Operating Manual

Because scanners vary widely in resolution, dynamic range, and ability to scan dense
films, evaluation is required to ensure that adequate scanning fidelity is achieved.
Depending on selected resolution many Megabytes are needed to store a single film, see
paragraph 16.12. Archiving of a digitised film, identical to CR- and DR images, is usually
done on an optical mass storage facility e.g.: CD-ROM, DVD etc. For uniform application
of film digitisation norm EN 14096 has been issued.
16.4 Computed Radiography (CR)
Digital radiography using storage phosphor plates is known as “Computed Radiography” or
CR for short. This “filmless” technique is an alternative for the use of medium to coarse-
grain X-ray films, see the graph in figure 27-16. In addition to having an extremely wide
dynamic range compared to conventional film, CR technique is much more sensitive to
radiation, thus requiring a lower dose, see figures 8-16 and 27-16. This results in shorter
exposure times and a reduced controlled area (radiation exclusion zone).
Two-step digital radiography
CR is a two-step process. The image is not formed directly, but through an intermediate
phase as is the case with conventional X-ray films. The image information is, elsewhere and
later, converted into light in the CR scanner by laser stimulation and only then transformed
into a digital image. Instead of storing the latent image in silver-halide crystals and
developing it chemically, as happens with film, the latent image with CR is stored (the inter-
mediate semi-stable phase) in a radiation sensitive photo-stimulable phosphor layer.
This phosphor layer consists of a mix of bonded fine grains of Fluor, Barium and Bromium
doped with Europium.
The CR imaging plate
The phosphor layer has been applied to a flexible carrier and been provided with a protec-
tive coating. An additional laminate layer mainly determines the mechanical properties
such as flexibility ( CR imaging plates are not as flexible as X-ray films). Such plates can
be used in a temperature range from -5° C to +30° C. Figure 2-16 shows the layered struc-
ture of this type of plate, which is generally called an imaging plate or sometimes
(wrongly) imaging screen.
Note: Screens in the world of NDT, made of lead or another metal, are used to intensify
the effect of incident radiation or to reduce the effect of (scattered) radiation.
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Image development
As a result of incident X-ray or gamma ray radiation on the storage phosphor, part of
its electrons are excited and trapped in a semi-stable, higher-energy state. This creates the
latent image. These trapped electrons can be released by laser beam energy. This stimula-
tion causes visible light to be emitted, which can then be captured by a PMT (Photo
Multiplier Tube). The wavelength of the laser beam (550 nanometres) and that of the emit-
ted visible blue light (400 nm) are of course different to separate the two .
The laser-scanning device used to scan (develop) the latent image contains the PMT and
its electronics, which digitises the analogue light signal that is generated. This process as
illustrated in figure 3-16 takes place in the phosphor scanner, or so called “CR scanner” or
“reader”. The plate is scanned in a linear pattern similar to the formation of a TV-image and
identical to the film digitisation process.
Scanners-Readers
There are various types of scanners. In the most professional scanners, all that needs to be
done is to insert the cassette in the input tray and the machine automatically completes the
processing cycle. After completion of this process, including erasing the latent image, the
cassette is released from the CR scanner and ready for re-use. Figure 4-16 shows a typical
tower-type (man-size) automated scanner.
In smaller and portable desktop scanner models intended for use at remote locations e.g. on
offshore platforms, the CR imaging plate is manually removed from the cassette and inser-
ted into the scanner, which slightly increases the risk of the plates being damaged.
In addition to flat bed CR scanners drum type scanners exist. Figure 5-16 shows an example.
This scanner can handle unlimited lengths of CR plates.
Protective coating
Storage phosphor layer
(thickness approx 200 micron)
Laminate
Flexible PET-carrier/base
Fig. 2-16. Structure of the CR imaging plate
Rotating
mirror
Red light 550 nm
A/D Converter
Display
Storage phosphor plate
Light guide
Blue light
400 nm
CR SCANNER OR READER WORK STATION
Laser
PMT
Fig. 3-16. Schematic
of CR imaging process
Fig. 4-16. Automated CR scanner
Fig. 5-16. CR drum type scanner