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RAD 231 - Unit 3

Fluoroscopy

QuestionAnswer
Define luminance Papp p46 Luminous intensity per unit of projected area of source or the light emitted or scattered from a particular source.
Which units measure luminance? Papp p45 Candela/m squared or NIT
What luminance should PRIMARY display monitors have? Greater than 170 nit
What luminance should SECONDARY monitors have? Greater than 100 nit
Describe CONE vision... Selman p259 PHOTOPIC (daylight vision) - located in central area of eye. Discern color/contrast, less sensitive to light; react best to bright/intense light, involve central vision, good visual acuity (fine detail); high spatial resolution; high speed of perception
Describe ROD vision. SCOTOPIC (night vis) - located periphery of retina. Color blind (except green). Respond low levels of light, low spatial resolution, peripheral vision, poor visual acuity (difficult perceive details); poor intensity discrimination; poor speed perception
Describe the anatomy of the eye. light travels through cornea, across anterior chamber, thru pupil across vitreous humor to retina (cones/rods)
Fluoro historical perspective Orth p342 Edison invented in 1896, handheld device, required radiologist to "dark adapt" eyes (20-30min in dark room, then wear red colored goggles) because so faint, high rad dose to patient and tech - now use image intensifiers to produce image
Define illuminance Papp p45 The amount of luminous flux incident per unit area, or the amount of light projected onto a given surface (brightness on the page, not the actual light source)
Define photometry Papp p45 The study of light and how it interacts with your eye
What is image contrast? Papp p46 the difference in pixel brightness values between various areas of the image. images with fewer shades of gray are considered HIGH CONTRAST, images with more shades of gray are considered LOW CONTRAST
What is subject contrast? Papp p46 distribution of tissue densities/physiologic changes present in anatomic part being imaged. Influenced by internal factors (tissue density) and external factors (rad quality, scatter, contrast agents, etc)
What is the purpose of fluoro? Dynamic imaging of organs
What are the components of a fluoro unit? x-ray tube (under table), movable table (carbon fiber), foot board, image intensifier, camera tube and viewing device
What affects the TV fields generated per second? based on incoming electrical current (typically 60Hz) 60 fields, 30 frames per second
What constitutes a TV video frame? two interlaced fields
How many lines are involved in each sequence of raster pattern? Orth p324-325 262.5 pLUS 262.5 EQUALS 525
How does fluoro differ from x-ray regarding x-ray exposure? lower tube current kVp (depends on exam) patient dose - increased due to exposure time
How does double contrast fluoro study differ from single contrast? Single contrast requires use of increased kVp due to higher density of contrast material. Double contrast needs less kVp due to presence of air with denser contrast material
What is the lux of 1 fc? 1 foot candle = 10.8 lux (reading room brightness)
Compare a sunny day in foot candles with an OR. sunny day = 10,000 fc; OR = 3,000 fc
Explain dark adaptation Began with first fluoro devices - due to using barium platinocide paper which fluoresces very dimly. Radiologist had to adapt the eyes to using ROD vision - 20-30 min in dark then wear red goggles. Exam performed in dark room.
What is the function of the image intensifier? Selman p259 To increase the brightness of the fluoroscopic image; increases it up to 1,000x
What components are contained within the image intensifier? Orth p343 Input phosphor, photocathode, electrostatic focusing lenses, accelerating anode, output phosphor. The device converts x-rays to electrons to light photons.
What type of enclosure does image intensifier have? Orth p345 a vacuum sealed glass envelope, surrounded by lead equivalent 2 mm housing.
What does image intensified fluoroscopy brightness depend on? kV (70-90) mA (0.5-5) anatomy being studied
Which factor cannot be changed during fluoro? patient thickness
What type of kVp and mA are typically used during fluoro exam? 70-90 kVp and 0.5 - 5mA
Describe the image intensifier input phosphor? Cesium Iodide is the scintillator, converts x-rays to light photons
Describe the Photocathode. Made from cesium antimony, it converts light from input phosphor into photo electrons. photo cathode has a concave surface. For each light photon, it creates many electrons pattern of photo electrons carries the latent image of the anatomy studied.
Describe the Electrostatic focusing lenses These are located inside the image intensifier and are charged w/25-35kVp to repel and focus the electron stream toward output phosphor. The charge controls the position of the focal point, where the image is reversed onto the output phosphor.
Why is the photocathode concave? To reduce distortion by maintaining the distance between all points on the input screen and the output phosphor.
What happens to the photoelectrons once they leave the photocathode? The accelerating anode attracts the electron beam to the output phosphor which consists of Zinc Cadmium Sulfide (znCdS); the output phosphor emits light when the electrons hit it. It’s about 2.5 - 5cm in diameter
What is the output phosphor? Zinc Cadmium Sulfide (ZnCdS); it's about 2.5-5cm in diameter
What is the principle of the image intensifier? Flux gain; Minification gain; Brightness gain; The ability of the unit to increase brightness and size of the image
What is the advantage of Cesium Iodide as the input phosphor? Increased resolution, increased contrast, increased DQE (60-65% conversion compared to 20% ZnCdS); less quantum noise; less lag
Explain how the electrostatic lenses work. PP p5 a series of charged electrodes that work to focus and accelerate the electrons across the tube to the output phosphor, contributing to brightness gain. Uses 25-35 kVp
Define FLUX GAIN. Orth p347 ratio of # of light photons/output phosphor to # of x-ray photons/input phosphor. Multiplication of photons in output screen (conversion efficiency) measurement of light photon gain from conversion FG = # output light photons / # of input x-ray photons
Define MINIFICATION GAIN. Orth p348 ratio of light from input phosphor to light at output phosphor (measurement of the increase in intensity/brightness) due to concentration of electrons to smaller surface MG = Input Diameter sq / Output Diameter sq
Define BRIGHTNESS GAIN. Orth p348 measurement of the gain of image brightness; ability of the II to increase the illumination level of the image; entire brightness change from input to output BG = MG x FG Usually 5,000 - 30,000 but decreases 10% per year (age/use)
What is the total brightness gain with a minification gain of 36 and a flux gain of 60? 36 x 60 = 2160 Brightness gain = 2160
What is the minification gain for an II with an input screen diameter 15cm and output screen 2.5cm? 15 sq / 2.5 sq 225 / 6.25 = 36 Minification gain = 36
What is the flux gain if there are 50 x-ray photons at the input screen and 4,500 light photons at the output phosphor? 4,500 / 50 Flux gain = 90
Define CONVERSION FACTOR. Orth p348/ PP p7 New definition for brightness gain. output phosphor illumination / input exposure rate CF = cd / m2 divided by mR/s CF from 50 - 300 (corresponds to BG 5000 - 30000)
What is multifield image intensifying? The ability of the system to change the diameter of the input phosphor (i.e. 6cm to 9cm)
Why does changing the diameter of the input phosphor change the size of the image? Changing the diameter of the input phosphor limits the field of view, but output surface remains the same.
