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DIP Exam 2
Terms in this set (39)
Preferred methods for measuring SID
Electronically locked in (detents)
built-in tape measure
Back-up methods for measure SID
Wingspans: fingertip to fingertip is about your height 6' = 72" SID
Decreased penumbra happens with an increase in SID because...
the beams recording the edges of the image are more parallel to the central ray
if we don't compensate for the changes in a decreased SID what is the result to our patient exposure?
"hot" image unnecessary increase in patient exposure
Inverse square law
when SID increases by a factor of 2, the area increases by that factor.
what is the formula for the inverse square law?
what is the formula for the direct square law?
What image qualities are not affected by SID?
What affect does OID have on subject contrast?
When you increase OID, the intensity of scatter radiation reaching the image receptor diminishes (positive impact)
how does OID affect overall exposure level at the IR?
as we increase OID, the intensity of the scatter decreases as well, if the OID is doubled, intensity of scatter is reduced to 1/4
how does OID affect penumbra and image sharpness?
the larger the OID, there is less sharpness of recorded detail.
the penumbral shadow is spread out, and blur at the edges increases
How does OID affect magnification?
the greater the OID, the greater the magnification of the image
Why doesn't OID have an affect on shape distortion
Shape distortion is affected by alignment of the tube, part, or IR
what are the major guidelines for determining proper field size?
XR field should never be larger than necessary and must never be larger than the size of the receptor plate
how does effective field size limitation reduce organ dose?
radiation exposure to a particular organ can be reduced to approximately 1/100th solely by collimating it outside of the primary x-ray beam
define off-focus radiation and which collimating devices are most effective at reducing it
off-focus radiation is produced when electrons are deflected to the parts of the X-ray tube outside the focal spot, and is responsible for "ghost images" at the edges of a collimated field.
what is the proper use of the positive beam limitation (PBL)
collimation devices with a secondary aperture reduce off-focus radiation
What are the risks associated with over-collimation?
can result in 'clipping' essential anatomy of interest and result in a repeated exposure
What is the guideline to prevent clipping?
always allow a least 1/2 in of light beyond each edge of the anatomy of interest
how does a larger field size contribute to scatter production and why subject contrast is reduced?
a larger field size will increase scatter, subject contrast will decrease because there are more degrading, undiagnostic, low-energy x-rays "clouding" up our image
how does a larger field size contribute to overall exposure level at the IR?
total exposure to that area is increased
Image qualities not affected by field size
how the line-focus principle is used to allow for high image sharpness while providing for adequate heat dispersion at the anode surface
the line focus principle uses steep anode level angles to project an effective focal spot that is much smaller than the actual focal spot. this allows for higher image sharpness, while providing for adequate heat dispersion the surface of the anode
state how the line-focus principle causes the image to be sharper at one end of the field than the other
because of the line-focus principle, the projected focal spot is smaller toward the anode end of the X-ray tube, and produces a sharper image at the end of the receptor plate
describe the cause the anode heel effect, and the factors that make it worse
the anode heel effect results in less radiation being emitted at the anode end of the X-ray tube. therefore, anatomy of variable thickness should be positioned with the thinnest portion toward the anode end of the X-ray tube
for different body parts or variable thickness, state the best end of the x-ray table to position the patients head end, in order to minimize the anode heel effect
"fat cat" place thicker body parts at the cathode end
Quantify the relationship between focal spot size and image penumbra
the size of the projected focal spot is directly proportional to penumbra, and inversely proportional to the sharpness produced in the remnant beam image
explain why geometrical penumbra occurs
penumbra is produced by rays from different portions of the focal spot recording the same detail edge in different locations at the receptor . as penumbra spreads, it also invades the umbral shadow of the image detail, causing the umbra to shrink
describe why the focal spot is uniquely considered as the controlling factor for sharpness
it is the only radiographic variable that does so, and therefore should be considered the controlling factor for image sharpness for the projected image at the detector. the small focal spot should always be engaged for smaller anatomical parts
explain why off-centering of the beam causes the same types of effects as off-angling
off-centering of either the central ray or the part in relation to each other diverging peripheral rays of the x-ray beam. these peripheral rays angle away from the central ray. the further they are from the CR, the more angled they are in relation to it.
explain the causes of elongation and foreshortening
elongation: always the result of the TUBE or IR being misaligned
Foreshortening: always a result of the PART being misaligned
explain how shorter SID becomes a contributing factor for distortion once it is already occurring due to misalignment
at a 40" SID, moving away from the CR in any direction results in approximately 2 degrees of beam divergence/inch at the tabletop
at a 72" SID moving away from the CR in any direction results in approximately 1 degree of beam divergence/inch at the table top
state the four geometrical objectives of radiographic positioning
used to maximize sharpness, minimize magnification, minimize shape distortion, or desuperimpose overlying structures
describe why a minimum of two projections must be taken as a general rule for diagnosis
in order to accurately ascertain the size, shape, and location of foreign bodies
list the three means of controlling the effects of motion
1. patient cooperation
2. immobilization of the part being examined
3. short exposure times
describe the effect of motion on penumbra and image sharpness
motion is the greatest enemy of image sharpness and its the most common cause of blurring
describe the effects of moderate and extreme motion on image contrast
motion causes extraneous densities from other structures to be superimposed over the anatomy of interest
define false images, and distinguish between the effects of motion and true distortion
a false image is a new image created by the interaction between anatomy present and the motion itself
A false image is not a distortion of the image of the real object, but a new image which does not represent the real object at all
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