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FLUOROSCOPY

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FLUOROSCOPY MINIFICATION GAIN SQUARE OF THE INPUT PHOSPHOR DIAMETER SQUARE OF THE OUTPUT PHOSPHOR DIAMETER Brightness gain is now defined as the ratio of the ... – PowerPoint PPT presentation

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Title: FLUOROSCOPY


1
FLUOROSCOPY
2
Since Thomas A. Edison invented the fluoroscope
in 1896, it has served as a valuable tool in the
practice of radiology.
3
A radiologic technique in which a fluoroscope is
used to visually examine the body or an organ. (A
fluoroscope utilizes an X-ray tube and
fluorescent screen, with the area to be viewed
placed between the screen and the tube.) This
immediate imaging, when coupled with an image
intensifier, is invaluable in situations such as
cardiac catheterization, thin needle biopsies of
tumors, and localization of foreign bodies.
4
TUBE ABOVE THE TABLE
5
TUBE UNDER THE TABLE
6
C-ARM FLUOROSCOPY
7
CONVENTIONAL FLUOROSCOPY
8
The kVp of operation depends entirely on the
section of the body that is being examined.
Fluoroscopic equipment allows the radiologist to
select an image brightness level that is
subsequently maintained automatically by varying
the kVp, the mA, or sometimes both. This feature
of the fluoroscope is called automatic brightness
control (ABC).
9
kVp DEPENDS ON THE BODY PART BEING EXAMINED
Examination kVp
Gallbladder 6575
Nephrostogram 7080
Myelogram 7080
Barium enema (air contrast) 8090
Upper gastrointestinal 100110
Small bowel 110120
Barium enema 110120
10
mA VARIES WITH THE BODY PART
  • USUALLY 1-5 mA

11
  • The principal advantage of image-intensified
    fluoroscopy over earlier types of fluoroscopy is
    increased image brightness. Just as it is much
    more difficult to read a book in dim illumination
    than in bright illumination, it is much harder to
    interpret a dim fluoroscopic image than a bright
    one.

12
Human Vision The structures in the eye that are
responsible for the sensation of vision are
called rods and cones. Light incident on the eye
must first pass through the cornea, a transparent
protective covering, and then through the lens,
where the light is focused onto the retina
13
When light arrives at the retina, it is detected
by the rods and the cones. Rods and cones are
small structures more than 100,000 of them are
found per square millimeter of retina. The cones
are concentrated at the center of the retina in
an area called the fovea centralis. Rods, on the
other hand, are most numerous on the periphery of
the retina. No rods are found at the fovea
centralis.
14
The rods are sensitive to low light levels and
are stimulated during dim light situations. The
threshold for rod vision is approximately 2 lux.
Cones, on the other hand, are less sensitive to
light their threshold is only approximately 100
lux, but cones are capable of responding to
intense light levels, whereas rods cannot.
15
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16
Cones are used primarily for daylight vision,
called photopic vision, and rods are used for
night vision, called scotopic vision.
17
Cones perceive small objects much better than
rods do. This ability to perceive fine detail is
called visual acuity. Cones are also much better
at detecting differences in brightness levels.
This property of vision is called contrast
perception. Furthermore, cones are sensitive to a
wide range of wavelengths of light. Cones
perceive color, but rods are essentially
color-blind. Under scotopic conditions, the
sensitivity of the eye is greatest in the green
part of the spectrum at about 555 nm.
18
During fluoroscopy, maximum image detail is
desired this requires high levels of image
brightness. The image intensifier was developed
principally to replace the conventional
fluorescent screen, which had to be viewed in a
darkened room and then only after 15 minutes of
dark adaptation The image intensifier raises
illumination into the cone vision region, where
visual acuity is greatest.
19
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20
IMAGE INTENSIFIER
  • The image-intensifier tube is a complex
    electronic device that receives the image-forming
    x-ray beam and converts it into a visible-light
    image of high intensity. The tube components are
    contained within a glass or metal envelope that
    provides structural support but more importantly
    maintains a vacuum. When installed, the tube is
    mounted inside a metal container to protect it
    from rough handling and breakage.

