Physics: Flouroscopy (Image Intensifier)

Front 1

Overview

Back 1

Image Intensifier

Front 2

What happens after the x-ray photons strike the input window

Back 2

The input phosphor absorbs the x-ray
photons and converts them into optical photons
(a phenomenon called luminescence)

Front 3

What happens to the optical photons next

Back 3

These op-tical photons are converted to photoelectrons
at the photocathode

Front 4

What is the next step

Back 4

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.

Front 5

Overview of image intensifier chain

Back 5

Front 6

What does the photocathode do

Back 6

creates electrons

Front 7

What does phosphor do

Back 7

creates light signal

Front 8

What are the components of an image intensifier

Back 8

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

Front 9

What are the layers of the input window

Back 9

Front 10

What is the shape and material of the input window

Back 10

The input
side of the image intensifier usually has a convex
shape and is generally made of aluminum

Front 11

Why is a convex shape used

Back 11

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.

Front 12

What is the thickness of the aluminum input

Back 12

1mm

Front 13

How are x-rays that move through the input window converted to light

Back 13

input phosphor

Front 14

What are the considerations when deciding on the thickness of the phosphor layer

Back 14

The thickness of the
input phosphor layer is a compromise between
spatial resolution and x-ray absorption efficiency.

Front 15

Does a thicker input phosphor layer reduce radiation

Back 15

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.

Front 16

Why does a thicker input phosphor reduce radiation

Back 16

However, with a thicker input phosphor layer,
more light photons are scattered laterally within
the phosphor layer, thus reducing the spatial resolution

Front 17

What are examples of thickness of the input phosphor layer

Back 17

urrently, the thickness of an input
phosphor layer typically measures between 300
and 450 mm, depending on the image intensifier
type and technology used

Front 18

What is the current phosphor of choice

Back 18

phosphor of choice is cesium io-dide (CsI:Na).

Front 19

How do you maximize the conversion of x-rays to electrons

Back 19

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

Front 20

Does the fact that CSI:Na having a high atomic number make it a good phosphor

Back 20

yes, it absorbs

Front 21

What are the layers of the input screen

Back 21

Front 22

What is the photocathode made of SbCs3

Back 22

SbCs3

Front 23

What should be done to maximize the sensitivity of the photocathode

Back 23

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

Front 24

What is the thickness and efficiency of the photocathode layer

Back 24

The
photocathode has a thickness of about 20 nm and
a photoelectron production efficiency of 10%–
15%.

Front 25

How many electrons are created from single photon

Back 25

Approximately 200 photoelectrons will be created for a single 60-keV x-ray photon absorbed
in the input phosphor

Front 26

How is CSI:Na put on to the supstrate of the input screen

Back 26

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.

Front 27

What doe the CSI crystals look like

Back 27

Front 28

What is the diameter of the CSI:Na needles

Back 28

5um

Front 29

After leaving the photocathode what happens to the electrons

Back 29

accelarated from the photocathode to the anode

Front 30

How are the electrons focused

Back 30

The accelerated photoelectrons are fo-cused down to the size of the output phosphor by
a series of electrostatic focusing electrodes.

Front 31

What do the focusing electrodes look like

Back 31

Front 32

What is the current produced by the electrons in the housing

Back 32

The total current
produced by these photoelectrons is approxi-mately 600 nA (600 ´ÿ 10-9 A).

Front 33

Why must the voltage of the electrodes be kept stable

Back 33

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.

Front 34

What is the anode covered with

Back 34

On the vacuum side of the output phosphor
surface, the anode of the electron optics system
has a thin aluminum film coating

Front 35

What is the function of the aluminum covering of the anode

Back 35

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

Front 36

What is the output phosphor

Back 36

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)

Front 37

How many light photons are created for each electron

Back 37

Approximately 2,000 lumi-nescence photons are generated for every acceler-ated 25-keV photoelectron.

Front 38

How is the image intensified

Back 38

Because every elec-tron was produced by one light photon, this rep-resents a luminescence gain of 2,000.

Front 39

What determines the temporal resolution of an image intensifier

Back 39

The luminescence decay time of
the output phosphor determines the temporal
resolution of the image intensifier.

Front 40

What happens if a strong magnetic field is close to the image intensifier

Back 40

the presence of strong
magnetic or electrical fields too close to the im-age intensifier will degrade image quality.

Front 41

Approximately 200 photoelectrons will be created for a single 60-keV x-ray photon absorbed
in the input phosphor.

Back 41

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.

Front 42

What is the advantage of a large field of view for an image intensifier

Back 42

A large
field of view allows one to visualize a larger area,
which can be very helpful in some clinical proce-dures.

