Digital radiography - is it for you?

Bruce Blakeley

Paper published in Insight, July 2004

Keywords: Computed Radiography, Imaging Plates

Abstract

Computed radiography is becoming more commonplace within the industrial NDT field. It offers several advantages such as defect recognition software, advanced analysis tools, faster exposure times, and lower energies - and yet manycompanies are still sceptical about its use. These companies are questioning whether they should wait for the new technology to improve before buying, or invest in the new technology now before being left behind. This paper attempts toanswer some of the common objections raised against digital, and to compare the relative merits of analogue (film) radiography against the various types of digital. It also examines some of the new terminology surrounding computedradiography and highlights some of the differences in technical skills required from radiographers and analysts.

Much of the resistance to computed radiography is from radiographers themselves, who are used to using film and developers. Many are not comfortable with the new digital technologies, and are suspicious of the advantages offered bydigital image processing. This paper attempts to dispel some of the myths regarding computed radiography, and takes an objective look at the various technologies on the market today.

1. Introduction

TWI has investigated a range of materials of various thicknesses, using a variety of digital radiographic techniques. These techniques include Film scanning, Computed radiography using Imaging Plates (IPs), and Digital radiographyusing flat panel detectors. When the digital radiograph is created using an imaging plate, the process is known as Computed Radiography (CR). The resultant image is still a digital radiograph.

The materials investigated by TWI include:

• 1-60mm Magnesium castings in the Magcast project
• 75mm steel in the Rail Inspect project
• Aerospace composite samples in the Nanoscan project.

All of these projects are €2M European funded CRAFT projects. TWI is also engaged in several joint industry projects which compare digital to film radiography. This paper draws on thecollective experience of these projects, and tries to answer the following questions:

• What are the different options?
• What are the advantages and disadvantages of the different technologies?
• How do they compare to film?
• What objections are typically raised by radiographers?
• What new skills will they have to learn?

1.1 Potential benefits

The potential benefits of digital radiography include ^[1] :

• Archiving
• Reporting
• Sharing information
• Digital enhancement
• Automated defect recognition
• Ease of automation
• No consumables, and no chemicals
• Faster exposure times
• Greater linearity and range.

1.2 The digital format

1.2.1 Pixels

The digital image is constructed by a matrix of square pixels; a typical digital radiograph may be comprised of many tens of Mega-pixels. This is also typical of digital photography, and indeed, digital radiography shares manysimilarities with digital photography. Both have a limited number of pixels, which are typically much larger than the grains in film - this can often lead to problems related to resolution and image sharpness. This problem can beovercome using projection magnification with a mini or micro-focus set. Digital radiography is also generally faster than radiographic film, just as digital camera chips are faster than photographic film. This has obvious healthbenefits for radiographers using the digital format.

1.2.2 Bits

Black and white digital photographs are normally constructed of 256 shades of grey alone. Eight bits are typically used, as the human eye cannot distinguish any smaller difference in shade. Digital radiography is normallyconstructed of 12 or 16 bit images that mean that, in theory, they contain more information than the human eye is capable of seeing. This extra information can be revealed by simple digital image processing, typically by adjusting thebrightness and contrast.

1.2.3 Compression

Compression is usually used in digital photography to reduce the size of the files, but can lead to the generation of artificial artefacts that could be mistaken for flaws. Figure 1 shows two digital images. The first image has been subjected to a typical compression value, while the second has been overly compressed. A closer examination of the first image reveals errant pixels, particularlyaround the tip of the beak. Digital radiographs are typically left uncompressed for this very reason. The combination of large pixel numbers, high bit rates and zero compression, leads to extremely large file sizes when dealing withdigital radiographs.

Figure 5 shows the equivalent Penetrameter sensitivity for flat panels (digital radiography - DR), imaging plates (CR phosphor) and film. ^[9] Both imaging plates and flat panels achieve the required 2% IQI sensitivity in a relatively short time in comparison to film, at low KVs. Flat panels are able to achieve film like quality in relatively little time, whereas thesensitivity of imaging plates is limited. However Agfa are in the process of producing more sensitive plates.

Table 3 details the current standards open for public comment at the time of writing. It is believed that the ASTM standards are very similar.

Table 3 Current standards

8.1 Zoom

When viewing a radiograph on screen it is important to note that there may be far more information within the image than the human eye is capable of seeing, and that the monitor screen is capable of displaying. A typical monitorscreen is made up of 1024 by 768 pixels, as shown in section 1.2.1, but the radiograph is likely to contain even more pixels than this, making it impossible to display all the image's pixels on the screen at one time without the use ofan extremely high resolution monitor. Such monitors do exist, but again the human eye is also limited in resolution. When analysing a digital radiograph the software's 'zoom' function should be set to a 1:1 ratio to ensure that onepixel on the monitor is displaying one pixel of the image. Magnifying the image to a larger size than this would be acceptable, but shrinking the image to less than a 1:1 size, would be insufficient, as not all the image's pixels wouldbe on display.

8.2 Bits

Most monitors are only capable of displaying 8 bits of black and white 'greyscale'. This is because the human eye is generally not capable of distinguishing any smaller difference. There are more expensive monitors on the marketthat are capable of displaying more than 8 bits, but their use is not essential, as the human eye may not be able to appreciate the difference. A graphics card capable of dealing with 12 to 16 bit greyscale images is essential however.

