CH 27 Contrast & CH18Grids Flashcards
Visibilty of detail
Image is visible to the human eye only because contrast permits it.
Image contrast
difference between adjacent densities/IR exposures
Subject contrast
in patient, differential absorption differences in densities.
Window width
Manipulating the contrast on digital systems
Contrasts main controlling factor?
kV
Larger differences between densities means
High contrast (less gray)
In regards to contrast what happens when kV decreases?
Contrast increases/High Contrast
High contrast means that the image has?
less densities, shorter scale
In regards to contrast what happens when kV increases?
Contrast decreases/Low contrast
Low contrast means the image has?
More denities, scale is longer
In regard to bone work what would kV be?
Lower to moderate kV.
Short scale is the same as?
High Contrast
Long scale is the same as?
Low contrast
In regards to soft tissue what would kV be?
Higher kV
Subject contrast
dependent on the amount of kV, type of tissue and tissue thickness
Use of a grid leads to
higher contrast, removes sctter, less densities, shorter scale, higher contrast
An increase of ___% (____kv) at 30-50kV will lead to a visible change?
4-5%, 1-3
An increase of ___% (____kv) at 50-90kV will lead to a visible change?
8-9%,4-8
An increase of ___% (___kv) at 90-130kV will lead to a visible change?
10-12%, 9-16kv
15 percent rule
Increasing kV by 15% will increase the density 2xs. Decreasing it by 15% will cut denisty in half.
Film base is made of
polyester that pocesses an inherent blue dye and fog.
Latitude is
The range of densities that can be viewed on the radiograph.
The geometric factors for detail are?
FSS (Focal Spot Size), SID (Source to IR Distance) and OID (Object to IR Distance).
What kinds of screens are there? (In order from slowest to fastest)
1)Nonscreen, 2)Slow/Detail/Extremity 3)Medium/Par 4)Fast/High speed
In order to help reduce motion, one should:
Use a faster exposure time/decrease in time.
Voluntary movements are reduced by:
clear instructions and communication
Involuntary movements causes:
Heartbeat, spasm, tremors, peristolces.
Mechanical movement is caused by:
x-ray tube/locks, bucky
Immobilization devices are
tape, sponges, sand bags, pigg o stat, mummy wrap
Penumbra formula:
(Focal Spot Size X OID)/SOD
What two types of distortion are there?
Size and Shape
Elongation is caused by?
Incorrect angle of IR (Excessive)
Foreshortening iss caused by
Angled part, or not sufficiently angled
When should a grid be used?
When greater contrast is needed.
The spaces between grid lines are
Radiolumenescent.
The grid lines are considered:
Radiopaque
What is the Moire Effect?
Grid error that occurs with Digital IR systems when the grid lines are captured and scanned parallel to the scan lines in the imaging plate reader.
What type of grid has lead strips running in only one direction?
Parallel grid
How is the focused grid designed?
It is designed to match the diverence of x-ray beam. So torward the middle the lines are parallel, however the outlines are more incline/slanted.
How does the air gap technique improve contrast?
It increases OID, which in turn helps reduce scatter. The same amount of scatter will be created but less of it will reach the IR.
The amount of scatter radiation increases with?
1)^Patient thickness 2) larger field sizes 3) Decrease in atomic number of the tissue
