The difference is how the latent image is created and how this image processing
is done. The basic CR imaging cycle has three steps [13]: (1) expose, (2) readout,
and (3) erase.
Inside the radiography cassette an image plate (IP)—or SPS—having a detective
layer of photostimulable crystals is available. The detective layer consists of a
family of phosphors BaFX:Eu2+ where X can be any of the halogens Cl, Br, or I (or
an arbitrary mixture of them) [17]. A typical SPS can store a latent image for a
considerable period of time. However, according to the American Association of
Physicists in Medicine [18], it will lose about 25% of the stored signal between
10 min and 8 h after an exposure resulting in the loss of energy through spontaneous
phosphorescence.
The phosphor crystals are usually cast into plates into resin material in an
unstructured way (unstructured scintillators) [10]. When the SPS is exposed to
the X-ray the energy of the incident radiation is absorbed and excites electrons
to high-energy levels (Fig. 2.2a, b). These excited electrons remain trapped at
unstable energy levels of the atom. The absorbed X-ray energy is stored in crystal
structure of the phosphor and a latent image is then created at these high-energy
states giving a spatial distribution of these electrons at the SP detector. This trapped
energy can be released if stimulated by additional light energy of the proper
wavelength by the process of photostimulated luminescence (PSL) (Fig. 2.2) [18].
After the X-ray exposure and the creation of the latent image, the SPS is scanned in
a separate CR reader device. The readout is a process that follows exposure of the
image plate and constitutes the second step of the CR imaging cycle. A red laser beam