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Filtration
Filtration/Beam Restriction Test
| Question | Answer |
|---|---|
| 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
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