interaction of radiation with matter

Question 1

when an x ray beam interacts with a patient radiation is –,–,–

Answer 1

transmitted
scattered
absorbed

Question 2

what results from transmitted radiation

Answer 2

form images

Question 3

what results from scattered radiation

Answer 3

staff doses

Question 4

what results from absorbed radiation

Answer 4

patient doses

Question 5

Energy fluence definition

Answer 5

energy crossing unit area in unit time, where the energy is the sum of each photon times its energy

Question 6

what does attenuation Describe

Answer 6

how photons are removed from the beam

Question 7

what mathematical relationship does attenuation follow

Answer 7

Exponential

Question 8

formula for attenuation

Answer 8

N=N_0exp(-ux)

Question 9

how to calculate transmitted fraction

Answer 9

N/N0

Question 10

if the linear attenuation coefficient of the material is lower how is the transmitted fraction affected

Answer 10

the transmitted fraction is greater

Question 11

what is the definition of the half value layer

Answer 11

the thickness of material which reduces the incident intensity to half i.e. a transmitted fraction of 0.5

Question 12

why are the transmitted fractions of real beams not exactly exponential at low debt

Answer 12

low energies are preferentially attenuated
the x-ray beam is broad, not narrow, so there is scatter from the irradiated volume

Question 13

how does the shape of the plot between the transmitted fraction and depth look for narrow beam compare to broad beam

Answer 13

broad beam more linear less exponential

Question 14

relationship between H VL and mu

Answer 14

HVL=0.693/mu

Question 15

winners 10th value layer used

Answer 15

usually for radiotherapy shielding calculation

Question 16

what does the exponential function of the remaining photons in depth assume

Answer 16

a narrow beam i.e. no scattered radiation is detected

Question 17

why is the beam not a pure exponential

Answer 17

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

Question 18

in the diagnostic energy range and increasing energy leads to a — in linear attenuation coefficient

Answer 18

decrease

Question 19

a decrease in linear attenuation coefficient means a — in half value layer

Answer 19

increase

Question 20

how is the linear attenuation coefficient related to the penetration of the radiation

Answer 20

smaller mu = more penetrating
greater mu = less penetrating

Question 21

what is the approximate half value layer for water which is approximately the same as soft tissue
what is the H VL for lead

Answer 21

30 mm (water)
0.1 mm (lead)

in the diagnostic range

Question 22

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

Answer 22

21 cm = 7HVLs
this thickness will attenuate the beam by (0.5)^7 = 1/128

Question 23

When is the density dependence of linear attenuation coefficient especially inconvenient

Answer 23

When one substance can exist in several different density states e.g. water ice steam

Question 24

what is the definition of mass attenuation coefficient

Answer 24

the linear attenuation coefficient over the density

Question 25

what is the difference between attenuation and absorption of radiation

Answer 25

attenuation is the removal of radiation from a beam Absorption is the taking up of energy from the beam by the irradiated material

Question 26

what is the absorbed dose D And what are the units

Answer 26

The energy Delta E absorbed in a mass Delta M D = delta E/ delta m units=Gy = J/kg

Question 27

what is a Gy

Answer 27

1 J per kilogram

Question 28

what does KERMA stand for

Answer 28

kINETIC ENERGY RELEASED PER UNIT MASS

Question 29

what is KERMA

Answer 29

The energy transferred from the x-ray beam to the electrons at a specified point

Question 30

what unit is KERMA measured in

Answer 30

Gy

Question 31

why is air KERMA not necessarily equal to the dose in air

Answer 31

because energy removed from the beam is not necessarily equal to the energy absorbed in that mass

Question 32

energy removed from the beam is not necessarily equal to the energy absorbed in that mass meaning that what isnt neccessarily equal

Answer 32

KERMA and dose in air

Question 33

why can air kerma be approximated to be equal to dose in air

Answer 33

because in the diagnostic energy range the electron ranges of very short there is negligible bremsstrahlung production by re-radiated secondary electron energies

Question 34

what is the definition of exposure

Answer 34

the amount of radiation which produces in air ions of either sign equal to 1 C per kilogram

Question 35

exposure only applies to ionisation in ---

Answer 35

air

Question 36

energy absorbed per unit mass, the absorbed dose, depends on the __ __ __ __

Answer 36

mass energy absorption coefficient

Question 37

what ratio is known for wide variety of substances and photon energy spectra

Answer 37

the ratio between the absorbed dose in a mass and the absorbed dose in air

Question 38

formula for absorbed dose

Answer 38

absorbed dose is the energy absorbed per unit mass D=mu_en/rho the linear energy coefficient over the density