Fluroscopy and II’s

Fluroscopy Taking real time x-ray images Requires very sensitive detector to limit the radiation needed Image Intensifier (II) is theoretically >1000x more sensitive, but in practice is x more sensitive.

The II Vacuum bottle so electrons travel unperturbed Input layer converts incident photons to electrons –Al window ~1mm gives the vacuum tube strength –CsI phosphor needles absorb ~400μm kV photons, emit 3000 light photons per absorption. –The Photocathode (thin layer Sb 2 S 3 ) emit electrons from absorbed light photons with 10-20% efficiency, and relatively low E Electron Optics focus the electrons as they are accelerated across the tube’s ~30kV potential to the anode As the now high E electrons hit the ZnCdS:Ag output phosphors they emit ~1000 photons per electron. The output phosphor grains are only a few μm in size to maintain high resolution, and the whole phosphor is small to amplify signal as the whole input window is focused down to the output window The output phosphor and anode are coated to the output window (glass) and this is coupled to a video system.

Input Window Sb 2 S 3 Al kV photons CsI crystals visible photons low E electrons

Electron Optics (pentode system) Anode Electrode 2 Electrode 1 Electrode 3 Photo Cathode electron spread

Output Window anode high E electrons ZnCdS crystals visible photons light absorber (reflections) Go to video system CCD

Things to know Curvature at the input window for strength and flat output window for coupling to video system mean there is a pincushion distortion inherent The images obtained are circular due to the vacuum tube input window shape. Needed for strength. Conversion Factor: the gain of the system. Deteriorates with time. FOV/Mag modes: alter the voltage to electron optics to change the focus. The higher the mag, the smaller the FOV and the more exposure needed. Peak efficiency of the system is ~60kVp, but less dose is delivered at higher kVp