X-ray flat panel detectors are widely used in security, industry, medical care, and other fields. In the medical field, they even cover all X-ray equipment except CT, including DR, DRF (dynamic DR), DM (mammography), CBCT (dental CT), DSA (interventional and vascular), C-arm (surgery), and so on. This is not only a popular science article but also a picture depicting the innovation, transformation, and development of X-ray detectors. In 1990, Robert Street and others from Xerox Corporation in the United States first proposed a detector implementation method using a PIN-structured amorphous silicon (a-Si) photodiode array combined with a two-dimensional amorphous silicon TFT (thin-film transistor) array for addressing, which is the earliest documented record of flat panel detectors. Subsequently, major imaging equipment companies conducted preliminary research on this technology. In the late 1990s, companies such as GE (in collaboration with Perkin Elmer), Thales, Siemens, Philips (which co-invested in Trixell), Varex, and Canon Medical developed amorphous silicon flat panel detectors. Around 2010, amorphous silicon flat panel technology further spread, and traditional film giants such as Carestream, Fujifilm, Konica, and Agfa also developed flat panel detectors. Meanwhile, South Korea's Viewworks and Rayence, as well as China's Shanghai Yirui and Jiangsu Kangzhong, successively launched their own amorphous silicon flat panel detectors. Due to its mature technology, good adaptability, and low cost, amorphous silicon has become the most mainstream flat panel detector. However, amorphous silicon flat panels are not the best choice in dynamic imaging application fields such as mammography, dentistry, and surgery. Therefore, in the mammography field, Hologic invented the amorphous selenium flat panel detector; in fields such as dentistry (CBCT) and surgery (C-arm), Dalsa first developed the CMOS flat panel detector, and China's Chengdu SenseTime has also successfully developed and mass-produced small and medium-sized CMOS detectors. In recent years, improved amorphous silicon IGZO detectors and direct conversion photon counting detectors have emerged. So far, the field of X-ray detectors has presented a "hundred flowers blooming" situation. However, unlike the "just starting" stage of core CT components, domestic X-ray flat panel detectors have successfully "entered the game". ## A Complete Guide to Indirect Conversion Detectors, with Additional Notes on the Bottleneck Technologies of Domestic Products ### X-ray Detector Development Roadmap (from the Internet) #### I. Two Major Technical Directions We know that amorphous silicon/IGZO/CMOS flat panel detectors belong to indirect imaging. Their principle is: converting X-rays into visible light, and by sensing the intensity of X-rays passing through the object, assigning different gray levels to the image, allowing people to observe the image. Therefore, the basic structure of an indirect conversion detector includes: a scintillator, a sensor and readout circuit, and a peripheral control circuit. The scintillator and sensor are the core parts, determining the main performance indicators of the flat panel detector. In contrast, direct conversion does not require a scintillator. After collecting X-rays, the photoconductive semiconductor material directly converts X-rays into electrical signals. Therefore, the basic structure of a direct conversion detector includes: a sensor and readout circuit, and a peripheral control circuit. The sensor (photoconductive semiconductor) is the core part. At present, indirect conversion detectors represented by mainstream amorphous silicon, CMOS, and new IGZO are absolutely the market mainstream, accounting for more than 90% of the market share of flat panel detectors. #### II. Indirect Conversion Detectors ##### 01. Scintillators with Great Differences According to the scintillator material of the detector, it can be divided into cesium iodide (CsI) flat panels and gadolinium oxysulfide (GdOS) flat panels. The imaging principles of the two are basically the same, but due to different detection materials, there are obvious differences in cost and performance: 1) Unlike cesium iodide, GdOS does not require a long-term evaporation and deposition process, has a simple production process, stable and reliable products, and its cost is 20% - 30% lower than that of CsI; 2) Compared with GdOS, needle-like cesium iodide crystals have a 30% - 40% higher X-ray conversion efficiency and smaller lateral light diffusion, thus having higher spatial resolution. Therefore, cesium iodide flat panels have higher DQE and clearer imaging. According to IHS Markit's prediction, in the field of scintillators, cesium iodide will further squeeze GdOS's market share due to its excellent performance. ###### Differences between GdOS and CsI Scintillators (from the Internet) The performance of scintillator raw materials and preparation processes have a crucial impact on light conversion rate, afterglow, spatial resolution, and other properties. The production process threshold is high, and it is difficult to control the mass production yield. Most detector manufacturers obtain scintillators through outsourcing, and few manufacturers build their own scintillator production lines. 