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Filtration

Filtration/Beam Restriction Test

QuestionAnswer
what is the effect of excessive scatter -poor image quality -loss of contrast/more grays -can cause too much density
what decreases as scatter increases contrast
-the simplest type of beam restriction device -projected light is not adjustable -problem with ghosting due to off focus radiation -attached at collimator lead aperature diaphragm
-low energy radiation is called what -radiation produced other than at focal spot/outside collimated area off focus/stem radiation
area of sharpness at center of an image umbra
area of unsharpness surrounding center if image penumbra
an extension off of lead diaphragm to further restrict the beam -cones-flared -cylinders-not flared(less penumbra)
first collimaters where what shape circle
what is PBL -positive beam limitation -automatic collimation(can make collimation smaller but not bigger)
what are the components of a collimator -entrance shutters -light source -mirror(45 degree) -moveable lead shutters
immoveable lead shutters, which reduces stem/off focus radiation entrance shutters
sits at 45 degree angle and must be at equal distance between the light source and the focal spot size mirror
-main portion of the collimator, which is what you actually adjust when collimating(allows anywhere from a square to a rectangle shape when collimating) -allows you to reduce amount of tissue irradiated moveable lead shutters
-plastic sheet attached to bottom of collimator -two black lines drawn on to form cross hairs, which is what is projected onto patient -light field should be congruent to field of radiation field center indicator
significant beam restriction results in what -less tissue irradiated=less scatter=less density
if collimating from 14x17 to 8x10 what must be done increase mass by 60%
if collimating from 14x17 to 10x12 what must be done increase mass by 40%
absorbs low energy long wavelength radiation, that would only contribute to skin dose, this is known as hardening of the beam filters
what materials are filters made of -aluminum -copper(electronic imaging)
where are the filters located between the source and patient at the collimator housing
measures amount of absorber(copper or aluminum)is required to reduce inital beam intensity by 1/2 or 50% HVL(half value layer)
inherent filtration(envelope and oil) plus added filtration layer total filtration
filtration requiremnts as defined by the FDA(CFR) -kVp less than 50= .5ml required -kVp between 50-70= 1.5ml required -kVp between 70-100= 2.5 required but 3.0 is recomended kVp greater than 100= 3.0
what affect does total filtration have on image quality -density/brightness decreases as the quantity of beam used decreases -contrast decreases with elevated kVp
used to even out density of body parts with unequal thickness compensating filters
when should you use a wedge filter -foot or t-spine xrays
when should you use a trough filter -chest xrays
Created by: bigad1982