Transcript Chapter 11 - RadTherapy

Chapter 11
Prime Factors
Prime Factors
• Prime factors: affect x-ray emission; under the
control of the radiographer.
– Miliamperage-second (mAs)
– Kilovoltage (kVp)
– Distance (d)
• Related to tube design and construction.
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Tube housing
Target material
Filtration
Voltage waveform
X-Ray Quantity
• X-ray quantity: measure of the number of
x-ray photons in the useful beam.
– AKA x-ray output, intensity, or exposure
– Unit Roentgen (R)
– Directly affected by:
• Miliamperage-second (mAs)
• Kilovoltage (kVp)
• Distance (d)
X-ray quality
• X-ray quality: measurement of the
penetrating ability of the x-ray beam.
• Describes the distance an x-ray beam
travels in matter.
– High energy x-ray photons travel farther in
matter- more penetrating
• Numerically represented by the Half-value
layer (HVL).
Half-Value Layer
• Half-value layer of an x-ray beam is that
thickness of absorbing material needed to
reduce the x-ray intensity to half its original
value.
• Affected by:
– Kilovoltage
– Filtration
• Not controlled from exposure to exposure
Milliamperage
• Milliamperage (mA): measurement of xray tube current.
– The number of electrons crossing the tube
from cathode to anode per second
– Directly proportional to tube current
• Ampere: equal to an electrical charge of 1
coulomb flowing through a conductor per
second.
• 1 Coulomb = 6.3 x 1018 electron charges.
Exposure
• Exposure time is directly proportional to the
number of electrons crossing the tube and is
therefore directly proportional to the number of
x-rays created.
• The number of x-rays that will be created at the
target is a product of the number of electrons
crossing the tube (tube current) and how long
the electrons are allowed to cross (exposure
time).
• Measured in mAs- primary controller of x-ray
quantity.
Density Relationship to mAs
• Radiographic film density is the degree of blackening of
an x-ray film
– Created by deposits of black metallic silver on an x-ray film that
has been exposed to light or x-ray and then processed
– Densities are the result of an x-ray exposure to the film and
intensifying screens.
• Film density is determined by the amount of silver
deposition in the emulsion due to:
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Film type
Exposure conditions
Exposure (mR)
Processing
Density Relationship to mAs
• If the exposure to a film is increased, the
density to that film will increase until the
point where the film reached its maximum
density (Dmax).
• mAs is used as the primary controller of
radiographic film density.
• By maintaining a specific exposure relative
to the speed of the image receptor,
consistent film density can be achieved.
Reciprocity Law
• The reaction of a photographic film to light is
equal to the product of the intensity of the light
and the duration of the exposure.
• The density on the x-ray film should remain
unchanged as long as the intensity and duration
of the x-ray exposure remains unchanged.
– Fails for exposures made at extremely short exposure
times (less than 1/100 second) or extremely long
exposure times (more than a second)
• Law failure in not very significant in diagnostic radiology
because exposures are seldom at those extremes
Kilovoltage
• Increasing the kilovoltage on an x-ray control
panel will cause an increase in the speed and
energy of the electrons applied across the x-ray
tube.
– Increased energy of the electrons results in the
production of x-ray photons with greater energy.
• Controls both the quantity and quality of the xray beam.
– Quantity: more interactions will occur at the target as
kVp increases.
– Quality: each electron has more energy resulting in a
beam with greater penetrability
Density Relationship to kVp
• Changes in kilovoltage create changes in
beam penetrability.
• kVp is the primary controller of the
differences in radiographic density.
– kVp should NOT be used to control
radiographic film density (contrast)
• As kVp increases, causes increase in
penetrability, which will result in less
contrast.
15 percent rule
• An increase in kVp by 15 percent will cause a doubling
in exposure, the same effect as doubling the mA or
doubling exposure time.
• Hypothetically, if kVp is doubled, the x-ray quantity would
increase by a factor of four, but this does not take into
account the increased penetrability of the bean with
increasing kVp.
– As a result, radiographic density is more significantly affected.
– To maintain exposure with changes in kVp, the 15 percent rule
can be applied,
– To maintain density, if the kVp is increased 15 percent, the mAs
must be reduced to one-half its original value.
Distance
• The intensity of x-rays varies greatly with
changes in distance.
• Measurement of the x-ray intensity is obtained
using a dosimeter.
• X-ray photons are most concentrated at the
target and from there they spread out in all
directions.
– X-ray intensity (quantity) begins to diminish.
• Photons that exit the tube port constitute the
primary useful beam.
Inverse Square Law
• Intensity of radiation at a given distance
from the point source is inversely
proportional to the square of the distance.
I1 / I2 = D22 / D12
I1 = original intensity
I2 = new intensity
D22 = original distance
D12 = new distance
Distance Relationship to Distance
• As the distance increases, intensity
decreases which causes a decrease in
exposure to the image receptor.
• Since mAs is the primary controller of xray intensity and radiographic film density,
mAs can be adjusted to compensate for
changes in distance.
Exposure (film density)
maintenance formula
• mAs should increase proportionally to the
square of the change when distance
increases. (direct square law)
mAs1 / mAs2 = D12 / D22
mAs1 = original mAs
mAs2 = New mAs
D22 = original distance
D12 = new distance
Density Relationship
to mAs, kVp & d
• The radiographer should select the kVp
based on the desired contrast, and adjust
mAs to provide the appropriate total
exposure to the receptor.