Front 43

What are the sources of brightness gain

Back 43

The brightness gain comes from two sources that
are completely unrelated: the minification gain
and the flux gain.

Front 44

What is the minification gain

Back 44

The minification gain is de-fined as the ratio of input area to the output area
of the image intensifier.

Front 45

What is the principle behind minification gain

Back 45

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.

Front 46

How does minfication gain change the characterisitcs of an image

Back 46

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.

Front 47

What does a smaller output window essentially do

Back 47

A smaller output window size will
just compress more photons into a smaller area,
producing a smaller but brighter image.

Front 48

What is the flux gain

Back 48

Flux gain is defined as the number of photons
generated at the output phosphor for every pho-ton generated at the input phosphor.

Front 49

What is the cause of flux gain

Back 49

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.

Front 50

What is a flux gain of 100

Back 50

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

Front 51

What is the total brightness gain

Back 51

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).

Front 52

Example of a brightness gain

Back 52

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.

Front 53

How are most image intensifiers specified

Back 53

converstion factor

Front 54

What is the conversion factor

Back 54

he conversion factor is defined as the output lu-minance level of an image intensifier divided by
its entrance exposure rate.

Front 55

What is the conversion factor a measure of

Back 55

It is a measure of how
efficiently an image intensifier converts the x rays
to light.

Front 56

What is a good rule of thumb for a conversion factor

Back 56

The conversion factor usual-ly equals to 1% of the brightness gain in value

Front 57

What happens too the conversion factor as an image intensifier ages

Back 57

onversion factors tend to deteriorate (decrease)
as image intensifiers age, resulting in higher pa-tient dose for older image intensifiers.

Front 58

What is the contrast ratio of an image intensifier

Back 58

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

Front 59

What is the contrast ratio a measure of

Back 59

veiling glare

Front 60

How is the contrast ratio usualy specified

Back 60

he contrast ratio is typically specified in two
ways: large area and small detail area.

Front 61

What is a techinque to measure contrast ratio

Back 61

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

Front 62

How is magnification changed in an image intensifier

Back 62

Changing the voltage applied to the electronic
lenses inside an image intensifier will change the magnification mode of the image intensifier.

Front 63

How does magnification work

Back 63

In a
magnification mode, a smaller area of the input
phosphor is used, giving the effect of zooming in
on the image.

Front 64

What happens to the brightness if magnification is used

Back 64

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.

Front 65

What happens to the radiation dose during magnification

Back 65

In fact, to maintain
the same noise level, the dose quadruples when
the magnification is doubled

Front 66

What are examples of dosage of radiation for given magnification modes

Back 66

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

Front 67

Do higher magnification modes increase spatial resolution

Back 67

yes

Front 68

What causes the dosage to go up in magnification mode

Back 68

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.

Front 69

Doubling magnification

Back 69

quadruple dose

Front 70

What are the artifacts that are caused by image intensifiers

Back 70

including lag, vignetting,
veiling glare, pincushion distortion, and S distor-tion.

Front 71

What is lag

Back 71

Lag is the persistence of luminescence after x-ray
stimulation has been terminated

Front 72

What does lag degrade

Back 72

Lag degrades
the temporal resolution of the dynamic image.

Front 73

What is the approximate lag of a modern flouro machine

Back 73

Current
image intensifier tubes have lag times of approxi-mately 1 msec.

Front 74

What is the main contributor to lag in modern flouro machines

Back 74

Therefore, lag in modern fluoro-scopic systems is more likely caused by the
closed-circuit television system than the image
intensifier

Front 75

What is vignetting

Back 75

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

Front 76

What causes veiling glare

Back 76

Scattering of light and the defocusing of photo-electrons within the image intensifier are called
veiling glare.

Front 77

What part of the image intensifier is effected by veiling glare

Back 77

Veiling glare degrades object con-trast at the output phosphor of the image intensi-fier.

Front 78

What is a way to determine if there is a lot of veiling glare

Back 78

the contrast ratio is a good
measure of determining the veiling glare of an
image intensifier

Front 79

What are the contributors of veiling glare

Back 79

X-ray, electron, and light scat-ter all contribute to veiling glare

Front 80

What is pincushion distortion

Back 80

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

Front 81

What does pincushion artifact look like

Back 81

Front 82

What does S-distortion look like

Back 82

Front 83

What is the cause of pincushion distortion

Back 83

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.

Front 84

What is the cause of S-distortion

Back 84

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

Front 85

What is resolution

Back 85

Resolution is the ability of the imaging system to differentiate small objects as separate images as
they are positioned close together.

Front 86

What is quantum mottling

Back 86

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

Front 87

What controls quantum mottling

Back 87

mA and this determines how low dose we can go