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Beam Restricting

Filtration and Beam Restriction

QuestionAnswer
Ability to image small objects that have high subject contrast. Spatial Resolution
Ability to distinguish between two similar tissues Contrast Resolution
Image detail or visibility of detail the sharpness of small structures
What are two principle ways of controling scatter radiation that reaches the IR? grids and beam restricting devices
What 3 factors influence the intensity of scatter reaching the IR? kVp, field size, patient thickness
What are three types of beam restricting devices? aperture diaphragm, cones and cylinders, variable aperture diaphragm (PBL)
Explain why kVp can have such an influence on the intensity of scatter reaching the IR? Increased kVp will allow for more penetration of beam and there will be lower patient dose (due to less attenuation) but an increase in scatter and decrease in image contrast
By decreasing the field size you _______ the level of scatter radiation. reduce
What is the simplest type of beam restricting device that is attached to the head of x ray tube? aperture diaphragm
Cones and cylinders restrict beam to a required size; modifications of aperture diaphragms
Also known as the variable aperture collimator; it is most common in diagnostic radiography PBL - positive beam limitation (automatic collimation)
What is the main purpose of beam restriction? reduce scatter to improve contrast and reduce patient dose
If collimation is increase, what will happen to pt. dose, density and contrast? decrease pt. dose, decreased density, increased contrast with proper compensation for density loss
If field size is increased (open up collimator), what will happen to pt. dose, scatter, density and contrast? increased patient dose, increased scatter, increased density and decreased scatter
What is the biggest problem with using lead aperture diaphragms? Problem with ghosting (shadow of anatomy)
Ghosting is due to what? off focus radiation (stem radiation) - electrons radiation does not hit target correctly
Where is the lead aperture diaphragm located? Between the anode and the patient
What type of test can be performed to see your collimators accuracy? 8-9 penny test; checks for collimator cutoff
Entrance shutters immovable circular piece of lead mounted below focal spot; purpose is to absorb any off focus or stem radiation
What is the purpose of the mirror in the PBL? project light image onto patient; sits at a 45 degree angle and sits exactly between the light source and focal spot siz
Moveable lead shutters controlled by tech; helps to reduce the beam
Main purpose of Filters. Harden the beam to reduce patient skin exposure
Filters have an affect on scatter. T/F False - they have nothing to do with scatter
What type of material do we use in filters? aluminum or can be a combination of aluminum and copper
How do filters reduce patient dose? absorbs low energy, long wavelength radiation that contributes to skin dose and leaves higher nergy, short wavelength beam which has more penetration.
Where is the filter located? between the sourece and patient at collimator housing
Measurement of the amount of absorber required to reduce intial beam intesnity by 1/2 or 50%. Half value layer
What is total AL filtration? inherent plus added AL layer
In total filtration what makes up the inherrent or built in element? envelope (gas or metal; houses anode and cathode) and oil
What are the requirements for the mm AL used if the kVp range was less than 50? .5 required
What are the requirements for the mm AL used if the kVp range was 50-70? 1.5 required
What are the requirements for the mm AL used if the kVp range was 70-100? 2.5 required; 3.0 recommened
What are the requirements for the mm AL used if the kVp range was greater than 100? 3.0 required
What affect do filters have on image quality (contrast and density)? density - decreases density as eliminates quantity of beam usedcontrast - lower contrast as leaves with higher kVp
What are two examples of compensating filters? wedge filter and trough filter
What is the purpose of the compensating filters? Even out density of body parts with unequal thickness
When radiating a foot how would you use the wedge filter? the thickest part of filter would be placed under the thinnest part
Created by: sr4095 on 2008-12-09



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