Factors controlling the X ray beam intensity Flashcards
what is intensity
total amount of energy contained in the X ray beam
what is the formula for calculating intensity
quantity x quality
define quantity
number of photons in the beam. primarily related to tube current and exposure time and less so by tube voltage (kVp)
what is quantity expressed as
tube current (mA) x time (sec) -> mAs
what is quality
average energy of the x ray beam
what is quality controlled by
the tube voltage
X ray beam is _____
heterogeneous
what are the factors controlling the X ray beam intensity
- tube voltage
- exposure time
- tube current
- filtration
-collimation - source receptor distance
as the tube voltage increases:
- number of photons generated increases (increased quantity)
- mean energy of photons increases (increased quality)
- maximum energy of photons increasesw
as exposure time increases:
- number of photons generated increases (increases quantity)
- mean energy of photons is unchanged (quality unchanged)
- maximum energy of photons is unchanged (quality unchanged)
as mA increases:
- the number of photons generated increases (icnreased quantity)
- mean energy of photons is unchanged (quality unchanged)
- maximum energy of photons is unchanged (quality unchanged)
what does filtration do
selectively removes long wavelength (low energy) X rays
what is involved in inherent filtration
- glass envelope
- immersion oil
- metal housing
- tube window
what is involved in added filtration
aluminum disks
how is total filtration calculated
inherent filtration + added filtration
as filtration increases:
- the number of photons decreases (reduced quantity)
- mean energy of photons increases (increases quality)
- maximum energy of photons is unchanged
what does collimation do
restricts the size and shape of the beam
- number of photons decreases
- mean energy of photons in unchanged
- maximum energy of photons is unchanged
as source to receptor distance increases:
- number of photons decreases (decreased quantity)
- mean energy of photons in unchanged (unchanged quality)
- maximum energy of photons is unchanged
what is the inverse square law
intensity of the beam varies inversely to the square of the source to receptor distance
what is density altered by
factors affecting quantity of the beam
what is contrast altered by
factors affecting quality of the beam
the amount of blackness of an image is related to what?
how many x rays reach the receptor
why is density controlled by mAs
because increasing mAs and kVp will increase the quantity and increasing quantity more x rays will reach the receptor -> the darker the image will be -> higher density
what is density primarily controlled by
mAs and less so by kVp
what is the mAs rule
mA and exposure time are inversely proportional
is mA is increased exposure time must be:
decreased to maintain the same density of the image
if mA is decreased exposure time must be ____
increased to maintain the same density of the image
what is contrast and what is it controlled by
- the difference in densities between light and dark regions of a radiograph
- mostly controlled by the voltage
high contrast ______ kVp
low
in high contrast, density differences between adjacent areas are ____, and there are fewer _____
greater; shades of grey
