Author(s): Xing Xueling, Ma Yongqiang, Li Lin, Qin Tokyo
Keywords Digital radiography; chest radiographs; lung nodules
Keywords Digital radiography; chest radiographs; lung nodules
Digital Radiography (DR) is an X-ray digital acquisition system that consists of a detector, a scan controller, a system controller, and an image monitor. Radiography,DR) is an X-ray digital acquisition system consisting of a detector, scan controller, system controller, system controller, and image monitor. Broadly speaking, DR includes digital X-ray machine, DR based on sensor screen + CCD, DR based on flat panel detector (Plat Panel Detector, PPD), DR based on line array scanning and so on. Depending on the detection technology, DR systems are divided into Direct Digit Radiography (DDR) and Indirect Digit Radiography (IDR).DDR refers to the use of flat panel detector technology to directly convert X-ray photons into digital information, which is the narrow sense of digital radiography. DDR is a photographic system that uses flat-panel detector technology to convert X-ray photons directly into digital information. Dual Energy Subtraction (DES) is based on this technology.
The concept of Subtraction Image (SI) was introduced as early as 1925, the method of film subtraction was invented by Ziedesdes plantes in 1934, and a digital image processor was designed by the Winsconsin University Project Organization in 1978 [1]. In 1980, the founder of digital imaging, Mistretta, announced the success of Digit Subtraction Angiography (DSA), an expression that defines itself precisely but also reveals the shortcomings of its application - it is not possible to subtract and isolate tissues other than blood vessels, such as soft tissues. This problem has led to the development of dual-energy subtraction. This challenge led to the development of dual-energy subtraction, which is based on the application of two X-ray photomass spectrometry techniques, and which has been in clinical use since 1983 in parallel with the digitalization of chest radiographs. However, the digital X-ray photography at this time due to its low spatial resolution of the image, limiting the scope of the clinical application of subtraction technology. In the new century, along with the invention of amorphous selenium (aSe) flat panel detector DR, dual-energy subtraction technology clinical application of research is its depth and breadth of the new [2~4].
1 amorphous selenium flat panel detector DR principle
Amorphous selenium (aSe) flat panel detector DR overcomes the drawbacks of low spatial resolution of the previous generation, this technology utilizes a photoconductive semiconductor material to capture the incident X-ray photons, directly converts the received X-ray photons into an electrical charge, and then by the thin-film transistor (TFT) arrays will produce an electrical signal to be read out to obtain the digitized X-ray image [5], the biggest advantage of this mode of operation is to completely overcome the non-direct conversion technology detector by the sensing screen or scintillator in the light scattering caused by the blurring effect of the image, and thus have a very high spatial resolution.
When the incident human X-ray photons by different tissues of the absorption of attenuation, the final role in the electronic dark combined with the selenium layer, due to the different strengths and weaknesses of the X-rays, the selenium layer photoconductor according to the size of the absorbed X-rays energy to produce proportional pairs of positive and negative charges, the top electrodes and the cumulative matrix of the high-voltage in the selenium layer generates an electric field, so that the positive and negative charges are separated from each other, the positive charges moved to the cumulative matrix until the capacitor stored in a thin-film transistor, matrix capacitor, and the capacitor is stored in the thin film transistor. The charge stored in the matrix capacitor is proportional to the intensity of the X-rays, and these charge signals are stored in the inter-polar capacitance of the TFTs. Each TFT and capacitance to form a pixel unit, each pixel area has a field effect tube, in the read out of the pixel unit electrical signals play a switching role. Thus ensuring that the scanning circuit in order to read one by one each matrix capacitance unit charge, and will be converted to digital signals further image formation. Direct conversion technology completely avoids the visible light scattering effect in non-direct conversion technology.
2 Dual-energy subtraction technology principle
Diagnostic X-ray film used is a low-energy X-ray beam, it is in the process of penetrating the human body tissues, the main photoelectric absorption effect and Compton scattering effect, photoelectric absorption effect of the intensity of the irradiated material is positively correlated with the amount of atoms, calcium, bone, iodine contrast agent and other high-density material attenuation of the X-ray photon energy of the main way. The Compton scattering effect is independent of the atomic weight of the irradiated material and is a function of the electron density of the tissue, occurring mainly in soft tissue. The images obtained from conventional radiographs contain a combination of both of these attenuation effects. Dual-energy subtraction radiography takes advantage of the difference in energy attenuation of X-ray photons between bone and soft tissues, as well as the difference in photoelectric absorption of substances with different atomic weights, which is more strongly reflected in the attenuation intensity of X-ray beams with different energies, whereas the intensity of the Compton scattering effect is negligible over a wide range of energies independent of the energy of incident X-rays, and the information about the two absorption effects is separated and selectively analyzed using digital photography. Using digital photography to separate the information of the two absorption effects, selectively removing the attenuation information of bone or soft tissue, and then obtaining a tissue characteristic image that can reflect the chemical composition of the tissue - pure soft tissue image and bone tissue image, i.e., the dual-energy subtraction mechanism.
3 Dual-energy subtraction radiography
31 Double-exposure method The double-exposure method refers to the use of different X-ray output energy (kVp) on the subject of two independent exposures to obtain two images or data, the image will be subtracted or data separation and integration of reconstruction for soft tissue density images, bone density images and ordinary chest radiographs, respectively [4~7]. The low-energy X-rays used have a peak value of 60-85 kVp, and the high-energy X-rays have a peak value of 120-140 kVp. The study of dual-energy subtraction of the chest began with the two-exposure method [5, 6], and was later applied to the film intensifier screen system, scanned projection radiography (SPR), computed radiography, computed radiography, and chest radiography (CRR). Later, it was applied to film intensifier screen system, scanned projection radiography (SPR), computed radiography (CR), and digital radiography (DR), but most of them were only reported in the research nature, and basically were not used in the clinic, which was mainly limited by the difficulty of shortening the time difference between two exposures to the required range, and the difficulty of eliminating the mis-coding between the two images caused by the displacement of the subject motion between two exposures [4 ~7]. This problem was not effectively solved until the introduction of direct digital radiography (DDR). Since this system uses a high-speed digital flat panel detector (DFP), the time difference between two exposures can be shortened to 200 ms, and the patient can complete the examination with one breath-hold, which reduces the miscoding to a large extent, and the quantum detection efficiency (DQE) of this system is also reduced. At the same time, due to the high quantum detection efficiency (DQE) of the system, the range of energy separation is large, and the low-energy and high-energy X-ray outputs are reduced to 60~80 kVp and 110~150 kVp without degrading the quality, especially the DFP, which turns the acquired information directly into visual images, and thus becomes a convenient and effective method of examination for chest X-ray filming [8].
32 Single-exposure method of dual-energy subtraction radiography The single-exposure method separates the energy of the remaining X-ray photons after they pass through the exposed object, yielding two images with different energies. The method was originally developed to eliminate the mis-coding problem of the two-exposure method, and was first proposed by Speller et al. in 1983. They achieved energy separation by iterating two film intensifier screen systems in a special dark box separated by a copper filter plate, with the lower-energy X-rays being imaged on the film in front, and the higher-energy X-rays being imaged through the filter plate on the film in the rear [2].Barnes Barnes et al. and Ishigakei et al. applied the single-exposure method to a variety of CR chest camera systems, replacing the double-film intensifier screen system with a double-layer image plate, whose post-processing of information resulted in an improvement in image quality [9, 10]. In the next 20 years, the application of a single exposure method in DR chest photography has made great progress, various forms of energy detectors have been used in the clinic [11].