21
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22
X-rays that exit the patient and are incident on
the image-intensifier tube are transmitted
through the glass envelope and interact with the
input phosphor, which is cesium iodide (CsI).
When an x-ray interacts with the input phosphor,
its energy is converted into visible light this
is similar to the effect of radiographic
intensifying screens. The CsI crystals are grown
as tiny needles and are tightly packed in a layer
of approximately 300 µm Each crystal is
approximately 5 µm in diameter. This results in
microlight pipes with little dispersion and
improved spatial resolution.
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24
The next active element of the image-intensifier
tube is the photocathode, which is bonded
directly to the input phosphor with a thin,
transparent adhesive layer. The photocathode is a
thin metal layer usually composed of cesium and
antimony compounds that respond to stimulation of
input phosphor light by the emission of electrons.
The photocathode emits electrons when illuminated
by the input phosphor. This process is known as
photoemission. The term is similar to thermionic
emission, which refers to electron emission that
follows heat stimulation. Photoemission is
electron emission that follows light stimulation.
25
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26
The image-intensifier tube is approximately 50 cm
long. A potential difference of about 25,000 V is
maintained across the tube between photocathode
and anode so that electrons produced by
photoemission will be accelerated to the anode
27
The anode is a circular plate with a hole in the
middle through which electrons pass to the output
phosphor, which is just the other side of the
anode and is usually made of zinc cadmium
sulfide. The output phosphor is the site where
electrons interact and produce light.
28
For the image pattern to be accurate, the
electron path from the photocathode to the output
phosphor must be precise. The engineering aspects
of maintaining proper electron travel are called
electron optics because the pattern of electrons
emitted from the large cathode end of the
image-intensifier tube must be reduced to the
small output phosphor. The devices responsible
for this control, called electrostatic focusing
lenses, are located along the length of the
image-intensifier tube. The electrons arrive at
the output phosphor with high kinetic energy and
contain the image of the input phosphor in
minified form.
29
The increased illumination of the image is due to
the multiplication of light photons at the output
phosphor compared with x-rays at the input
phosphor and the image minification from input
phosphor to output phosphor. The ability of the
image intensifier to increase the illumination
level of the image is called its brightness gain.
The brightness gain is simply the product of the
minification gain and the flux gain.
30
BRIGTNESS GAIN (B.G.)
B.G. Minification gain x Flux gain
MOST INTENSIFIERS 5,000 20,000
31
The interaction of these high-energy electrons
with the output phosphor produces a considerable
amount of light. Each photoelectron that arrives
at the output phosphor produces 50 to 75 times as
many light photons as were necessary to create
it. This ratio of the number of light photons at
the output phosphor to the number of x-rays at
the input phosphor is the flux gain.
32
FLUX GAIN
OF PHOTONS AT THE OUTPUT PHOSPHOR OF PHOTONS
AT THE INPUT PHOSPHOR
33
The minification gain is the ratio of the square
of the diameter of the input phosphor to the
square of the diameter of the output phosphor.
Output phosphor size is fairly standard at 2.5 or
5 cm. Input phosphor size varies from 10 to 35 cm
and is used to identify image-intensifier tubes.
34
MINIFICATION GAIN
SQUARE OF THE INPUT PHOSPHOR DIAMETER SQUARE OF
THE OUTPUT PHOSPHOR DIAMETER
35
Brightness gain is now defined as the ratio of
the illumination intensity at the output
phosphor, measured in candela per meter squared
(cd/m2) to the radiation intensity incident on
the input phosphor, measured in milliroentgens
per second (mR/s). This quantity is called the
conversion factor and is approximately 0.01 times
the brightness gain. The conversion factor is the
proper quantity for expressing image
intensification.
36
Image intensifiers have conversion factors of 50
to 300. These correspond to brightness gains of
5000 to 30,000.
37
Internal scatter radiation in the form of x-rays,
electrons, and particularly light can reduce the
contrast of image-intensifier tubes through a
process called veiling glare.
Advanced II tubes have output phosphor designs
that reduce veiling glare.
38
Multifield Image Intensification Most image
intensifiers are of the multifield type.
Multifield image intensifiers provide
considerably greater flexibility in all
fluoroscopic examinations and are standard
components in digital fluoroscopy. Trifield tubes
come in various sizes, but perhaps the most
popular is 25/17/12 cm.
39
When a switch is made to the 17-cm mode, the
voltage on the electrostatic focusing lenses
increases this causes the electron focal point
to move farther from the output phosphor.
Consequently, only electrons from the center
17-cm diameter of the input phosphor are incident
on the output phosphor. The principal result of
this change in focal point is to reduce the field
of view. The image now appears magnified because
it still fills the entire screen on the monitor.
Use of the smaller dimension of a multifield
image-intensifier tube always results in a
magnified image, with a magnification factor in
direct proportion to the ratio of the diameters.
A 25/17/12 tube operated in the 12-cm mode
produces an image that is times larger than
the image produced in the 25-cm mode.
40
The portion of any image that results from the
periphery of the input phosphor is inherently
unfocused and suffers from vignetting, that is, a
reduction in brightness at the periphery of the
image.
41
MAGNIFICATION MODE RESULTS IN
  • Better spatial resolution
  • Better contrast resolution
  • Higher patient dose

