Volume 5, Number 2
April 2006
Film radiography is the dominant volumetric nondestructive testing technique utilized throughout the world. Film is lightweight, flexible, and suitable for diverse applications with a proven track record of more than one hundred years. However, film does have disadvantages. It requires significant amounts of time (approximately 20 minutes) for processing that must be done in a darkroom using chemicals that have to be legally disposed of. In addition to a limited shelf life, processed film requires a storage area in which the temperature and humidity are controlled — a significant expense. In contrast, digital radiography requires none of the above. Radiographic images can be generated, optimized, analyzed, stored and distributed in digital format.Computed Radiography
Computed radiography is a transitional technology between film and direct digital radiography. A reusable, flexible, photo stimulable phosphor (PSP) plate is loaded into a cassette and is exposed in a manner similar to traditional film radiography. The cassette is then placed in a laser reader where it is scanned and translated into a digital image. Depending on the resolution required and image size, the process of digitizing may take from one to five minutes. Once digitally captured, the image may be stored on a computer or other electronic media. Archiving is made easier and the images can be electronically distributed to others for viewing.
Direct Digital Radiography
A direct digital system is different from computed radiography in that it digitizes the photon radiation that passes through an object directly into an image that can be displayed on a computer monitor. Amorphous silicon, charge coupled device (CCD), and complementary metal oxide semiconductor (CMOS) are the three principle technologies used in direct digital imaging. Direct digital system images are available for viewing and analysis in seconds as compared to the minutes required in computed radiography systems. The increased processing speed is a result of the unique construction of the pixels in a direct digital system; an arrangement that also allows an image resolution that is superior to computed radiography and most film applications.
How Do CMOS Pixels Work?
Making a direct digital image involves converting X- or gamma radiation into a digital signal, a multi-step process. A conversion screen is exposed to radiation and converts the radiation into light. The pixel, in direct contact with the screen, converts the light into electrons that are then stored in a capacitor. When the capacitor discharges, the current is sent to an amplifier. Each CMOS pixel is configured with its own amplifier, a significant improvement over CCD detectors in which only one amplifier is available for each row of pixels in the array. Timing and control circuitry for the CMOS detector is resident on the chip, making additional circuitry adjacent to the panel unnecessary. The last step of the process is to perform analog to digital conversion of the signal that is done right at the sensor. In this process, the analog signal is converted into binary code. The binary signal is then sent directly to the computer.
A CMOS imager can be used with either X- or gamma radiation. Compared to traditional film radiography, the exposure times are significantly shorter, thereby reducing the duration of radiation exposure to the radiographer. The object to be imaged may be placed directly on the panel, or the panel may be positioned on or adjacent to the object. The source is energized and the detector is started. The image is captured and displayed on the computer monitor ready for analysis.
Digital Image Processing and Analysis
A digital system with more than an eight-bit processor is capable of providing an image with better contrast than that achieved with film. Image sensitivity meets or exceeds film characteristics. The highest spatial resolution available in a CMOS detector at this time is 39 microns (0.0015 in.) and linear array detectors typically have a pixel size of 80 microns (0.0032 in.).
Analysis software for digital images offers a wide range of functions not available for film radiography. For example, images that are over or under-exposed are less problematic with digital systems because of the increased latitude of the collected data. In addition, the ability to enlarge or magnify portions of an image on the computer screen improves the technician’s ability to interpret areas of interest. Automatic algorithms can also be programmed to detect specific discontinuity types. The inspection process can be fully automated and integrated with the manufacturing process allowing near real-time, zero defect manufacturing. Archiving an image is a simple save function from a computer pull-down menu.
Using Digital Imaging Systems In the Field
Flat panel digital imagers are fragile when compared with film. However, that does not limit their use to laboratory applications. Flat panel digital imaging systems have been used in field environments as diverse as those found on Alaska’s North Slope and in Saudi Arabia. Often, sheet metal housings are fabricated to protect the imager. The flat panel imager may be housed in a case suspended through a system of springs. The springs act as shock absorbers that prevent the panel from contacting the casing as well as to prevent direct impact to the panel. Since panels are heavier than film, sufficient means are necessary to support the panel in the proper position during an exposure. Methods to achieve this can be as simple as a series of shock cords and nylon straps.
Corrosion under insulation (CUI) is a major concern on insulated piping systems in power plants, refineries, and transportation pipelines (Fig. 1). When water infiltrates the galvanized outer wrap, the contact zone between the insulation and pipe becomes a perfect location for corrosion. Hand-held C-arm systems that consist of a lightweight arm that can reach around pipe are frequently used for detection of CUI. One side supports the imager, and the other supports a pulsed X-ray tube (Fig. 2). These lightweight and portable systems are deployed using a tangential radiography technique (Fig. 3). When looking at an image of the intersection of exterior insulation and the pipe surface, the undamaged external surface of the pipe will appear as a straight black line. If corrosion is present, or if water is present in the insulation, the pipe surface appears rough or irregular and the water may appear as slight fogging in the insulation. Dark scale, a byproduct of the corrosion process, may also appear in the image.
Care in Daily Use. Here are some tips technicians should keep in mind when using any digital radiographic system.
- Make a daily check of the panel and housing to ensure that the imager is clean and properly aligned and that all electrical connections are secure.
- Carry the imager with both hands to avoid dropping or jarring it.
- Be aware of your footing and surroundings.
- Return the imager to a padded storage container for transportation purposes.
- Carry an extra set of communication and power cables.
Digital imaging is the future of industrial radiography. Although film radiography is a well-established technology that will undoubtedly be used for years to come, digital imaging technology continues to improve and in time, can be expected to exceed film applications. TNT
Raymond R. Shepard is the Training Manager/Quality Specialist for Kakivik Asset Management LLC, an NDT and corrosion engineering services company in Anchorage, Alaska. He is a former Professor of welding and NDT at the University of Alaska, Anchorage. (907) 770-9418, <rshepard@kakivik.com>.
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