Computed Radiography: Keep it Short and Simple
Industrial radiography, also known as X-ray inspection, is being continuously challenged to adequately replace conventional standardized film methods with computed radiography (CR). Radiography is one of the oldest nondestructive inspection methods used in various stages of manufacturing, fabrication and preventative maintenance programs for initial defect detection, corrosion monitoring and product conformity.
The adage “A picture is worth a thousand words” refers to a concept that a complex idea can be conveyed with a single still image. This adage achieves the main goal of visualization, specifically making it possible to absorb large amounts of data quickly in a single view. In nondestructive testing (NDT), we have historically used an analog image (film/radiograph) to be worth a thousand words. Now technicians can call this dynamic and prized possession a digital radiograph.
This method of inspection has many inherent challenges that the seasoned film radiographer must be aware of, and if not addressed properly, these conditions can be detrimental to the overall inspection results. The characterization and evaluation of this image requires additional training, over and above current requirements found in SNT-TC-1A, NAS 410 and CP-189 for the typical industrial radiographer.
CR, as defined by ASTM E1316 (photo stimulated luminescence method), is a two-step radiographic imaging process. First, a storage phosphor imaging plate is exposed to ionizing radiation. Second, the luminescence from the plate’s photostimulable luminescent phosphor is detected, digitized and presented on a high-resolution computer screen.
Managing this process, with the objective to achieve the optimum digital image, is the challenge faced by radiographic practitioners. The overall success is based on the digital image acquisition system utilized and variables that are consistent with typical radiographic conditions.
Implementing CR begins with a fundamental understanding of the radiographic principles that have been practiced for decades with film radiography and commissioned by the aerospace, automotive, construction, energy, oil and gas, and shipping industries. Certified radiographers are required to understand these principles, along with specific knowledge in the manufacturing and fabrication process to correctly interpret the acquired digital image. Geometric principles attributed to the setup are critical in the evaluation of hidden mechanical or fatigue failure and for assessment of repair work.
The utilization of the proper image quality indicators (IQIs) to ensure technique validation and contrast sensitivity, along with resolution, is both essential and mandatory based on most industry specification and code compliance. Integration of an operator defined quality control program to monitor equipment performance and maintain records of data collected is essential to ensure a stable continuous process. System and scanner performance tests will vary from manufacturer to manufacturer but must include those quality control checks specified by the manufacturer and be modeled to meet the technician’s requirements. If, at the time of inspection, significant equipment malfunctions are found, the technician may be required to perform more frequent testing to ensure good image quality.
Step 1: SetupGeometric parameters such as source-to-film distance (SFD) and object-to-film distance (OFD); as well as IQI, area of interest, scatter and alignment are variables that remain similarly consistent as experienced in film radiography. Items that require attention with a greater appreciation for the acquisition of the digital image pertain to the secondary scatter of the technique, knowing the inherent characteristics of the specimen under evaluation, using the correct IQI, optimizing the exposure based on the energy levels and the actual exposure time.
The rule of thumb exposure approach-X-ray or gamma-was to optimize sensitivity and resolution by using the lowest energy possible, thereby maximizing contrast and leading to longer exposure times. This approach is being debated routinely, as longer exposures have tendencies to saturate (overexpose) the imaging plates. Successful results have been obtained on thicker components using CR with significantly lower radiation doses from reduced exposure time.
Step 2: Imaging PlatesChoosing the proper imaging plate will be dependent on a number of factors, including the scanner that is used, the required resolution, nature of the specimen and quality of radiation energy. The best approach is to ensure that image quality-which is influenced by the scanner and imaging plate system-achieves the quality requirements of the technician.
The favorable choice would be to use a scanner that accepts both rigid cassettes, as well as the flexible plates since hardware being evaluated may require an imaging plate to conform to its configuration. Imaging plates and industrial film have a commonality in that the latent image is stored and is not visible until processed. The latent image on film is revealed when developed; the latent image on a CR imaging plate is not visible until a CR scanner or reader acquires the image. The need for chemicals to develop a latent image stored on imaging plate is not required.
The requirement for multiple films and exposures with film to satisfy a wider range of thicknesses is no longer necessary as the wider latitude is accomplished with CR imaging plates. With film radiography each specific film type was identified by one of four classes, ranging from special through class three. With computed radiography, the same imaging plate could theoretically qualify for all four classes due to the wider dynamic range of computed radiography vs. that of a normal film or screen system.
Silver bromide crystals are utilized in the matrix of film, where the latent image is acquired based on the relative exposure to radiation. In CR, a thin phosphor layer of fine-grained barium fluorohalide crystals captures the energy within the phosphor creating a latent image. Short radiographic exposures vs. long exposures can be compensated by adjusting the signal sensitive parameters of the scanner.
The use of lead screens as energy intensifiers in film radiography is now relegated to absorbing scattered soft radiation in CR, as the intensification is not applicable to the phosphor. Imaging plates can theoretically be reused thousands of times if they are handled carefully. Handling under industrial conditions may result in damage after a few hundred uses. Reusable phosphor plates are environmentally safe but need to be disposed of according to local state and federal regulations.
Step 3: Processing of the Imaging PlatesCR is considered an environmentally friendly process since there are no acids, caustic chemicals or effluents containing heavy metals as required when processing the analog film or radiograph. There is also no longer a need for dark rooms as required in the processing of film. Processing CR images can be accomplished in subdued light and can be easily adapted to customer locations for onsite processing.
