Radiation Quality
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Transcript Radiation Quality
Radiation Quality
Chapter 4
X-ray Intensity
• Intensity: the amount of energy present per unit
time per unit area perpendicular to the beam
direction at the location of interest.
The number of photons reaching the detector per
second is a measure of beam intensity (photons/cm2).
Exposure Rate: (mR/hr); dose rate: (cGy/min)
• Intensity of photon beam at the tumor
Depends primarily on the original strength of the
beam.
Reduced by beam divergence and attenuation.
Beam Divergence
• The area over which radiation spreads, is
proportional to the square of the distance
from the source.
• Inverse square law: intensity is inversely
proportional to the square of the distance
from the source.
I1 / I2 = D22 / D12
Or
I2 = I1(D1/D2)2
Beam attenuation
• Attenuation: the removal of energy from the beam.
• X-rays interact with charged particles through the
electromagnetic fields associated with the electric and
magnetic fields of electrons and nuclei.
When a beam passes through matter, energy is removed from
the beam.
• Photons will either…
Interact with the filter/attenuator
Be absorbed by the material
They deposit all their energy in the filter
Direction changed or scattered
Unaffected by the filter
Transmission
• Transmission: the ratio of beam intensity
I to IO.
• As the filter thickness increases, the
intensity of the attenuated beam drops.
• Transmission = I/ IO
IO: initial intensity at the detector before
filtration
I: the final beam intensity after filtration
Transmission
• Photon source with single energyattenuation of the beam:
I = IOe-μx
• x = thickness of the filter
• μ = linear attenuation coefficient, (length-1)
• e = base of natural logarithm (2.718)
• Each millimeter of thickness added to the
filter reduces the beam by a constant
percent
Transmission Example
• Attenuation coefficient (μ) = 0.2 mm-1
• Thickness (x) = 3mm
• IO = 2000 photons
I = IOe-μx
I = 2000 * e(-0.2 *3)
I = 2000 * 0.549
I = 1098 photons
Linear Attenuation Coefficient
• Linear attenuation coefficient (μ ): a
function of the filter material and the
energy of the photon beam.
Represents a probability per unit thickness
that any one photon will be attenuated.
• Half-Value Layer determined by μ
Monoenergetic / Homogenous
• Monoenergetic/homogenous: all photons in
the beam have the same energy
μ: remains unchanged for all filter thicknesses or
number of photons removed (number of photons
change)
Higher μ higher probability of interaction smaller
HVL
• More easily reduced in intensity
• An exponential function produces a straight line
on semi-log graph paper.
Polyenergetic / Heterogenous
• Polyenergetic/heterogenous: broad range of
photon energies (bremsstrahlung).
μ: each energy has a different value.
• On semi-log graph paper, the slope changes as
filter is added due to beam hardening.
• Beam Hardening: the effective energy of the
beam increases as it passes through the filter.
Only occurs in a polyenergetic/heterogeneous beam.
Half-Value Layer
• Half-value layer (HVL): the thickness required
for a particular material to cut the beam’s
intensity in half.
HVL = 0.693/ μ
Used to describe the beam’s penetrability
Convenient to characterize different bremsstrahlung
beams using their attenuation characteristics.
• Materials used to specify beams HVL changes
with energy range
Diagnostic/superficial: mm Al
Orthovoltage: mm Cu
MeV: mmPb
Homogeneity Coefficient
• Homogeneity Coefficient:
HC = 1st HVL/2nd HVL
• As HC 1: the more alike individual
beam energy are.
• 1st HVL = 2nd HVL = 3rd HVL…
monoenergetic beam
Equivalent Energy
• Equivalent Energy: represents the
average energy of the beam
The energy of the monoenergetic beam that
would have an HVL equal to the first HVL of
the bremmsstrahlung beam in question.
Attenuation Coefficients
• Linear attenuation coefficient (μ): gives the
probability that a given photon will be attenuated
in a unit thickness of a particular attenuator.
– Photon energy & material dependant
• Mass attenuation coefficient (μ/ρ): probability
of interaction per unit mass length when
μ/ρ << 1, (cm2/g)
– Decreasing the density of the material will cause
much less attenuation.