The full range of shades can be seen by adjusting the brightness and contrast, even if a lower quality monitor is used. Figure 6 is a radiograph of a 1-10mm magnesium stepwedge taken on an Imaging Plate. The single 16 bit image has been separated into three 8 bit images by adjusting the brightness and contrast of the original image, thenexporting as a jpeg. Many radiographers and technicians are suspicious of changes made when magnifying the image or by simply adjusting the brightness and contrast, as they believe that this is 'manipulating' the image, which couldlead to 'false' information being displayed on screen. This is not the case when using such simple functions as zoom, brightness and contrast and sharpening - in this case you are actually removing information to allow the remaininginformation to be seen clearly. However, some processes do genuinely distort the image - these functions include embossing and some edge enhancements. It is important to understand which functions in your software change the image, andwhich merely adjust the image to make viewing easier.

When adjusting the size, brightness and contrast for optimal viewing it may be beneficial to use your IQI wires or Penetrameter. Common sense would dictate that any adjustment that reveals more wires or holes would be better thanone that doesn't. Several critics of digital radiography have suggested that both wire/Penetrameter IQIs and duplex wires be used simultaneously. One for sensitivity and one for resolution, as the resolution of digital images can besuspect due to the limited pixel-pitch.

8.4 Quantifiable measurements

One of the advantages of digital radiographs is that exact quantifiable measurements can be taken. In Figure 7 a simple line measurement has been taken. The operator has drawn a line across a stepwedge, and the software has measured the greyscale value of every single pixel along the line. This is an extremely simple exampleof what measurements can be obtained.

8.5 Other functions

There are various other functions and filters that can be applied to digital radiographs. These tools can be used to aid defect detection or to reveal hidden detail within the image. Figure 8 shows three images of a valve. The first is the original unfiltered image. The second is the same image after sharpening, while the third has been embossed in the Region Of Interest (ROI).

8.6 Automatic defect recognition

Automatic defect recognition is used to analyse images for defects without the intervention of a human operator. A high degree of repeatability is required for automatic defect recognition, which means that it is usually employed onproduction lines using flat panel detectors. ^[10] The source is typically stabilised in some manner to provide consistent doses of radiation for each component examined.

Each image is subtracted from a 'golden image' of a perfect component. The algorithm then examines pre marked 'danger areas' where faults are suspected. Small errors in aligning the two images can lead to errors and false positives.The two images are first 'registered' to ensure that they are correctly aligned pixel to pixel before subtraction. This technique is often used in castings such as automotive aluminium wheels.

9. Computed tomography

Computed tomography is used to recreate three dimensional views or 'slices' through an object. Figure 9 shows a photograph of a radiographic system used to create tomographs. Essentially an object is placed on a turntable, which is rotated in a series of incremental steps, taking a digital radiograph, with a flat panel,at each step. These images can then be combined either as simple 'frames' to create a movie of the rotating component, or can be processed by a more complex algorithm to produce a three dimensional virtual object as shown in Figure 9. This image can be rotated to view from any angle, or can be 'sliced' to reveal new details within the object.

Fig. 9. Computed Tomography
Courtesy of Agfa

10. Radiographers and Technicians

Technicians involved in traditional film radiography have not welcomed the advances made in digital radiography, and have often shown a deep mistrust of the new technology. This mistrust and scepticism was also apparent whenphotographic digital cameras were first introduced on to the market, and yet today there are very few photographers who do not own one. Many radiographers do not trust the new technology, and are particularly sceptical about thebenefits offered, such as greater linearity, and even some of the simplest tools such as brightness and contrast.

The skills required to create digital and film radiographs are very different, as digital and computed radiography depends upon a high degree of computer literacy. The new skills required from radiographers include image processing,multi-media, networking and filing/archiving. There is a very real risk of losing experienced personnel if radiographers fail to adapt to the new technology.

11. Conclusion

There are currently three methods in which a digital radiograph can be created. The first is to take a traditional film radiograph and to scan it into a PC in much the same way as a flatbed scanner can be used to scan documents andphotographs. The second technique uses phosphor coated Imaging Plates (IPs) to create the digital radiograph. This process is known as Computed Radiography (CR). The final technique employs flat panel detectors, which capture the imagedirectly.

• If your image requires a fine-grain/high resolution film, the present digital sensors may lack the required resolution, for example with fine cracking; but keep an eye on the market as they are improving daily.
• If your application requires a medium-grain film and you want to lower exposure doses, you require a robust system and/or to be able to bend around pipes - consider Imaging Plates.
• Finally if your application is for flat items or castings, and you require a high throughput, full automation or real-time, consider flat panel.

In general digital radiography appears to be lagging behind digital photography by five to ten years. Five to ten years ago people asked whether digital cameras would ever take off...

12. Acknowledgements

The author would like to thank Eric Deprins of Agfa for his assistance and technical expertise.

13. References

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2. E Deprins, Agfa Antwerp, 'By personal communication' 2003
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8. SR Amendolia et al, 'Comparison of imaging properties of several digital radiographic systems', Nuclear instruments and methods in physics research A466 (2001) 95-98
9. Agfa official web site ndt.agfa.com/BU/NDT/index.nsf/EN/radviewdigitalsystems.htm
10. AG Vincent et al, 'Defect detection in industrial casting components using digital X-ray radiography', BINDT Insight Vol 44 No 10 October 2002.