NDT.net • June 2004 • Vol. 9 No.06
2nd MENDT Proceedings

Digital Radiography in NDT Applications

Eric Deprins
Agfa Gevaert N.V.
Mortsel, Belgium
Tel : *32 3 444 8279
Fax : *32 3 444 8243
eric.deprins.ed@belgium.agfa.com

Abstract

30% of all film radiography could be replaced by today's technologies in the field of digital radiography. Only few of these applications have indeed replaced film. The choice to go digital depends on cost, quality requirement, workflow and throughput.
Digital images offer a lot of advantages in terms of image manipulation and workflow. But despite the many advantages, a lot of considerations are needed before someone can decide to convert his organization from conventional to digital radiography. This paper gives an overview of all different modalities that can be used in digital radiography with today's technologies, together with the experiences of the pioneers of digital radiography. Film Scanning, Computed Radiography and Direct Radiography by using of different kinds of flatpanel detectors all have their specific application fields and customers. What is the status of the technology today, which advantages brings digital radiography, and which are the limitations radiographers have to consider when replacing film by digital systems.

1. Introduction

As experienced NDT professionals know, multiple NDT methods can be required to fully meet the demands of a particular inspection application. The same has been true for radiography, where a wide assortment of films has been developed for the specific quality and throughput requirements. Today, the options for radiography include not only film, but with recent technological advantages, it is now possible to meet a wide range of NDT inspection applications with digital solutions that are reliable and cost effective. More and more applications can be covered by the improving image quality of digital radiography systems, and together with the digitisation of radiographic applications, come the enhancements in workflow, that are made possible by availability of digital images.

2. The Digital Radiography System

2.1 The Workstation

Before describing the details of digital detectors, the most important component of a digital radiography system should be covered. The performance of the workstation and the accompanying software will determine the efficiency of a digital system. Where new developments in digital technology always follow the developments of the medical industry, it should not be forgotten that NDT applications have completely different needs than hospitals, and that radiographic workflows should be adapted to the necessities of the industry.

Fig 1: RADView Digital System
The main functions of the workstation are described as :

  • Control digital image acquisition
  • Display and analyse digital images
  • Manage information and data
  • Control the output

2.1.1. Digital image acquisition
One and the same workstation software should be able to control any kind of acquisition modality. Albeit film scanning, Computed Radiography or Direct Radiography, a lot of different applications will offer a home for one of these modalities, depending on throughput requirements, working conditions or required image quality.

2.1.2. Image display and analysis
Besides the user friendliness of the common imaging tools like contrast enhancement, sharpening, pan, scroll and zooming, which make evaluation of digital images so much easier than viewing films on a light box, is now the development of the software more moving into the direction of accurate measurements and workflow. The RADView workstation will soon be communicating with external application databases, such as quality surveillance databases of refineries or production control packages in serial manufacturing plants.

Also, more advanced measuring tools will be available in the software. In co-operation with BASF and BAM, Agfa will soon start offering a sophisticated wall thickness measurement software tool, specially designed for on-stream applications. The software takes the source - to - object distance and the nominal pipe diameter as a reference, and will calculate the rest wall thickness at a certain predefined area.

Smoother line density profile measurement by averaging different lines, angle measurement and area measurement also make part of the broader range of possibilities of today's digital radiography systems.

2.1.3. Manage information and data
The strength of digital radiography systems lies not only in the imaging possibilities, but also in the management of data associated with a certain image. All data can be entered, manually or automatically, in the same record in the database of the workstation. In the more sophisticated CR systems, data can be coupled to an image at the time of exposure, using a predefined worklist. (see below in the section CR Systems).

Most important, the database is designed this way that original image data is always preserved, prohibiting image manipulation as long as the original data is not securely saved. This is needed in order to use the images in a later stage as evidence material, which would of course be impossible if the original content of an image has been modified.

2.1.4. Controlling the output
As the workstation is a Windows 2000 compatible computer, all common peripherals can be used to communicate with the outside world, including printers, networks, etc. The software itself foresees in easy-to-use options to create reports or write images on a CD or DVD.

