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FLUOROSCOPY X-RAYS IN MOTION “Viewing dynamic studies of the human body” HISTORY • Thomas Edison, 1896 • Screen (zinc-cadmium sulfide) placed over patient’s body in x-ray beam • Radiologist looked directly at screen • Red goggles-30 minutes before exam • 1950 image intensifiers developed PRESENTLY…. • Fluoro viewed at same level of brightness as radiographs (100-100 lux) • X-ray tube under table/over table or in c-arm • Image intensifier above patient in carriage • Carriage also has the power drive control, spot film selection and tube shutters RED GOGGLES? The eye • Light passes thru the cornea • Between the cornea and lens is iris • Iris acts as a diaphragm • Contracts in bright, dilates in dark • Light hits lens which focuses the light onto the retina where the cones and rods await • Cones- central • Rods - periphery RODS • Sensitive to low light • Used in night vision (scotopic vision) • Dims objects seen better peripherally • Color blind • Do not perceive detail CONES • Less sensitive to light (threshold of 100 lux) • Will respond to bright light • Daylight vision (phototopic vision) • Perceive color, differences in brightness • Perceive fine detail IN FLUOROSCOPY • The winner is…… • CONES!! FLUORO X-RAY TUBES • Operate at .5 to 5mA. Why do they operate at such low mA stations? • They are designed to operate for a longer period of time with higher kVp for longer scale contrast. • kVp dependent on body section • kVp and mA can be controlled to select image brightness • Maintaining (automatic) of the brightness us called ABC or ABS or AGC (control,stabilization gain control) Fluoro X-ray Tubes • Fixed…may be mounted no closer than 15 inches or 38 cm to patient • Mobile may be brought no closer than 12 inches or 30 cm to patient IMAGE INTENSIFIER RECEIVE REMNANT XRAY BEAM, CONVERT IT TO LIGHT…INCREASE THE LIGHT INTENSITY 5000-30,000 TIMES THE SEQUENCE • Beam exits the patient • Hits the input phosphore(cesium iodide CsI tightly packed needles…produce excellent spatial resolution) • Converts x-rays to visible light The sequence cont. • Hits photocathode (Cesium and antimony components) • Emits electrons when struck by light (photoemission) The sequence cont • The potential difference within the image intensifier tube is a constant 25,000 volts • Electrons are accelerated to anode • Anode is a circular plate with hole for electrons to go thru. • Hits output phosphor which interact with electrons and produce light The Electron Path • MUST BE FOCUSED FOR ACCURATE IMAGE PATTERN • Electrostatic lenses (focusing devices) • Accelerate and focus electron beam • “The engineering aspect of maintaining proper electron travel is called electron optics” Continuing the sequence • Electrons hit output phosphor (zinc cadmium sulfide) with high kinetic energy producing an increased amount of light • Each photoelectron at the output phosphor has 50-75 more light photons FLUX GAIN • Ratio of number of light photons at the output phosphor to the number of xrays at the input phosphor • Flux gain = # of output light photon # of input x-ray photons MINIFICATION GAIN • Ratio of the square of the diameter of the input phosphor to the square of the diameter of the output phosphor OR • # of electrons produces at large input screen ( 6 inches) squared, compressed into the area of small output screen ( 1 inch) squared • • • • Try the math 6 inches squared = 36 1 inch squared =1 Minification gain = 36 BRIGHTNESS GAIN • Minification gain x flux gain • Increases illumination level of an image • Ratio of the intensity of the illumination ot the output phosphor to the radiation intensity at the input phosphor • Brightness gain of 5000-30,000 • Maintaining (automatic) of the brightness us called ABC or ABS or AGC (control,stabilization gain control) CONVERSION FACTOR • Ratio of intensity of illumination at the output phosphor (measured in Candela per meter squared) to the radiation intensity at the input phosphor (mR per sec) • Cd/mr squared mr/s MULTIFIELD IMAGE INTENSIFICATION Allows focal point change to reduce field of view and magnify the image Some facts about multifield image intensifiers • Standard component on most machines • Always built in in digital units • Most popular is 25/17 • Trifield tubes are 25/17/12 or 23/15/10 MULTIFIELD IMAGE INTENSIFICATION • Numeric dimensions refer to the input phosphor (25/17) • Smaller dimension (25/17) result in magnified images • At 25-all photoelectrons are accelerated to output phosphor MULTIFIELD IMAGE INTENSIFICATION • Smaller dimension – voltage of focusing lenses is increased • Electron focal spot moves away from the output. • Only the electrons from the center of input strike the output PROS CONS • Only central • Minification gain region of input is is reduced = used dimmer image • Spatial • To compensate resolution is must increase better (think of mA it as the umbra!) • Increase patient • Lower noise, dose higher contrast resolution VIGNETTING: REDUCTION OF PERIPHERAL BRIGHTNESS COMING SOON • • • • • • Coupling (Vidicon, Plumbicon) Fiber Optics Lens coupling Beam splitting Modulation Size of the video signal directly proportion to the light intensity received by x-ray tube. The signal received by the TV tube is modulated Image recording • Cassette loaded spot film • When recording image, the radiographic mA goes from a low mA to high mA. Why? • Photospot camera • Patient dose increases with size of film DIGITAL FLUOROSCOPY • Bushong, Chapter 27 pgs 437441 • Reference: Chapter 12, Fauber pg 302-303 to include figure 126 QUESTION Why is it easy to convert a conventional fluoro unit to a digital one? Facts about digital fluoro • Image acquisition is faster • Can post process • Similar equipment to a conventional fluoro room except • two monitors • Operates in radiographic mode DF and radiographic mode • Hundreds of mA vs 5 mA • Due to the high generator required for DF • the x-ray beam is pulsed progressive fluoroscopy PULSED PROGRESSIVE FLUOROSCOPY • Generator can be switched on and off rapidly • Interrogation time • Tube switched on and meets selected levels of kVp and mA • Extinction time • Time required for the tube to be switched off • Each must have times of less than one 1 ms. CCD • Instead of a vidicom or plumbicom (see figure 27-8, pg 440 Bushong) • Discuss Box 27-1 on pg 441 FPIR (pg 440-442) • Flat panel Image receptor • Replacing CCD’s • Made of cesium Iodide pixel detectors • Lighter, smaller than image intensifiers • No cassette needed FPIR CONT. • Improvement to image as the spatial resolution is uniform and distortion free • High DQE • Improved contrast • Rectangular image • See page 442 Box 27-2