A
radiographic screen containing a special class of phosphors which
when exposed to X-rays, stores the latent image as a distribution of
electron charges, the energy of which may later be freed as light by
stimulation with a scanning laser beam. The light is directed to a
photomultiplier tube, and the output electrical signal is digitized.
The final result is a digital projection radiograph. The
photostimulable phosphor plate is also known as an imaging plate,
storage phosphor imaging plate, and digital cassette. The technique
has also been termed computed radiography (CR) (after the
introduction of the imaging plate in 1981 by the Fuji company, who
named the new technique FCR).
The photostimulable phosphors in the imaging plate have a property
termed phosphorescence or photoluminescence (see luminescent screen)
which in this context means they are able to store X-ray energy and
later, when stimulated by (laser)light, free the energy as emitted
light. The phosphors used in radiography are mixtures of three
different barium fluorohalides doped with europium as an activator;
BaFI:Eu2+, BaFCl:Eu2+, and BaFBr:Eu2+. To prepare the imaging plate
for an X-ray exposure, the plate is exposed to intense light to
erase any previous image. For X-ray imaging, the plate is placed in
a cassette and is used just like a film screen cassette with
standard radiographic euipment. When exposed to X-rays, the europium
atoms in the phosphor crystalline lattice are ionized (converted
from 2+ to 3+), liberating a valence electron. These electrons are
raised to a higher energy state in the conduction band (see solid
and photoconduction for an explanation of conduction band). Once in
the conduction band, the electrons travel freely until they are
trapped in a so-called F-centre in a metastable state with an energy
level slightly below that of the conduction band, but higher than
that of the valence band. The number of trapped electrons is
proportional to the amount of X-rays absorbed locally. The trapped
electrons constitute the latent image. Due to thermal motion, the
electrons will slowly be liberated from the traps, and the latent
image should therefore be read without too much delay. At room
temperature, the image should, however, be readable up to 8 hours
after exposure.
Reading of the exposed imaging plate is performed by scanning the
plate with a small (50–200 mm) dot of light from a helium-neon
laser. The laser light stimulates the trapped electrons up to the
conduction band, where they are free to move to the europium atoms,
thereby leaving the high energy conduction band to return to the
lower energy valence band. The transformation of europium from the
3+ to the 2+ state therefore involves liberation of energy, and this
is done by emission of light. Since there is a larger energy
difference between the conduction band and the valence band than
between the conduction band and the F-centres, the (green) light
emitted has a higher energy than the (red) laser light needed to
stimulate the trapped electrons. The difference in wavelength
between the two lights is critical for detection of the emitted
light. By using a filter that absorbs red light but is transparent
to green light, the emitted light is selectively detected. The laser
beam scans the imaging plate in a transverse direction while the
plate is moved past the scanning beam. The emitted light is
collected using a light guide and is fed to a photomultiplier tube
where the light is converted to an electrical signal which is
amplified to an electric output signal. This signal is digitized,
and the image is stored in a computer as a digital matrix, each
pixel having a gray scale value determined by the amount of light
emitted from the corresponding dot on the imaging plate.
The imaging plate has a much wider dynamic range than film-screen
systems, with a linear characteristic curve (see digital radiography
(I), Fig. 2), giving the system a much wider exposure latitude than
film-screen systems. Because of certain pre-scan operations
performed prior to the actual read-out of the imaging plate, an
automatic gain control is achieved; overexposed images are recorded
with equal "brightness" as underexposed images. The required amount
of radiation to the plate is, however, in average the same as needed
with film-screen systems. Due to the wide exposure latitude and
"automatic gain control", doses may be reduced, but at the cost of
increased noise. The uniform density despite over- and underexposure
is one of the great benefits of the system as compared to
conventional film-screen systems; almost no retakes due to incorrect
exposures are necessary. Additional benefits are those common to all
digital techniques, including postprocessing such as changing window
level and width, exact measurement of distances, angles, and areas,
zooming, panning, and not the least, digital archiving and
communication (see PACS).
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From: www.amershamhealth.com
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