1) In terms of scintillator materials, China does not produce gadolinium oxysulfide scintillator materials, which are mainly imported from Japan; however, Jiangxi Dongpeng New Materials has produced cesium iodide crystal powder with a purity of up to 99.999%, achieving independence and self-sufficiency. 2) In terms of scintillator preparation, Jiangsu Kangzhong is the earliest enterprise in China to develop cesium iodide detectors, and Shanghai Yirui is the earliest manufacturer in China to develop gadolinium oxysulfide detectors. Both have mastered the direct growth technology of cesium iodide, and currently, their technology is almost close to the international advanced level. ##### 02. Sensors are the Core At present, due to its cost-performance advantage, amorphous silicon flat panels will still be the mainstream choice for static flat panel detectors and large-sized dynamic flat panel detectors in a short time. However, from the perspective of technological development trends, detectors are developing towards higher sensitivity, lower noise, and higher frame rates. Therefore, CMOS, IGZO, and other technologies have become the focus of research and development by major manufacturers. From the perspective of application fields, there is no substitution relationship between the four technologies, but more enterprises have laid out amorphous silicon technology. The future market potential lies in IGZO and CMOS technologies with better performance. Because IGZO technology can greatly improve the response speed and scanning rate of pixels, it is mainly used in high-speed, large-sized dynamic flat panel detectors, such as DSA and dynamic DR; CMOS flat panels have higher resolution and are mainly used in high frame rate, small and medium-sized dynamic flat panel detectors, such as mammography, dentistry, surgery, and industrial non-destructive testing. ###### Detectors Based on TFT Technology LCD, that is, liquid crystal display, is widely used in mobile phones, tablets, and TVs, becoming the basis for our interaction with the virtual world. Among them, the most famous and best-performing one is TFT-LCD. TFT-LCD, namely thin-film transistor liquid crystal display. The essence of TFT is a switch, that is, each pixel of the liquid crystal display relies on TFT for switching and driving. In the early 1990s, the maturity of TFT technology made TFT-LCD quickly grow into a mainstream display. The goal of developing TFT technology in the future is larger size and lower cost. Whether it is the currently widely used amorphous silicon, or the promising IGZO and flexible detectors, they are all based on TFT panel technology. According to the semiconductor medium, TFT flat panel detectors can be divided into amorphous silicon/IGZO: ###### Amorphous Silicon/Flexible Detectors The so-called amorphous silicon flat panel means that the channel of the TFT device is made of amorphous silicon material, and amorphous silicon can be deposited on a large-area glass substrate. It has the characteristics of large area, mature and stable technology, good response to the energy spectrum range of ordinary radiation, stable and reliable materials, good environmental adaptability, etc., and can meet the needs of both static and dynamic detectors. It is the most mainstream X-ray flat panel detector sensor technology. At present, amorphous silicon technology has entered a red sea. Almost all detector enterprises produce amorphous silicon flat panels, making them more like a "daily necessities". Parameters are only one aspect, and quality and reliability are more important, even the most important. Flexible detectors are currently a relatively cutting-edge X-ray detector technology. The so-called flexible detector means that its TFT substrate is a flexible substrate, that is, using a thin and soft material (such as optically transparent polyimide) instead of the traditional glass substrate to make a flexible flat panel that can be deformed, bent, and not easy to break. It has the characteristics of ultra-narrow border, light weight, impact resistance, and not easy to damage. Due to the performance difference between the flexible substrate and the glass substrate, the technology and process of the flexible substrate detector are more complex, and the cost is higher. At present, it is only used in specific scenarios, such as mobile medical care. With the continuous improvement of flexible technology and process and the continuous reduction of cost, it can be expanded to more mainstream application scenarios in the future. At