IMAGE CHARACTERISTICS FOR RADIOGRAPHY

1. PRINCIPLES OF COMPUTED RADIOGRAPHY
HISTORICAL PERSPECTIVE
 The real forerunners to the CR systems of
today were developed in the 1970s, when
researchers started to look for ways to
improve on the inefficient light connection
and the resultant suboptimal image quality
produced with a full-field illumination
method. their attempts led to the
development of scanned storage phosphor
for systems, in which a focused beam of
light stimulated the SP detector point by
point, and a photodetector was placed close
to the stimulation point to collect as much of
the locally emitted luminescence as
possible.
 These efforts culminated in the introduction
of the first commercial CR system in 1981
(by Fuji Photo Film Co.). since that time,
numerous manufacturers have researched
and produced commercial systems that use
the PSL effect, and not just for medical
imaging
STRUCTURE AND MECHANISM
Energy Storage
 A photostimulable phosphor imaging plate is
a rigid sheet with several layers that are
designed to record and enhance
transmission of the image from a beam of
ionizing radiation. the layers include a
protective layer, a phosphor layer, a base
layer (support) made of polyester, antistatic
(conductor) layer, and a reflective (light
shield) layer.
 The protective layer simply insulates the
imaging plate from handling trauma.
 the phosphor layer holds the
photostimulable phosphor, which is the
active component in the plate.
 The base layer is simply a support on
which to coat the other layers.
 The antistatic layer grounds the plate to
eliminate electrostatic problems and absorb
light to increase sharpness.
 Finally, the reflective layer prevents light
from erasing data on the imaging plate or
looking through the backing, decreasing the
spatial resolution.
 The IP has lead backing that reduces
backscatter x-rays. this improves the
contrast resolution of the image receptor.
 The imaging plate is loaded into a cassette
that looks much like a radiographic film and
intensifying screen.
 Consequently, computed radiography
cassette are sometimes referred to as
“filmless cassettes”
 The PSP, barium fluorohalide, is fashioned
similarly to a radiographic intensifying
screen. because the latent image occurs in
the form of metastable electrons, such
screens are called storage phosphor
screens (SPSs).
 The SPS appears white because the small
PSP particles (3-10 µm) scatter light
excessively, such a scattering is called
turbid.
 PSP particles are randomly
positioned throughout a binder.

2.  SPSs are mechanically stable,
electrostatically protected, and fashioned to
optimize the intensity of stimulated light.
SOme SPSs incorporate phosphors grown
as linear filaments that enhance the
absorption of x-rays and limit the spread of
stimulated emission.
 In order for CR to function, the imaging
plate material must have the ability to store
and release the image information in a
usable form. The most common phosphors
with characteristics favorable for CR are
barium fluorohalide bromides and iodides
with europhium activators, (BaFBr:Eu and
BaFi:Eu).
 The halides are approximately 85 percent
bromide and 15 percent iodide.
 Some other compounds that may be used in
the PSP are BaSrFBr:Eu, RbBr:T1, RbCI,
and CsBr:Eu
Photostimulable Luminescence (PSL)
 When these electrons return to the ground
state, visible light is emitted.
 Over time, these metastable electrons return
to the ground state on their own.
 However, this return to the ground state can
be accelerated or stimulated by exposing the
phosphor to intense infrared lightfrom a laser.
Hence the term photostimulable
luminescence from a photostimulable
phosphor (PSP).
Notes:
 During photostimulable luminescence:
o X-rays interacts with the material in
the imaging plate most specificallythe
phosphor.
o Once it interacts the electrons within
the phosphor will go to an excited
metastable state.
o In order for them to be accelerated or
stimulated, we use the laser light in
order for the electrons to go back to
their ground state. Thus, emitting the
light needed for the image.
Energy Release
 The sequence of events engaged in producing
a PSL signal begins when an x-ray beam
exposes a PSP, the energy transfers results in
excitation of electrons intoa metastable state.
 Approximately 50% of these electrons return
to their ground state immediately,resulting in
prompt emission of light.

3.  The remaining metastable electrons return to
the ground state over time.
 This causes the latent image to fade and
requires that the Imaging Plate must be read
soon after exposure.
 CR signal loss is objectionable after
approximately 8 hours.
 The next step in CR imaging is stimulation. The
finely focused beam of infrared light with a
beam diameter of 5O to 100 µm is directed at
the PSP.
o As laser beam intensity increases, so
does the intensity of the emitted
signal.
 The diameter of the laser beam determines
the spatial resolution of the CR imaging
system.
Note:
 That as the laser beam penetrates, it
spreads. The amount of spread
increases with PSP thickness.
 The thicker the phosphor is, the more
your laser beam penetrates and
spread.
 The third step in this imaging process, which is
detecting or reading the stimulated emission.
 The laser beam causes metastable electrons to
return to the ground state withthe emissionof
a shorter wavelengthlightin the blue regionof
the visible spectrum.
 Through this process, the latent image is made
visible.
o Some signal is lost as the result of:
1) Scattering of the emitted light
2) The collection efficiency of the
photodetector
o Photodiodes (PDs) are the light
detectors of choice for CR.
 The stimulation cycle of PSL signal acquisition
does not completely transition all metastable
electrons to the ground state. Some excited
electrons remain. (Final stage - erase)
 If residual latent image remained, ghosting
could appear on subsequent use of the IP.
 Any residual latent image is removed by
flooding the phosphor with very intense white
light from a bank of specially designed
fluorescent lamps.

