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87 Cards in this Set

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Overview
Image Intensifier
What happens after the x-ray photons strike the input window
The input phosphor absorbs the x-ray
photons and converts them into optical photons
(a phenomenon called luminescence)
What happens to the optical photons next
These op-tical photons are converted to photoelectrons
at the photocathode
What is the next step
The photoelectrons are ac-celerated by the electric field produced by the
strong electric potential difference of the image
intensifier and are collected at the output phos-phor. Each accelerated electron produces many
optical photons at the output phosphor.
Overview of image intensifier chain
What does the photocathode do
creates electrons
What does phosphor do
creates light signal
What are the components of an image intensifier
an input window, an input phos-phor and photocathode, several electrostatic fo-cusing lenses, an accelerating anode, an output
phosphor screen, and a protective vacuum case
What are the layers of the input window
What is the shape and material of the input window
The input
side of the image intensifier usually has a convex
shape and is generally made of aluminum
Why is a convex shape used
The convex shape not only minimizes the
patient distance thus maximizing the useful en-trance field size (2), but it also gives the image
intensifier better mechanical strength under at-mospheric pressure.
What is the thickness of the aluminum input
1mm
How are x-rays that move through the input window converted to light
input phosphor
What are the considerations when deciding on the thickness of the phosphor layer
The thickness of the
input phosphor layer is a compromise between
spatial resolution and x-ray absorption efficiency.
Does a thicker input phosphor layer reduce radiation
yes, A thicker phosphor layer has higher x-ray absorp-
tion efficiency, which means more x-ray photons
can be absorbed and converted to light photons
in the phosphor layer. A thicker phosphor layer
requires fewer x-ray photons to generate the
same amount of light photons at the image inten-
sifier output window, thus reducing patient dose.
Why does a thicker input phosphor reduce radiation
However, with a thicker input phosphor layer,
more light photons are scattered laterally within
the phosphor layer, thus reducing the spatial resolution
What are examples of thickness of the input phosphor layer
urrently, the thickness of an input
phosphor layer typically measures between 300
and 450 mm, depending on the image intensifier
type and technology used
What is the current phosphor of choice
phosphor of choice is cesium io-dide (CsI:Na).
How do you maximize the conversion of x-rays to electrons
To maximize the conversion efficiency from x-ray photons to photoelectrons, the mass attenua-tion coefficient of the input phosphor material
should be matched with the spectrum of the x
rays emerging from the patient. CsI:Na matches the x-ray spectrum and is well suited
Does the fact that CSI:Na having a high atomic number make it a good phosphor
yes, it absorbs
What are the layers of the input screen
What is the photocathode made of SbCs3
SbCs3
What should be done to maximize the sensitivity of the photocathode
To maximize the conversion effi-ciency from light photon to photoelectron, light
emitted from the input phosphor should match
the sensitivity spectrum of the photocathode
What is the thickness and efficiency of the photocathode layer
The
photocathode has a thickness of about 20 nm and
a photoelectron production efficiency of 10%–
15%.
How many electrons are created from single photon
Approximately 200 photoelectrons will be created for a single 60-keV x-ray photon absorbed
in the input phosphor
How is CSI:Na put on to the supstrate of the input screen
In addition to its high absorption efficiency,
CsI:Na can be evaporated onto the substrate in
crystal needle form. These needles act like light
pipes, in a manner similar to the light propaga-
tion in a fiber-optic faceplate, thus reducing cros
scatter inside the phosphor screen and yielding better spatial resolution.
What doe the CSI crystals look like
What is the diameter of the CSI:Na needles
5um
After leaving the photocathode what happens to the electrons
accelarated from the photocathode to the anode
How are the electrons focused
The accelerated photoelectrons are fo-cused down to the size of the output phosphor by
a series of electrostatic focusing electrodes.
What do the focusing electrodes look like
What is the current produced by the electrons in the housing
The total current
produced by these photoelectrons is approxi-mately 600 nA (600 ´ÿ 10-9 A).
