Development and Technology of Direct Digital Radiography Systems (DR)

Development and Technology of Direct Digital Radiography Systems (DR)

Based on the composition materials and working principles of the detectors, DR is mainly divided into three major technologies: CCD, one-dimensional scanning, and amorphous flat panels (amorphous selenium, amorphous silicon + cesium iodide / amorphous silicon + gadolinium oxide).

1. CCD: Due to physical limitations, experts generally believe that large-area flat panel imaging CCD technology is inadequate, and CCD devices have certain gaps in image quality compared to amorphous silicon/selenium flat panel devices, but they have a relative price advantage; there are still a few manufacturers in the world using this technology, such as Swissray.

2. One-dimensional scanning: Also known as one-dimensional line scanning technology, invented by the Russian Academy of Sciences’ Institute of Nuclear Physics, it is produced by ZTE Aerospace in China; it has the advantages of low radiation dose and relatively lower equipment cost compared to flat panel technology, but it also has fatal flaws such as long imaging time (several seconds), low spatial resolution (initially 1mm/lp), and low X-ray utilization efficiency; the imaging quality is poor, and patients receive a lot of unnecessary radiation.

3. Amorphous flat panels: Amorphous selenium/amorphous silicon; mainly composed of an amorphous selenium layer (a-Se) / amorphous silicon layer (a-Si) plus a thin-film transistor (TFT) array.

1. a-Si (amorphous silicon flat panel detector) — Two-step digital conversion technology, where X-ray photons first convert to visible light and then are detected by photomultiplier tubes to convert into digital signals. Mainstream manufacturers include Philips, Siemens, GE, etc. Due to different coating technologies, it is further divided into amorphous silicon + cesium iodide flat panels and amorphous silicon + gadolinium oxide flat panels.

2. a-Se (amorphous selenium flat panel detector) — A so-called direct detection technology, where X-ray photons are converted into electrical signals in the selenium coating layer and directly converted into digital signals. Currently, only Hologic in the United States possesses the core of this technology, and DRs from Kodak and domestic manufacturers like Youtong use this type of detector.

The technological advancement of DR is closely related to the development of imaging plate technology. The technological development of flat panels is reflected in two aspects: size and dynamic response time. The cesium iodide/amorphous silicon flat panel has incomparable advantages in these two aspects over other technologies and is currently the most mature and mainstream technology, with major leading manufacturers in the world using this technology.

* Cesium Iodide / Amorphous Silicon (CsI) + a-Si + TFT: When X-rays hit the CsI scintillation crystal layer, the energy of the X-ray photons is converted into visible photons, which excite photodiodes to generate current; this current integrates on the capacitance of the photodiode itself to form stored charge; the amount of stored charge for each pixel is proportional to the energy and number of incident X-ray photons in the corresponding range; comprehensive levels of imaging speed, image quality, and work efficiency are very high.

* Gadolinium Oxide / Amorphous Silicon (Gd2O2S) + a-Si + TFT: The working process is similar to the above, except that cesium iodide is replaced by gadolinium oxide; due to technical reasons, its original image is 12 Bit / 4096 gray levels, and A/D conversion is 14 Bit; the process cost is lower, but the overall technical level is inferior to cesium iodide panels.

* Amorphous Selenium a-Se + TFT: Incident X-ray photons generate electron-hole pairs in the selenium layer, and under the action of an external bias electric field, electrons and holes move in opposite directions to form current, which integrates in the thin-film transistors to become stored charge; the amount of stored charge for each transistor corresponds to the energy and number of incident X-ray photons; the process cost is lower, but it has poor absorption of incident X-rays, and the overall technical level in terms of imaging speed and stability is inferior to amorphous silicon flat panels.

