Resident Physics Lectures

Fluoroscopic Fluoroscopic ImagingImaging

George DavidAssociate Professor of RadiologyMedical College of Georgia

The Bad Ol’ DaysRadiology directly viewed fluorescent

screenscreen covered with lead glass for

protectionlow light levels

10-30 minute dark adaptation required by wearing red goggles

Human eye light receptorsrods (scotopic vision)

respond to very low light levels night vision peripheral vision

sensitive to blue-green wavelengths

poor visual acuitypoor gray shade detection

cones (photopic vision)high direct vision acuityblind at low illumination levels

Image Intensifiersevacuated glass

envelopevacuum tube

          

                              

Image IntensifierGlass tube

2 to 4 mm thickcurved bottomlead lined

protects operator from stray radiation

lined with “mu” metal protects image tube from

defocusing stray magnetic fields

Image Intensifier Components

input phosphorinput phosphorx-rays to light

photocathodephotocathodelight to electrons

electrostatic focusing electrostatic focusing lenslenssteer those electrons

accelerating anodeaccelerating anodespeed up those

electronsoutput phosphoroutput phosphor

electrons to light

Input Phosphor

cesium iodide (CsI) CsI crystal needles

perpendicular to substrate

minimizes lateral light diffusion or scattering

improves resolution

typical image tube resolution 3 - 5 line pairs / mm

x-rays ==> light

Input PhosphorK-Edge of phosphors

CS ==> (36 keV) I ==> 33.2 keV well suited to average

fluoro beam energy 30 to 40 keV

absorbs ~ 2/3 of incident beam

energy

absorption

Photocathode

attached directly to input phosphorminimizes light diffusion

photoemissive metallight causes emission of

photoelectronsphotoelectrons## photoelectrons

emitted proportional to incident light from input phosphor

light photons ==> electrons

Electrostatic Focusing LensSeveral electrodes plated to

inside of glass envelope+ voltage applied to

electrodeseach electrode at different voltagevoltages determine magnification mode

focuses each point of input phosphor to a point on output phosphor inverts & reverses image

lenses

+

+

+

+--

Accelerating Anodein neck of image tube+ 25 - 35 kV charge

accelerates electronsfaster electrons produce more

light when they strike output phosphor

Output PhosphorSmall viewable fluorescent

screen 0.5 - 1 inch diameterconverts electron’s kinetic

energy to light~ 50 fold increase in # light

photons over input phosphor

OutputPhosphorelectrons ==> light

Output Phosphorthin aluminum layer on back of

output phosphorprevents screen’s light from

going back through tube and reaching input phosphor

-X

OutputPhosphor

Aluminum

Output phosphor viewingdirect

uses lenses & mirrorstelevision

high quality closed circuit television chain

Image Intensifier - TV Coupling Using a Lenslens couplingFiber Optic

Fiber Optic II - TV Couplingcannot record image directly

from image tubeAll recording done from TV

ImageIntensifier

Tube

TV

Fiber Optic Bundle

Image Tube ParametersBrightness Gain

ratio of II brightness to a “standard” screen

Conversion Factorlight output per radiation rate input

Change in time10% decline in brightness / year typicalmust increase patient exposure to get

same light intensity

II Gain (Intensification Factor)

Output phosphor brightness-------------------------------------- “standard” screen brightness

typically ~ 10,000

II Gain (Intensification Factor)Brightness gain = minification gain X flux gain

Minification gain making image smaller also makes it brighter

Flux gain acceleration of electrons toward output phosphor

MinificationMinification Factor =Minification Factor =

Diameter of (effective) input phosphor---------------------------------------------- Diameter of output phosphor

Minification Factor Changes with Magnification Mode!Effective input phosphor diameter

decreases with magnificationso does image tube gain

A/B

A

B

Minification

Minification Factor =Minification Factor =

Diameter of (effective) input phosphor--------------------------------------------- Diameter of output phosphor

Area Reduction FactorArea Reduction Factorgain proportional to area reduction factorequals [minification factor] 2

OR

Area of (effective) input phosphor----------------------------------------- Area of output phosphor

Minification Gainimage brighter because output screen

smaller than input screenchanges with magnification mode (9”,

6”, etc)changes by about 2X for each mag

mode typically 81 for 9” mode (output phosphor about 1” diam)36 for 6” mode16 for 4” mode

Highest magnificationLowest Minification gain

Lowest magnificationHighest Minification gain

Flux GainCaused by high voltage of anode

acceleration of electrons in tubeDoes not change with magnification modetypical value ~ 50

ContrastRatio of brightness at center of image

with & without blocking centerTypically 10:1 to 20:1Degrades over time

ImageIntensifier

Tube

ImageIntensifier

Tube

Light Meter

Lead

Other II CharacteristicsLagLag

persistence of illumination after irradiation

insignificant for modern tubesDistortionDistortion

electron steering better in center than in periphery unequal magnification

straight lines appear bent pincushion effect

Vignettingloss of brightness in image

peripherycaused by

periphery displayed over larger area of input screen decreases brightness

poorer periphery focus

Multi-Field Image TubesDual, 3X, 4X field sizes commonImage focused by adjusting voltage on focusing

electrodes (electronic lenses)By law, collimators must cone in during mag

operation X-ray field should match imaged field

ImageIntensifier

Tube

ImageIntensifier

Tube

9 “ 6”

MagnificationAdvantages

Magnifies anatomy improves spatial

resolution

Disadvantagessmaller field of view increased radiation

intensity (but less tissue exposed) decreased minification

gain

ImageIntensifier

Tube

ImageIntensifier

Tube

9 “ 6”

Large Field of View II’sApplications

digital imaging angiography digital spots

12”, 14”, 16” availableConstruction

metal often used instead of glass for strength

Advantage large field of view

Disadvantage expensive bulky / heavy

Flat Panel Digital

Technology