Download RAD 216 ADVANCED IMAGING MODALITIES THOMAS A. EDISON

RAD 216 ADVANCED
IMAGING MODALITIES
IMAGE INTENSIFICATION
AND TV IMAGING
SYSTEMS
THOMAS A. EDISON
is credited with the
development of
fluoroscopy. His work
enabled physicians to see
DYNAMIC or moving
images of internal
structures of the body.
EARLY FLUOROSCOPY
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FLUOROSCOPY CIRCA 1920’s
FLUOROSCOPY CIRCA 1955
http://time.com/4155549/vintage-x-ray-photos/
EARLY FLUOROSCOPY
relied on the radiologist’s ability to
DARK-ADAPT the eyes in order
to take advantage of SCOTOPIC
(ROD)VISION. This was
achieved by wearing red goggles
for at least 15 minutes.
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EARLY FLUOROSCOPY
took place in a darkened room. The
patient stood behind a leaded glass
plate coated with a fluorescent
material. The radiologist could see
the internal anatomy by staring very
closely at the glass plate.
IMAGE-INTENSIFIED
FLUOROSCOPY
came into common use in the 1960s
as a by-product technology used
during the Vietnam conflict (nightvision equipment). As used in
radiology, it is the ability to convert
a few photons of x-ray into many
photons of light.
THE IMAGE INTENSIFIER
TUBE
is the heart of the image intensifier
system. It consists of a large vacuum
tube approximately 50 cm (20 in.) in
length. Many are 25 cm (10 in.) in
diameter at the largest end and 2.5
cm (1 in.) at the other end.
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IMAGE INTENSIFIER TUBE
25 cm
2.5 cm
50 cm
IMAGE INTENSIFIER
COMPONENTS
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•
•
•
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INPUT PHOSPHOR
PHOTOCATHODE
FOCUSING LENSES
ANODE
OUTPUT PHOSPHOR
INPUT PHOSPHOR
consists of CESIUM
IODIDE crystals. They
convert the x-rays
emerging from the patient
into light photons.
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PHOTOCATHODE
is bonded directly to the input
phosphor. It is
PHOTOEMISSIVE. In
other words, it releases
electrons when stimulated
by light. It is made of
CESIUM and ANTIMONY
COMPOUNDS.
ELECTROSTATIC FOCUSING
LENSES
direct the electrons toward the
anode end of the image
intensifying tube. This is
accomplished by applying
an electric charge to the
plates.
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THE ANODE
attracts the electrons from the
cathode end of the image
intensifier tube. However,
the electrons actually pass
through a hole in the anode
to collide with the...
OUTPUT PHOSPHOR
which is made of ZINC CADMIUM
SULFIDE. The output phosphor
converts the electrons into visible
light. The conversion from x-rays to
light is many times brighter than
simply using a large intensifying
screen.
IMAGE INTENSIFIER TUBE
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MULTIFIELD IMAGE
INTENSIFIER TUBES
are tubes designed to provide
magnification capability. This is done
by decreasing the field of view (FOV).
The voltage applied to the electrostatic
focusing lenses is increased to refocus
the electrons being directed to the
output phosphor.
MULTIFIELD IMAGE
INTENSIFIER (normal mode)
MULTIFOCUS IMAGE
INTENSIFIER (mag. mode)
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MAGNIFICATION FACTOR
The amount of magnification an
image intensifier tube is
capable of can be calculated as
the normal diameter
(nd)/magnification diameter
(md)
VIGNETTING
is the loss of detail and brightness seen
on the periphery of the fluoroscopic
image due to the curve of the input
phosphor. Because only the central
portion of the input phosphor is used
in magnification mode, the image is
sharper and less distorted.
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VIGNETTING
VIGNETTING
PINCUSHION AND
S-DISTORTION
Two type of fluoroscopic image distortion
that results from the shape of the input
phosphor (pincushion) or severe magnetic
irregularities affecting the way that electron
travel toward the output phosphor (sdistortion).
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PINCUSHION ARTIFACT
S-DISTORTION
MAGNIFICATION vs IMAGE
BRIGHTNESS
When an image intensifier tube is
switched to magnification mode, less
of the input phosphor is utilized
(smaller FOV). The result is fewer
electrons striking the output phosphor,
resulting in a darker image (all other
factors unchanged).
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MAGNIFICATION vs IMAGE
BRIGHTNESS
To correct for this loss of brightness,
the x-ray generator increases the mA,
permitting more electrons to be
generated. The tradeoff, however, is
increased patient and occupational
dose until the system is returned to the
normal viewing mode.
MEASURES OF IMAGE
INTENSIFICATION EFFICIENCY
There are two measures which
determine an image intensifier’s
ability to convert x-rays into light.
These are MINIFICATION GAIN
and FLUX GAIN.
MINIFICATION GAIN
is defined as the ratio of the
square diameter of the input
phosphor to the square diameter
of the output phosphor:
(di/do)2
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EXAMPLE
The diameter of the input
phosphor is 9 inches. The
diameter of the output
phosphor is 1 inch.
Therefore, the minification
gain is
92/12 = 81
FLUX GAIN
The ratio of the number of light
photons created at the output
phosphor to the number of x-ray
photons striking the input
phosphor. This can vary by as
much as 3000:1
BRIGHTNESS GAIN
is the product of minification gain
and flux gain. Therefore, if an
image intensifier tube has a
minification gain of 81 and a
flux gain of 3000, the brightness
gain is 243,000.
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VIEWING THE
FLUOROSCOPIC IMAGE
Early image-intensified fluoroscopy relied
on a MIRROR SYSTEM to view the
image. Unfortunately, this method
permitted only the fluoroscopist to see the
image because of the mirror’s small size.
Present day image-intensified systems use
CLOSED-CIRCUIT TV.
A FEW WORDS ABOUT
IMAGE INTENSIFICATION
Because of its high efficiency, the
fluoroscopic current on image-intensified
systems is between 1.5 and 5 mA. This
permits a lower patient dose per unit
time. However, prolonged use of
fluoroscopy during procedures actually
increases patient dose when compared to
routine overhead imaging.
A FEW WORDS ABOUT
IMAGE INTENSIFICATION
QUANTUM MOTTLE is almost always
unavoidable when using high kVp
techniques during image-intensified
fluoroscopy. It is the result of fewer Xray photons being generated as a
consequence of the very low
fluoroscopic mA being used.
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QUANTUM MOTTLE
The resulting fluoroscopic image
appears grainy. To offset the
grainy appearance, fluoro mA
could be manually increased, but
at a cost of higher patient and
occupational dose.
QUANTUM MOTTLE
If the problem cannot be corrected by
increasing fluro mA, then the problem
is system noise which may be due to
faulty settings on the vidicon tube (to be
discussed in another lecture). There
will always be system noise, but should
not be so severe as to prevent visibility.
A FEW WORDS ABOUT
IMAGE INTENSIFICATION
Image densities are reversed on
fluoroscopic images compared to
radiographic images. For example,
barium appears dark and air appears
brighter during fluoroscopic
examinations of the colon.
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RADIOGRAPHIC vs.
FLUOROSCOPIC
FLUOROSCOPY VS.
RADIOGRAPHY
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