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As nondestructive examination (NDE) operations using computed radiography (CR) continue to rise in popularity, many trainers are finding it difficult to separate common myths from fact when educating newcomers to the technology. The end-result of a successful radiographic capture is a high-quality image that helps professionals to scrutinize the structural composition of a subject in order to make educated conclusions on its integrity. Final perceived image quality results from the cascaded effects of many factors relating to the equipment and techniques used in radiographic applications. As CR technologies continue to become more powerful and advanced, it is necessary to maintain regular training programs at all levels of expertise. Proper education ensures that jobs are completed on time, within budget and, most importantly, safely and accurately.
Specifications and RequirementsIndustry rhetoric presents examiners with a plethora of specifications when choosing the correct equipment to meet their testing needs. Sell-sheets are littered with details on material parameters such as spot size, pixel size, bit depth, resolution and sharpness. While these figures certainly aid in the comparison of various products on a rudimentary level, they do little to provide significant insight into a system’s true capability to meet the needs of the customer. In fact, not a single specification, on its own, can guarantee that a system will achieve the radiographic sensitivity required for a particular examination task.
A training course should include insight into learning how to correctly use specifications to make informed judgments on the feasibility of a project. There are many parameters that contribute to a system’s final perceived image quality and radiographic sensitivity, such as the signal-to-noise ratio, contrast-to-noise ratio and modulation transfer function. While these factors do, in fact, combine to produce a quality image, there are a few points that must be taken into consideration when weighing the importance of each.
Because of the inherent competitive interactions between each parameter, NDE students must learn where to make educated trade-offs between specifications to achieve desired functionality, and how to tweak the specifications within their control in order to create successful synergies.
Maximizing MTFThe modulation transfer function (MTF) is the percentage of the signal that can be transferred through the system as a function of spatial frequency. Because the MTF value is commonly associated with final image quality, many beginners are quick to reference only the overall MTF of the radiography system when formulating a testing technique. In reality, each individual system component has its own MTF characteristic. This is true for X-ray sources, imaging plates, CR readers, display monitors and even the observer’s eye. Some MTF parameters are static and others such as laser power, spot stability, photostimulated luminescence and electronic bandwidth can be controlled by manipulating design or optimization within a system.
The cascading effect of each component’s characteristics on the overall system MTF makes it crucial that educators spend time informing students of their effects on overall image quality.
Laser Spot SizeA commonly misunderstood figure in computed radiography specifications is the laser spot size. Solutions are often advertised with exact diametric figures to shed favorable light on a certain piece of equipment.
Students must take into account that a ‘spot’ does not represent a crisp, clear circle with a defined boundary. Because of this imperfect circular geometry, manufacturers do not all conduct measurements using the same formula. Therefore, comparing systems based on spot size alone will not provide a meaningful comparative analysis. Other factors, such as scatter within the phosphor layer or depth of field, affect the correlation between spot size and overall image quality. A properly trained test engineer will evaluate spot size while taking into account all other considerations when reaching system feasibility conclusions.
Pixel SizeNewcomers to computed radiography are quick to succumb to the misperception that a small pixel size always results in higher quality imagery. In most cases the “clock rate” of a system is constant, resulting in larger pixels produced during fast scans and smaller pixels produced with slow scans. While smaller pixels often provide more detailed images, this is not always the case.
A point of diminishing returns exists, after which a smaller pixel can actually result in a less acceptable image. This is caused by the spot maintaining a longer dwell time on the subject, which can lead to overall image degradation through excessive penetration and spread. Long high-resolution readouts may result in wasted time, large file sizes and system stress with very little or no actual improvement in image quality.
Exposure and SNRWhen conducting radiographic examinations, exposure values have a large effect on image quality. Producers of CR equipment tend to market the ability of radiographic equipment to reach the highest signal-to-noise (SNR) classification of CR systems as set by ASTM International and EN standards. But because most products on the market are capable of reaching these levels, it is important to also look at low-exposure performance of CR equipment when determining performance and reliability.
Image noise is decreased as more “quanta” are able to reach the imaging plate, leading to cleaner images as a result of longer exposures. Because of this, it is easy to cover up negative low-exposure performance figures due to the fact that increased exposure can alleviate inherent deficiencies in a product’s performance (SNR). Radiographers must be taught to seek out equipment that performs well without the aid of increased radiation because not all examinations require high exposure levels. Students will need to gain an understanding of low-level and high-level exposure performance data when making competent purchase decisions based on SNR specifications.
Success Through Proper TrainingComputed radiography system performance is a function of many competing parameters such as the modulation transfer function, spot size, pixel size and signal-to-noise ratio. It is easy for beginners to become overwhelmed by marketing sell-sheets and lose sight of which specifications are critical to overall system performance.
There is no single magic button to fall back on when referencing performance data to determine purchase requirements. Therefore, it is critical that NDE trainees are properly educated on the importance of radiographic technique as well as the many parameters in system design and their individual effects on overall performance. Promoting the development of educated decision-making techniques as opposed to spec-based rationalizations will aid in the long-term development of a successful career in nondestructive examination. ndt