Author: Xing Lin Dongjing
Digital x-ray photography; Chest film; Pulmonary nodules
Digital x-ray photography; Chest film; Pulmonary nodules
Digital Radiography (DR) is a digital X-ray acquisition system, which consists of a detector, a scanning controller, a system controller, a system controller and an image monitor. Generalized DR includes digital X-ray machine, DR based on intensifying screen and +CCD, DR based on flat panel detector (PPD), DR based on linear scanning, and so on. According to different detection technologies, DR system can be divided into direct digital radiography (DDR) and indirect digital radiography (IDR). DDR refers to a photographic system that directly converts X-ray photons into digital information by using flat panel detector technology, which is a narrow sense of digital X-ray photography technology. Dual Energy Subtraction (DES) of digital photography is based on this technology.
As early as 1925, the concept of subtraction image (SI) was put forward. 1934, Ziedesdes plantes invented the film subtraction method, 1978, Winsconsin university organized and designed the digital image processor [1]. Mistretta, the founder of 1980 digital image, announced the success of digital subtraction angiography (DSA). DSA defines itself accurately, but it also exposes the shortcomings of its application-it can't subtract and separate tissue images other than blood vessels, such as separating bone tissue images from soft tissues. This problem has started the research of dual-energy subtraction technology, the core of which is to use two kinds of X-ray photons for photography. As early as 1983, the chest radiograph digitization technology was studied in clinical application. However, the low spatial resolution of digital radiography at this time limits the clinical application of subtraction technique. Since the beginning of the new century, with the invention of amorphous selenium (aSe) flat-panel detector DR, the clinical application research of dual-energy subtraction technology is brand-new in depth and breadth [2 ~ 4].
DR principle of 1 amorphous flat panel detector
Amorphous Sun (aSe) flat panel detector DR overcomes the shortcoming of low spatial resolution of the previous generation. In this technology, incident X-ray photons are captured by photoconductive semiconductor materials, and the received X-ray photons are directly converted into charges, and then the generated electrical signals are read out by thin film transistor (TFT) arrays to obtain digital X-ray images [5]. The biggest advantage of this working mode is that it completely overcomes the image blur effect caused by light scattering in the intensifying screen or scintillator of indirect conversion technology detector, so it can be used as a reference.
When X-ray photons incident on the human body are absorbed and attenuated by different tissues and finally act on the selenium layer with overlapping electrons, the photoconductor of the selenium layer produces positive and negative charge pairs proportional to the absorbed X-ray energy due to different X-ray intensities. The high voltage between the top electrode and the battery matrix generates an electric field in the selenium layer, which separates the positive and negative charges, and the positive charges move to the battery matrix until the capacitor is stored in the thin film transistor. The charge stored in the matrix capacitor is proportional to the X-ray intensity, and these charge signals are stored in the electrodes of the TFT. Each TFT and capacitor form a pixel unit, and each pixel area has a field effect transistor, which plays a switch role when reading the electrical signal of the pixel unit. So as to ensure that the scanning circuit sequentially reads the charges of each matrix capacitor unit one by one, and converts the electrical signals into digital signals, thereby forming an image. Direct conversion technology completely avoids the visible light scattering effect in indirect conversion technology.
Principle of dual-energy subtraction technology
Diagnostic radiography uses low-energy X-ray beams, which mainly produce photoelectric absorption effect and Compton scattering effect in the process of passing through human tissues. The intensity of photoelectric absorption effect is positively correlated with the atomic weight of irradiated substances, which is the main way for high-density substances such as calcium, bone and iodine contrast agent to attenuate X-ray photon energy. Compton scattering effect has nothing to do with the atomic weight of the irradiated substance, but is a function of the electron density of the tissue, which mainly occurs in soft tissues. The images obtained by conventional X-ray photography contain the comprehensive information of the above two attenuation effects. Dual-energy subtraction photography takes advantage of the different attenuation modes of X-ray photon energy between bones and soft tissues, and the differences of photoelectric absorption effects of substances with different atomic weights. This difference between attenuation and absorption is more strongly reflected in the variation of attenuation intensity of X-ray beams with different energies, and the intensity of Compton scattering effect has nothing to do with the energy of incident X-rays in a large range, so digital photography is used to separate the information of the two absorption effects. The dual-energy subtraction mechanism is to selectively remove the attenuation information of bones or soft tissues, and then obtain tissue characteristic images that can reflect the chemical composition of tissues-pure soft tissue images and bone tissue images.
3 dual-energy subtraction photography
3 1 double exposure method Double exposure method refers to the method that subjects are exposed twice independently with different X-ray output energy (kVp) to obtain two images or data, which are reconstructed into soft tissue density images, bone mineral density images and ordinary chest radiographs through image subtraction or data separation and integration [4 ~ 7]. The peak value of low-energy x-ray is 60~85 kVp, and the peak value of high-energy x-ray is 120~ 140 kVp. The study of chest dual-energy subtraction began with double exposure method [5,6], and was later applied to film intensifying screen system, scanning projection photography (SPR), computer photography (Cr) and digital photography (DR). But most of them are only research reports, which are basically not used in clinic. The main reason is that it is difficult to shorten the time difference between two exposures to the required range and eliminate the wrong coding between two images caused by the motion displacement of the subject between two exposures [4 ~ 7]. It was not until the appearance of direct digital radiography (DDR) that this problem was effectively solved. Because the system adopts 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 greatly reduces the error coding. At the same time, due to the high detectable quantum efficiency (DQE) and wide energy separation range of the system, the output of low-energy and high-energy X-rays is reduced to 60~80 kVp and110 ~150 kvp without reducing the quality. In particular, DFP directly converts the collected information into visible images, thus becoming a convenient and effective chest X-ray examination method [8].
32 One-time exposure method Dual-energy subtraction photography One-time exposure method is to separate the energy of the remaining X photons after passing through the exposed object to obtain two images with different energy. This method was originally proposed by Speller et al. in 1983, in order to eliminate the problem of double exposure. They stacked two sets of film intensifying screen systems in a special cassette, separated by a copper filter. X-rays with lower energy are imaged in the front film, and X-rays with higher energy are imaged in the back film through the filter, thus realizing energy separation [2]. Barnes et al. and Ishigakei et al. applied the one-time exposure method to various CR chest photography systems, and replaced the double-film intensifying screen system with a double-layer image board. The post-processing function of information improves the image quality [9, 10]. In the next 20 years, the application of one exposure method in DR chest photography has made great progress, and various forms of energy detectors have been applied in clinic one after another [1 1].