Post on 30-Dec-2015
description
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