Exposure Class Flashcards
(35 cards)
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
