The see-through look... digital radiography gets under the skin

TWI Bulletin, July/August 2004

Computed radiography is becoming more commonplace within the industrial NDT field.

Bruce Blakeley has worked for TWI since April 2003. As senior project leader in the NDT department, he is responsible for two collaborative projects, 'Robot Inspector' and 'Magcast'. This paper is based on a presentation given at the British Institute for NDT annual conference 2003.

It offers several advantages such as defect recognition software, advanced analysis tools, faster exposure times, and lower energies - and yet many companies 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. In this paper Bruce Blakeley attempts to answer some of the common objections raised against digital, and compares the relative merits of analogue (film) radiography against the various types of digital. It also examines some of the new terminology surrounding computed radiography 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 by digital 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.

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 radiography using 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 Euro € 2M European funded CRAFT projects. TWI is also engaged in several joint industry projects which compare digital to film radiography. This paper draws on the collective 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?

Potential benefits

The potential benefits of digital radiography include:

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

Creating a digital radiograph

There are three main technologies used to create a digital radiograph. These are:

• High resolution scanning of film
• Phosphor Imaging Plates (IPs)
• Flat panel detectors

Each has its advantages and disadvantages which will be discussed in turn. It should be noted that the digital radiographs created using the phosphor imaging plates, described in the next section, are typically referred to as 'Computed Radiography', while the flat panel detectors are described as 'Digital Radiography'. The only difference is the method of creation. Once the image is displayed on screen there is absolutely no difference between the two.

High resolution scanning of film

This method is used when a digital image is required, but a traditional film has been used to capture the radiographic image. There are several reasons why this may be desirable. The most obvious include archiving and communication, but there are more technical reasons why this may be needed. A digital radiograph of a low-density component may contain much more information than the human eye is capable of seeing. This is also true with a film radiograph, although the effect will not be linear. This information may still be valuable and can be extracted by digitising the film much in the same manner as a paper document can be 'scanned' into a PC using a flat bed scanner. There are several service providers in the UK who can digitise films. The results are typically burnt on to a CD and posted back to the client. The pixel-pitch of such scans may vary, but are typically around 25-50mm.

Imaging plates

The use of phosphor Imaging Plates (IP) to capture a digital radiograph is normally referred to as 'Computed Radiography'. The imaging plate itself is a flexible white plastic sheet that is used in much the same way as film ( Fig.1). It is placed in a light proof cassette with lead screens as normal, then placed in exactly the same position as you would when using film - it is even possible to bend the flexible plate around a pipe. The exposure time is then calculated. The typical exposure dose is approximately 70% of the kVs used for an Agfa D7, and 10% of the exposure. The current plates on the market will give a similar radiographic quality of a D7, but newer plates on the market have the potential of delivering quality levels of a D4/D5.

Once the plate has been exposed it is removed from its lightproof cassette in a darkened room. There is no need for a 'dark room' as such. The plate is wrapped around a drum, which is then placed in the scanner ready for digitisation. The scanning procedure can take between three and 12 minutes depending on the size of the plate and the required resolution.

Very briefly, the latent image is created when X-ray quanta hit the grain structure of the plate. The energy is absorbed by the atom, without creating a visible image. This energy is then released in the scanner by a laser. This laser releases a quantity of blue light, which is detected by a Photo Multiplier Tube (PMT). The amount of blue light is a linear measure of radiographic 'density' at this point. The typical pixel-pitch of such scanners is 50-150mm. The voltage across the PMT can be varied as 'gain'. A high value of gain during scanning will lead to a darker radiograph, which can be used to reduce exposure times. This will of course lead to lower sensitivity. A lower value of gain will result in a lighter image. In order to achieve the required greyscale value it would be necessary to expose the plate for a longer period, which would increase the sensitivity.