present, China's Yirui and Kangzhong have launched related products. ###### IGZO Detectors Amorphous silicon detectors have not "dominated the world" but have become synonymous with "low-end flat panels" because of their fatal shortcomings: the electron mobility is only 0.5-1.0 cm²/VS, resulting in poor low-dose DQE, large image noise, and low resolution. Thus, another type of IGZO detector was born, bringing higher resolution and higher refresh rate. In fact, a more rigorous statement is Oxide TFT, that is, oxide TFT. It is called IGZO, namely indium gallium zinc oxide, because IGZO is the most successful representative among them. IGZO flat panels also belong to TFT technology flat panels, but their control of display pixel driving is upgraded from TFT to faster IGZO. This is because the electron mobility of IGZO is 20 to 50 times that of amorphous silicon. Therefore: 1) higher pixel readout speed and frame rate can be obtained; 2) the size of the transistor can be greatly reduced, the pixel density can be increased, the image resolution can be higher, and the low-dose DQE can be improved. IGZO flat panels not only inherit all the advantages of amorphous silicon flat panels, such as easy large-area manufacturing and low cost but also have higher performance, such as lower noise, higher acquisition speed, and higher resolution. They are ideal large-sized high-speed dynamic flat panel detectors and can be widely used in X-ray fluoroscopy equipment such as DSA and DRF. In the field of IGZO flat panels, Varex took the lead in introducing and mass-producing this IGZO technology, and China's Yirui has also initially realized the mass production of IGZO flat panel detectors. ###### Detectors Based on CMOS Technology CMOS, namely complementary metal-oxide-semiconductor, is the basic unit of chips. CMOS technology sounds advanced, but it is actually close to us. Almost all mobile phone cameras are based on CMOS image sensor chips, which are similar to CMOS flat panels. If making TFT detectors is like making a TV, then making CMOS detectors is more like making a chip. Because, unlike the glass substrate of amorphous silicon/IGZO detectors, the substrate of CMOS detectors is single crystal silicon, whose electron mobility is 1400 cm²/VS, which is an important material for making wafers. **Q1: Why are electron mobility mentioned for amorphous silicon, IGZO, and CMOS? Is it so important?** No matter what technology, we hope the electron mobility is as fast as possible. Therefore, high electron mobility allows a single transistor to be made smaller, making the display pixel smaller and the image spatial resolution higher. To give a vivid example: if you want to transport enough coal to another city within a specified time, if the train speed is slow, you need multiple tracks and multiple trains to transport at the same time, which is obviously very space-consuming (low spatial resolution); if the train speed is fast enough, a single small train can handle it (high spatial resolution). So, "fortunately" CMOS detectors are more expensive, otherwise, there would be no place for other technologies. **Q2: Then why are CMOS detectors more expensive?** The substrate of amorphous silicon/IGZO detectors is cheap glass or plastic, while the substrate of CMOS detectors is expensive single crystal silicon wafers; TFT-based detectors can be prepared at low temperatures, while CMOS-based detectors have higher and more complex preparation process temperatures. For example, a 65-inch LCD TV is only more than 3,000 yuan, while a piece of Intel i9 CPU (size: 37.5mm x 37.5mm) is also as high as more than 3,000 yuan. The area of a 20*20cm CMOS flat panel is more than 1,000 times that of an i9 CPU. So, CMOS flat panel detectors are more expensive, but everyone knows that expensive things have their advantages. The so-called CMOS flat panel detector refers to integrating photodiodes, addressing circuits, and more importantly, amplifiers (the biggest difference from amorphous silicon/IGZO detectors) on a wafer, and transmitting the signal to the outside after amplifying it. Therefore, it has significantly better low-dose DQE and higher acquisition speed than amorphous silicon detectors. Due to the faster electron mobility of single crystal silicon, an amplifier circuit can be added next to the photodiode to amplify the signal before transmitting it to the outside, which is the biggest difference between CMOS flat panels and amorphous silicon or IGZO flat panels. Therefore, CMOS detectors have significantly better high resolution, high acquisition speed, and high and low dose DQE than amorphous silicon/IGZO detectors. However, due to the size limitation of semiconductor wafers, 8-inch wafers are widely used in CMOS flat panels. The size of flat panels is generally 13*13cm and 15*12cm, and they can also be spliced, but the process is more complex. CMOS flat panels have obvious advantages in the