 cm away from the origin of the detector arc.

3) The CT of the detector composed of multiple modules is configured, and a certain gap exists between the modules.

Disclosure of Invention

In view of the above technical deficiencies, the present invention aims to provide a method for realizing CT precise alignment, which can calculate and realize the position of the focus of the bulb and the precise alignment of the iso-channel.

The technical scheme adopted by the invention for solving the technical problems is as follows: a method for realizing accurate CT alignment comprises the following steps:

1) scanning the slender metal pin to obtain scanned projection data;

2) assuming that the tube focus is at the desired position;

3) calculating the corresponding iso channel position according to the geometric layout of the CT;

4) according to the geometric layout of the CT, calculating the angle information of the direction of the incident beam of each channel of the detector, namely the included angle between the beam and the y-axis;

5) calculating the gravity center value of the metal projection values of all channels of each view relative to the light beam angle of each channel by using the scanned projection data, wherein the value represents the angle of the light beam direction in which the projection gravity center is positioned;

6) averaging the projection gravity center values under all views to obtain the angle of the light beam direction corresponding to the iso channel；

7) Calculating a new position f of the x-axis of the focal point of the bulb;

8) repeating 3) to 7) until a specified iteration number is reached, or the error of the x-coordinate of the focus of the bulb obtained by two adjacent calculations is less than a specified value, or the error of the value of the iso channel obtained by two adjacent calculations is less than a specified value;

9) calculating the adjustment amount d of the focal position according to the calculated x-coordinate of the focal point of the bulb tube, and mechanically adjusting the focal position of the bulb tube;

10) repeating 1) to 9) until the error between the value of the iso channel obtained by scanning and calculation and the expected value of the design is less than a specified value, and finishing the accurate alignment of the CT.

Preferably, the number of iterations specified in the step 8) is 30, the error of the x-coordinate of the focal point of the bulb obtained by two adjacent calculations is less than 1.0e-4 mm, and the error of the value of the iso channel obtained by two adjacent calculations is less than 1.0e-3 channels.

Preferably, in the step 10), the precise alignment of the CT is completed until the error between the value of the iso channel obtained by scanning and calculation and the expected value of the design is less than 1.0e-3 channels.

Preferably, the iso-channel and corresponding focus position are computed according to conventional methods, set to the initial focus position, to speed up the iterative computation process.

Preferably, the focus position of the bulb before mechanical adjustment is set as the initial focus position to speed up the iterative calculation process.

Preferably, the average of the focal position of the bulb before mechanical adjustment and the desired position is set as the initial focal position to speed up the iterative calculation process.

The invention has the following beneficial effects and advantages:

1. the method can be suitable for the traditional CT system and the modern complex CT system, and realizes the accurate alignment of the position of the bulb focus and the iso channel.

2. By utilizing the advantages of the position of the bulb tube focus and the accurate alignment of the iso channel, the method can improve the spatial resolution of the CT system and reduce the frequency aliasing artifact.

Drawings

FIG. 1 is a diagram of the iso channel and the position f and adjustment d of the bulb focus;

fig. 2 is a projected value curve of the metal pin;

fig. 3 is an angle of a channel in a maximum projection value neighborhood of the metal pin and a difference thereof;

FIG. 4 is an iterative computation process of iso channel and focus position;

FIG. 5 is a calculation of the iso channel;

FIG. 6 is a flow chart of CT fine alignment;

FIG. 7 is a flow chart of CT fine alignment using a focal position calculated according to a conventional method as an initial value;

FIG. 8 is a flow chart of CT fine alignment with the focus position before mechanical adjustment as an initial value;

fig. 9 is a flowchart of CT fine alignment using an average of the focus position before mechanical adjustment and the desired position as an initial value.

Detailed Description

The present invention will be described in further detail with reference to the attached drawings, but the present invention is not limited to the scope of the present invention.

During CT alignment, the operator mechanically adjusts the position of the focal point of the bulb in the x-axis to bring the position of the iso channel to the desired ideal value. This procedure requires scanning an eccentrically placed elongated metal pin (cylinder of about 5mm diameter) with a circular cross-section to obtain scanned projection data, which consists of n views of projection data. According to the projection data, the position of the iso channel and the position coordinate of the focus of the bulb tube on the x-axis can be accurately calculated by combining the angle information of the specific light beam direction of each channel of the detector and an iterative method.

