Chapter 11 Flashcards
Primary factors of Exposure Technique
kVp - kilovoltage peak
mA - milliamperage
Time - seconds
mAs - milliamperage seconds
What mA is used for small filament?
100mA
Benefits of using small filament
Will use the small focal spot giving you sharpness and good spatial resolution
What mA is used for large body parts?
300 mA
What happens when large filament is used?
Will use large focal spot, giving you large penumbra, crappy spatial resolution - but good enough for large body part
What type of relationship does mA and s have
Inverse Proportional Relationship
Numbers can be adjusted to come out with the same dose (Resiprocity Law)
What kind of relationship is kVp and Receptor Expsoure (RE)
Direct - Increasing kVp will affect RE by making xrays stronger
Relationship between kVp and scatter
Increasing kVp, increased scatter
How do you get rid of scatter?
Grid
How does scatter affect contrast?
Lowers contrast
How does changing kVP affect xray transmission?
A higher kVP increases the number of xrays reaching the IR, therefore increasing transmission
What is the Inverse Square Law?
Relationship between distance of the xray beam and intensity
Initial Intensity (final distance)^2
——————— = ————————
Final intensity (Initial distance)^2
If distance doubles, divide dose by 4
If distance halves, times the dose by 4
What are the standards xray SIDs?
40 inch and 72 inch
What is the Direct Square Law?
Used when you change SID in order to maintain an optimum exposure image
Relationship between mAs and distance
Initial mAs Intl Dis ^2
————— = —————-
Final mAs Fin Dis ^2
What happens when SID increases/decreased (spatial resolution and magnification)?
Increases:
Spatial resoltuion increases and magnification decreases
Decreases:
Spatial resolution decreases and magnification increases
How do you fix a noisy image?
mAs
Grid conversion factors
No grid –> 1
5:1 –> 2
6:1 –> 3
8:1 –> 4
10:1 –> 5
12:1 –> 5
16:1 –> 6
Grid conversion equation
New mAs New grid factor
————- = ———————–
Old mAs Old grid factor
Where does mA come from?
Step down transformer
What is attenuation?
Decrease in intensity as the photon passes through matter
What is the gray blanket on an image?
Scatter
What is the relationship between kVp and attenuation?
Inverse
What happens if the grid is tilted more than 3 degrees from perp
Grid cutoff
What does it mean if a problem has the IR speed?
No grid was used.
What is a good magnification factor?
1.1 to 1.3
What is the line focus prinicple?
The relationship between the actual focal spot on the anode surface and the effective focal spot.
Decreasing the size of the anode angle, decreases the effective focal spot
Magnification Factor Equation
SID/SOD
Relationship between field size and collimation
Inverse
A large field size - decrease collimation
A smaller field size - increase collimation
What are the effects of a large field size?
Increases the amont of tissue irradiated
Increasing scatter
Increasing the amount of radiation reaching the IR
Lowering contrast
What are the effects of using tube filtration?
Decreases x-ray quantity
Filtration absorbs low-energy photons that do not contribute to the image.
Reduces patient dose by removing these photons
Relationship between mAs and part thickness
Thinner parts - need less mAs
Thicker parts - need more mAs
Guideline for thickness and mAs
For every change in part thickness of 4-5 cm, adjust the mAs by a factor of 2
What exposure technique factors affect spatial resolution?
Focal Spot Size
SID
OID
Central Ray
Patient thickness
Motion
What exposure technique factors affect contrast?
kVp
OID
Grid use
Collimation
Tube filtration
Patient thickness
What exposure techniques affect RE?
kVp
mAs
Grid
OID
SID
CR Angle
Collimation
Tube filtration
Patient Thickness
How does SID affect prenumbra?
The greater the SID, the less prenumbra
