Exam Study Session 9/18 Flashcards
What is good geometry?
Position detector so it does not measure scatter photons
What is ideal filtration?
Filtration that would remove all low energy xrays
Quality of xray beams is specified by_________&___________ for dx & _________&___________ for mega voltage X-rays
KVP (voltage) / HVL MV & PDD in water
HVL must be measured under___________ Condition?
Describe those conditions
“Good geometry”
Narrow beam and large distance between absorber and detector to avoid scatter radiation
Filter that must be arranged in proper order of highest atomic # closest to the target
Thoreaus filters ( tin, copper, aluminum, patient)
Half value layer equation
HVL = ln2/u(mew)
Produced during the decay process, will consist of single energy (written ev, kev, MeV)
Mono-energetic beams
Produced with x-ray generators will consist of multiple energies(written in v, kv, mv)
Poly energetic beams
Kvp output???
Kerma stands for?
Energy released in matter or kinetic energy released per unit mass
Energy transfer via radiative photons (bremsstrahlung )
Radiative kerma
Collisions between photons and charge particles results and electrons which dissipate energy vehicle collisions leading to ionization and excitation
Collisional kerma
Units of KERMA
Gy
?Defined in any medium for indirectly ionizing radiation?.
KERMA
How does KERMA relate to absorbed dose?
In the build up region kerma is higher than absorbed dose. Absorbed dose is greater than kerma after Dmax
Why is absorbed dose higher after build up region?
Absorbed dose takes into account back scatter.
How to find energy fluence?
(#Ofphotons x energy of photons) + (# Of photons x energy of photons)
1 Gy = 1 J/kg
What are limits to exposure (R)
Only valid for photons( xray & gamma ray)
3 mev limit
Only for air ionization
Occurs when the #and energy of charged particles (electrons) are equal entering & exiting the measurement volume
Charged particle equilibrium
What is CPE used to measure?
Exposure in ion chamber
How does radiation equilibrium and charge particle equilibrium differ?
Radiation equilibrium takes into account photons and electrons when charged particle equilibrium only accounts charged particles
Measure of ionization produced in air by photons
Exposure
1 Roentgen=_______C/kg of air
2.58x10^ -4
Steps of exposure & ion chamber
- Photons passes thru air and liberate electron through photon interactions.
- Electrons will then cause ionizations (anions and cations
- Ion collection plate with voltage applied to have anions go to the positive side and cations go to the negative side)
- This starts occurrent and a charge of either sign can be measured with an electrometer.
Guard electrode does what?
Insulator, prevents leakage, defines collection volume, prevents contamination
How does air density affect ion chamber readings:
Higher temp, molecules spread out (air density is lower)
Low reading
What air conditions would cause a high reading for an ion chamber
Low temp and high density (low temp causes more molecules to pack together and therefore cause more reactions within the ion chamber
FAC corrections
Air density changes
Ion recombination
Air attenuation of photons
Secondary chambers calibrated against FAC chambers use calibration factors
Ion chamber undesirable effect that arises from irradiation of chamber stem & cable
Stem effect
What type of chamber has negligible stem effect
Fully guarded
Stem effect varies w/?
Energy
What chamber has a collection volume .6cm^3
Farmer chamber
What chamber is well guarded and has 3 electrodes
Farmer chamber
What are the 3 electrodes of a farmer chamber
Central electrode - collector
Thimblewall - tissue equivalent ( pure graphite)
Gaurd electrode
What chamber uses a build up cap?
Thimble chamber/ farmer chamber
What is a type of thimble chamber and what makes it unique?
Farmer chamber - has three electrodes
What is a build up cap and purpose?
Solid air shell made of (polytrichlorofluorethlene)
Measure dose of different energies instead of in the build up region
Why is it hard to measure dose in the build up region
Steep gradient
Which of the following measure dose in the build up region
Extrapolation chamber
Other names for Parallel plate chamber?
plane parallel / pancake chamber
How do Parallel plate & extrapolation chamber differ & similar
Differ- extrapolation chamber has vaned electrode spacing. Parallel has even 2mm spacing
Similar - they measure dose in build up region.
Which of the following devices are preferred for detecting a lost source
GM counter
Which of the following used for in vivo measurements
TLD or diode
‘When positive and negative ions recombine the result in a charge created that never reaches collecting electrode
Ion recombination
What helps reduce ion recombination
Bias voltage approx 300 V
Which of the following would be a primary standard for ion chamber
FAC free air ion chamber
For Dosimeters, sensitivity to radiation is an ____
Ideal characteristic
Is it ideal for a dosimeter to have no energy dependence?
yes
Why is no saturation limit ideal for dosimeter?
because it could continue to read and not get maxed out
What dosimeter has the best spatial resolution
Film
Dosimeters with material with effective water equivalent Z is an ______
ideal characteristic
Which dosimeter is not reusable
film
Low cost, little processing for dose readout and reusable are
ideal characteristics for dosimeters
What uses temperature rise as a dosimeter
calorimeter
Dosimeter that is not sensitive, difficult to set up and has low spatial resolution
calorimeter
What is a chemical dosimeter
Ferrous sulfate or fricke dosimeter
TG 21 was based what machine
Cobalt
Which is more simplistic and in use still, TG 21 or TG 51
TG 51
Which TG is based on exposure to air kerma
TG 21
Which TG is based on absorbed dose in water
TG 51
Increase in source size affect on penumbra
direct
increase SSD affect on penumbra
direct
Increase SDD ( put collimator or shield closer to the skin) has what affect on penumbra
Indirect
Fricke dosimeter uses what chemical change?
