Transcript FLUOROSCOPY

FLUOROSCOPY
Since Thomas A. Edison invented the
fluoroscope in 1896, it has served as a valuable
tool in the practice of radiology.
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.
TUBE ABOVE THE TABLE
TUBE UNDER THE TABLE
C-ARM FLUOROSCOPY
CONVENTIONAL FLUOROSCOPY
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).
kVp DEPENDS ON THE BODY PART
BEING EXAMINED
Examination
kVp
Gallbladder
65–75
Nephrostogram
70–80
Myelogram
70–80
Barium enema (air contrast)
80–90
Upper gastrointestinal
100–110
Small bowel
110–120
Barium enema
110–120
mA VARIES WITH THE
BODY PART
• USUALLY 1-5 mA
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.
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
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.
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.
Cones are used primarily for
daylight vision, called photopic
vision, and rods are used for night
vision, called scotopic vision.
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.
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.
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.
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.
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.
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
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.
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.
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.
BRIGTNESS GAIN (B.G.)
B.G. = Minification gain x Flux gain
MOST INTENSIFIERS: 5,000 – 20,000
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.
FLUX GAIN
# OF PHOTONS AT THE OUTPUT PHOSPHOR
# OF PHOTONS AT THE INPUT PHOSPHOR
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 imageintensifier tubes.
MINIFICATION GAIN
SQUARE OF THE INPUT PHOSPHOR DIAMETER
SQUARE OF THE OUTPUT PHOSPHOR DIAMETER
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.
Image intensifiers have conversion factors
of 50 to 300. These correspond to
brightness gains of 5000 to 30,000.
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.
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.
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.
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.
MAGNIFICATION MODE RESULTS IN:
• Better spatial resolution
• Better contrast resolution
• Higher patient dose
MULTIFOCUS IMAGE INTENSIFIER
FLUOROSCOPIC DATA AQUSITIONIMAGE INTENSIFIED SYSTEM
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X-RAY TUBE
PATIENT
IMAGE INTENSIFIER
OUTPUT PHOSPHOR
CAMERA
MONITOR
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).
CAMERA ATTACHED TO
THE OUTPUT POSPHOR
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.
CAMERA
Image intensifiers and television camera tubes are
manufactured so that the output phosphor of the imageintensifier 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
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.
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 beamsplitting 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.
The video signal is amplified and is transmitted by
cable to the television monitor, where it is
transformed back into a visible image.
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
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
result—“two-on-one.” Four-on-one and six-on-one modes
are also available, with the images becoming successively
smaller.
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.