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beam restriction
| Question | Answer |
|---|---|
| scatter production | kvp |
| kvp | kilo volts peak- energy of beam |
| volume of tissue | collimation and patient |
| types of tissue | air-filled structures (black). muscle, fat, pathology and scatter (greys). bone- more (whites) |
| purposes of beam restriction | to reduce s/s to improve contrast/ quality and reduce patient dose |
| affects of excessive s/s | lose contrast as there will be more greys. if excessive amount it can cause to much density |
| types of beam restriction devices | lead aperture diaphargms which are simplest but are heavy and attach at the head of the collimator |
| disadvantage of pb aperture diaphragms | projected field is not adjustable and problems with ghosting due to off-focus radiation |
| off-focus radiation | radiation produced anywhere except focal spot |
| umbra | |
| penumbra | |
| off-focused or stem radiation | produce anywhere other than the focal spot |
| affect on image from off-focused or stem radiation | ghosting or shadowing and patient skin dose |
| cone | flared |
| cylinder | not flared but straight |
| PBL | positive beam limitation or automatic beam limitation |
| collimator | entrance shutters, light source, mirror, moveable lead shutters. |
| mirror | must be 45 degrees. projects the light field onto the patient same or content to radiation feild |
| moveable pb shutters | are used to collimate in |
| components of collimator | entrance shutters, filter, focal spot, mirror, lamp, moveable pb shutters |
| entrance shutters | are immovable which helps absorb off-focus or stems radiation |
| filter | within collimator housing |
| focal spot to mirror and mirror to lamp should be what? | the same distance from each other |
| field center indicators (cross hairs) | plastic sheet attached to the bottom of the collimator. two black lines drawn on it. light projects field center on patient |
| penny test | is done to check for proper alignment of collimation. should be done at 40 SID which is the standard of 2% of the SID |
| two accessory devices that help scatter are? | grids and collimation |
| technical factor consideration/ signi. beam restriction | less tissue irradiated =less s/s= less density if collimating from 14x17 to 8x10,incr. mas 60% if collimating from 14x17 to 10x12, incr. mas 40% |
| what is the type of material are filters made out of? | aluminum and copper |
| aluminum | which we use. reduces low frequency wavelength (absorbs) |
| copper | ALARA, absorbs even more |
| what is attached at the exit window? | the collimator is screwed on there |
| what is the purpose of filters? | harden the beam to reduce patient skin dose |
| where are filters located? | between source and patient at collimator housing |
| how do filters work? | absorbs low energy, long wavelength radiation that would just contribute to skin dose. leaves higher energy, short wavelength beam which has more quality (penetration) |
| filters (measurement) | half value layer - amount of absorber required to reduce initial intensity by 1/2 or 50% |
| HVL | half value layer |
| what material is used for filtration? | aluminum |
| how much pb is in the walls and ceilings | 1/16 in the wall and ceiling has 1/32 |
| inherent = | built-in |
| total Al filtration = | inherent (glass envelope and oil) + added Al layer |
| filtration amount for aluminum | kvp range mm AL <50 .5 req 50-70 1.5 req 70-100 we use 2.5 req/3.0 req >100 3.0 req |
| affect on image quality (density/brightness)& contrast | density/brightness - decrease as eliminates quantity of beam used (not visible). if you over filtrate you can see it with the naked eye, you will see a lose in contrast. contrast - lower contrast as leaves with higher kVp |
| compensating filters | save patient skin dose. purpose is to even out density of body parts with unequal thickness |
| in CR and DR we have a fixed (built-in) algorithm equalization true or false? | true |
| to make up for tissue differences we use what? | an aluminum wedge or trough filter |
| aluminum wedge filter is used with? | t spine and foot |
| trough filter can be used with? | Chest x ray |
| compensating filters can be put where? | on collimator more radiation compensates for tissue densities |
| compensating filters are NOT the same as total filtration true or false? | true |
Created by:
eckoultd1972
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