2. First generation fluoroscopes
• Consisted of an x-ray tube, an x-ray table, and a fluoroscopic screen.
• The fluorescent material in the screen was copper activated zinc
cadmium sulfide that emitted light in the yellow-green spectrum.
• A sheet of lead glass covered the screen, so the radiologist could stare
directly into the screen without having the x-ray beam strike his eyes.
• Fluoroscopic examinations were carried out in a dark room.
• The solution came in the form of the x-ray image intensifier.
5. • After an x-ray beam passes through the patient, it enters the image
intensifier tube.
• The input screen absorbs x-ray photons and converts their energy into
light photons. The light photons strike the photocathode, causing it to
emit photoelectrons.
• As the electrons flow from the cathode to anode, they are focused by
an electrostatic lens, which guides them to the output screen.
• The electrons strike the output screen, which emits the light photons
that carry the fluoroscopic image
6. The image is carried in the following order
x-ray
photons
light
photons
next by
electrons
light
photons.
7. Input Phosphor
• The input fluorescent screen in image intensifiers is cesium iodide
(Csl).
• Csl is deposited on a thin aluminium substrate during the deposition
process the crystals of CsI grow in tiny needles perpendicular to the
substrate.
• When one of these needle-shaped crystals absorbs an x-ray photon
and produces light, the light will be transmitted to the photocathode
with little scattering.
8. • 4 physical characteristics of cesium iodide make it superior to the
older zinc -cadmium sulfide screens.
• The vertical orientation of the crystals
• A greater packing density
• More favorable effective atomic number
• Packing density of cesium iodide is three times greater than that of
zinc-cadmium sulfide.
9.
10. Photocathode
• The photocathode is a photoemissive metal.
• When light from the fluorescent screen strikes the
photocathode,photoelectrons are emitted.
• The numbers are proportional to the brightness of the screen.
11. Electrostratic Focusing Lens
• The lens is made up of a series of positively charged electrodes that
are usually plated onto the inside surface of the glass envelope.
• These electrodes focus the electron beam as it flows from the
photocathode toward the output phosphor.
12. Point inversion
• Electron focusing inverts and reverses the image.
• This is called a point inversion because all the electrons pass through
a common focal point on their way to the output phosphor.
13.
14. • Each point on the input phosphor is focused to a specific point on the
opposite side of the output phosphor.
• The input phosphor is curved to ensure that electrons emitted at the
peripheral regions of the photocathode travel the same distance as
those emitted from the central region.
• The image on the output phosphor is reduced in size.
• This is one of the principal reasons why it is brighter.
15. Accelerating Anode.
• Located in the neck of the image tube.
• Its function is to accelerate electrons emitted from the photocathode
toward the output screen.
16. Output Phosphor
• Is silver activated zinc-cadmium sulphide.
• Crystal size and layer thickness are reduced to maintain resolution in
the minified image.
• The number of light photons is increased approximately 50-fold.
17.
18. • A thin layer of aluminum is plated onto the fluorescent screen to
prevent light from moving retrograde through the tube and activating
the photocathode.
• The glass tube of the image intensifier is about 2 to 4 mm thick, and is
enclosed in a lead-lined metal container.
• The lead lining protects the operator from stray radiation.
19. Viewing the output of an image intensifier
• Directly- Mirror optical system
• Indirectly- Closed circuit television.
20. Mirror optical system
• Light is reflected and focused several times.
• This can be done with only minimal loss of brightness.
•Disadvantages-
• Image is only visible in a small viewing angle.
• Freedom of movement is limited by the mirror.
• Only one observer can view the image.
21.
22. Closed circuit television
• In modern systems, Viewing the output of an image intensifier is done
via a closed circuit television chain.
• We could record the film image from the TV camera signal.
• It is better to expose the film directly to the output phosphor of the
image intensifier tube.
23. • To maintain continuous TV viewing while exposing the film, we must
split the light from the II output into two paths at the time of film
exposure.
• During routine fluoroscopy, all the light output of the II is directed to
the TV camera.
• When film mode is selected, a semi transparent mirror is positioned
in the light beam.
26. • In some systems, the image is coupled to the TV camera by a
fiberoptic bundle.
• Fiberoptic coupling precludes filming directly from the output
phosphor of the image intensifier tube.
• A fiber face plate is a bundle of fine optically shielded glass fibers
(several thousand per mm2) that is a few millimeters thick.
27. BRIGHTNESS GAIN -Two methods
• The first compares the luminance of an intensifier output screen to
that of a Pattersontype B-2 fluoroscopy screen when both are
exposed to the same quantity of radiation.
• The brightness gain is the ratio of the two illuminations:
• Intensifier luminance
• Brightness gain = Patterson B-2 luminance
28. • Pattersontype B-2 fluoroscopic screens, however, vary from one batch
to another, and deteriorate at an unpredictable rate with time.
