Nondestructive testing - X-ray detection real-time imaging technology resolution

As an emerging non-destructive testing technology, X-ray real-time imaging detection technology has entered the practical application field of industrial product testing. Like other detection technologies, X-ray real-time imaging detection technology requires a set of equipment (hardware and software) as support to form a complete detection system, referred to as X-ray real-time imaging system. X-ray real-time imaging system uses X-ray machine or accelerator as the ray source. After the X-ray passes through the detected object, it is attenuated and received by the ray receiving/conversion device and converted into analog signal or digital signal. Using semiconductor sensing technology, computer image processing technology and information processing technology, the detection image is directly displayed on the display screen, and the computer program is used for evaluation, and then the image data is saved to the storage medium. X-ray real-time imaging system can be used for non-destructive testing of metal welds, metal or non-metallic devices .

2 Basic configuration and influencing factors of X-ray real-time imaging system

The X-ray real-time imaging system is mainly composed of an X-ray machine, an X-ray receiving and converting device, a digital image processing unit, an image display unit, an image storage unit and detection tooling.

2.1 X-ray machine

The energy range of the X-ray machine should be selected according to the material and thickness range of the workpiece to be inspected, and a certain amount of energy reserve should be left. For operations that require continuous inspection, a DC constant voltage forced cooling X-ray machine should be selected. The focal spot size of the X-ray tube has a great influence on the quality of the inspection image. A small focal spot can improve the system resolution. Therefore, a small focal spot X-ray tube should be selected as much as possible.

At present, the small focus X-ray flaw detectors that the flaw detector factory can provide are: 160 kV constant voltage X-ray system, focus size ≤ 0.4mm×0.4mm; 225 kV constant voltage X-ray system, focus size ≤ 0.8mm×0.8mm; 320 kV constant voltage X-ray system, focus size ≤ 1.2mm×1.2mm; 450 kV constant voltage X-ray system, focus size ≤ 1.8mm×1.8mm. The focus should not be too small. If the focus is too small and the cooling is not good, the focus is easy to "burn out".

2.2 X-ray receiving and conversion device

The function of the X-ray receiving and converting device is to convert invisible X-rays into visible light. It can be a radiation-sensitive device such as an image intensifier, an imaging panel, or a linear scanner. The resolution of the X-ray receiving and converting device should be no less than 3.0LP/mm.

The X-ray receiving and converting device subsystem is also called the image imaging system. It can be divided into two types according to the current imaging technology level. One is the traditional imaging system based on the image intensifier. The image intensifier is a vacuum tube. The ray input screen is made of thin aluminum or titanium materials. The base of the screen is coated with sodium (Na)-cesium iodide (CsI) as the input scintillator (CsI: Na). It can convert the invisible X-ray image into a visible light image, and then convert the visible light image into a corresponding electron beam through the action of the photocathode plate. The electron beam is accelerated and focused on the fluorescent output screen (ZnCdS: Ag scintillator material) under the action of high voltage, thereby forming a visible detection image. The output screen is equipped with a focusing optical lens and a CCD (charge-coupled device) camera at the rear end, and the analog signal of the visible image is collected and input into the image acquisition card for A/D conversion, and then input into the computer for image processing. Currently, there are three types of image intensifiers available for selection, namely Φ225mm (9"), Φ150mm (6"), and Φ100mm (4"), according to the diameter of the input screen. The Φ225mm (9") image intensifier has a larger diameter, a wider field of view, and a longer detection length at one time, but has lower clarity and a higher price. The Φ100mm (4") image intensifier has a smaller diameter, a lighter weight, is easy to carry, and has higher clarity, but has a narrow field of view, a shorter detection length at one time, and lower work efficiency. It is usually advisable to choose a Φ150mm (6") image intensifier. Commonly used CCD cameras include those with a chip of 1/2" and a resolution of 752×582 lines and those with a chip of 1/3" and a resolution of up to 1000×752 lines. Currently, CCD cameras with higher resolution have also recently been launched on the market.

