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133 Cards in this Set

  • Front
  • Back
path of x-ray beam from
e- to patient
electron beam strikes target
goes through primary collimators
-this creates a forward beaked beam
therefore, we use a flattening filter housed on a carousel to flatten the beam
next the photons pass through the ion chamber
then through secondary collimators
past slot for wedges, block compensators
then to the patient :)
path of e- beam to patient
electron beam passes by the target but does NOT hit it
goes through primary collimators
then hits the scattering foil housed on a carousel which spreads out the e- beam
next it goes through secondary collimators
then through the accessory mount for electron applicator
then to the patient :)
is the operations center for the linac
it supplies the timing pulse that initiates each pulse of radiation
control console
gantry rotates around it
treatment couch
major parts of the modulator cabinet
DC power supply, pulse forming network (PFN), thyratron
mayor parts of the stand include
magnetron or klystron, circulator, part of the waveguide
what defines the dose rate?
the PRF-pulse repetition requency
what does that thyratron control?
controls the HV pulses that are supplied by the DC power supply
what does electron gun do?
produces electrons for acceleration
magnetron does___
klystron does____
magnetron PRODUCES RF waves
klystron AMPLIFIES OR MAGNIFIES RF waves
job of waveguide
transports the wave to the accelerating structure
circulators does what
is interposed to prevent reflected power from the accelerator reaching themagnetron or klystron and this reflected power is absorbed by a load connected to the circulator
accelerator structure aka
accelerating waveguide
what happens in the accelerator structure/waveguide?
electrons derive energy from microwaves in the accelerator
the AFC (automatic frequency control) does what?
drives the magnetron or klystron tuner at the resonant frequency of the accelerating structure
bending magnet does what
bends electron beam 270 degrees
electron beam passes through
scattering foil, dual ionization chamber, and finally collimation system
photon beam passes through
target creating photons through bremsstrahlung, flattening filter, ionization chamber, collimation system
treatment head contains:
bending magnet (sometimes)
target
scattering foil/flattening filter
ion chambers
collimators
ODI
monitor unit is what?
the amount of charge collected by the ion chamber through which the linac beam passes.
it represents an amount of dose delivered to a certain reference point and is independent of dose rate
attenuation is?
removal of photons from radiation beam (what blocks do)
absorption is?
conversion of photon energy to electron motion (in body)
____delivers the dose not the ____
electrons deliver dose
not photon
electron motion causes
biological damage
a photon is ___ ionizing
indirectly ionizing
if thickness of absorber increase then the number of photons removed form beam will
INCREASE
linear attenuation coefficient definition
equal to the fraction of photons removed from eh beam per cm of absorber
dimensions= per cm
def. of HVL
thickenss of absorber required to attenuate half of the incident radiation
def. of TVL
thickness of material to reduce transmission to 1/10 or 10%
___determines the # of e- available
density
as density INC, ___ # of tragets available= ___ photoelectric and pair production
Increase, Increase
linear attenuation coefficient of a material depends on
density of material
how is mass attenuation coefficient different from linear attenuaton coefficient
mass att. coef. eliminates the effects of density variations
mass attenuation coefficient definition
gm/cm^2
coeff determined by (for MAC)
dividing mass attenuation by the number of electrons per gram
atomic attenuation coefficient=
atoms/cm^2
mass energy absorption coefficient def.
reflects the amountof energy transferred to charged particles
the only portion of energy from teh beam of photons which is not abosrbed is
bremsstrahlung (however bones might cause this to occur)
if attenuation coefficient is 2.4x10-1/m
transmission of 10m thick is ??
9%
if attenuation coefficient is 2.4x10-1/m
trnasmission of 20m thick is ??
.8%
if attenuation coefficient is 2.4x10-1/m
attenuation is?? of 30m thick?
attenuation is 99.9%
if attenuation coefficient is 2.4x10-1/m
trnamision of 5m thick is?
30%
if attenuation coefficient is 2.4x10-1/m
HVL of this material is???
2.8875m

formula= .693/u
if attenuation coefficient is 2.4x10-1/m
TVL is?
9.583m

formula= 2.3/u
if attenuation coefficient is 2.4x10-1/m
transmission of 4 HVL
6.25
if attenuation coefficient is 2.4x10-1/m
transmission of 2TVL?
.1
how do you quantify radiation
measure number of photons= fluence
fluence=
numer of photons to cross a given area
fluence rate aka
flux density
fluence rate def.
number of photons to cross a given area IN A GIVEN TIME
energy per unit area=
energy fluence
energy flow=
energy fluence rate
energy fluence rate aka
energy flux density
Exposure is
measure quanity through air
Exposure formula
Exposure =(change in electrical charges)/ (change in mass)


=C/kg
what happens when a photon beam passes through air?
primary e- are produced
-they produce secondary e-
if left to themselves , ions would recombine
what do you need to measure exposure?
isolate charged particles produced
determine the total charge, (change A), produced by ionization as particles lose their energy through air
need electronic equilibrium
divice to directly measure exposure
free air ion chamber
how is electronic equilibrium achieved?
achieved when the # of electrons set in motion inside a volume is equal to the # of electrons come to rest in thevolume or
those that sneak in =those that sneak out
saturation voltage is?
when voltage across electrodes is large enough for all ion pairs to be collected before they recombine
exposure measures
amount of ionization produced in air.....NOT ABSORBED DOSE IN MEDIUM
exposure is defined only for___ not ___
electromagnet radiation NOT charged or uncharged particles
free air chamber exposure limit is
3MeV
density of air=
.001293gm/cm^3 at STP
Q=charge=
1.6x10^ -19
to measure Exposure with free air ion chamber use this equation
Exposure= (NxQ)/(change in Lxp)