Typical CR scanners use a laser-emitting red light and scan the imaging plate. When the imaging plate is put through the scanner, the scanning laser beam causes the electrons to relax to lower energy levels, emitting light that is detected by a photo-multiplier tube, which is then converted to an electronic signal. The electronic signal is then converted to discrete digital values and placed into the image processor pixel map-more ionizing radiation captured in a given region of the imaging plate the greater amount of blue light is emitted. Each pixel intensity value is proportional to the absorbed dose during the exposure.
When scanning, there is often a misconception in obtaining the optimum image. The practice of “scan at the highest resolution possible” does not always yield the highest resolution in the acquired image and should be carefully evaluated. Many factors, such as sensitivity, resolution (both spatial and contrast), sharpness, system noise, pixel intensity values and all other parameters specified in the radiographic technique should be considered prior to scanning.
The combination of imaging plates, scanner and software used should be classified and documented to ensure repeatability along with image quality. During the processing of the image, the computer applies various algorithms and displays an image on a computer monitor. Erasing the image plate is critical in removing any ghost images from the plate that may interfere with the subsequent image.
Erasing an image plate is accomplished by simply exposing the plate to a room-level fluorescent light. Most CR scanners automatically erase the image plate after laser scanning is complete. Achieving the optimum quality image in the raw data form without any post processing and/or a software enhancement converting the image is the key objective that closely represents the film quality that has been accomplished for decades.
Step 4: Post-ProcessEvaluating digital images is similar to evaluating film images, as they are required to be viewed in subdued lighting by qualified and experienced personnel. CR’s unique property-as compared to film systems-is the ability to display a wider range of visible gray-scale levels for a defined range of material thickness, particularly when image processing is used.
Achieving a good signal-to-noise ratio (SNR) in the area of interest will yield a quality digital image as required for inspection applications, similar to the contrast and resolution achieved for film images. Contrast and brightness are prominent terms used in film radiography and are restated as window and leveling when evaluating digital images.
Window (contrast) and leveling (brightness) adjustments are temporarily assigning different gray values instead of those gray values originally assigned because darker pixels can be lowered in gray scale value while lighter pixels can be raised. This ability allows areas of interest to be optimized for interpretation in single image.
Most CR imaging software packages have a range of enhancing abilities, however, no one single filter or combination of filters can produce a magic button for improved image quality. There are many parameters, such as edge enhancements, high pass, sharpen, unsharp mask and many others that compete with each other, and are traded off against each other. An improvement in one parameter can degrade another, resulting in decreased sensitivity and resolution. Achieving a significant sharpness parameter within a digital image, also known as spatial frequency response, will not result in a good image without good contrast-to-noise ratio (CNR) and SNR. Filters applied may increase or decrease these ratios.
Analyzing the radiograph for good CNR and SNR and other factors determining image quality can be demonstrated by most software platforms. These critical functions should be evaluated and procedures documented by the cognizant engineer or NDT Level III establishing the CR program. Using a magnifier loupe to evaluate and grease pencils to mark film are no longer required as all CR evaluation stations have these functions electronically and can measure reliably based on an image calibration point.
To achieve optimum performance of any CR program, even for the experienced radiographic practitioner, establishing an adequate training program with complete understanding of all post processing parameters is paramount. With all nondestructive inspection programs, operators are required in most cases to establish a quality control program that routinely validates an inspection process.
Using a phantom, a recommended traceable standard, at prescribed intervals allows for routine evaluations of the CR system operation. Parameters for variation with control limits should be documented and agreed on by the practitioner along with the technician. Phantom process verification can be accomplished by contrast, spatial resolution/unsharpness, converging line pair, geometric distortion, laser beam function and laser jitter, scan line integrity, blooming or flare, slippage, shading, erasure, IP artifacts and SNR.
Step 5: Final and FileFinal acceptance of the image is the same as film radiography and is the responsibility of the certified technician. Reference radiographs for defect severity and comparison are being updated to digital formats for ease of interpretation. Preserving the raw image is critical and maintaining its origin is a requirement by most industry codes, specifications and end users. It is the operator’s responsibility to maintain this data set to ensure recall in the original format as needed.
Post-processing enhancements, along with annotations, are required to be traceable to the original raw data file. Film packages, missing film, storage damage that were film related issues will now be lost images, damaged files, files that cannot be opened due to outdated software unless a proper data file protocol is established by the end user.
Take ChargeIs computed radiography the complete film replacement at this time? Is there one nondestructive examination that achieves 100% defect detection? Is ultrasonic phased array better than all other conventional ultrasonic techniques? The answer by experienced NDT practitioners would be unanimously no. All nondestructive inspection modalities have their inherent abilities and some prove to be better than others.
CR has improved significantly and should be credited for enhanced radiographic process, environmentally compliant, reduced radiation exposures, cost effectiveness over time and improved communication with today’s digital tools. The continued excuses such as resistance to change, cost to implement, low computer skill sets, lack of specifications, lack of training, perceived detection limits and associated factors are impeding the transition from film radiography to CR.
With the economic and environmental factors alone, the cost of film radiography is positioned to escalate. Implementing a CR program with solid fundamental practices, a commitment to integrity, along with a solid training program, will overcome these challenges. This same blueprint of practical perseverance was implemented when film radiography was first introduced decades ago. NDT practitioners must now step up and take charge. NDT