Factors controlling the X-ray beam intensity Flashcards
Intensity:
Total amount of energy contained in the x-ray beam
Intensity: equation
Quantity X Quality
o Quantity:
Number of photons in the beam. Primarily related to tube current and exposure
time and less so by tube voltage (kVp)
Quantity
Expressed as:
tube current (mA) X time (sec) → mAs
oQuality:
Average energy the x-ray beam.
Quality
▪ Controlled primarily by the
tube voltage (kVp)
X-ray of beam is —.
heterogeneous
Factors
controlling the X-ray beam
intensity (6)
- Tube voltage
- Exposure time
3.Tube current - Filtration
5.Collimation - Source-receptor distance
As tube voltage increases:
(3)
o Number of photons generated increases
(increased quantity)
o Mean energy of photons increases
(increased quality)
o Maximum energy of photons increases
As exposure time increases:
(3)
o Number of photons generated increases
(increased quantity)
o Mean energy of photons is unchanged
(quality unchanged)
o Maximum energy of photons is unchanged
(quality unchanged)
As mA increases:
(3)
o Number of photons generated increases
(increased quantity)
o Mean energy of photons is unchanged (quality
unchanged)
o Maximum energy of photons is unchanged
(quality unchanged)
Filtration
Selectively removes
long wavelength (low energy) x-rays.
Total Filtration =
inherent filtration + added filtration
Inherent Filtration
(4)
o Glass envelope
o Immersion oil
o Metal housing
o Tube window
Added Filtration
(1)
o Aluminum disks
As filtration increases:
(3)
o Number of photons decreases (reduced quantity)
o Mean energy of photons increases (increased
quality)
o Maximum energy of photons is unchanged
Collimation
Restricts the
size and shape of the beam.
Collimation
(3)
o Number of photons decreases
o Mean energy of photons is unchanged
o Maximum energy of photons is unchanged
Inverse Square Law
Intensity of the beam varies inversely to the square of the source-to-receptor distance.
Inverse Square Law
One method of calculating the new beam intensity when changing the source-toreceptor distance:
If distance is doubled (8” to 16”) =
If distance is tripled (4” to 12”) =
If distance is halved (16” to 8”) =
new intensity will be ¼ (inverse of 22)
new intensity will be 1/9 (inverse of 32)
new intensity will be 4x (inverse of 1/22)
As source-to-receptor distance increases:
(3)
o Number of photons decreases (decreased quantity)
o Mean energy of photons is unchanged
(unchanged quality)
o Maximum energy of photons is unchanged
Increase in: Tube Voltage (kVp)
Quantity:
Quality:
increase
increase
Increase in: Tube current (mA)
Quantity:
Quality:
increase
no change
Increase in: Exposure time (s)
Quantity:
Quality:
increase
no change
Increase in: mAs
Quantity:
Quality:
increase
no change
Increase in: Distance
Quantity:
Quality:
decrease
no change
Increase in: Filtration
Quantity:
Quality:
decrease
increase
DENSITY – altered by factors
affecting — of the beam
Quantity
CONTRAST – altered by factors
affecting — of the beam
Quality
Amount of blackness of an image → Related to
how many x-rays reach the receptor
Density
o Primarily controlled by —, and less so by —
mAs
kVp
Primarily 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)
mAs rule:
mA and exposure time are inversely proportional.
If mA is increased, exposure time must be — to
maintain the same density of the image.
decreased
oIf mA is decreased, exposure time must be — to
maintain the same density of the image.
increased
Example: The mA has been increased from 5 to 10. In order to
keep the same density in this image, the exposure time should
be reduced from 6 to _____
?
Contrast
o The difference in densities between light
and dark regions of a radiograph
Contrast is
o Primarily controlled by the
voltage
High Contrast
(Short Gray Scale)
(2)
▪ Low kVp (long wavelengths, less penetrating)
▪ Density differences between adjacent areas
are greater; fewer shades of gray.
Low Contrast
(Long Gray Scale)
(2)
▪ High kVp (short wavelengths, more
penetrating)
▪ Density differences between adjacent areas
are more subtle; more shades of gray
Effect of kVp
on contrast
Low kVp:
High kVp:
High contrast (low gray scale)
Low contrast (large gray scale)
