Rapid industrial and technological growth throughout the world makes it necessary to develop new materials along with advanced Non-Destructive Testing (NDT) Methods to ensure their quality without premature failures. The complex modern system, where materials are required to perform consistently with optimum efficiency, demands stringent quality control of engineered components. With the increase in power of personal computing systems, all inspection data are being digitized to analyze signals effectively. Radiography is not an exception. New advanced radiography systems have been developed that are gradually replacing conventional film radiography. Fluoroscopy systems, image plate based computer radiography systems and flat panel technology are new additions in digital radiography. The advantages of digital radiography are enormous in terms of speed, dynamic range, reusability, etc. Also, digital radiography information can be shared with any part of world in nearly instantly through the internet.
The image can be modified without disturbing the content of the image. In this paper, different digital radiography systems and advantages of image processing in digital radiography are discussed.
Digital radiography is a method of obtaining a digitized radiographic image of the object, which can be displayed on a monitor. There are different types of systems which can produce digital images. Each system has its own distinct operational principle. One of the many advantages of digital radiography is that it does not need wet chemical processing and there are no associated chemical hazards with the process. Another advantage is that the amount of radiation required to produce a digital image is significantly less than conventional film radiography. In digital radiography, an image capturing device is used in place of x-ray film to transform radiation photons into useful information. Later, the image, recorded in the detector, is transferred into a suitable viewing medium. This virtual image can be processed later to improve contrast, and brightness and many filtering operations can be carried out to enhance the image. Finally the image can be stored in hard disc, CD, or any other storage device.
The oldest system for digital radiography is fluoroscopy. The main drawback of this system was poor resolution and limited brightness. The fluoroscopic image can be coupled with image intensifiers to improve image quality. In the mean time, special phosphor plated image plates were developed which can be directly read in image scanners to produce a better image with less radiation. This system gained wide acceptance for medical use. The latest addition in digital radiography is the Flat Panel Detector (FPD) or Digital Detector Array (DDA), which is a two dimensional array of transistor base detectors. The size of each detector determines the resolution of image.
In fluoroscopy, penetrating radiation passes through the object to produce an image on screen which is viewed concurrent with radiation. It allows the radiographic interpretation to be performed simultaneously with the progress of the event. The arrangement of the source, object, and the screen is similar to conventional radiography. The most important event in fluoroscopy is the conversion of radiation to light by means of a fluorescent screen. The light may be viewed directly, amplified, or converted to video signal. The most popular screen used is ZnCdS (zinc cadmium sulphide) and Gd2O2S (gadolinium-oxy-sulphide). The screen must have high conversion efficiency, low unsharpness and useful spectral emission. The schematic representation of fluoroscopy is shown figure 1. Because of radiation hazards, the screen is not directly viewed. The image is transferred from the screen onto the viewing window with the help of a reflecting mirror. The advantage of fluoroscopy is the low brightness of the screen and high internal unsharpness of the screen. In modern systems, the screen is coupled with an image intensifier to improve brightness and visibility of the image. In spite of using advanced technology, image sharpness is still a concern and in many applications magnification is used to get fine details.