Special Imaging Methods

T.O. 33B-1-1 6-47 particularly effective when X-ray sources with small focal spots are used (mini-focus and micro-focus sources as described in paragraph 6.2.5.2.3). Geometric enlargement can be realized by moving the radiographic object away from the detector, toward the radiation source. Moving their inspection object halfway between the X-ray source and detector produces a magnification factor of two. If the object is closer to the X-ray source (say 1/10th of the source- detector distance, then the magnification is ten times. Useful radiographic images have been produced with small microfocus sources with geometric magnifications of 10X or more. Although this discussion has covered geometric magnification using only a film detector, this special method can be used with any imaging detector, including radioscopic detectors, as discussed in the next sections. Recognize that as magnification increases the inspected volume of the object decreases. Therefore, more radiographic views may be needed for a complete inspection. 6.6.3 Special Imaging Methods. Conventional film radiography has its own capabilities and its own limitations. The capabilities of film radiography have been covered thoroughly in the previous sections. Consider some of the limitations of film radiography. Film takes a long time to process, and the results of the inspection cannot be known until the film is processed. Therefore, in some situations, a need exists to provide more rapid means of imaging. There are many alternatives to the use of conventional film for recording radiographic images. These include fluoroscopy, electronic imaging systems, Polaroid radiography, radiographic paper, and photothermographic film. The following paragraphs discuss the advantages and capabilities of these imaging systems. Most provide for more rapid imaging than is available using conventional film radiography. However, each of these methods also has its own limitations. Special imaging methods that are not included in authorized inspection manuals SHALL NOT be used without written approval of the appropriate depot engineering activity. 6.6.3.1 Radioscopy. The oldest non-film imaging method involves the use of fluorescent screens to produce a visible image. These phosphor screens fluoresce (emit visible light) in proportion to the amount of radiation striking them. Thus, an instantaneous visible image is produced, and the results may be instantly read using a now outdated method called fluoroscopy. Modern, prompt-view or real-time radioscopic systems make use of closed-circuit television systems to bring these images out to a safe viewing location, where a bright television image can be viewed. Radioscopy is defined in ASTM standards as "the electronic production of a radiological image that follows very closely the changes with time of the object being imaged" (see reference 9). For tutorial information about radioscopy, see reference 10. 6.6.3.2 Fluorescent Screens. The light-emitting fluorescent screen can be viewed directly to see the prompt X-ray image. However, this method is rarely used now because closed-circuit television methods can provide a safer, more efficient environment to view the low-light level signal from the fluorescent screen. The fluorescent screen light signal can be detected by sensitive television cameras such as the image orthicon. In some systems, the weak light signal from the fluorescent screen is amplified by using a light-image intensifier tube between the fluorescent screen and the television camera. 6.6.3.3 Image Intensifiers. Image intensifiers are specially designed evacuated electronic tubes that intensify the image on a very fine grain fluorescent screen. The input signal, fine grain screens that are used in image intensifiers do not produce sufficient light to be viewed and employed for direct fluoroscopy. Therefore, an image intensification system is employed as shown in Figure 6-20. The fluorescent screen is backed by a photo-emissive layer that produces electrons in proportion to the number of visible light photons emitted by the fluorescent screen. A series of focusing and accelerating electrodes propel these electrons toward a second and much smaller fluorescent screen. This second screen has very high detail resolution. This screen is typically viewed by a light-sensitive vidicon or other television camera and displayed on a television monitor. The image intensifier provides the immediate imaging capability of the fluorescent screen while providing higher brightness and detail resolution in a safe area remote from the radiation. However, resolution is still less than that obtained with Class 4 radiographic films. Also, no permanent record is provided unless a photograph or video tape is made. The X-ray image intensifier is widely used as part of a radioscopic X-ray inspection system. Other radioscopic systems are described in reference 10.