2.2 Film Digitising

The recent classification of film digitising systems has led to the enhancement of the existing film digitisers. Still the digitiser will scan original radiographic images with a resolution of 50 um, at a very high accuracy like before. But previously, the scanner was limited to a maximum optical density of 4.0, and the contrast sensitivity of 0.02 was not guaranteed in the density range 3.5- 4.0, which made scanner classify itself as a class A scanner. Currently, the scanner in the RADView assortment will reach optical density of 4.7, with all the conditions met for a full blown class B scanner.

As before, the system employs a HeNe laser beam, which sweeps across the film by a polygon mirror system. The F-Theta lens avoids distortions of the image, by keeping the optical distance of the laser beam unchanged at all spots of the scanned area. The logarithmic amplification process guarantees high signal to noise ratios to up to 4.70 D.

Every scanner is calibrated and characterized at the time of shipment, and a unit-specific LUT is delivered with each machine. This guarantees an artifact-free scanning at highest possible precision, repeatability and speed. A 14 x 17 inch film can be digitized in as little as 7 seconds.

This is a cost-effective solution for anyone who wants to digitize films for handling archives, for easy image transfer or for using the advanced viewing features of the RADView system.

2.3 Computed Radiography

Computed radiography uses a reusable imaging plate in place of the film. This plate employs a coating of photostimulable storage phosphors to capture images.

When exposed to X-rays, electrons inside the phosphor crystals are excited and trapped in a semi-stable higher-energy state. The CR reader scans the plate by means of a laser beam.
The laser energy releases the trapped electrons, causing visible light to be emitted. This light is captured and converted into a digital bit stream which encodes the digital image.

  • No more retakes
    The storage phosphors on the Imaging plate have an extremely wide dynamic range. This gives a high tolerance for varying exposure conditions and more degrees of freedom in selecting the exposure dose. As a consequence, the need for retakes is drastically reduced.
  • Dose Reduction
    The wide exposure latitude of the Imaging plates allows, in many cases, the visualisation of all diagnostic information with only one exposure. In this way, the use of Imaging plates results in a substantial reduction of the dose load. Also the fact that the sensitivity is about 10 times higher than the sensitivity of conventional film results in shorter exposure times and thus significant dose reduction.
  • Long Lifetime
    NDT Imaging plates are protected by an EBC (electron-beam-cured) top coat. EBC top coating is an Agfa-proprietary technology for hardening a prepolymer lacquer coat into a high-density polymer on top of the phosphor layer. This results in plates with superb protection from mechanical wear and extensive immunity to chemical cleaning solutions. Superior durability of the RADView Imaging plate is thus secured.
  • Image Quality
    The recent efforts done for medical CR systems to be used in mammography applications, have also found their way into the NDT applications. Where until recently the image quality could be compared with D7 or D8, are we now comparing to D5 and even D4 image quality.

2.3.1. Stationary CR Scanner (RADView CR Tower)
The newest CR scanner in Agfa's assortment has been designed for utmost convenience in digitising 8" x 10" (20 x 25cm) and 14"x 17" (35 x 43cm) imaging plates. The practical cassette system limits the handling of the plates to a minimum. The cassette is placed in the scanner's input tray. The internal mechanism takes the plate out of the cassette, transports it to the scanner unit, erases the plate after scanning and places the plate back in the cassette. The cassette is then unloaded from the scanner, ready for the next exposure. The specially designed NDT cassettes with built-in lead sheets avoid any unnecessary plate handling, increasing their lifetime by 3 times. The scanner provides a reliable, cost effective solution for low volume and mobile applications where a small footprint is required. The system offers ease-of-use and low maintenance, ensuring reliable and repeatable system operation.

