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37 Cards in this Set
- Front
- Back
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what does beam restriction do; what does this result in |
limits the exposure area; results in decreased exposure to the patient and a reduction of scattered radiation |
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what can happen if we modify the primary beam |
decrease the fraction of the photons that might be scattered in the body. |
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increasing kVp does what to the fraction of radiation reaching the film |
increases |
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what happens to the fraction of radiation scattered as the area and/or volume of irradiated tissue increases |
increases |
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if we increase scattered radiation, what happens to patient exposure |
increases |
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what happens if we half the width of a square beam |
it reduces the exposure area to 1/4 and the area to 1/4 |
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what do beam restriction devices serve to do |
remove as much off focus radiation as possible |
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when does off focus radiation arise |
when unfocused electrons strike parts of the anode other than the target area itself; can amount to as much as 25% of the on target radiation |
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how is leakage radiation controlled |
shielding; there will always be a certain amount of leakage |
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what are the types of BLD's |
1 aperture diaphragms 2 cones 3 variable aperture beam-limiting devices (collimators) |
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aperture diaphragms |
-metal plates with a hole cut in it |
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are aperture diaphragms totally effective in removing unwanted radiation |
no, because x-rays originate from the focal spot at innumerable points and go in all directions |
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what are cones |
metal tube that flares outward and away from the tube aperture |
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how do cones limit a beam |
by virtue of its upper opinion only, acting as an aperture diaphragm |
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what is an extension cylinder |
a modification of the cone |
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what does the extension cylinder look like and do |
has openings of equal size at both ends so the lower opening acts as an aperture diaphragm |
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what is the result of the longer distance of the lower aperture from focal spot |
the beam limitation is better than the cone |
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what is the problem with a flared cone |
excessive area of the circular field required to cover a rectangular field |
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what does collimator mean |
to direct waves to where they are parallel |
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what does the collimator do with x rays |
they limit the be a to the desired area; radiation continues to diverge after leaving the collimator |
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what does a collimator look like |
a box-like apparatus equipped with two or more sets of adjustable diaphragms, stacked on above the other |
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what are collimators equipped with |
an illuminated field and beam-centering device, has calibrated scales to allow aperture adjustments for films of various sizes at different SIDs |
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what is a positive beam limiting device (PBLD) |
automatic bucky tray that adjusts to the size of the cassettes |
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what are the requirements of the computation of film coverage |
1 distance from source to aperture 2 SID 3 diameter of aperture |
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if calculating with a flared cone, use.. |
measurements TO the top opening |
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if calculating with a cylinder cone, use.. |
lower aperture |
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SAD/SID =? |
Diameter of aperture/field coverage size |
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what are 2 other quality enhancement methods |
1 anode heel effect 2 compensating filters |
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if the center of the film is 100%, what will the film below the cathode be? what will the end of the film below the anode be? |
below cathode, film will be 105%; below anode, film will be 80% |
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why is the intensity of the beam under the anode "cut-off or attenuated" |
because it has to traverse the heel of the anode to exit |
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what factors control the anode heel effect |
1 angle of anode decreases, AHE increases 2 AHE decreases as SID increases 3 AHE decreases as field size decreases 4 thickest anatomical part should be under cathode end |
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what are compensating filters used for |
to make a more uniform density, thereby improving visibility of detail and in some cases, the need to 2 exposures/images |
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gradient wedges |
1 used for hands and feet 2 placed between part and IR |
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boomerang filters |
1 used for shoulders and face 2 placed between part and IR |
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ingot filters |
1 used for lateral cervical spine 2 mounted on the collimator apparatus |
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ACBE filters |
1 used for decubitus views when doing ACBE 2 mounted on the collimator apparatus |
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trough filters |
1 used for PA chest 2 mounted on the collimator apparatus |