Why must the voltage of the electrodes be kept stable
the high voltages on the electrodes
must be kept very stable to guarantee the image
quality, since ripple in the voltage will be noticed
as periodic variation in image diameter.
What is the anode covered with
On the vacuum side of the output phosphor
surface, the anode of the electron optics system
has a thin aluminum film coating
What is the function of the aluminum covering of the anode
This alumi-num film allows electrons to pass through, but it
is opaque to light photons generated on the fluo-rescent screen. It stops these photons from being
scattered back into the image intensifier and ex-posing the photocathode. The film also serves as
a reflector to increase the output luminance
What is the output phosphor
The output phosphor of the x-ray image intensi-fier, which typically is called P20, is a fluorescent
compound made of silver-activated zinc-cad-mium sulfide (ZnCdS:Ag)
How many light photons are created for each electron
Approximately 2,000 lumi-nescence photons are generated for every acceler-ated 25-keV photoelectron.
How is the image intensified
Because every elec-tron was produced by one light photon, this rep-resents a luminescence gain of 2,000.
What determines the temporal resolution of an image intensifier
The luminescence decay time of
the output phosphor determines the temporal
resolution of the image intensifier.
What happens if a strong magnetic field is close to the image intensifier
the presence of strong
magnetic or electrical fields too close to the im-age intensifier will degrade image quality.
Approximately 200 photoelectrons will be created for a single 60-keV x-ray photon absorbed
in the input phosphor.
Approximately 2,000 lumi-nescence photons are generated for every acceler-ated 25-keV photoelectron. Because every elec-tron was produced by one light photon, this rep-resents a luminescence gain of 2,000.
What is the advantage of a large field of view for an image intensifier
A large
field of view allows one to visualize a larger area,
which can be very helpful in some clinical proce-dures.
What are the sources of brightness gain
The brightness gain comes from two sources that
are completely unrelated: the minification gain
and the flux gain.
What is the minification gain
The minification gain is de-fined as the ratio of input area to the output area
of the image intensifier.
What is the principle behind minification gain
Because the number of
photoelectrons leaving the photocathode is equal
to the number striking the output phosphor, the
number of photoelectrons per unit area at the
output phosphor increases.
How does minfication gain change the characterisitcs of an image
The minification gain
does not improve the statistical quality of the
fluoroscopic image. It will not change the con-trast of the image, but it will make the image ap-pear brighter.
What does a smaller output window essentially do
A smaller output window size will
just compress more photons into a smaller area,
producing a smaller but brighter image.
What is the flux gain
Flux gain is defined as the number of photons
generated at the output phosphor for every pho-ton generated at the input phosphor.
What is the cause of flux gain
The flux
gain results from the acceleration of photoelec-trons to a higher energy so that they generate
more fluorescent photons at the output phos-phor.
What is a flux gain of 100
Each light photon generated at the input
phosphor will generate approximately 100 pho-tons at the output phosphor, resulting in a flux or
luminance gain of 100
What is the total brightness gain
The total brightness gain
of the image intensifier is the product of minifi-cation gain and flux gain (total brightness gain =
flux gain ´ minification gain).
Example of a brightness gain
The minification gain for a 23-cm image
intensifier with an input entrance field size of 22
cm (380 cm2) and a 2-cm output window (3.14
cm2
) is approximately 120. With a flux gain of
approximately 100, the total brightness gain for
this image intensifier would be approximately
12,000.
How are most image intensifiers specified
converstion factor
What is the conversion factor
he conversion factor is defined as the output lu-minance level of an image intensifier divided by
its entrance exposure rate.
What is the conversion factor a measure of
It is a measure of how
efficiently an image intensifier converts the x rays
to light.
What is a good rule of thumb for a conversion factor
The conversion factor usual-ly equals to 1% of the brightness gain in value
What happens too the conversion factor as an image intensifier ages
onversion factors tend to deteriorate (decrease)
as image intensifiers age, resulting in higher pa-tient dose for older image intensifiers.