Detector Technology	Manufacturers	Representative Manufacturers	Technical Features	Remarks
Amorphous Silicon + Cesium Iodide (CsI + a-Si + TFT)	France Trixell (Joint venture of Philips/Siemens/Thomson)	Philips Siemens	Special process of Csl columnar crystal structure scintillator coating; excellent X-ray absorption, effectively reduces visible light scintillation, small pixel size, high resolution, fast imaging speed, and excellent image quality; overall technical level is very high, recognized as the most mature and high-end DR flat panel technology in the world.	Complex process makes it difficult to produce large-area panels, using four small panels spliced into a 17″×17″ large panel, with image stitching compensated by software.
GE (acquired EG & G’s industrial panel technology for medical use)	GE	Non-columnar crystal structure ordinary Csl coating, serious visible light scintillation phenomenon, significant energy loss; lower process cost; but effective size is small, pixel size is larger, refresh speed is slower, and image quality is poor.	Their flat panels use industrial panel technology; during operation, a lot of heat is generated, requiring a special water cooling device.
Varian	Wandong, Shanghai Medical, Changqing, Pantai	Varian flat panel has too small a field of view, with a very narrow application range.	Significant limitations and poor image quality
Amorphous Silicon + Gadolinium Oxide (Gd2O2S + a-Si + TFT)	Japan Canon USA Varian	Canon Toshiba Shimadzu	Uses gadolinium oxide (Gd2O2S) as a scintillator material to complete the conversion process from X-ray photons to visible light. Fast imaging, lower cost, but generally lower gray level dynamic range (below 12 bit), compared to other high-end 14 bit products, the image diagnostic quality is insufficient; energy loss is more serious than Trixell.	Commonly known as “Canon panel”

Amorphous Selenium	USA Hologic (acquired D.R.C company’s DirectRay technology) New Medical Technology	Hologic Kodak Zhuhai Youtong Shenyang Neusoft Beijing Dongjian	Defects of amorphous selenium flat panels include poor temperature adaptability and slow imaging speed. Hologic flat panels have strict environmental requirements, easily damaged by freezing, leading to dead pixels (many domestic users’ panels have dead pixels); long imaging time and insufficient stability of image quality. New Medical Technology has made some progress in technology, improving the sensitivity of its amorphous selenium detectors to temperature and slow imaging speed, but it still cannot guarantee stable image quality, and the damage rate of panels during use remains high; their “bedside type” panels can meet the requirements for converting existing X-ray equipment in small hospitals to DR.	Immature technology; unstable image quality; the main technology owner Hologic has exited the international DR system market due to issues with its selenium coating technology, and New Medical Technology has shifted focus to producing portable, low-requirement DR panels.
One-dimensional scanning	Russian Academy of Sciences Nuclear Physics Research	ZTE Aerospace	Uses slit-type line scanning technology and high-sensitivity line array detectors. The flat fan-shaped X-ray beam emitted by the tube passes through the human body to reach the detector, obtaining a line of signal data, and with the help of the scanning mechanism, the tube and detector move uniformly from top to bottom, scanning line by line, and after computer processing and reconstruction, a flat digital image is obtained.	Full name “Multi-wire proportional chamber one-dimensional line scanning technology”, with drawbacks including excessive exposure time, low pixel matrix, and spatial resolution.
Fisher Company	Uses a strip CCD structure detector technology, consisting of a scintillator that converts X-ray photons to visible light and four CCDs, completing data collection using line scanning.

CCD (CsI/Gd2O2S + lens/fiber optics + CCD/CMOS)	Canada IDC Germany Imix Russia Electron Switzerland Swissway Netherlands Nucletron South Korea T.I.T.C South Korea Raysis USA Phoxxo France Staford	X-rays first pass through a visible light conversion screen made of scintillator or phosphor, converting X-ray photons into visible light images, which are then sent to the optical system via lenses or fiber optics, where CCD captures and converts them into image electrical signals.	Outdated technology, poor image quality; cannot compete with TFT panel technology, facing elimination.
CMOS (CsI/Gd2O2S + CMOS)	CaresBuilt Tradix	Constrained by the poor spatial resolution of indirect energy conversion, although composed of a large area matrix of low-resolution CMOS probes, it still cannot effectively compete with the advantages of TFT panels.	Very outdated technology, poor image quality; has begun to be phased out.

