Resident Physics Lectures Fluoroscopic Imaging (year 1) George David Associate Professor of Radiology Medical College of Georgia

Early Fluoroscopy Radiologist directly viewed fluorescent screen screen covered with lead glass for protection Low light levels 10-30 minute dark adaptation required by wearing red goggles

Human eye light receptors rods (scotopic vision) respond to very low light levels night vision peripheral vision poor visual acuity poor gray shade detection cones (photopic vision) high direct vision acuity blind at low illumination levels

Image Intensifiers evacuated glass envelope vacuum tube                                          

Image Intensifier Input Output x-rays ==> light X-rays Small viewing screen

Image Intensifier Components input phosphor x-rays to light photocathode light to electrons electrostatic focusing lens steer those electrons accelerating anode speed up those electrons output phosphor electrons to light

Input Phosphor Cesium iodide (CsI) x-rays ==> light CsI crystal needles perpendicular to substrate minimizes lateral light diffusion or scattering improves resolution typical image tube resolution 3 - 5 line pairs / mm

Input Phosphor K-Edge of phosphors absorbs ~ 2/3 of incident beam CS ==> (36 keV) I ==> 33.2 keV well suited to average fluoro beam energy 30 to 40 keV absorbs ~ 2/3 of incident beam absorption energy

light photons ==> electrons Photocathode light photons ==> electrons attached to input phosphor minimizes light diffusion Photo-emissive metal light causes emission of photoelectrons # photoelectrons emitted proportional to incident light from input phosphor

Electrostatic Focusing Lens lenses + - Several electrodes plated to inside glass envelope + voltage applied to electrodes voltages determine magnification mode focuses each point of input phosphor to a point on output phosphor inverts & reverses image Side-side Top=bottom Black-white

Accelerating Anode in neck of image tube + 25 - 35 kV charge accelerates electrons faster electrons produce more light when they strike output phosphor

electrons ==> light Output Phosphor Small viewable fluorescent screen 0.5 - 1 inch diameter converts electron’s kinetic energy to light ~ 50 fold increase in # light photons over input phosphor Output Phosphor electrons ==> light

Output Phosphor thin aluminum layer on back of output phosphor prevents screen’s light from re-entering tube & reaching input phosphor Output Phosphor - X Aluminum

Output phosphor viewing direct uses lenses & mirrors television high quality closed circuit television chain

Image Tube Parameters Brightness Gain Conversion Factor ratio of II brightness to a “standard” screen Conversion Factor light output per radiation rate input Deterioration over time 10% decline in brightness / year typical must 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 Electrons +

Minification Minification gain 12” mag mode 9” mag mode Minification gain Area of (effective) input phosphor ----------------------------------------- Area of output phosphor Smaller image Less magnification More minification Larger gain Larger image More magnification Less minification Smaller gain

Minification Gain image brighter because output screen smaller than input screen changes 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” mode 16 for 4” mode Highest magnification Lowest Minification gain Lowest magnification Highest Minification gain 6” 12” 9”

Auto-Brightness Generator senses image brightness and modulates beam intensity to hold brightness constant Lowest magnification Highest Minification gain Lowest dose Highest magnification Lowest Minification gain Highest dose 12” 6” 9”

Flux Gain Caused by high + voltage of anode acceleration of electrons in tube Does not change with magnification mode typical value ~ 50

Contrast Ratio of brightness at center of image with & without blocking center typically 10:1 to 20:1 Light Meter Lead

Image Tube Contrast degrades over time depends on collimation tube properties input phosphor photon absorption output phosphor electron absorption light backscatter from output phosphor to input phosphor Light Meter Lead

Other II Characteristics Lag persistence of illumination after irradiation insignificant for modern tubes Distortion Image intensifier has rounded bottom Electron steering better in center than in periphery unequal magnification straight lines appear bent pincushion effect

Vignetting loss of brightness in image periphery caused by periphery displayed over larger area of input screen decreases brightness poorer periphery focus

Multi-Field Image Tubes Dual, 3X, 4X field sizes common Image 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 9 “ 6”

Magnification Advantages Disadvantages Magnifies anatomy improves spatial resolution Disadvantages smaller field of view increased radiation intensity (but less tissue exposed) decreased minification gain 9 “ 6”

Digital Fluoroscopic Receptors Flat Panel Digital Technology Now used for angiography / cardiology and some portable C-arms No spatial distortion