Ch3-Basic Physics of Medical Imaging.
Screen Film
Radiography
1-Screen Film Radiography
The
imaging principle:
·
Patient's body
is composed of different materials with different attenuation properties.
·
If an X-ray incident
on the patient, the high-density material (Bone) will absorb a high amount of radiation,
while the low-density material (Air) will absorb less amount of radiation.The
transmitted X-ray photons will be received by imaging receptors to represent
the attenuation properties of different materials.
The
role of screen in the imaging process:
·
The imaging
receptor differs from the past up to today, starting from screen film
combination up to digital receptor.
·
If The
transmitted X-Ray photons interact with the film directly, you have to expose
the patient too much to receive the required number of photons for optimum
image quality.
·
The role of the screen
is to receive the X-Ray photon and convert it to hundreds of light photons
·
The screen is in
front of the film to absorb the incident x-ray photon, however, there is
another screen on the back side of the film to detect any X-ray photons passing
from the front screen.
·
The light photons
are reflected back towards the film from the back screen.
·
The screen is
composed of phosphor material (fluorescent material) which has a high Z number,
High density, and favorable K-edge.
·
The fluorescent
material converts the X-ray photon to visible light photons immediately (used
in screen film radiography), while phosphorescent material requires energy to
release the light photons (used in Computed radiography).
·
The conversion
of X-Ray photons to light photons is called intensification, so the screen is
called intensifying screen.
·
There are
different materials that can be used as an intensifying screen. although
tungsten has a higher Z = 74, rare earth materials (gadolinium Gd Z=64 and
lanthanum La Z=56) are commonly used. The reason is that Gd and La have
K-absorption edges at 50 Kev and 39 Kev respectively, while Tungsten at 70 Kev.
The mean energy of the X-ray beam used in radiography is close to the
K-absorption edge of Gd and La which increases the absorption efficiency of X-ray
photons.
·
The conversion
efficacy of rare earth material is 20 %, and 5% for tungstate.
·
The higher
sensitivity of rare earth material causes lower usage of KV and mAs, Lower
patient exposure, and lower tube loading.
·
The intensifying
screen is maximum when the X-Ray energy is just above the K-Edge energy.
·
If the Kv
increase, the mean energy increase which causes less attenuation and less
screen sensitivity.
·
Calcium
tungstate screen releases blue light after x-ray exposure, while rare earth
material emits green light.
Screen
thickness affecting image quality and patient dose.
·
If the screen
thickness increase, the higher absorption of X-ray photons, as the probability
of passing X-Ray photons without interaction will be less.
·
Not many X-ray
photons are required to get the optimum image quality in the case of thicker
screens, so the patient radiation exposure will be less.
·
In the thicker
screen, the light photons diffuse over a long distance before reaching the film
·
The film will
receive the light photons from a location different from the site of light
photon production inside the screen.
·
This mechanism
happens with a thicker screen causing a loss in resolution of the X-ray image.
·
The thinner
screen enables the light photons to diffuse over a short distance which has
less effect on image resolution, however patient radiation exposure increases
to compensate for the loss of X-ray photon absorption.
·
The selection of
screen size must be compromised between image resolution and patient dose.
·
Thin screen is
preferable in high-detail imaging procedures (breast imaging, wrist, and
ankle), while a thicker screen can be used for abdomen, chest, and pelvis
imaging.
The
film and light photons interaction mechanism:
·
The film is in
contact with the screen to receive the light photons by the emulsion.
·
The emulsion is
the sensitive part composed of grains that contain silver bromide crystals.
·
The bromide is
excited by light photons to release an electron.
·
As silver
bromide is in a crystal structure, so sensitive spot is present which traps the
released electron.
·
The sensitive
spot will attract the silver positive ions to form an exposed sensitive spot.
·
Many sensitive
spots are formed in the emulsion to form a latent image which has to be
converted to the real image.
·
More silver ions
will be attracted to the sensitive spots when higher radiation exposure
happens.
·
The more
radiation incident on the screen, the more the light photons are received by
the film emulsion, the more bromide ions excitation, the more electrons are
released, and the more silver ions are attracted.
·
The latent image
is formed by silver ions accumulation in the sensitive centers.
·
To convert the
latent image to the real image, we have to blacken the silver ions and remove
the unexposed crystals (grains).
·
The degree of
blackening depends on the number of silver ions accumulated in the sensitive
spots.
Film
Developing
·
Developing
process occurs by adding a source of electrons (phenidone solution) that
converts the silver ion into a black metallic silver.
·
These electrons
invade all exposed and exposed grains.
·
The electrons
invade the unexposed grains to form the background fog of the film.
Fixing
·
The film is
immersed in fixer solution so that the unexposed and unreduced silver halide is
removed to render the image stable in white light.
·
The fixer
solution contains:
Neutralizer:
acetic
acid to stop ongoing development after removal from developer solution
Clearer:
ammonium
or sodium thiosulphate is used to clear the undeveloped silver halide grains
from the film
Preserver:
sodium
sulphite
Hardener:
aluminum
chloride shrinks and hardens the emulsion to produce the final film.
Automatic
processors
·
An automatic
processor:
1.
Unloads the film from the cassette
2.
Transports the film through the processing chemicals in the right order
3.
Produces a developed, hardened, and dried film
The
role of Film screen contact
·
Film should be
in direct contact with the screen
·
Inadequate
film-screen contact usually produces blurring in only a portion of the image
area.
The
Film composition:
·
Supercoat
protection layer
·
Base polyester
layer that gives the film the physical stability
·
Adhesive adhere
layers together
·
Emulsion
radiation and light sensitive layer
Screen
Film combination:
· No screen films
·
Only film is
used to detect the X ray photons directly.
·
High radiation
exposure (air KERMA) is required to get a proper image quality
· High
resolution is produced as no light diffusion
· Single Screen Film
·
The film is
attached to on screen in the front side
·
Each Xray photon
is absorbed and converted to hundreds of light photons
Double-screen film
·
the film is
sandwiched between a double screen.
·
Double emulsion
layer is used
·
the back screen
is thicker than the front one to absorb the X-ray photons that passed without absorption.
·
One-third of X-ray
photons are absorbed by the front screen.
·
Half of the light
photons produced, are absorbed by front screen
·
The back screen
is used to absorb the X ray passed and reflect the produced light towards the
emulsion
· Double screen film is a highly sensitive
combination but affects image sharpness.
· Reduced the property of motion un-sharpness,
as less exposure time is required.
· Reduce the tube loading and less power
and acquisition parameters can be used for imaging.
Film
Sensitivity
· Film sensitivity (Speed) express how much
radiation exposure is required to get an image.
· High Sensitivity (speed) require less
radiation exposure than low speed.
· High sensitivity (speed) films are chosen
when the reduction of patient exposure and heat loading of the x-ray equipment
are important considerations.
· Low sensitivity (speed)
films are used to reduce image noise. The relationship of film sensitivity to
image noise is considered in the section titled, "Image Noise."
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