application of small and medium-sized dynamic X-ray equipment, such as dental CBCT, surgical C-arm, and mammography machines. In the field of CMOS flat panels, Dalsa and Varex are currently the two largest CMOS detector manufacturers in the world. China's Chengdu SenseTime and Shanghai Yirui have mastered this technology and have mass production capabilities, and Jiangsu Kangzhong is also in research and development. Among them, Chengdu SenseTime has developed the first domestic CMOS flat panel detector and has realized mass production. Chengdu SenseTime, a detector enterprise spun off from Beijing Nano Vision, has participated in the research and development of static CT dedicated detector chips and components. It is a high-tech enterprise focusing on solid-state imaging chips and detector modules. Its main business includes CMOS flat panel detectors, CT detectors, photon counting detectors, etc. At present, its CMOS flat panel detectors are the most advanced in China, and almost all domestic dental CBCT manufacturers have registered their products. #### 3. Direct Conversion Detectors Compared with indirect conversion X-ray detectors, direct conversion X-ray detectors do not need to convert X-rays into visible light through a scintillator. Instead, electron-hole pairs are generated when X-rays irradiate the sensor material. Since no visible light is generated, there is no influence of lateral light diffusion, and thus higher spatial resolution is achieved. Its sensor material has a high atomic number, large X-ray absorption coefficient, and high carrier mobility; and only a millimeter-level thickness is needed to basically completely absorb X-rays of hundreds of kilovolts, without any delay or afterimage. At present, direct conversion X-ray detectors mainly include two types: energy-integrating detectors represented by amorphous selenium materials, and photon counting detectors represented by cadmium telluride (CdTe), cadmium zinc telluride (CZT), and single crystal silicon (Si). ##### 01. Amorphous Selenium Detectors Amorphous selenium technology is unique to Hologic and is still under patent protection. Similar to amorphous silicon flat panels, amorphous selenium flat panels are also made based on TFT technology. Historically, there was a famous battle between amorphous silicon and amorphous selenium. As a result, amorphous silicon detectors became the market mainstream. The reason why amorphous selenium flat panels have become "unpopular" is that: 1) selenium has poor absorption performance for X-rays, and thermal crystallization will lead to performance attenuation, so the process stability needs to be further improved; 2) the starting bias electric field of amorphous selenium detectors is as high as several thousand volts, which will cause irreversible damage to TFT switches, making the service life of amorphous selenium detectors not long; 3) amorphous selenium flat panels are very sensitive to temperature, and their use conditions are also limited to a certain extent; 4) amorphous selenium films cannot be made thick, which is not suitable for the detection of high-energy X-rays. At present, in the field of breast X-ray imaging, amorphous selenium is almost the "king", and high-end mammography machines basically use amorphous selenium flat panels; however, due to price reasons, there are also a large number of cost-effective mammography machines focusing on early screening that use amorphous silicon flat panels. In addition, in the field of breast tomographic imaging, with high frame rate, high resolution, high and low dose DQE, and other performances, CMOS flat panels have become a better choice for breast TOMO/DBT. ##### 02. Photon Counting Detectors Photon Counting Detectors, derived from direct X-ray conversion technology in high-energy physics, are recognized as the next generation of X-ray imaging technology. Different from other detectors based on scintillators, photon counting detectors are based on semiconductor materials. Their principle is to set a threshold to extract photons with signal amplitude exceeding the threshold from dark noise below the threshold, which can eliminate false counts caused by dark current and achieve true zero noise; direct imaging avoids light scattering caused by scintillators, enabling higher spatial resolution and density resolution. In addition, compared with the monochromatic imaging of integrating detectors, photon counting detectors can realize multi-color imaging of multi-energy spectrum sampling points of rays, thus having material分辨 ability. In the future, X-ray imaging will gradually develop from 2D and 3D to 4D, and from black and white to color. CdTe/CZT has long been recognized as a promising semiconductor material for hard X-ray absorption, which can effectively absorb X-rays in the range of 10-140 keV and provide good energy resolution even at room temperature. At present, companies such as GPS, V