Note: Currently, experts worldwide generally recognize the mature technology of amorphous silicon + cesium iodide flat panel detectors; Trixell’s flat panel detectors, due to their stable and excellent imaging characteristics and good environmental adaptability, have become the first choice for DR equipment; with the adoption of the world’s best flat panel detector technology, coupled with high-quality tubes and excellent mechanical performance, along with powerful professional-grade post-processing workstations, Philips/Siemens has become a globally recognized top brand in DR systems.

1. Detectors: For direct digital X-ray photography technology, the image quality is determined not only by the type of technology used in the flat panel but also by important factors such as DQE, acquisition matrix, acquisition gray levels, spatial resolution, and minimum pixel size; each factor is crucial; with the same image size, the larger the acquisition matrix, the smaller the pixel size, the higher the image resolution, allowing for better display of fine tissue structures.

(1) Material/Technology Type: Cesium iodide/amorphous silicon is mainstream; among them, Trixell flat panels are the best.

(2) Effective Size: The mainstream sizes are 17×17in or 14×17in; 17×17in can meet 99% of patients including obese patients, allowing for single exposure imaging; while 14×17in cannot meet 23% of patients, requiring double exposure, increasing patient radiation damage and the workload of technicians.

(3) Pixel Matrix: The mainstream is 2.5K×3K or 3K×3K.

(4) Pixel Size: 143μm/200μm; the size of the pixel directly affects the fineness of the image.

(5) Spatial Resolution: The determining factors are the size of the detector and quantum noise, which are the physical determinants (of course, software can interpolate algorithms to obtain smaller pixel numbers, but this is not the true signal of the image, but a calculated result); in addition, the quality of the rays is a factor that cannot be ignored. Among all flat panels, Trixell flat panels have the largest size and the least quantum noise.

(6) Gray Levels: The mainstream is 14 Bit / 16,384 gray levels, with only a few companies like Canon having original images of 12 Bit / 4096 gray levels, with A/D conversion to 14 Bit.

(7) Detector Quantum Efficiency (DQE): It is the efficiency of converting input signals into output signals, and high DQE is the basis for potential dose reduction. Digital flat panel detectors have the characteristic of having higher DQE compared to screen-film X-ray photography. At the same radiation dose, the DQE of amorphous selenium is lower than that of amorphous silicon; amorphous silicon detectors outperform amorphous selenium detectors in dose reduction.

(8) External Devices: Whether water cooling or other devices are needed.

2. X-ray Tube: Quality and lifespan of the rays;

(1) Focus

(2) Heat capacity

(3) High-speed rotation, anode rotation speed

(4) Beam collimator

3. High Voltage Generator:

(1) Power, frequency

(2) Output range

(3) KV adjustment

(4) Shortest exposure time

4. Control Console:

(1) Automatic exposure control, anatomical site photography: generally available.

(2) Workstation screen: 19in is mainstream; 17in is gradually being phased out.

(3) Operating system: Personal computer-level Windows system or professional server-level UNIX system; anyone with a little computer knowledge understands that the latter has incomparable stability and high processing capability compared to the former.

(4) Hard disk: Generally 60~80G; there are ordinary IDE hard disks and high-speed SCSI hard disks; the latter has the fastest response speed and longest lifespan, especially in image processing, showing the advantages of high-speed multi-channel.

(5) Time from exposure to diagnostic image display: Generally required to be ≤10s, with a few able to reach within 5s; an important indicator of the computer system’s working capability in the inspection workstation.

(6) Image quality control function: Generally available, whether good or bad.

