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beam restriction

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
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