What is the contrast ratio of an image intensifier
The contrast ratio of an image intensifier is de-fined as the brightness ratio of the periphery to
the center of the output window when the center
portion of an image intensifier entrance is totally
blocked by a lead disk
What is the contrast ratio a measure of
veiling glare
How is the contrast ratio usualy specified
he contrast ratio is typically specified in two
ways: large area and small detail area.
What is a techinque to measure contrast ratio
One method of determining detector (intensifier) contrast is to take the ratio of
the bightness in the open field at a given exposue o the bightness underneath a lead disk coveing
10 percent of the useful cental imaging aea in a second exposure. Contrast atios for modem image
intensifiers exceed 15:1
How is magnification changed in an image intensifier
Changing the voltage applied to the electronic
lenses inside an image intensifier will change the magnification mode of the image intensifier.
How does magnification work
In a
magnification mode, a smaller area of the input
phosphor is used, giving the effect of zooming in
on the image.
What happens to the brightness if magnification is used
Because the input field size is re-duced, the exposure to the input phosphor must
be increased to maintain a constant brightness
level at the output phosphor.
What happens to the radiation dose during magnification
In fact, to maintain
the same noise level, the dose quadruples when
the magnification is doubled
What are examples of dosage of radiation for given magnification modes
The image
intensifier exposure rate is typically set to 30 mR/
sec for the 25-cm mode, 60 mR/sec for the 17-cm
mode, and 120 mR/sec for the 12-cm mode
Do higher magnification modes increase spatial resolution
yes
What causes the dosage to go up in magnification mode
With automatic bightness control (ABC) the mA is automaically increased when the unit is used in the 6-inch node o compensate for the decreased bightness.
Doubling magnification
quadruple dose
What are the artifacts that are caused by image intensifiers
including lag, vignetting,
veiling glare, pincushion distortion, and S distor-tion.
What is lag
Lag is the persistence of luminescence after x-ray
stimulation has been terminated
What does lag degrade
Lag degrades
the temporal resolution of the dynamic image.
What is the approximate lag of a modern flouro machine
Current
image intensifier tubes have lag times of approxi-mately 1 msec.
What is the main contributor to lag in modern flouro machines
Therefore, lag in modern fluoro-scopic systems is more likely caused by the
closed-circuit television system than the image
intensifier
What is vignetting
A fall-off in brightness at the periphery of an im-age is called vignetting. Vignetting is caused by
the unequal collection of light at the center of the
image intensifier compared with the light at its
periphery. As a result, the center of an image in-tensifier has better resolution, increased bright-ness, and less distortion
What causes veiling glare
Scattering of light and the defocusing of photo-electrons within the image intensifier are called
veiling glare.
What part of the image intensifier is effected by veiling glare
Veiling glare degrades object con-trast at the output phosphor of the image intensi-fier.
What is a way to determine if there is a lot of veiling glare
the contrast ratio is a good
measure of determining the veiling glare of an
image intensifier
What are the contributors of veiling glare
X-ray, electron, and light scat-ter all contribute to veiling glare
What is pincushion distortion
Pincushion distortion is a geometric, nonlinear
magnification across the image. The magnifica-tion difference at the periphery of the image re-sults from the projection of the x-ray beam onto
a curved input surface
What does pincushion artifact look like
What does S-distortion look like
What is the cause of pincushion distortion
The magnifica-tion difference at the periphery of the image re-sults from the projection of the x-ray beam onto
a curved input surface.
What is the cause of S-distortion
Electrons within the image intensifier move in
paths along designated lines of flux. External
electromagnetic sources affect electron paths at
the perimeter of the image intensifier more so
than those nearer the center. This characteristic
causes the image in a fluoroscopic system to dis-tort with an S shape
What is resolution
Resolution is the ability of the imaging system to differentiate small objects as separate images as
they are positioned close together.
What is quantum mottling
Quantum motle is a grainy appearance in an image caused by statistical fluctuation of absorbed X-ray photons. Mottle is more visible in a high resolution, high contrast system.
The number of photons in an X-ray image cannot be decreased indefinitely even though patient
radiation doses would also be decreased. A large decrease In the number of X-ray photons would
cause a seious deterioration of image quality
What controls quantum mottling
mA and this determines how low dose we can go