(7) Image processing software and upgrades: Vendors generally provide free upgrade services within the usage period; image processing software developed exclusively for medical diagnostic needs is particularly important and is one of the important criteria for judging the level of DR equipment.

(8) DICOM 3.0 and functions: Generally available.

(9) External storage devices: DVD or CD-RW burning.

(10) Image output: Output in digital form to cameras and PACS systems.

(11) Network transmission speed: 100m/ms or 1000m/ms; the latter has a faster transmission rate.

5. Tube Stand and Diagnostic Bed: Requires ergonomic design and compliance with clinical needs.

(1) Tube stand

(2) Tube rotation

(3) Automatic electromagnetic locking and angle and distance display functions

(4) Requirements for diagnostic beds

(5) Grid

6. After-sales service:

(1) Free maintenance: Generally, the whole machine has a one-year warranty.

(2) Detector warranty: Generally two years.

(3) PACS system connection and related software and hardware required for connection: Generally provided for free.

(4) Operation and maintenance manual: Requires detail.

(5) On-site application and maintenance training services: Generally provided for free.

(6) Operating rate: Generally required to be above 95%.

(7) After-sales service response time and maintenance years after warranty: Generally required to arrive at the fault site within 24 hours after receiving the maintenance notice; provide more than 10 years of maintenance service after the warranty period.

7. Radiation Safety Protection

Must comply with international radiation safety protection standards, with radiation safety protection testing certificates or certifications from authoritative organizations such as the US FDA or the European Community; although all equipment on the market has relevant certifications, different flat panel technologies and tubes vary greatly in this regard, with PHILIPS being the best, having the lowest exposure dose among all DR products, providing maximum protection for patients and staff.

III. Principles for Purchasing DR System Equipment

1. Overall Evaluation Principle: The true mission of DR is to achieve revolutionary high efficiency by changing the workflow of flat film while ensuring image quality; user evaluations of equipment should also be based on this, considering maintainability, failure rates, prices, overall costs, and subsequent costs. As a system device, a comprehensive overall evaluation is required, without being misled by a manufacturer’s claims about a certain component or indicator or term; comprehensive considerations should include image quality, work efficiency, usage costs, and after-sales service.

1. Image Quality: High-quality and stable imaging quality is one of the primary reasons for purchasing DR equipment and is the physical basis for improving diagnosis and treatment levels; it involves distortion, signal-to-noise ratio, resolution, clarity, detail display, etc.; mainly determined by flat panel technology, tube ray quality, computer, and image software processing capabilities; among them, flat panel technology is the core factor (material type, effective size, pixel matrix, pixel size, gray levels, DQE, spatial resolution, stability, etc.).

2. Work Efficiency: Reducing labor intensity and changing the workflow to improve efficiency is one of the main functions of DR and an important reference for purchasing such equipment; it involves dynamic range, imaging speed, data transmission/processing speed, and many other aspects; because many unnecessary work procedures are omitted, the normal output rate should be 2-3 times that of traditional screen/film systems.

3. Usage Costs: The largest cost is the maintenance and usage cost of the flat panel; the immature technology of amorphous selenium flat panels leads to a high scrap rate, resulting in expensive maintenance costs; imaging time is also longer, during which too much information is lost, leading to high time costs.

4. After-sales Service: Requires timely and complete; before purchasing, it is essential to consider the differences in after-sales service quality brought about by technical and brand differences; it is advisable to choose mainstream mature products from globally recognized large manufacturers; the high maintenance frequency of amorphous selenium equipment due to its immature technology is a factor that must be considered before purchase.

2. Actual Needs: Do not be misled by a manufacturer’s portrayal or claims about a certain component’s excellent performance / a certain outstanding technical indicator / a unique application, but start from the actual needs of the department in your hospital, comprehensively considering the overall performance of the equipment, image quality, usage costs, and after-sales service.

