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Beam Restricting
Filtration and Beam Restriction
| Question | Answer |
|---|---|
| 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
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