fluoroscopy

early fluoro units

screens composed of zinc cadmium sulfide

• emits yellow-green light

required dark adapting

dark adapting

required wearing dark goggles for 20-30 minutes before the fluoro exam

parts of human vision

cones

• daylight and central vision

rods

• night vision and peripheral vision

when was the image intensifier introduced

1948

what is an image intensifier

electronic device that converts x-rays to visible light with higher energy

under-table x-ray tube

most common configuration

tube and collimator are under the tabletop

image intensifier is above

mA technical factors in fluoroscopy

0.5 mA - 5 mA

• low mA, long periods of time

image intensification

creates a brighter image

5 parts of the image intensifier tube

input screen

photocathode

electrostatic lenses

accelerating anode

output screen

input screen

converts x-ray photons to light (yellow-green)

anywhere between 6 and 23 inch convex screens

made of glass, titanium, steel, or aluminum

coated with cesium iodide crystals

• 0.1-0.2 mm layer

• columnar

• tightly packed

conversion efficiency

how effectively light is created from each incoming photon

AKA quantum yield

typically 66%

what is the purpose of a curved input screen

to reduce light spread to increase resolution

photocathode

absorbs light from input screen and emits electrons

made of photoemissive material

• cesium antimony compounds

has a small protective coating between the input screen to prevent chemical reactions between the layers

photoemission

the process of turning light into electrons

electrostatic focusing lenses

series of metal bands of ring with increasing positive charges

focuses the electrons toward the anode ring

• become narrower

accelerating anode

located in front of the output screen

positively charged

• attracts electrons and accelerates them toward the output screen

• increase in energy of the electrons is called flux gain

• potential difference is 25 kV

what happens to the image as it travels from the input screen to the accelerating anode

it is reversed and inverted

output screen

converts electrons into light (green)

made of zinc cadimum sulfide or cesium iodide

diameter of 1 inch

thin aluminum coating prevents backflow of light

newer units use a fiber-optic disc instead of a screen

types of magnification tubes

multi-field

dual-field

triple-field

quad-field

multi-field image intensifiers

have more than one input screen size

have a “mag mode”

focal point

point where the electrons cross

moves based on voltage applied

• 25-35kV

higher voltage moves focal spot closer to the input screen causing a magnified image

dual field magnification

the optical system can only see the central part of the image

reduces FOV to improve resolution and magnify image

mA is automatically increased

range of magnification for multifield iis

1.5x - 4x

magnified images and pt dose

a magnified image increases pt dose

more x-rays used

ABC boosts technique

smaller field =

improved resolution

increased radiation dose

brightness control

maintains brightness of image by automatically adjusting exposure factors for thicker body parts

monitors current flowing between cathode and anode (the intensity at the output screen)

types of brightness control

ABC - automatic brightness control

ADC - automatic dose control

ABS - automatic brightness stabilization

automatic gain control

adjusts the current flowing to the display monitor

doesn’t adjust technical factors

most common type of brightness control

automatic brightness control (ABC)

automatic brightness control

automatically maintains image density and contrast

• rad selects brightness level desired and ABC adjusts technique to produce that level throughout the exam

image lag

ii systems have slow response time to adjusting to changes in tissue density

total brightness gain

brightness gain = minification gain x flux gain

may deteriorate as much as 10% per year as screens age

brightness gain

the ability of an ii to increase image illumination

minification gain

increase in image brightness or intensity

occurs as electrons are compressed into smaller areas

gain in intensity equals the ratio of the diameter of the input and output screens

same number of light photons are concentrated on a smaller screen, creating a bright image

flux gain

anode accelerates electron toward output screen

acceleration increases kinetic energy

electrons arrive at output screen with increased energy

50-75 times more

conversion factor equation

output phosphor illumination / input exposure rate

conversion factor

how much light is given off at the output screen per x-ray photon at input screen

measures how effectively ii converts x-ray energy to light energy

ranges from 50-300

output screen attachments

TV/video tubes

CCDs

CMOSs

TV/video tubes

old technology

coupling device sends signal from output screen to viewing monitor

uses a beam splitter used to send image data to spot film and cine camera

image must be converted to electrons to be sent to viewing devices

converts light from output phosphor to electrical signal and then sent to TV monitor for viewing

charge coupled device (CCD)

coupled to output phosphor by optic cables

light strikes CCD

CCD releases electrons proportional to the incident light

stores latent image

emits signal in a raster pattern

benefits of CCD

faster discharge time

no image lag

operates at lower voltages

high contrast images

high resolution

high SNR
high DQE which decreases technique = decreased pt dose

data element of CCD

DELs

complimentary metal oxide semiconductor

CMOS

coupled to output screen by optic cables

converts light from output screen to electronic signal

lower image quality

each DEL contains its own readout components

viewing monitors

electron gun sprays pulsed stream of electrons onto screen phosphor in raster patter

phosphor crystals emit light to transmit a visual image

what is the weakest link of the fluoro system

viewing monitor

contrast limitations

degradation between input and output screens

deteriorates about 10% per year

background fog

incident photons striking output screens

what determines detail in fluoro

geometric factors

greatest resolution limitation

video monitor

distortion types

pincushion

vignetting

veiling glare

pincushion

distortion of lines

caused by curvature of input screen and flat output screen

electrons flare out at outer edges

vignetting

brightness varies from center of images to edges

a consequence of pincushion

veiling glare

light scatter from output screen degrades contrast

factors contributing to quantum mottle

video noise

radiation output

beam attenuation

conversion efficiency

minification gain

flux gain

brightness gain

flat panel digital fluoro

replaces image intensifier

increased SNR

resolution not as high as a radiograph

less pt dose

improved contrast

last image hold

less distortion

advantages of digital fluoro

reduces patient dose

higher DQE

higher SNR
durable

reduced artifacts

less geometric distortion

types of fluoro equipment

over-table x-ray

fixed c-arm

mobile c-arm

mini c-arm

bi-plane

• floor mounted (like in IR/cath)

mm lb for apron and gloves

0.5

mm lb for bucky slot shield

0.25

SSD for fixed unit

15 inches

SSD for mobile unit

12 inches

maximum tabletop exposure rate

10 R/min