1. If you are a larger hospital with a high patient flow, purchasing equipment has always emphasized brand names, and you tend to prefer leading or advanced products, it is recommended to choose between Philips dual panels and Siemens dual panels (of course, the single panel DR from these two brands is also the first choice). Philips’ entire series and most of Siemens use Trixell 4600 flat panels (17×17″ cesium iodide/amorphous silicon flat panels), which are recognized as top products.

2. If your hospital is relatively sensitive to equipment prices but has certain technical pursuits, you might consider GE, as well as other manufacturers using cesium iodide/amorphous silicon panels besides Philips and Siemens, such as Beijing Wandong, Shanghai Zhongke, and USA Changqing. GE’s panels are also cesium iodide/amorphous silicon panels, 14×17″, but not Trixell’s, rather GE’s self-produced panels based on certain industrial panel technology; its main drawback is that due to high heat generation, it requires water cooling, which can increase failure rates and quantum noise.

3. If you are not very concerned about details and just need a flat panel DR, with low price being the most important, then Canon panels (i.e., gadolinium oxide/amorphous silicon panels) and amorphous selenium panels are also good choices. Brands using Canon panels include various Japanese brands (Toshiba, Shimadzu, etc.) and some Siemens models; the drawback of Canon panels is slightly lower parameters (slightly poorer images), but they are lightweight, so bedside DR machines generally use them. Many manufacturers also use amorphous selenium panels: Anke, Kodak, etc.; the drawback of amorphous selenium panels is their extremely high return rate, but they are cheaper than cesium iodide/amorphous silicon panels.

4. If the hospital has a high demand for cost-effectiveness, it is strongly recommended to choose CCD-DR. Undoubtedly, among all types of DR, CCD-DR has the lowest price. The main drawbacks of CCD-DR are two: geometric distortion in images (due to the presence of optical systems), and higher X-ray doses during imaging. The biggest advantage is its low cost. In situations where one does not want to spend too much money but wishes to buy DR, CCD-DR is the first choice. Manufacturers producing CCD-DR include Beijing Wandong, Swissray, IMIX, etc.

2. Pursuing the highest cost-effectiveness: Low price and high quality are the highest pursuits of users.

3. Try to purchase products and services from professional large manufacturers and conduct preliminary research and investigation.

IV. Evaluation of Various Manufacturers and Products in the DR System Equipment Market

First Tier: Philips full series DR, Siemens high-end DR (those using Trixell panels are high-end products; for market segmentation, Siemens also has low-end products using Canon panels); recognized as the best in the world, with the highest overall level of flat panel technology, tube quality, mechanical performance, and workstation processing capability, with excellent image quality, work efficiency, usage costs, and after-sales service.

Second Tier: GE full series DR; its cesium iodide/amorphous silicon panels are transformed from certain industrial panel technology for medical use, with a slightly smaller effective size of 14×17″, and lower technical indicators such as pixel size and resolution, resulting in poorer image quality.

Third Tier: Other DR products using cesium iodide/amorphous silicon panels, such as Pantai, Changqing, Wandong, etc.; as the main components of DR equipment, the flat panel technology they use is still good, which is the main reason for their ranking in the third tier; however, due to their low-quality tubes, poor mechanical performance, and low-level operation and post-processing workstations, their overall performance cannot compete with the first two tiers.

Fourth Tier: Low-end DR from Siemens using Canon panels, and Japanese DR from Shimadzu/Toshiba; the overall technical level of the flat panels is poor, and image quality is subpar; mostly used for bedside machines with low diagnostic requirements.

Fifth Tier: DR from Kodak, Anke, Youtong, etc., using Hologic amorphous selenium panels; their flat panels have low manufacturing costs but high return rates due to technical shortcomings; Hologic has gradually exited the DR system equipment market, while Kodak and others focus on low-end small hospitals at the cost of lowering diagnostic requirements.

Sixth Tier: DR with CCD flat panels, currently produced mostly by small companies, due to inherent technological deficiencies, their application range is shrinking and will inevitably be eliminated; however, there is still some market space in clinic-type medical institutions.