How does the fluoro system control brightness? Orth p348, Selman p268 via Automatic Brightness Control, Automatic Brightness Stabilization and Automatic Gain Control; Brightness/luminance of image has to be continuously adjustable as II is moved from radiopaque to radiolucent areas (abdo to lungs) or latter will black out
How does brightness control work? PP p8 maintains image brightness automatically by varying the kVp, mA or both
Where is brightness control? outside Image Intensifier - in pickup tube
Why is the image magnified during multifield imaging? Mag mode uses less diameter on input phosphor, draws focal point to input phosphor when mag mode is activated (fig 26.6 p346 Orth/fig 20.5 p263 Selman) resulting larger field of view on output phosphor (magnified image) controlled by electrostatic lenses
Why is the field of view smaller during mag mode? Mag mode reduces space used on surface of input phosphor Selman p263 fig 20.5 Orth's figure is confusing
Why is patient dose increased? Because brightness reduced due to smaller surface area of input phosphor being utilized (fewer x-rays, fewer photoelectrons, same surface of output phosphor), must increase mA to compensate
What type of image quality results from magnification? more photons within smaller area, more information
Why is brightness reduced during magnification? reduction of photoelectrons from input phosphor (smaller surface area being utilized We do not see these brightness changes, due to ABC
describe magnification factor. Orth p349 magnification expressed when using distance or selecting input phosphor MF = SID / SOD MF = IW / OW beam is collimated and increased voltage to lenses MF = Input Screen diameter / input screen diameter during magnification
how does magnification mode affect minification gain? MG is reduced because there are fewer photoelectrons reaching the output phosphor, resulting in the appearance of magnification
How does using mag mode affect electrostatic lenses? by increasing the voltage (25-35 kVp) across the lenses, causing the electron beam to tighten and narrow, drawing the focal point towards the input phosphor
What are the magnification capabilities of the image intensifier? 1.5 to 4 X magnification
How does mag mode affect patient dose? reduced collimation - less tissue is irradiated Increased mA - brightness reduced, compensate with more radiation (better spatial resolution, less noise better, contrast resolution) Overall, increased patient dose to irradiated tissue (2x the radiation)
Describe electrostatic focusing lenses. Orth p346 located along inside of II and charged with 25-35kV to repel and focus the negative photoelectrons; focal point reverses image; allows for mag mode to occur
Describe output phosphor. Orth p346-347 ~2.5-5 cm diameter; made of silver-activated zinc cadmium sulfide ZnCdS which converts electrons back to visible light (50-75 x more light photons from one electron) to increase image intensity and brightness; light coating of Al to prevent leaking back
What is radiant flux? Papp p45 Radiant energy that strikes/crosses surface per unit of time or radiant energy emitted by a source per unit time; Measured in WATTS
SOLVE: What is the magnification for an image viewed with 25/17/15 trifield II tube with an input screen diameter 25cm using 15cm diameter area? PP p9 input screen diam / input screen diam during magnification 25 / 15 = 1.667
SOLVE: The distance between patient and x-ray tube is 57cm. The distance between the tube and input screen is 78 cm. What is the mag factor? PP p9 MF = SID / SOD 78 - 57 = 21 = SOD 78 / 21 =
SOLVE: What is the magnification for an image view with an image intensification tube where the input screen diameter is 9" and 6" diameter is used in mag mode? MF = SID / SOD = IW / OW = full field input / mag mode selection 9 / 6 = 1.5MF
What is the maximum entrance exposure rate during fluoro? Papp p155 The intensity of beam at tabletop cannot exceed 10 R/min or 88 mGy/min air kerma rate with ABS; 5 R/min or 44 mGy/min air kerma rates without ABS
What are the types of camera tubes used in fluoro? Papp p149 Selman p264 PP p12 Orthicon - old technology not used much now Plumbicon - uses lead oxide as target phosphor - SHORT lag time, LOW BRIGHTNESS Vidicon - uses antimony trisulfide as target phosphor - LONGER lag time (helpful in GI studies), HIGH BRIGHTNESS
Describe CCD. PP p13 Papp p149 Orth p353 Charge Coupled Device solid state device that converts visible light to electrons; replaces vidicon type camera tubes PROS - smaller, longer life, rugged, more sensitive
Describe camera tube. PP p12 Orth p352 Papp p149 fig7.5 Camera tube = video tube = video pickup tube = pickup tube converts light > electrical signal > video signal; Parts include WINDOW, SIGNAL PLATE, TARGET (We See Tim); includes electron gun and focusing coils; diameter of tube = output phosphor
Created by: Larobbins