The other is an imaging system based on linear array scanning detectors (LDA-linear diode arrays). LDA contains a large number of electronic components and imaging points, mainly composed of light-emitting crystals, photodiode arrays, front-end data acquisition systems, etc. X-ray scintillator materials (common crystals are yttrium, GdWO4 and CsI based on phosphor screens) can convert X-rays into visible light. The crystals are installed on the surface of many photodiodes and arranged according to certain rules to form photodiode arrays (large-scale integrated circuits ). According to the scanning method, they are divided into line scanning (line array) and surface scanning (surface array). Array detectors are expensive, and linear array detectors are mostly used at present. Linear array scanning detector LDA imaging system is divided into two types according to the combination method. One is that the LDA imaging system is directly combined with the image acquisition card. The analog image collected by the LDA imaging system is sent to the acquisition card for A/D conversion, and then processed by computer image. Its working principle is basically the same as that of the image intensifier, but the resolution of the LDA imaging system is much higher than that of the image intensifier imaging system. The other is the combination of the LDA imaging system and the CMOD (complementary metal-oxide-semiconductor (transistor)) sensor to complete the entire process of radiation photoelectric conversion and digital acquisition in one step. This imaging system is called the LDA-CMOS radiation digital direct imaging system. The LDA-CMOS radiation digital direct imaging system is currently at an advanced level among various imaging systems. The conversion method of the LDA-CMOS radiation-digital direct imaging system greatly reduces the signal interference during long-distance transmission and conversion, and the pixel size of the photoelectric array is very small, so the spatial resolution is greatly improved.

The cost of the linear array detector-CMOS direct digital imaging system is much higher than that of the image intensifier imaging system. Based on price factors, the image intensifier imaging system is often used for the X-ray real-time imaging system of ordinary products, while the linear array detector-CMOS direct digital imaging system can be used for products with higher requirements. If the linear array detector-CMOS direct digital imaging system is used, the X-ray machine will not be limited by the small focus, and the cost of the X-ray machine is relatively low. Because it is a line scan, the pixels are scanned line by line, and there is almost no geometric unsharpness. Therefore, the image clarity is greatly improved; however, due to the line-by-line scanning, the imaging detection speed is slow. There is now a surface array imaging board abroad that can greatly improve the clarity of the image and the detection speed, but it is expensive and is now mostly used in customs container high-energy ray detection devices.

2.3 Image Processing Unit

The image processing unit should have image data acquisition and processing functions. The image data acquisition method can be an image acquisition card or other digital image synthesis device. The image acquisition resolution should not be less than 768×576 pixels, and the ratio of horizontal resolution to vertical resolution should be 4:3; the dynamic range, that is, the grayscale level should not be less than 256 levels.

The image acquisition card is installed in the computer. Its main function is to perform A/D conversion, converting the analog signal collected by the imaging system into a digital signal that can be recognized by the computer and become a digital image. The acquisition resolution of commonly used image acquisition cards is mostly 768×576 pixels, and the dynamic range is 8bit=256 gray levels. With the development of technology, the current high-resolution image acquisition resolution can reach 1k×1K, and the dynamic range can reach 12bit=4096 gray levels. If a high-resolution image acquisition card is selected, the system resolution can be greatly improved, but the price is higher. Imaging software is usually provided with the card.

2.4 Image processing software

Image processing software should have basic functions such as noise reduction, brightness contrast enhancement, edge enhancement, etc. Image processing software should be able to adapt to the technical standards specified by the corresponding inspection products, and have image geometric dimension calibration and measurement and defect location functions; the error between the calibrated defect position and the actual position in the inspection image should be ≤2mm, and the measurement accuracy of a single defect is ±0.5mm.

Basically, two types of image processing software are needed. One is control software, whose function is to control the imaging system by sending commands through the data bus. These commands include workpiece motion instructions, calibration of imaging devices, obtaining images from acquisition cards, image plane size calibration, real-time image acquisition, synchronous processing of images and image storage. According to video technology theory, image acquisition speed of 25 frames per second is considered real-time imaging. If only a census is conducted on the workpiece, instructions such as image acquisition can be disabled. The other is imaging software, whose function is to display images on the computer, evaluate the defect level according to the quality standard of the workpiece being inspected, generate workpiece inspection database files, output evaluation reports, and then save the inspection images and database files to storage media such as CDs at the same time. If the acquisition resolution of the inspection image is very high and the dynamic range of the acquisition is large, the data capacity of the image is large. Therefore, the imaging software should also have a data compression function. Since the inspection image is an important technical data, lossless compression should be adopted, and it should have good decompression and playback functions. Image processing software is usually provided to the user by the X-ray real-time imaging system development unit. The user unit with conditions can also develop image processing software by itself.