V. Certain Technical Essentials of DR System Equipment

1. The so-called energy subtraction of a certain manufacturer

1. The essence of energy subtraction is to use two different exposure conditions to expose the same substance in succession, obtaining separate images of lower density and higher density materials. Currently, it is mainly applied in chest imaging, attempting to overcome the defect of rib obstruction of some lung tissues on flat films.

2. The ultimate goal of energy subtraction is to hope to see lesions obscured by ribs. So, what kind of patients should this be done for? Who decides to expose the patient twice? A patient receives a check-up request from the clinical doctor’s office to the radiology department for imaging, involving three roles (patient, clinical physician, technician) who have no foresight or decision-making power regarding this; they cannot foresee the need for a second exposure.

3. In fact, at the final diagnostic stage of the conventional flat film workflow, the diagnosing doctor faces three possibilities: first, there is a soft tissue lesion behind the rib, but it is not visible, and the doctor has no reason to request a second exposure for such a patient; second, there is indeed no soft tissue lesion behind the rib, and a second exposure is unnecessary; third, the range of soft tissue lesions behind the rib exceeds the width of the rib, and can be seen against the lung tissue, indicating the need for further detailed examination, but from qualitative, quantitative, and locational perspectives, as well as the feasibility of technical implementation (this technology requires two exposures in a very short time), it clearly exceeds the capabilities of DR, necessitating the use of CT and other devices.

4. The proposal of this technology is inspired by a certain lung lesion that has been confirmed by CT to be obscured by the rib, which was not detected on flat film. Therefore, the technology proposer believes that if the ribs are removed, the lesions can be displayed. This is a typical retrospective thinking, merely aimed at solving the problem without considering whether the actual environment allows for such a solution.

2. The so-called tissue balancing of a certain manufacturer

1. Tissue balancing is to display tissues with significant density differences on the same image; essentially, it involves observing low-density and high-density tissues within relatively narrow gray levels separately, which can be achieved by adjusting gray levels and contrast on PACS diagnostic workstations. This so-called “advanced technology” is, like the former, a word game with almost no practical application value.

2. For doctors using diagnostic workstations, adjusting gray levels and contrast to observe tissues of different densities is a very natural thing.

3. For hospitals without diagnostic workstations, as they still face film images, this technology may be useful. However, this adjustment requires some time; even in an ordinary city-level hospital, the technician’s workload is already high, and there is simply no time to do such things; in larger hospitals with higher traffic, the possibility of implementing such time-consuming post-processing is almost nonexistent.

4. In summary, the first two so-called “new technologies, new applications” are merely deceptive word games, essentially highlighting their differences from other products to attract user attention. The digital images provided by DR are merely a preliminary screening tool, providing images that are tissue overlap images, and its most important function remains the same as traditional flat films. If the density of lesions is small but still discernible from normal tissues on DR images, the final diagnosis still requires further CT and other examinations. Digital flat films currently cannot, and in the future will not, meet the clinical requirements for quantitative, qualitative, and locational demands for most lesions. The true mission of DR is to achieve revolutionary high efficiency by changing the workflow of flat films while ensuring image quality, and it is neither intended nor capable of replacing CT or other diagnostic devices.

3. The distinction between indirect digital radiography (IDR) and direct digital radiography (DDR)

1. Indirect Digital Radiography (IDR) is a digital photography technology that uses silicon semiconductor indirect collection of X-particles, employing a two-step digital conversion process where X-ray particles first convert to visible light and then are detected by photomultiplier tubes to convert into electrical signals. It consists of Gd2O2S:Tb or CsI as the X-ray conversion screen, or scintillator, where X-rays pass through the reflective layer to reach the scintillator, exciting visible photons; the visible light is transmitted to photodiodes, which trigger field-effect transistors to produce output signals. There is some energy loss in these conversion processes, but it has a high X-ray absorption efficiency.