4. Notes:
 Crystals has both: VALENCE BAND and
CONDUCTION BAND
 Electrons within the valence band will be
transformed and pump and become exciteddue to
introduction of energy through x-rays.
 These excitedelectrons will be trapped inthe color
center of the phosphor crystals.
 Once they are trapped here and if they are
exposed to the laser light, they are transported
into the luminescence center.
 The PSP is sufficiently sensitive that it can
become fogged by background radiation.
 The laser light used to stimulate the PSP is
monochromatic.
 A solid-state laser is the stimulating source of
choice. The resulting emission has a
polychromatic spectrum.
 The emitted light intensity is many orders of
magnitude lower than that of the stimulating
light.
 Solid-state lasers produce longer wavelength
light and therefore are less likely to interfere
with emitted light.
 Even so, optical filters are necessary to allow
only emitted light to reach the photodetector
while blocking the intense stimulated light.

5. Digitalization
-emitted light from the IP (Imaging Phosphor) is
channeled into a funnel-like fiber-optic
collection assembly and is directed at the
photodetector, PMT (Photomultiplier tube), PD
(Photo Diode), or charge-coupled device
(CCD).
Note: How does this x-ray become a digital
signal? From x-rays, it becomes light and it
becomes a digital signal. In Order for the x-
rays to become a signal, it must undergo
through the wires.
Note: After the laser light is produced, there is
an emitted light which will be filtered through by
your optical filter ands photodetector.
 Before Photodetection occurs, the light
is filtered so that none of the long-
wavelength stimulation light reaches the
photodetector and swamps emitted light.
(Note: short wavelength lang ang
ginadawat sa photodetector that is why
mayroon tayong tinatawag na optical
filter)
 In this case, emitted light is the signal
and stimulating light is the noise;
therefore, proper filtering improves the
signal-to-noise ratio. (Note: Two results
of the film that was exposed in laser
light --- emitted light and stimulating
light. Optical filter only allows the
emitted light. Dapat mas madami ang
signal compared to the noise.)
 The output of the photodetector is a
time-varying analog signal that is
transmitted to a computer system that
has multiple functions. The time-varying
analog signal from the photodetector is
processed for amplitude, scale, and
compression. This shapes the signal
before the final image is formed. (Note:
From x-rays, it becomes light and then
it becomes an analog signal)
 Then the analog signal is digitized, with
attention paid to proper sampling (time
between samples) and quantization
(the value of each sample).
 The image buffer is usually a hard
disc. This is the place where a
completed image can be stored
temporarily until it is transferred to a
workstation for interpretation or to an
archival computer.
 The computer of the CR readers is in
control of the slow scan and the fast
scan. This control works off the
computer clock in gigahertz (GHz).
Note: From x-rays, it becomes light, it
becomes a metastable energy within the
electrons. The electrons are emitted in the form
of light. Light is passed through the optical filter
and detected by the photo detector. It begins to
be an analog signal and then, it becomes a
digital signal through analog-to-digital
converter (ADC).

6. Note: Because of the sampling and
quantization and shipping of the signal, it
becomes a digital signal and then, computer
compliment.
Energy is neither created nor destroyed.
 Kung unsa tong energy na naa sa x-
rays or the value of the energy or the
strength of the x-rays, it is the same pag
mahimo na siyang light.
 When x-rays interact with the imaging
platespecifically sa phosphor, it
becomes a metastable energy. Gihimo
niya na excited ang electrons. It puts the
electrons into a metastable state.
 After that, your electrons will emit that
energy in the form of emitted light. The
light is passed through the optical filter
which filters both emitted and stimulated
light, but ang ginatanggap niya lang
talaga mostly is the emitted light.
 It passes through the photodetector,
after it becomes detected by the
photodetector, it becomes an analog
signal, then after quantization and
sampling, shipping, scaling, amplitude,
compression, it becomes a digital signal.
And then, imaged is displayed in the
monitor.
 The heart of the computed radiography
is the imaging plate.

7. Film-Screen vs Computed Radiography
Film-Screen CR
Exposure
Medium
Film and
intensifying
screen
Phosphor and
light (both
emitted and
the laser light)
Processing
Involves the
use of an
automatic
processor with
the use of
chemistry
Uses an
automatic
reader.
Processing
Time
Longer
processing
time
Shorter
reading times
Evaluation
A viewbox or
negatoscope is
needed to
access the
images.
The images
pop up in the
monitor after
reading the
image plate.
Archiving
 Requires
a huge
space
for
storing
and
ordering
of films.
 Hard
copy
 Only
needs a
small
space
for the
device
storage.
 Uses
PACS
 Soft
copy
Availability
Only few
medical
facilities utilize
this.
Widely
available