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Fluoroscopic Unit (Bushong, chapter 21)

By Deborah Ford,2014-07-09 08:05
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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

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