2. Direct Digital Radiography (DDR) is a digital photography technology that uses so-called direct X-particle technology, where X-ray particles are converted into electrical signals in the selenium coating layer; it does not produce visible light but only conducts electrons, theoretically avoiding the generation of scattered rays, thus having no energy loss in the photoelectric conversion process. However, due to the poor X-ray absorption efficiency of the selenium layer, the actual conversion efficiency is not good, and imaging time is long.

3. Regardless of the type of technology used in the flat panel, the goal is to obtain the most realistic diagnostic images possible; given the current feasible production process levels, amorphous silicon indirect digital conversion technology is the best choice for producing flat panels, which is why large medical equipment manufacturers like PHILIPS, SIEMENS, and GE use amorphous silicon panels; especially Trixell panels, whose unique process results in imaging quality far superior to that of amorphous selenium panels and other amorphous silicon panels. (The Trixell panel’s CsI scintillator layer, due to its crystal structure, does have some light scattering during signal conversion, resulting in slight energy loss, but it has little impact on the final image quality; its high quantum detection efficiency (DQE) allows for high-quality images at lower dose exposures; due to fast imaging, it can be used in fluoroscopy and time subtraction fields, greatly increasing the range of X-ray examinations.)

4. The quality of radiographic images is formed by the joint action of many factors; merely emphasizing how much energy loss occurs in a single conversion process cannot guarantee high-quality diagnostic images; it also depends on the actual conversion efficiency and final image quality, and one cannot only look at the theoretical values of a certain technology.

5. Almost all world-class experts and scholars recognize that amorphous silicon panels have better imaging quality stability than amorphous selenium panels.

4. The so-called direct energy conversion of amorphous selenium panels without energy loss

1. Using photoconductive material amorphous selenium panels does not produce visible light but only conducts electrons, avoiding energy loss from scattering and refraction, which is beneficial for improving image clarity.

2. Theoretically, amorphous selenium panels do not have energy loss in the photoelectric conversion process, but this does not mean their conversion effect is excellent, nor does it mean their imaging quality is high; in fact, at low radiation doses, their imaging quality is difficult to meet diagnostic needs; that is to say, to obtain high-quality images, amorphous selenium panels require very high radiation doses; only under high-dose exposure can their imaging quality barely match that of amorphous silicon panels represented by Trixell at low doses. This is contrary to the environmental requirements of reducing radiation exposure for patients and staff.

3. Using selenium as a photoconductor can directly convert light signals into electrical signals, theoretically avoiding the process of converting visible light into electronic signals, thus preventing scattering; however, the low absorption rate of the selenium layer for incident X-rays results in a loss of much original information; the so-called “direct conversion” process is also very slow, which not only affects work efficiency but also leads to significant information loss; therefore, under low-dose conditions, image quality cannot be guaranteed, and a very high radiation dose is required to obtain effective diagnostic images. Analyzing its working process reveals that the claim that “amorphous selenium is direct conversion with no energy loss” is purely taken out of context; it merely avoids the energy loss of the photoelectric conversion process of amorphous silicon panels, but it is certainly not without energy loss; on the contrary, due to its low absorption rate for X-rays, the slow speed of converting X-ray particles into electrons, and the slow final imaging speed, it leads to a large amount of information loss, and its image quality is significantly inferior to that of amorphous silicon panels (especially Trixell panels), which can only compensate for its excessive information loss by increasing radiation doses. Another fatal flaw is that the selenium layer is particularly sensitive to temperature, with very poor stability and usability, greatly limiting its usage conditions, and it is prone to damage and has a high return rate.

4. Overall, the technology of amorphous selenium panels is still very immature, specifically manifested in the need for high radiation doses, poor stability, and extremely high return rates. Its representative manufacturer Hologic has encountered insurmountable technical difficulties and has exited the DR system equipment market.