• After completing this lecture, the student should be able to:
• Describe the principle of operation of the image intensifier.
• know limitation of the sharpness, and resolution of a image
• Explain how design features can be used to improve image
• Name the events that take place when imaging with image
intensifier is performed.
It is possible to
with naked eyes,
image produce a
• Thomas Edison, 1896
• Screen (zinc-cadmium sulfide) placed over patient’s
body in x-ray beam
• Radiologist looked directly at screen
• Red goggles-30 minutes before exam
• 1950 image intensifiers developed
• In early fluoroscopic techniques, x rays emerging from
the patient impinged directly on a fluoroscopic
• Light was emitted from each region of the screen in
response to the rate at which energy was deposited
by the incident x rays. The light image on
• The fluoroscopic screen was viewed by the radiologist
from a distance of 10 or 15 in.
• A thin plate of lead glass on the back of the
fluoroscopic screen shielded the radiologist
from x radiation transmitted by the screen.
Using this direct fluoroscopic technique:
The radiologist perceived a very dim image
with poor visibility of detail.
Radiologists had to “dark adapt” their eyes
by remaining in the dark for extended
periods in order to view the images.
• Light hits lens which focuses the light onto the
retina where the cones and rods await
• Cones- central
• Rods - periphery
• Sensitive to low light
• Used in night vision (scotopic
• Dims objects seen better
• Color blind
• Do not perceive detail
• Less sensitive to light
(threshold of 100 lux)
• Will respond to bright light
• Daylight vision (phototopic
• Perceive color, differences in
• Perceive fine detail
• Fluoro viewed at same level of brightness as
• X-ray tube under table/over table or in c-arm
• Image intensifier above patient in carriage
• Carriage also has the power drive control, spot film
selection and tube shutters
Principles components of an image intensifier tube;
1. The input phosphor, is 25-57 cm in diameter, exposure of
input phosphor to x-ray beam that has been transmitted
through the patient causes it to emit light.
2. Photocathode layer, which release electrons in number
related to intensity of light (electronic image is
3. In the body of the Image Intensifier tube the electron are
accelerated through a potential difference of 25-35 kv
and focused electronically onto a circular output
phosphors 2.5-3.5 cm in diameter, which is coated of
zinc-cadmium sulphite, which fluoresces when
bombarded with electrons.
4. The light image so produced may up to 9000 times
brighter than the input phosphor image.
Fluoroscopic equipment uses electronic image
intensifiers to provide real-time (dynamic)
• Fluoroscopy is used for the dynamic
evaluation of functional disorders and
guidance during routine surgical procedures,
• Fluoroscopy is used during interventional
(Fluoroscopic Equipment (cont
.Direct fluoroscopy should no longer be used
Direct” fluoroscopy does not use electronic image“
amplification. The real-time image is viewed on a
fluorescent screen in a completely darkened room
and requires the fluoroscopist to dark adapt for
. approximately 20 minutes before the examination
Improper attention to these requirements can
significantly increase the radiation dose to patients
. and users
(Fluoroscopic Equipment (cont
:-All fluoroscopic units
• shall display the instantaneous values of x-ray tube voltage (kV
peak), tube current (mA) and accumulated fluoroscopic exposure
time at the control or to the user.
should be provided with a Dose-Area Product meter or a measuring system
.to indicate patient exposure
The dose rate at the image intensifier input phosphor shall not exceed the
. relevant IEC recommended values
)Fluoroscopy : dynamic (real time) imaging (cont
Light amplifier tubes,
in combination with a
are the most widely
Output phosphor image quality
1. Geometric unsharpness may be minimized by
positioning the intensifier input phosphor as close
as possible to the patient.
2. Spreading of light within the input phosphor is
minimized by using needle-shaped caesium iodide
micromcrystals. Light emitted with each crystal is
internally reflected within the crystal and exits only
at its end.
3. Due to limitation of electronic focusing, it results in
a slight divergence of and the electrons hit output
phosphor over a finite area rather than a point.
Output phosphor image quality –con-B. Noise;
1. Quantum mottle, loss of image quality due
quantum effect is minimized by ensuring that the
input phosphor has a high quantum detection
2. Structures mottle, input phosphor, photocathode
and output phosphor display a graininess, which
produce grainy appearance on the output phosphor
Alignment of caesium iodide
crystal on the input phosphor
focusing in an image
tents to deteriorate
toward the edges.
The light-pipe principle operating in a
caesium iodide crystal. Lateral spread of
light is minimized by internal reflection.
--Output phosphor image quality –con
The output phosphor image is only 2.5-3.5 cm in
diameter and must be magnified before it is
displayed to observer.
• The electron image or pattern created as electrons
from the photocathode, should be duplicated in
miniature at output phosphor, any deviation from
this pattern results distortion of the intensified