The latent image can be erased by placing the imaging plate on a normal light box for five minutes. Once the image has been fully erased the plate can be reused. In general the plates are very robust and are ideal for industrial purposes where a more delicate sensor may be damaged or adversely affected by dirt. The plates can be cleaned with isoproponal cleaning fluid to remove dirt and grease from handling. Grease from one's hands can seriously affect the final image of the radiograph. The manufacturers claim that the plates have a theoretical working life of many tens of thousands of exposures, but industrial users have claimed that the practical working life is more likely to be many hundreds of exposures.

The plates are particularly sensitive to lower energies, as they were originally designed for the dental and medical field, and have been adapted for industrial NDT. This sensitivity to low energies is advantageous, but there can be problems with scatter in thicker walled objects. The effective use of filters and screens is essential.

Flat panel detectors

There are two forms of flat panel detectors - Amorphous Silicon, and the newer more expensive Amorphous Selenium. Fig.2 is a cut away view of Agfa's Amorphous Selenium flat panel detector. These panels are capable of achieving film like quality, but with a typical pixel-pitch of 100mm.

The panel is connected directly to a PC for power and control. This enables the system to be used in real-time, but can make site work difficult, as the panels are often delicate. There are flat panel detectors on the market that are protected in an industrial enclosure with their own PC card and battery power supply for site work, but panels are more normally used in automated production lines.

The average lifetime of these panels should be taken into account when considering their use. Overexposure, too high energies, and failure to allow the panels to 'rest' between exposures can severely decrease their effective life span. The price and specification should also be taken into consideration when considering the use of these panels. In particular, the pixel-pitch of 100-150m can be a problem. This may be overcome by projection magnification, but this is combined with loss of imaging area. However, the price and specification of these panels are improving rapidly. When considering purchasing such a panel it is important to note that the cost and quality of these panels varies tremendously.

Density and grey-scale values

The density of a film radiograph is based on the logarithmic ratio of the incident light from the viewer and the transmitted light. That is to say a radiograph with a density of four reduces the light intensity by a factor of 10 000. Obviously a digital radiograph is not placed on a viewer, so the same evaluation of radiographic lightness and darkness cannot be used.

Digital radiographs typically use greyscale values instead. A 12 bit image will have a range of greyscale values from 0 to 4095. Zero being black and 4095 being white. These numbers are hardly convenient, and the situation becomes even more unwieldy when using 16 bit. The author proposes that a simple percentage scale be used - 0% representing a completely white pixel, and 100% representing a black pixel. In this manner the higher values of percentiles represents a darker image, as is consistent with radiographic density. The values would not be affected by the number of bits.

Comparison with film

Table 1 compares film, imaging plates and flat panels with such properties as sensitivity and pixel-pitch. Film and Amorphous Selenium are capable of achieving high sensitivity over a range of energies. Film has a far greater resolution than flat panels, while the flat panels require a much lower exposure. This is also seen in digital photography, where a traditional film camera is far more able to capture sharper images, but requires far more light to be able to do this. Imaging plates have the least sensitivity, but have the best resolution of the various digital techniques.

Table 1 Comparison with film - 1

Table 2 details some of the other advantages and disadvantages of film, imaging plates and flat panel detectors.

Table 2 Comparison with film - 2

Fig.3 shows the equivalent Penetrameter sensitivity for flat panels (digital radiography - DR), imaging plates (CR phosphor) and film. Both imaging plates and flat panels achieve the required 2% sensitivity in a relatively short time compared to film. Flat panels are able to achieve film-like quality in relatively little time, whereas the sensitivity of imaging plates is limited. However manufacturer Agfa is in the process of producing more sensitive plates.

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 when photographic 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 the benefits 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 depend 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.

Conclusion

There are currently three methods by 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 and photographs. 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 uses flat panel detectors, which capture the image directly.

• If your image requires a fine-grain/high resolution film, the present digital sensors may lack the required resolution, 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 be able to accommodate curved surfaces 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 a flat panel.

In general digital radiography appears to be lagging behind digital photography by five to 10 years. Five to 10 years ago people asked whether digital cameras would ever take off.