• Because of this,ICRU, has recommended a second method of
evaluation.
• The conversion factor is a ratio of the luminance of the output
phosphor to the input exposure rate:
cd/m2
• Convers1on factor =mRisec
• Candelas-Output screen luminance
• The conversion factor usually equals about 1% of the brightness gain.
29. • The brightness gain of an image intensifier comes from two
completely unrelated sources-
• Minification gain
• Flux gain
• Brightness gain = Minification gain x Flux gain
30. Minification Gain
• The brightness gain from minification is produced by a reduction in
image size.
• The quantity of the gain depends on the relative areas of the input
and output screens.
• Minification gain = (d1/d2)2
31. Flux Gain
• For each light photon from the input screen, 50 light photons are
emitted by the output screen.
• Flux gain increases the brightness of the fluoroscopic image by a
factor of approximately 50.
32. one light photon from the input
screen eject one electron from the
photocathode.
The electron is accelerated to the
opposite end of the tube, gaining
enough energy to produce 50 light
photons at the output screen
34. Contrast
A 1/4in. thick lead disc
is placed over the
center of the input
screen
The input phosphor is
exposed to a specified
quantity of radiation
Brightness is
measured at the
output phosphor
35. • Contrast is the brightness ratio of the periphery to the center of the
output screen.
• Contrast ratios range from approximately 10:1 to better than 20:1.
• Two factors tend to diminish contrast-
1) The input screen does not absorb all the photons in the x-ray beam.
• Some are transmitted through the intensifier tube, and a few are
eventually absorbed by the output screen.
36. • These transmitted photons contribute to the illumination of the
output phosphor produce a background of fog that reduces image
contrast.
• 2) Retrograde light flow from the output screen.
• Most retrograde light flow is blocked by a thin layer of aluminum on
the back of the screen.
38. Lag
• Persistence of luminescence after x-ray stimulation has been
terminated.
• With older image tubes, lag times were 30 to 40 ms.
• With Csl tubes, lag times are about 1ms.
39. Distorsion
• The electric fields that accurately control electrons in the center of
the image are not capable of the same degree of control for
peripheral electrons.
• Peripheral electrons tend to flare out from an ideal course, do not
strike the output phosphor where they ideally should.
• The result is unequal magnification, which produces peripheral
distortion.
• The amount of distortion is always greater with large intensifiers.
41. Vignetting
• The center of the output screen is brighter than the periphery.
• The peripheral image is displayed over a larger area of the output
screen, and thus its brightness gain from minification is less than that
in the center.
• A fall-off in brightness at the periphery of an image is called
vignetting.
42. MULTIPLE-FIELD IMAGE INTENSIFIERS
• Resolve the conflicts between image size and quality.
• They can be operated in several modes, including a 4.5- in., a 6-in., or
9-in. mode.
• Field size is changed by applying a simple electronic principle:
• The higher the voltage on the electrostatic focusing lens, the more
the electron beam is focused.
43.
44. • In the 9-in. mode, the electrostatic focusing voltage is decreased.
• The electrons focus to a point, or cross, close to the output phosphor,
and the final image is actually smaller than the phosphor.
• In the 6-in. mode the electrostatic focusing voltage is increased, and
the electrons focus farther away from the output phosphor.
• After the electrons cross, they diverge, so the image on the output
phosphor is larger than in the 9-in. mode.
By a radiologist who had dark-adapted his eyes by wearing red goggles for 20 to 30 minutes prior to the examination.
without distorting their geometric configuration
Substrate by a process called vapour deposition
Phosphor thicknesses have been reduced
comparably from approximately 0.3 mm to 0.1mm
(commonly a combination of antimony and cesium compounds).
The anode has a positive
potential of 25 to 35 k V relative to the photocathode,
Because the electrons are greatly accelerated, they emit more light photons from the output screen than were originally present at the input screen.
This can be done with only minimal loss of brightness.
the resulting image would be degraded by the television chain.
(often termed"partially silvered")
defined as the luminous intensity
so a conversion factor of 100 is about the same as brightness gain of 10,000
Most x-ray image intensifiers have an input screen from 5 to 9 in. in diameter.
output screen approximately 1 in. in diameter.
intensifier
has a gain of 81.
in the same way that scattered x-ray photons produce fog and reduce contrast in a radiographic image
the further .an electron is from the center of the intensifier, the more difficult it is to control.
This is the cine image of a wire screen
the wires curve out at the periphery; this effect is most noticeable at the corners.
In summary, the center of the image intensifier
screen has better resolution, a
brighter image, and less geometric distortion
The 9-in. mode
is used when it is necessary to view large
anatomic areas. When size is unimportant,
the 4.5- or 6-in. mode is used because of
better resultant image quality.
In the 6-in. mode, the optical
system "sees" only the central portion of
the image, the part derived from the central
6 in. of the input phosphor
