Factors controlling the X-ray beam intensity Flashcards
What is the intensity of the beam mean?
Total amount of energy contained in the x-ray beam
What does the quantity mean?
Number of photons in the beam. Primarily related to tube current and exposure time
What is the equation for intensity?
Intensity: Quantity X Quality
What is quantity expressed as?
tube current (mA) X time (sec) → mAs
What is quality?
Average energy the x-ray beam
What is quality of beam controlled by?
tube voltage (kVp)
What is quantity of beam controlled by?
tube current and exposure
time
X-ray of beam is heterogeneous or homogeneous?
heterogeneous
What are the factors that control x-ray beam intensity?
- Tube voltage
- Exposure time
- Tube current
- Filtration
- Collimation
- Source-receptor distance
As the tube voltage increases what happens to the…
Number of photons
Mean energy
Maximum energy
o Number of photons generated increases
o Mean energy of photons increases
o Maximum energy of photons increases
As tube voltage increases the beams become more or less efficient?
more
As the exposure time increases what happens to the…
Number of photons
Mean energy
Maximum energy
o Number of photons generated increases
o Mean energy of photons is unchanged
o Maximum energy of photons is unchanged
As the tube current (mA) increases what happens to the…
Number of photons
Mean energy
Maximum energy
o Number of photons generated increases
o Mean energy of photons is unchanged
o Maximum energy of photons is unchanged
If double the mA you must _____ the time to have the same energy/number of photons
half
What does filtration do?
Selectively removes long wavelength (low energy) x-rays
What are the inherent filtrations in the x-ray machine?
o Glass envelope
o Immersion oil
o Metal housing
o Tube window
What are the added filtrations in the x-ray machine?
aluminum disks
As the filtration increases what happens to the…
Number of photons
Mean energy
Maximum energy
o Number of photons decreases
o Mean energy of photons increases
o Maximum energy of photons is unchanged
What does collimation do?
Restricts the size and shape of the beam.
Adding a collimator does what to the…
Number of photons
Mean energy
Maximum energy
o Number of photons decreases
o Mean energy of photons is unchanged
o Maximum energy of photons is unchanged
What is the inverse square law for the source-to-receptor distance?
Intensity of the beam varies inversely to the square of the source-to-receptor distance.
What does the inverse square law say if the distance is doubled the intensity will be…
1/4
What does the inverse square law say if the distance is tripled the intensity will be…
1/9
What does the inverse square law say if the distance is halved the intensity will be…
4x
As the source-to-receptor distance increases what happens to the…
Number of photons
Mean energy
Maximum energy
o Number of photons decreases
o Mean energy of photons is unchanged
o Maximum energy of photons is unchanged
What is the density altered by?
altered by factors affecting Quantity (number of photons) of the beam
- exposure time and mA
What is the contrast altered by?
altered by factors affecting Quality (average energy) of the beam
- kVp and filtration
The amount of blackness of an image is related to…
how many x-rays reach the receptor
What is density primarily controlled by?
mAs
- less so by kVp
Why does increasing mAs and kVp increase the density?
Because increasing mAs and kVp will increase the quantity. Increasing quantity more x-rays will reach the receptor → the darker the image will be (higher density)
If mA is increased, exposure time must be _____________ to maintain the same density of the image
decreased
If mA is decreased, exposure time must be ___________ to maintain the same density of the image
increased
What is contrast?
The difference in densities between light and dark regions of a radiograph
What is contrast primarily controlled by?
voltage
What is better low or high contrast?
depends on what you are looking for
High contrast has what kind of gray scale?
short gray scale
- fewer shades of gray
Low contrast has what kind of gray scale?
long gray scale
- more shades of gray
High contrast images are caused by what kind of kVp?
Low kVp (long wavelengths, less penetrating)
Low contrast images are caused by what kind of kVp?
High kVp (short wavelengths, more
penetrating)
If a patient has a big mandible do you need to raise or lower the kVp?
higher kVp
High kVp: ____ contrast (large gray scale)
Low
Low kVp: ____ contrast (low gray scale)
High