N=ions collected
Q=charge=1.6x10^ -10
p= density of air= .001293
change in L=volume of air
change
Exposure SI=
C/kg
t/F:
C/kg is larger than R?
true
1R=? C/kg
2.58x10^-4 C/kg
?R=1C/kg
3759R
kerma stands for?
kinetic energy released in matter
Kerma unit is?
J/kg
kerma definition
sum of all the initial kinetic energy transferred from photons or neutrons to charged particles per unit mass of absorbing material
absorbed dose=
energy retained in the medium
this creates biological effects by ionizing our water molecules
absobed dose equation
Dose= ED/M

ED=mean energy imparted to the volume element of
M=mass
absorbed dose SI unit=
Gy
1Gy=?J/kg
1Gy=1J/kg
1rad=?Gy

1rad=?cGy

1Gy=?rad
1rad=.01Gy

1rad=1cGy

1Gy=100rad
average energy to create one ion pair in air=
33.97eV/ion pair
1 R= ? ion pairs/gm of air
1.61x10^12
1R=?rad
1R=.876rads
1Sv=?rem
1Sv=100rem
dose equivalent equation
H=DxQxN

D is absobed dose
Q is quanity factor
N=1
convert exposure(C/kg) to dose (J/kg)
(1ip/1.6x10^-19)x (33.97/ip) x (1.6x10^-19J/eV)=33.97J/C
the air equivalent wall does what?
attenuates photons at the same rate as air
ionization will be a maximum when
wall thickness equals range of e-'s scattered into the cavity
mast maximum thickness the wall acts as
an attenuator
electrometer measures
current, charge , resistance
what do yo do o measure mass of air in chamber?
send off to be calibrated
must be calibrated ever 2 years
wall thickness is= ___ for a ___beam
1mm for a 300kvp beam
what do you do to a chamber when used for cobalt?
add a cobalt cap
Temperature pressure corrections EQUATION
TPcf= (760/PmmHg) x (273+t degrees C/295)

P=pressure
is an ion chamber sealed
no...never!
electrometer measures
current, dose rate, accumulated charge, integrated dose over the irradiated time
Calibration factors units=
R/C or R/uC
central electrode is charged to ? V?
300V by electrometer
examples of condenser chambers
victoree Rmeter, pocket dosimeters
write an equation which relates, u, the linear absorption and, (en u), the mass energy absorption coefficient
Uen=(u/p)

p=density
two marameters which are necessary to define the Exposure from a beam of ionizing radiation
density of air
below 3Mev
Mass of collection volume
# or charges of one sign in a collection volume
attenuation after 3HVL
87.5% attenuated
attenuation after 6HVL
98.4375% attenuated
attenuation after 2TVL
99%attenuated
attenuation after 5TVL
99.999% attenuated
thermoluminescent is
the emission of light by the application of heat
glow curve
signal received from TLD material may be a function of either heating time or temp
glow curve is most stable at
200 degrees C
humps on the glow curve graph represent
dosimetry peaks at low number then 100 then 200 degrees C
atomic # of LiF

of soft tissue

of air
8.18

soft tissue 7.4

air 7.65
primary instruments to measure

Exposure

dose
Exposure= free air ion chamber

dose=calorimeter
secondary instruments
ionization chamber
tertiary methods
TLDs, chemical dosimetry, diodes, film
what happens to the response of LiF at 1000R?
supralinear region

= response is independent of exposure rate until very high rates are reached
what is used for personnel monitoring, patient dose confirmation, monitoring environmental levels of radiation?
TLD's
density of a film equation
D=log10 (Bo/B)

Bo is with out film=high reading
B is transmitted with film=low reading
characteristic curve aka
H&D curve
gamma of film=
slope of characteristic curve
gamma determines
minimum difference in exposure which will be detectable
latitude of film=
straight line portion of H&D curve
latitude def.
range of exposures over which densities will like on the linear portion of the curve
speed of sensitivity=
reciprocal of exposure required to produce density of 1
speed of diagnostic
333 or 1/30mGy or 1/.003Gy
speed fo therapy film
33 or 1/30cGy or 1/.03Gy
why is diagnostic faster?
it has a smaller exposure to create a density of 1
why at lower energy is the speed higher
photoelectric effect
T/F
the absorbed radiation may produce a chemical change in the absorbing medium. the amount of this change may be used to measure dose
True
Fricke dosimetry is?
relatively insensitive doses of 5,000-50,000rads needed before effect is measurable
T/F
almost all energy absorbed from radiation turns into heat
true
what is a calorimeter
device which measures teh rise in temp. of an irradiated medium
only method to directly measure dose
calorimeter
p-type is what
has 1 open spot in valence band
n type is what
has a donor e-
diodes flow from __to__
n to p
types of diodes

type we use
silicon=Si
Germanium=Ge

we use Si cause it operates at room temp.