The CR cassettes have a programmable chip that can be programmed with a handheld identification station (or ID station). This creates a greatly improved workflow: The ID Station is programmed by the main workstation, and contains a worklist. This worklist can be created at the workstation, or can come from an outside application. A team goes out with an amount of cassettes and the ID station, and every time before an exposure is made, the data associated with the exposure is copied from the ID station into the cassette on which the exposure is made. At the end of the day, the cassettes are brought to the scanner, which has the ability to read the image and the chip at the same time, making sure the data in the chip is copied in the right fields in the database, together with the image. The cassettes are identified at the time of exposure, not at the time of scanning, avoiding human mistakes in data input.

2.3.2. Mobile CR Scanner (RADView CR 100)
Applications that require special sizes of plates or require a mobile scanner cannot use a stationary scanner like CR Tower. Especially for these operations, Agfa offers a compact and all-round CR scanning solution.

After exposure, the plates are manually removed from the cassette and inserted into the scanner for readout.

The RADView CR 100 scans custom sizes and shapes up to 14" (35cm) wide.

2.4. Direct Radiography

DR systems are designed to improve inspection efficiency on two levels:

  • First, the systems provide the opportunity for more rapid inspection flow due to immediate feedback.
  • Second, within and between inspection facilities, these systems can enhance workflow through networked distribution of the diagnostic images.

With Direct Radiography systems, your facility will be able to use the most convenient location for inspections and be assured that they will be completed quickly and accurately. After acquisition, which takes a few seconds, the images can be viewed on the monitor immediately, and can be forwarded wherever they are needed. And because the images are digital, multiple copies of the image data are always identical.

2.4.1. Amorphous Selenium Flatpanels
These panels use an amorphous selenium-coated, thin-film transistor (TFT) array to capture and convert X-ray energy directly into digital signals without the use of scintillators or phosphor. Because of the absence of scattering and the optimal signal-to-noise ratio, the image quality approaches the quality of medium-grain film.

The limitation of a Se detector is the narrow temperature range requested by the Amorphous Selenium. Either in operation or in storage/transport conditions, the detector needs to remain within a temperature range between 5°C and 30 °C, in order to avoid destruction of the Selenium layer. Selenium is also sensitive for ghost images if higher energies are applied (>180 kV). These limitations make that Se panels are only applicable in very specific applications, where conditions are controlled very strictly.

2.4.2. Amorphous Silicon Flatpanels
Amorphous Silicon (aSi) Flatpanels use a Scintillator, consisting of Cesium-Iodide or Gadolinium Oxusulfide, which converts incident X-rays into visible light. This light is converted into an electrical charge by an array of amorphous Silicon sensors. Earlier developments of aSi detectors showed a high noise level which made aSi only applicable for real time applications. The recent progress of the technology, together with software tools that allow averaging multiple frames, the signal-to-noise ratio improved in such a way, that the aSi panel's image quality exceeds that of CR systems and approaches that of aSe detectors. Moreover, aSi is much less sensitive to environmental conditions, which makes it useable in outdoor and uncontrolled applications.

2.4.3. Limitations of DR panels (1)
While flatpanel detectors are often described as the radiography detectors of the future, long time suppliers of these detectors are now gathering significant experience data about the lifetime of these panels. The reality shows that, due to experiences gained in the medical market, this lifetime has been drastically overestimated and can be, dependent on the duty cycle and the doses applied on the detectors, extremely short. In both static and in real time systems, we see that detector exchange after 10-12 months shows not to be exceptional, however, we always have to see this related to the application where the panels are used. Important however is, before considering using flatpanel detectors in your production environment, to make a detailed assessment of the risks involved.
Besides this issue, there remains the problem of the dead pixels. Inherent to the production process, every panel shows more or less pixels that do not work, and these appear as black dots in the images. In most cases, the software can eliminate these dead pixels, but still you have to realise that they are there, and that in the image these pixels have been replaced by fake spots.

Both flatpanel manufacturers and system suppliers that need to take their responsibility in these issues. For the researchers of the detectors remains the challenge to find a solution for both issues, as these put a serious question to the future of Direct Radiography.

References and footnotes

1. Flächendetektoren - Die Detektoren der Zukunft ? Ein Erfahrungsbericht, Dr. Matthias Purschke, Agfa NDT Pantak Seifert.

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