42
MULTIFOCUS IMAGE INTENSIFIER
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44
FLUOROSCOPIC DATA AQUSITION-IMAGE INTENSIFIED
SYSTEM
  • X-RAY TUBE
  • PATIENT
  • IMAGE INTENSIFIER
  • OUTPUT PHOSPHOR
  • CAMERA
  • MONITOR

45
Two methods are used to electronically convert
the visible image on the output phosphor of the
image intensifier into an electronic signal
  • Thermionic television camera tube
  • The solid state charge-coupled device (CCD).

46
CAMERA ATTACHED TO THE OUTPUT POSPHOR
47
The television camera consists of cylindrical
housing, approximately 15 mm in diameter by 25 cm
in length, that contains the heart of the
television camera tube. It also contains
electromagnetic coils that are used to properly
steer the electron beam inside the tube. A number
of such television camera tubes are available for
television fluoroscopy, but the vidicon and its
modified version, the Plumbicon, are used most
often.
48
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49
CAMERA
50
Image intensifiers and television camera tubes
are manufactured so that the output phosphor of
the image-intensifier tube is the same diameter
as the window of the television camera tube,
usually 2.5 or 5 cm. Two methods are commonly
used to couple the television camera tube to the
image-intensifier tube
51
The simplest method is to use a bundle of fiber
optics. The fiber optics bundle is only a few
millimeters thick and contains thousands of glass
fibers per square millimeter of cross section.
One advantage of this type of coupling is its
compact assembly, which makes it easy to move the
image-intensifier tower. This coupling is rugged
and can withstand relatively rough handling. The
principal disadvantage is that it cannot
accommodate the additional optics required for
devices such as cine or photospot cameras.
52
To accept a cine or photospot camera, lens
coupling is required. This type of coupling
results in a much larger assembly that should be
handled with care. It is absolutely essential
that the lenses and the mirror remain precisely
adjusted because malposition results in a blurred
image. The objective lens accepts light from the
output phosphor and converts it into a parallel
beam. When an image is recorded on film, this
beam is interrupted by a beam-splitting mirror so
that only a portion is transmitted to the
television camera the remainder is reflected to
a film camera. Such a system allows the
fluoroscopist to view the image while it is being
recorded.
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54
The video signal is amplified and is transmitted
by cable to the television monitor, where it is
transformed back into a visible image.
55
Image Recording
  • The conventional cassette-loaded spot film
  • The photospot camera is similar to a movie camera
    except that it exposes only one frame when
    activated

56
During fluoroscopy, the cassette is parked in a
lead-lined shroud so it is not unintentionally
exposed. When a cassette spot-film exposure is
desired, the radiologist must actuate a control
that properly positions the cassette in the x-ray
beam and changes the operation of the x-ray tube
from low fluoroscopic mA to high radiographic mA.
Sometimes, it takes the rotating anode a second
or two to be energized to a higher speed. The
cassette-loaded spot film is masked by a series
of lead diaphragms that allow several image
formats. When the entire film is exposed at one
time, it is called one-on-one. When only half
of the film is exposed at a time, two images
resulttwo-on-one. Four-on-one and six-on-one
modes are also available, with the images
becoming successively smaller.
57
The photospot camera does not require significant
interruption of the fluoroscopic examination and
avoids the additional heat load on the x-ray tube
that is associated with cassette-loaded spot
films. The photospot camera uses film sizes of 70
and 105 mm.
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