Fe 2+ to Fe 3+
Fricke dosimeter uses what to determin change
Spectrophotometer
TLD that is most common
Lithium Fluroide (LiF)
How do TLDs work?
Electrons excited, become trapped in conduction band, when heated they are released and recombin in valence band, and release light
Why is LiF used in TLDS
the atomic number is close to tissue equivalent and thermo luminescence
For TLDs light released is _________ to the radiation dose
proportional
Annealing removes what and is used for what dosimeter?
residual affects from exposure and TLDS
heating plus slow cooling to remove residual effects of exposure to TLD to “wipe slate clean”
Annealing process
Standard pre-irradiation annealing process for LiF
1 HR @ 400 degrees celsius
24 HR @ 80 degrees celsius
??What removes the second peak on the glow curve for TLD and annealing process?
24 Hr @ 80 degrees celsius
TLD is used for
In Vivo measurements
Which dosimeter has advantages of reuable, good tissue equivalence and small?
TLD
What are limitations to TLD
No immediate readout, low spatial resolution and fragile
Which dosimeter works like a photomultiplier tube?
scintillator
Which dosimeter fluoresces when struck by ionizing radiation
Scintillator
Which dosimeter uses a P and N type junctions
Silicon Diodes
What dosimeter is similar to how an ion chamber works by inducing a current, but it uses a solid instead of air?
Silicon Diodes
What is the area called of a silicon diode where the P region and N region meet?
Depletion region
Which dosimeter is utlizied in dosimetric QA
Silicon Diode
Which dosimeter has immediate readout
Silicon Diodes
Advantages to silicon diodes
- immediate readout
- small
- sensitive
- durable
- used for QA (depth dose and profile)
Which Dosimeter has the limitation of if wires are twisted (directional dependence)
Silicon Diodes
Limitation of what dosimeter?
* energy dependence in photons beams
*if wires are twisted it wont read (directional dependence)
* sensitive to thermal affects
* radiation induced damage
Silicon Diodes
What are disadvantages of radiographic film
*require developing, *dependant on photon energy
* not tissue you equivalent
* more sensitive to low energy (photoelectric effect)
*Not tissue equivalent
What has these advantages?
*high spatial resolution
*permanent record
*inexpensive
Radiographic Film
Disadvantages to Radiochromic film
Expensive
and insensitive to radiation (requires 10-50Gy)
What is insensitive to radiation and requires 10-50Gy
Radiochromic film
A film that is not based on silver halide but a radiosensitive monomer
Radiochromic film
What would be used for QA and light radiation field.
Radiochromic film
MOSFETs stands for
Metal oxide semiconductor-field effect transistor
What has immediate dose reading and reusable.
Also is small and Llghtweight
MOSFETs
Tx couches weight limit
typically 450-550 pounds
Older couch tops had web called
tennis racket
Why are new couches all carbon fiber?
Low Z material and therefore easier to treat thru
Magnetron or kylstron:
Used for high energy and more expensive
Klystron
Magnetron or klystron: magnifies or amplifies microwaves
Klystron
Magnetron or klystron: require low energy microwave source (RF driver)
Klystron
Magnetron or klystonon: electrons get exposed to a magnetic field which converts them into microwaves
Magnetron
Magnetron or klystron: Converts the energy of a high voltage pulse into a high power microwave
Klystron
What type of gas is in the accelerator tube
SF6
What part of the linac forms the dose rate
modulator
For the electron gun, what energy are electrons introduced
50Kv
What type of target is in a linac?
Transmission target
What type of target is in an xray tube
Reflection target-rotating anode
What are the conditions at which a MU is defined
amount of time to deliver 1 cGy at 10x10 cm FS at DMax at 100cm away
What collimators move in the linac
Secondary
Do primary collimators move?
no
Max field size set by primary collimators
40 x 40
Why are secondary collimators move divergently with the beam?
to help reduce transmission penumbra
How does secondary collimators moving divergently help reduce transmission penumbra
Keeps beam parallel to the collimator edges to help minimize penumbra
What type of penumbra caused by variable transmission of beam through nondivergent collimator edge
Transmission penumbra
What penumbra is due to the finite dimension of the source/ focal spot
Geometric penumbra
Factors that influence geometric penumbra
beam energy, lateral transport of electrons in tissue
Which penumbra is the spread of dose distriubtion near field borders
Physical penumbra
Does ion chamber go before the flattening filter?
No
Which of the following linac uses a Physical source?
Cobalt
Average energy of cobalt machine?
1.25 me V
Cobalt 60 is housed in what and is what size?
Stainless steel capsule & 1-2 cm in diameter
How many modes of decay does cobalt 60 have & what are they?
2 modes: 1.17 mev / 1.33 mev
Cobalt loses now much activity per month?
1%
Betatron is also called
Donut
Xray output is________ to the filament current
Very sensitive
Xray output increase____________ with tube current, and________of the to tube voltage
Proportionally / squared
The higher the energy of the electron bombarding the target the more__________ the direction of the xray emission
Forward
