Download Fluoroscopic Unit (Bushong, chapter 21)

Fluoroscopic Unit (Bushong, chapter 24)
 fluoro for dynamic studies; visualizes the motion of internal structures and fluids.
 spot films: radiographs taken while fluoroing.
 fluoro mA = less than 5 mA
- during image intensified fluoro, tube current is normally 2 to 4 mA.
- pt. dose is high (more than w/rad examinations).
 fluoro kVp determined by section of body being examined.
 automatic brightness control: (ABC) allows the radiologist to select an image brightness
level that is subsequently maintained automatically by varying the kVp or the mA or
sometimes both.
-also called automatic brightness stabilization (ABS); automatic exposure control (AEC);
automatic gain control (AGC).
Describe: the sequence of events from initial x-ray to output of image on an image intensifier
tube.
Tube – Primary radiation – Patient – remnant radiation – Image Intensifier ( Input
phosphor= photo cathode = electrons = electrostatic lenses to anode = output phosphor)
Beam of light split by beam spliter sent to recording devices (video, photospot, cine)
Or through fiber optics – to camera tube – to control unit to TV monitor
1. Image Intensifier ): II provides ‘d image brightness.
 illumination meas’d in lamberts (L) & millilambets (mL) (1L = 1000 mL)
 human eye is sensitive to a wide range of illumination; radiographs and fluoro visualized
under illumination levels of 10-1000 mL.
 II raises illumination into cone vision region where visual acuity is greatest.
Bushong
Define flux gain, minification gain, and brightness gain;
explain the relationship – show the formula BG = MG X FG
a. II tube ≡ electronic device that receives remnant x-ray beam-converts it to light-‘s
light intensity.
 contained in glass envelop for vacuum and support
 x-rays that exit pt are incident on the II
-incident x-rays are transmitted to the glass envelop and interact w/ input
phosphor (cesium iodide)
-converted to light
-light activates photo cathode (thin metal layer composed of cesium and antimony
compounds
 =‘s photoemission (21:356): (electron emission p/ light stimulation)
-# of electrons emitted by the photocathode is proportional to intensity of light
falling on it.
-# of electrons proportional to intensity of incident x-rays.
 25,000 volts is maintained across the tube between photcathode and anode.
 near anode is output phosphor where the electrons interact and produce a burst of
light.
 accurate image pattern = precise electron pattern from photocathode to output
phosphor.
 electron optics: engineering aspects of electron travel; electrons focused as visible
light.
 electrostatic focusing lenses responsible for control; located along length (50cm) of II
tube.
 electrons arrive at output phosphor and contain the image of input phosphor in
minified form.
 output phosphor: zinc cadmium sulfide crystals.
-photoelectrons boosted to 50-75 times as many light photons necessary. to
create it.
 flux gain: ratio of # of light photons @ output phosphor to # @ input phosphor.
 minification gain: ratio of the sq of the diameter of the input phosphor to the
square of the diameter of the output phosphor.
 brightness gain: ability of II tube to  the illumination;
-brightness gain=minification gain x flux gain
 standard sizes: output phosphor: 2.5-5 cm
input phosphor: 10-35 cm
 brightness gain of most II is 5000-20,000 and it decreases with tube age and use.
Explain multifield image intensification. How does vigneting occur?
b. Multifield Image Intensification:  flexibility w/fluoro; standard w/digital fluoro.
 dual-focus: diameter of input phosphor of II tube = 25 cm/17cm
 trifocus: diameter of input phosphor of II tube = 25/17/12 or 23/15/10.
 @ 25 cm mode photoelectrons form entire input phosphor accelerate to output
phosphor.
 @ 17 cm mode, voltage on the electrostatic focusing lenses is ‘d = focal pt moves
away from output phosphor.
-only electrons from center of 17cm diameter of input phosphor are incident on
output phosphor = magnification.
-magnification =‘s 1.5  w/25/17 tube.
-mA is  to maintain brightness =‘ing  dose to pt (more x-ray photons pre unit
area are used to form an image).
 vignetting: a reduction in brightness at the periphery of an image; inherently
unfocused portion of any image resulting from the periphery of the input phosphor.
2. Viewing Systems
a. Optical Monitoring: mirror optics system
 system of optical lenses and mirrors that magnifies the image from the output phosphor
onto a viewing glass.
 disadvantage: limited viewing--one person at a time; significant amount of light lost so
full advantage of II tube not taken.
b. TV Monitoring: output phosphor of II tube is coupled directly to a TV camera tube.
 vidicon: TV camera tube used most often in fluoro.
-has sensitive input surface the same size as the output phosphor of II tube.
-TV camera tube converts the light image to electrical signal sent to TV monitor.
 advantages: able to control brightness level & contrast electronically; multiple
observers; image can be stored.
3. Recording systems (e.g., videotape)
4. Portable (mobile) – fluoroscopic units – c-arm