2.5 Image display unit

The image display adopts black and white mode, the display dot pitch is not more than 0.26mm, the display should be progressive scanning, the refresh rate is not less than 85Hz, and the image evaluation can choose 17~19″ display to make the observer's field of vision more comfortable.

2.6 Image Storage Unit

The test images can be stored in digital discs and other media. The stored digital images and valid information cannot be modified or deleted. The retained digital images should also contain the original acquisition data. For important test technical data that needs to be stored for 3 to 30 years, CD-R disposable discs should be selected (the storage period of CD-R discs can be up to 50 years), and CD-RW rewritable discs cannot be selected. [page]

2.7 Basic Computer Configuration

For an independent X-ray real-time imaging system, at least two computers should be configured, one for image acquisition and image processing, and the other for image evaluation and report printing, etc. The two computers are connected by cables. The basic configuration of computer hardware requires Pentium III 600 or above, 256M memory, 20G hard disk, and floppy drive, CD-ROM drive, printer and burner; the software environment is required to run under the Windows 2000 operating system.

2.8 Inspection tooling or assembly line

In order to realize continuous inspection of workpieces, necessary inspection tooling equipment or assembly lines should be available, and they should have high mechanical precision.

2.9 Selection of X-ray Real-time Imaging Detection System

A practical X-ray real-time imaging detection system is actually a selective combination of the basic configuration of the above X-ray real-time imaging system and multiple influencing factors. Different combinations will have different costs and usage functions. The user unit can choose an X-ray real-time imaging system suitable for the unit based on the basic configuration and influencing factors of the above X-ray real-time imaging system, combined with the unit's product characteristics and product technical quality inspection standards as well as its own economic conditions.

3 Resolution of X-ray real-time imaging system

3.1 System Resolution

The quality characteristics of X-ray real-time imaging systems can be evaluated by a number of technical performance indicators, such as system resolution, sensitivity, maximum withstand voltage, system stability, system continuous working time, image acquisition and image processing speed, detection efficiency, image one-time detection range (length × width), image dynamic range, system anti-interference, system working life, system price-performance ratio and other indicators. Among them, system resolution is an important indicator. Changes in each subsystem in the system will cause changes in the comprehensive performance of the system resolution. Therefore, grasping the comprehensive indicator of system resolution is equivalent to grasping the key to the X-ray real-time imaging system. The system resolution indicator is a comprehensive reflection of the performance of the entire X-ray real-time imaging system. The higher the system resolution, the better the technical performance of the system. The system resolution is a reflection of the objective performance of the system equipment. It is only related to the composition and performance of the system, and has nothing to do with the detection process method. Therefore, the system resolution is also called the inherent resolution. As the system equipment ages, the system resolution will also decline. Therefore, the system resolution should be tested regularly. The system resolution can be tested directly in the system using a resolution test card.

3.2 Test method for resolution of real-time imaging system

Place the resolution test card close to the center of the input screen surface of the X-ray receiving conversion device (such as image intensifier), with the line pair grid perpendicular (or parallel) to the horizontal position, and perform transillumination according to the following process conditions, and image on the display screen:

(1) The distance from the X-ray tube focus to the image intensifier input screen surface is not less than 700 mm;

(2) The tube voltage is not more than 40kv;

(3) The tube current is not greater than 2mA;

(4) The image contrast is moderate.

Observe the image of the test card on the display screen. If you see a group of line pairs that are just separated by the grid bars, the resolution corresponding to this group of line pairs is the system resolution. The unit of the system resolution is "line pair/millimeter" (LP/mm).

System resolution can also be expressed in terms of system clarity (in mm), and the conversion relationship between them is "half of each other's reciprocal".

3.3 The role of system resolution

After the system equipment configuration is determined, the system resolution is a determined parameter. In the real-time imaging detection process, the system resolution is usually used as a known parameter to determine other detection parameters. For example, the image detected by real-time imaging is usually an enlarged image, and the system resolution factor (or system unsharpness) is required to determine the image magnification:

Mopt represents the optimal magnification in the real-time imaging detection process, Ui is the system unsharpness, and d is the focal size of the radiation source, which are all known numbers. Substituting them into the formula can calculate Mopt, and setting Mopt =M, the detection magnification M can be quickly determined.

