Direct digital radiography, or DDR, refers to the technology, which use one-dimensional or two-dimensional x-ray detectors to directly turn x-ray image information into a digital signal under the control of a computer with image processing functions.
Direct digital photography is a forward step of the digital general X-ray photography. Comparing with the indirect way of storage phosphor digital photography, it conclude four characteristics: (1) smaller doses to patients; (2) higher dynamic range, quantum detection efficiency; (3) cover a larger range of contrast, image layers are richer; (4) with enhanced image resolution, faster, more efficient work.
Digital Radiography, or DR, which is composed by the electronic magazine, scan controller, system controller, video monitor display device etc., is to turn x-ray photon directly into digital images through the electronic magazine, and a broad sense, direct digital X-ray photography. The narrow sense of direct digital radiography usually refers to using the image of direct conversion flat panel detector to directly convert into digital radiography of technology. It is the true direct digital X-ray camera system and the current trend .It will help to improve the quality of medical diagnosis, and promote modernization of the hospital management level.
Technical terms for the DR, to determine its image quality is not only flat panel detector type used, as well as plate DQE and MTF, acquisition matrix, collecting gray scale, spatial resolution, the smallest pixel size and other important factors.
Indirect digital photography (IDR): flat panel detector based on amorphous silicon photodiode array as the core of the medical x ray image detector.
Imaging process: x line -visible light -electric charge image -digital image.
The basic structure of amorphous silicon flat panel detector:
1. Cesium iodide scintillator (x line - the process of visible light).
2. Amorphous silicon photodiode array (visible - the image of the process of charge)
3. Line drive circuit.
4. Image signal read circuit.
Imaging principle: first, at the top of the cesium iodide detector scintillation crystal turn the incident x-ray into visible light. Then amorphous silicon photodiode array, under the visible light layer of cesium iodide, make the photodiode generate current, which will convert visible light into electrical signals, forming their own storage charge in the photodiode capacitance. Changes in the charge of each pixel and the intensity is proportional to the incident x-ray, while the array will be the continuous x-ray image is converted to a number of rows and columns form the total array of images. The density matrix determines the spatial resolution of the image. Under the unity control of central timing controller, the line driver circuit living in the line direction and the signal living out the direction read out the charge circui line by line, which will be converted to a serial pulse sequence, and quantize quantify the digital signal. the digital signal form the x-ray digital Image after being send to the image processor through interface circuit.
 

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