What 3 beam geometry test that are commonly performed?
- field size accuracy
- light field congruency
- perpendicularity of central ray
How is field size accuracy done for manual collimation?
By setting a field size and making an exposure, then measuing the length/width of the field on the image
What is the acceptability for each dimension in a manual collimation field accuracy test?
Each dimension should be within 0.5 of the expected value
How is field size accuracy done for automatic collimation?
A cassette is placed in a table bucky to activate the automatic collimation and then exposure done. Length/width is then measured with difference in SID taken into account
What does light congruency of an x-ray field test?
That there is no radiation exposure outside the collimated area of the image
How is a light congruency test conducted?
Radiopaque objects (pennies) are placed to outline the lightfield projected onto a cassette.
- One exposure is made, then the collimator is open to cover the cassette and another exposure is made.
- Evaluation done to determine the distance between the center of the light field and center of the x-ray
What is the acceptability limit for light congruency of the x-ray field?
It must not exceed 2% of the SID
What happens if the CR is not perpendicular in the transverse direction?
Grid cutoff will cause an increase in patient dose
How is beam perpendicularity tested?
A test tools with two radiopaque balls, one above the other is used to determine the perpendicularity
What is the acceptable limit for beam perpendicularity?
The angulation should not exceed 3 degrees
What is the actual focal spot?
The area of the target bombarded with electrons
What is the effective focal spot?
The apparant size as seem from the film plane
How does the size of the FS vary?
It becomes larger when higher mA is used, especially at low KV
What is the enlargement of the focal spot called?
Blooming
What does NEMA suggest the FS size be?
May be 45-50% larger than the stated size, but no smaller
What tools are used to test focal spot?
- pinhole camera
- star test phantom
- slit camera
- Focal spot test tool
How does a pinhole camera work to test FS size?
Provides an actual (magnified) image of the FS but is rarely used now
How does a start test phantom use to measure FS size?
A device with a radial arrangement of lead strips is taped to the collimator in the center of the beam and then radiographer
What does the resulting image of a start test phatom show?
A blur zone, which can be measure and then converted to FS size
How does a slit camera work to measure FS size?
Provides an actual (magnified) image of the FS but is easier to use than pinhole camera
How does the focal spot test tool work to measure FS size?
A bar resolution test pattern is used to evaluate wheter the FS spot has changed
What are two grid problems evaluated by techs?
- Accuracy of centering
- homogeneity of exposure
What does off centering for bucky grids cause?
A subtle increase in patient dose, because the loss of density is compensated for by increasing exposure
How is grid uniformity performed?
Radiograph the grid on tabletop and inspect the image for irregularities and variation in density
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Visual Qualities of an Image25
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Geometric Qualities of an Image22
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Factors Affecting Radiographic Quality134
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Technique Charts40
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AEC34
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Creating the Digital Image53
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Image Identification5
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Criteria For Image Eval17
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Image receptors21
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Conventional Fluoroscopy38
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Digital Fluoroscopy19
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Imaging Processing & Display95
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Quality Control33
