Ch.7 Principles of Exposure & Image Quality Flashcards
Prime vs Geometric factors
Grainy appearance caused by lack of photons (low mAs/kVp)
Quantum Mottle
a step-wedge aluminum device used to measure optimal exposure settings for a particular body part
Penetrometer
Range of differences in the beam intensity after attenuation (affected by kVp & tissue density)
Subject Contrast
Actual anatomic area shown in the image
Umbra
“unsharp edges” of the body part on the image
-Goal: reduce as much as possible by using a small focal spot
Penumbra
any movement (involuntary/ voluntary) that will cause blurring on the image
Motion
kVp, mAs, SID
Prime Factors of exposure
key words: Contrast, Penetration, Quality
kVp
key words: Quantity, Image Density “brightness”
mAs
Key words: Intensity, Beam Density
* Intensity is INVERSELY PROPORTIONAL to distance
SID
all three (____) affect Quantity, HOWEVER, ____ & _____ affect Quality
kvp,mAs,SID
kVp & Filtration
Distance increase = Intensity decreases x4
Distance decrease = Intensity increases x4
how SID is inversely proportional to Distance
high kVp = low contrast “long-scale” (gray appearance)
Low kVp = high contrast “short scale” (B&W appearance)
Affects of High vs. Low kVp
Contrast, Density, Distortion, Spatial Resolution
Geometric Factors
Controlled by kVp
- key factor in detail visibility
- Fog (scatter) & collimation heavily influence
Contrast
increase kVp = reduce Contrast (create long scale) (gray)
Decrease kVp = increase Contrast (create short scale) (B&W)
how to adjust contrast
controlled by mAs
- overall darkness of the image
- affect detail visibility
Density
overexposure= too dark
underexposure = too light
how density looks on an image
Controlled by: OID, SID, CR angle, IR position, Body position
- unequal magnification of different portions of the same object
- 2 Types: shape & Size
Distortion
unequal magnification of the actual shape of the structure
- Foreshorten (appears short)
- Elongate (appears Longer)
- Occurs when subject & IR are parallel, but the CR is angled
Shape Distortion
magnification/enlargement caused by OID
- occurs when OID increased or SID decreased
- controlled by placing body part close to IR while using the longest SID practical
Size Distortion
Controlled by: SID, OID, Focal spot size, Motion, & quantum mottle
- recorded image detail
-key words: “detail” “definition” “sharpness”
Spatial Resolution