3.4 System resolution index

According to the different configurations of the X-ray real-time imaging detection system, the X-ray real-time imaging detection system can be managed at three levels: A, AB, and B. The system resolution index of level A can be set at ≥1.4LP/mm, which is used for X-ray real-time imaging detection of ordinary products, such as automobile aluminum alloy wheels, refractory bricks of iron-making blast furnace linings, and food cans; the system resolution index of level AB can be set at ≥2.0LP/mm, which is used for the detection of more important products, such as the detection of butt welds of boiler pressure vessels and pressure pipelines, and the detection of automobile parts and electronic components; the system resolution of level B is set at ≥3.0LP/mm, which is used for the detection of important products, such as nuclear industry products and aerospace equipment.

4 Image Modulation Transfer Function

From the perspective of information theory, the pixels of an image are the information carriers of the detected image. Pixels can have different gray levels. The number of pixels and the combination of gray distribution constitute the detected information. The quality (or information) of the detected image can be reflected by the transfer characteristics of the system itself. Based on Fourier series and image information theory, the modulation transfer function is proposed as the basis for evaluating system quality or image quality (information).

4.1 Modulation Factor (MTF)

Modulation is a concept originally used in radiology . When applied to X-ray detection, it is called contrast. The ability to reproduce the contrast of the image of the detected object is characterized by modulation, which is defined as the ratio of the difference between the maximum grayscale and the minimum grayscale in the image to the sum of the maximum grayscale and the minimum grayscale, expressed as MTF:

Where:

MTF --- Modulation (0≤MTF≤1)

I1 --- Maximum grayscale

I2 - minimum grayscale

The relationship between modulation and resolution can be expressed by the modulation transfer function MTF curve:

Figure 1 Modulation transfer function MTF curve

The modulation transfer function can be represented by an MTF curve, with the horizontal axis being resolution and the vertical axis being contrast. The smaller the resolution, the larger the MTF; the larger the resolution, the smaller the MTF. When the resolution reaches a certain level, the MTF approaches zero (the line spacing indicating the image resolution is so small that it is almost indistinguishable). The later this level is reached, the higher the image resolution. In X-ray real-time imaging detection, the grayscale is usually set to 8bit (28), or 256 levels. Experiments have shown that the lowest grayscale that a normal person's eye can distinguish is 5%. The resolution corresponding to an MTF of 8% is usually taken as the image limit resolution.

4.2 Transfer function of MTF function

The modulation transfer function has three functions: first, the information provided by the MTF curve is objective; second, the contrast and resolution can be measured; third, the information reflected by the MTF can be transmitted, that is, the image quality at each stage of the system is reproducible, and this transmission can be obtained by a simple method. Therefore, the MTF function is a more objective and comprehensive method for evaluating image quality. The MTF function is usually used to explain the configuration of the system or the phenomenon of image quality, and the MTF function provides a theoretical basis for image processing.

4.3 Basic methods to improve the resolution of X-ray real-time imaging systems

To improve the system resolution, the configuration of the system equipment should use subsystems with high MTF as much as possible, and the MTFs of the subsystems should match each other as much as possible. If one subsystem has a low MTF, it will affect the resolution of the entire system (this can be called the "barrel effect"); reduce the number of subsystems as much as possible, and use integrated devices as much as possible.

5. Prospects of X-ray Real-time Imaging Technology

After more than ten years of efforts, X-ray real-time imaging detection technology has become mature as an emerging non-destructive testing technology in China. Its detection image quality is comparable to that of radiographic film. In addition, due to the use of optical discs as storage media, the detection cost is greatly reduced, which is welcomed by users. Many unused units are also eager to try it. However, the one-time investment in the equipment of the X-ray real-time imaging system is large, especially some key components such as industrial image intensifiers, high-definition CCD cameras, LDA linear array scanning detectors, and CMOS sensors are basically not domestically produced. Therefore, the price of the equipment of the entire system cannot be reduced, which has become a constraint on the development of X-ray real-time imaging technology. Real-time imaging technology has many similarities with digital camera technology. Nowadays, digital cameras have entered ordinary people's homes, and digital technology has entered all walks of life. In the face of the arrival of the digital age, our users have high hopes for my country's non-destructive testing research and development units, hoping that the X-ray real-time imaging detection system can be localized as soon as possible, and the price can be reduced, so that the digital X-ray real-time imaging detection technology can enter a wider range of applications.