|NDT.net June 2004 Vol. 9 No.06|
| 2nd MENDT Proceedings
Digital Radiography in NDT ApplicationsEric Deprins
Agfa Gevaert N.V.
Tel : *32 3 444 8279
Fax : *32 3 444 8243
Abstract30% 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. IntroductionAs 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 WorkstationBefore 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.
2.1.1. Digital image acquisition
2.1.2. Image display and analysis
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
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
2.2 Film DigitisingThe 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 RadiographyComputed 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.
2.3.1. Stationary CR Scanner (RADView CR Tower)
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)
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 RadiographyDR systems are designed to improve inspection efficiency on two levels:
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
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
2.4.3. Limitations of DR panels (1)
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 footnotes1. Flächendetektoren - Die Detektoren der Zukunft ? Ein Erfahrungsbericht, Dr. Matthias Purschke, Agfa NDT Pantak Seifert.