Exposure Class Flashcards
dry plaster cast, wet plaster, fiberglass - mAs adjustment?
2x dry, 3x wet (or base exposure on thickness measurement), no change fiberglass
Controlling factor for contrast?
Kilovoltage
What are the influencing factors for contrast?
OID, Grid Ratio, Collimation (Part thickness, contrast media)
Controlling factor for density?
mA and seconds
For every change in part thickness of 4 cm, mAs adjustment?
x2
How is anode heel effect most visible?
w/ short SID and large field size
What is HIGH contrast?
Few densities w/ great differences between them (SHORT SCALE)
What is LOW contrast?
Many densities w/ few differences between them (LONG SCALE)
Film contrast vs Subject contrast?
Film contrast is inherent property manufactured into film and how it should be XRed, subject contrast is result of absorption char of tissue and kvp
How does increasing kvp change contrast?
inc kvp, Dec contrast: higher penetrating, lower absorption, more transmission and few density diffs in tissues…. also increases scatter: fog and unwanted density decrease contrast
Increasing tube filtration effect on contrast?
increases penetrating power of beam (kvp), inc scatter, dec constrast
exposure factor modifications for peds?
fast exposure times 0-5: 25% mas 6-12: 50% mas
exposure factors for soft tissue?
increase contrast to penetrate ST foreign bodies
Focal spot size range?
.5 - 1.2 mm
What factors effect geometric unsharpness?
focal spot size, SID, OID
Line pairs?
line pairs per mm : greater resolution and recorded detail
What is resolution?
ability of digital imaging sys to distingusih between 2 adjacent structures
Spatial resolution?
smallest detail that can be detected in an image
Contrast resolution?
ability of imaging sys to distinguish between small objects having similar subject contrast
compensation in mAs when significantly collimating?
increase 30-50%
When to use grids?
when part is thicker than 9-10 cm, and more than 60 kvp needed
Grid frequency?
number of lead lines per unit length (25-45 lines/cm or 60-110 lines/inch)
Grid ratio?
ratio of height of lead strips to distance between them (4:1 to 16:1)
Convergent point on grid?
on focused grid, if lines were drawn to focal point of grid … points drawn along imaginary line on length of grid: convergent line
Focal distance?
distance between grid and convergent line/point; helps determine SID
Focal range?
recommended SIDs to be used w/ focused grid
Stationary grids?
wafer grid, grid cassette, grid cap
Moving/reciprocating grids?
Potter-bucky diaphragm; moves during exposure laterally
What is the contrast improvement factor?
K : contrast w/ grid / contrast w/o grid
What is grid cutoff?
decrease in number of transmitted photons reaching IR bc of grid misalignment (higher grid ratio: higher potential for grid cutoff)
Upside-down focused grid cutoff?
focused grid upside down on IR (loss of density on edges of film…grid lines opp angle of divergent beam)
Off-level cut off?
XR beam angled across lead strips, tube or grid angled…happens w/ parallel or focused
Off-center cutoff?
focused grid: lateral decentering: tube angled not aligned side/side w/ center of grid
Off-focus cutoff?
loss of density on periphery of image when proper SID not used
Photoelectric effect vs compton effect?
Photoelectric: incoming photon knocks inner-shell electron and is completely absorbed - secondary photon created (smaller percentage at higher kvp) Compton: incoming photon ejects outer shell electron and changes direction (greater percentage at higher kvp) (absorption vs scattering)