The invention compares the calculation results of the iso channel by simulation calculation, as shown in fig. 5, aiming at a CT system according to a specific geometric layout, according to the conventional method, the iso channel is 330.8081; according to the calculation method of the present invention, the iso channel is 330.8297; the actual iso channel is 330.8299. The error between the result of the calculation method and the actual result is-0.0002 channels, and the result of the calculation method is more accurate than that of the traditional method.

As shown in fig. 1, the alignment of the CT system involves a ball tube and a detector, which rotate around a rotation center iso, scan an eccentrically placed metal pin, send the collected scan projection data to a computer or a specific processor for processing and accurate calculation, obtain an actual iso channel and an actual position of a focus of the ball tube, further obtain an adjustment amount of the focus position of the ball tube by comparing the actual iso channel with an expected iso channel, then mechanically adjust the focus position of the ball tube, continue to scan the metal pin, and repeat so as to make the iso channel reach the expected position, and finally realize the accurate alignment of the CT system.

The method is characterized in that according to projection data obtained by scanning, the position of the iso channel and the position coordinate of the focus of the bulb tube on the x-axis are accurately calculated by combining the angle information of the specific light beam direction of each channel of the detector and an iterative method.

As shown in fig. 6 and fig. 1, the specific scanning and calculating process is as follows:

1) as shown in fig. 1, the elongated metal pin is scanned resulting in scanned projection data as shown in fig. 2.

2) The bulb focus is assumed to be at the desired position.

3) And calculating the corresponding iso channel position according to the geometric layout of the CT.

4) From the geometry of the CT, the angular information of the direction of the incident beam of each channel of the detector, i.e. the angle of the beam with the y-axis, is calculated, as shown in fig. 3.

5) Using the scanned projection data, a centroid value of the metal projection values of all channels of each view with respect to the beam angle of each channel is calculated, which represents the angle of the beam direction in which the projection centroid is located.

WhereinRepresents the view i, the projection value of channel j,denotes view i, angle of direction of channel j, m denotes total number of channels,the angle of the beam direction in which the center of gravity of the projection of view i is located is indicated.

6) Averaging the projection gravity center values under all views to obtain the angle of the light beam direction corresponding to the iso channel。

7) The new position f of the x-axis of the tube focus is calculated.

Where D represents the distance from the focal point of the bulb to iso.

8) Repeat 3) to 7) until a specified number of iterations is reached (e.g., 30), or the x-coordinate error of the tube focus calculated in two consecutive iterations is less than a specified value (e.g., 1.0e-4 mm). Or the error of the value of the iso channel obtained by two adjacent calculations is less than a specified value (for example, 1.0e-3 channels), as shown in fig. 4.

9) And calculating the adjustment amount d of the focal position according to the calculated x-coordinate of the focal point of the bulb tube, and mechanically adjusting the focal position of the bulb tube.

Wherein d is the amount of adjustment,the desired focal position represents the focal position corresponding to the beam angle of the desired iso channel.

10) Repeat 1) through 9) until the error between the value of the iso channel obtained by scanning and calculation and the expected value of the design is less than a specified value (e.g., 1.0e-3 channels), completing the accurate alignment of the CT.

In order to accelerate the iterative calculation process, various methods can be used to set the initial position of the focal point of the bulb. As shown in fig. 1 and 7, after scanning the metal pin, the iso channel and the corresponding focus position can be calculated according to a conventional method and set as an initial focus position. The traditional method calculates the iso channel according to the projection value of the metal pin in each channel and the number of the channels where the pin is located, the calculation process ignores angle information of a specific channel direction, the result is not accurate, but the method is also approximate or approximate to a real result, and therefore the method can be used as an initial value of a focus position to accelerate an iteration process.

The 2 nd method for setting the initial position of the tube focus, as shown in fig. 1 and 8, can set the tube focus position before mechanical adjustment as the initial focus position to speed up the iterative calculation process.

In the 3 rd method for setting the initial position of the tube focus, as shown in fig. 1 and 9, the average value of the tube focus position before mechanical adjustment and the expected position can be set as the initial focus position to accelerate the iterative calculation process.