factors controlling x ray beam intensity Flashcards
intensity
total amount of energy in the beam
intensity: quantity x quality
Quantity:
Number of photons in the beam. Primarily related to tube current and exposure time and less so by tube voltage (kVp)
Expressed as: tube current (mA) X time (sec) → mAs
increased mA=more e released
Quality:
Quality: Average energy the x-ray beam.
▪ Controlled primarily by the tube voltage (kVp)
X-ray of beam is heterogeneous (bremmsstrhaghlung)
factors controlling beam intensity
- Tube voltage
- Exposure time
3.Tube current - Filtration
5.Collimation - Source-receptor distance
tube voltage effect on intensity (increased)
As tube voltage increases:
oNumber of photons generated increases (increased quantity)
oMean energy of photons increases (increased quality)
oMaximum energy of photons increases
Exposure time effect on intesity
As exposure time increases:
oNumber of photons generated increases(increased quantity)
oMean energy of photons is unchanged (quality unchanged)
oMaximum energy of photons is unchanged(quality unchanged)
tube current effect on intensity
As mA increases:
oNumber of photons generated increases(increased quantity)
oMean energy of photons is unchanged (quality unchanged)
oMaximum energy of photons is unchanged(quality unchanged)
filtration effect on intensity, types of filtration
Selectively removes long wavelength (low energy) x-rays.
Inherent Filtration: Glass envelope, Immersion oil, Metal housing, Tube window
Added Filtration: Aluminum disks
As filtration increases:
oNumber of photons decreases (reduced quantity)
oMean energy of photons increases (increased quality)
oMaximum energy of photons is unchanged
why would low energy rays be filtered
not useful, only increase dosage
Collimation effect on intensity
Restricts the size and shape of the beam.
oNumber of photons decreases
oMean energy of photons is unchanged
oMaximum energy of photons is unchanged
Source-to-receptor distance follows what law
Inverse Square Law
Intensity of the beam varies inversely to the square of the source-to-receptor distance.
inverse square law examples
doubled
tripled
halved
If distance is doubled (8” to 16”) = new intensity will be ¼ (inverse of 22)
If distance is tripled (4” to 12”) = new intensity will be 1/9 (inverse of 32)
If distance is halved (16” to 8”) = new intensity will be 4x (inverse of 1/22)
As source-to-receptor distance increases:
oNumber of photons decreases (decreased quantity) divergence
oMean energy of photons is unchanged (unchanged quality)
oMaximum energy of photons is unchanged
DENSITY – altered by factors
affecting?
quantity
CONTRAST – altered by factors
affecting?
CONTRAST – altered by factors
affecting Quality of the beam
Density
primarily controlled by?
Why?
oAmount of blackness of an image → Related to how many x-rays reach the receptor
oPrimarily controlled by mAs, and less so by kVp
Why? 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)
density mAs/exposure time rule
mA and exposure time are inversely proportional.
o If mA is increased, exposure time must be decreased to maintain the same density of the image.
o If mA is decreased, exposure time must be increased to maintain the same density of the image.
Contrast
primarily controlled by?
o The difference in densities between light and dark regions of a radiograph
o Primarily controlled by the voltage
high and low contrast diagram
High Contrast (Short Gray Scale)
kVp/wave?
density differences?
▪ Low kVp (long wavelengths, less penetrating)
▪ Density differences between adjacent areas are greater; fewer shades of gray.
Low Contrast (Long Gray Scale)
kVp/ waves?
densities?
▪ High kVp (short wavelengths, more penetrating)
▪ Density differences between adjacent areas are more subtle; more shades of gray.
Effect of kVpon contrast
Low kVp: High contrast (low gray scale)
High kVp: Low contrast (large gray scale)
which is low/high contrast?
left: low
right: high
