interaction of radiation with matter Flashcards
when an x ray beam interacts with a patient radiation is –,–,–
transmitted
scattered
absorbed
what results from transmitted radiation
form images
what results from scattered radiation
staff doses
what results from absorbed radiation
patient doses
Energy fluence definition
energy crossing unit area in unit time, where the energy is the sum of each photon times its energy
what does attenuation Describe
how photons are removed from the beam
what mathematical relationship does attenuation follow
Exponential
formula for attenuation
N=N_0exp(-ux)
how to calculate transmitted fraction
N/N0
if the linear attenuation coefficient of the material is lower how is the transmitted fraction affected
the transmitted fraction is greater
what is the definition of the half value layer
the thickness of material which reduces the incident intensity to half i.e. a transmitted fraction of 0.5
why are the transmitted fractions of real beams not exactly exponential at low debt
low energies are preferentially attenuated
the x-ray beam is broad, not narrow, so there is scatter from the irradiated volume
how does the shape of the plot between the transmitted fraction and depth look for narrow beam compare to broad beam
broad beam more linear less exponential
relationship between H VL and mu
HVL=0.693/mu
winners 10th value layer used
usually for radiotherapy shielding calculation
what does the exponential function of the remaining photons in depth assume
a narrow beam i.e. no scattered radiation is detected
why is the beam not a pure exponential
spectrum of energies is emitted from an x-ray tube and each energy has its own value of linear attenuation coefficient Because mu is a function of photon energy
there beam is composite
in the diagnostic energy range and increasing energy leads to a — in linear attenuation coefficient
decrease
a decrease in linear attenuation coefficient means a — in half value layer
increase
how is the linear attenuation coefficient related to the penetration of the radiation
smaller mu = more penetrating
greater mu = less penetrating
what is the approximate half value layer for water which is approximately the same as soft tissue
what is the H VL for lead
30 mm (water)
0.1 mm (lead)
in the diagnostic range
If the anterior posterior dimension of the patient is 21 cm and the H VL in the diagnostic range for water is approximately 30 mm
how much will the beam be attenuated by
21 cm = 7HVLs
this thickness will attenuate the beam by (0.5)^7 = 1/128
When is the density dependence of linear attenuation coefficient especially inconvenient
When one substance can exist in several different density states e.g. water ice steam
what is the definition of mass attenuation coefficient
the linear attenuation coefficient over the density
what is the difference between attenuation and absorption of radiation
attenuation is the removal of radiation from a beam
Absorption is the taking up of energy from the beam by the irradiated material
what is the absorbed dose D And what are the units
The energy Delta E absorbed in a mass Delta M
D = delta E/ delta m
units=Gy = J/kg
what is a Gy
1 J per kilogram
what does KERMA stand for
kINETIC ENERGY RELEASED PER UNIT MASS
what is KERMA
The energy transferred from the x-ray beam to the electrons at a specified point
what unit is KERMA measured in
Gy
why is air KERMA not necessarily equal to the dose in air
because energy removed from the beam is not necessarily equal to the energy absorbed in that mass
energy removed from the beam is not necessarily equal to the energy absorbed in that mass meaning that what isnt neccessarily equal
KERMA and dose in air
why can air kerma be approximated to be equal to dose in air
because in the diagnostic energy range the electron ranges of very short there is negligible bremsstrahlung production by re-radiated secondary electron energies
what is the definition of exposure
the amount of radiation which produces in air ions of either sign equal to 1 C per kilogram
exposure only applies to ionisation in —
air
energy absorbed per unit mass, the absorbed dose, depends on the __ __ __ __
mass energy absorption coefficient
what ratio is known for wide variety of substances and photon energy spectra
the ratio between the absorbed dose in a mass and the absorbed dose in air
formula for absorbed dose
absorbed dose is the energy absorbed per unit mass
D=mu_en/rho
the linear energy coefficient over the density
