Chapter 11 Flashcards
define 15% rule
- The 15% rule states that changing the kVp by 15% has the same effect as doubling the mAs or reducing the mAs by 50%
- Maintaining or adjusting exposure to the IR can be accomplished with kVp
define body habitus
the general form or build of the body, including size
what are the 4 different types of body habitus
- sthenic
- hyposthenic
- hypersthenic
- asthenic
define sthenic
define hyposthenic
define hypersthenic
define asthenic
define direct square law
provides a mathematical calculation for adjusting the mAs when changing the SID.
define exposure maintenance formula
provides a mathematical calculation for adjusting the mAs when changing the SID.
define inverse square law
- states that the intensity of the x-ray beam is inversely proportional to the square of the distance from the source
- The relationship between distance and x-ray beam intensity
define magnification factor (MF)
- indicates how much size distortion or magnification is demonstrated on a radiographic image.
- MF = SID / SOD
define object to image receptor distance (OID)
- When distance is created between the object radiographed and the IR
- decreased beam intensity may result
define source to image receptor distance (SID)
- The distance between the source of the radiation and the IR
- affects the amount of radiation reaching the patient
- the intensity of the radiation varies at different distances
define source to object distance (SOD)
- the distance from the x-ray source (focal spot) to the object being imaged
- SOD = SID - OID
what are the 3 primary exposure factors?
- mA
- seconds
- kVp
does mAs control quality or quantity?
- quantity
- controls the number of electrons boiled off the filament
does mAs and exposure value have a direct or indirect relationship?
direct
What happens to the exposure value when mAs is increased?
- exposure increases
what happens when the exposure value when mAs is decreased?
- exposure decreases
true or false: In order to get a certain mAs, the milliamperage or time can be adjusted to get the same byproduct
true
- when we look for a mAs of 20, different methods can be applied:
100 mA x 0.2s = 20 mAs
200 mA x 0.1s = 20 mAs
what type of relationship does milliamperage and exposure time have?
- indirect
- when adjusting mAs and time, they have an indirect relationship in order to get a specific mAs
- 50 mA x 200ms (0.2s) = 10 mAs
- 200 mA x 50ms (0.05s) = 10 mAs
- the variables depend on the type of examination being done and the type of tissue density (pediatrics, elders, etc.)
Can mAs control brightness level when using digital IR?
- no
- need to make sure your exposure is correct in order for the Image to be diagnostic
How is the brightness level affected when thr mAs is too low?
- image is bright, but there’s an increased on quantum noise
How is the brightness level affected when the mAs is too high?
- exposes the patient with too much radiation
define kVp
- affects the exposure to the IR because it alters the amount and penetrating ability of the x-ray beam
- more kVp increases kinetic energy
true or false: Increasing or decreasing the kVp changes the amount of radiation exposure to the IR and the subject contrast produced within the image.
true
Can kVp control brightness level when using digital IR?
-no
- has the same affect as mAs
what controls image brightness?
- computer processing
What is the formula to increase kVp by 15%?
- multiply kVp by 1.15
- divide mAs by 2
what is the formula to decrease kVp by 15%?
- multiply kVp by 0.85
- multiply the mAs by 2
what is a disadvantage of high kVp?
- increase of scatter
- higher chances of photoelectric
- possible lower contrast (due to scatter)
what is a desired technique when doing examinations?
- a high kVp and a law mAs
- helps reduce patient dose
name the secondary factors
- focal spot size
- SID
- OID
define focal spot size
- affects the sharpness of the beam
- small filament = increased spatial resolution (lower mA setting)
- large filament = decreased spatial resolution (higher mA setting)
what focal spot size is desirable?
- large focal spot
- spreads out heat better
true or false: Modern radiographic x-ray generators are equipped with safety circuits that prevent an exposure from being made if that exposure exceeds the tube-loading capacity for the focal spot size selected.
true
whats the formula for inverse square law?
- I1/I2 = D2^2/D1^2
true or false: Increasing the SID requires that the mAs be increased to maintain exposure to the IR, and decreasing the SID requires a decrease in the mAs to maintain exposure to the IR.
true
what are the standard SID distances?
40, 48, 72 inches
what other components does SID effect?
- image distortion (magnification)
- spatial resolution
true or false: As SID increases, size distortion (magnification) decreases and spatial resolution increases; as SID decreases, size distortion (magnification) increases and spatial resolution decreases.
True
what does it mean when the MF is greater than 1?
there is a percentage of magnification
