Measurement, Dose Cal, Radiation Measurement Flashcards
What is Fluence Rate?
Beam intensity as a function of time.
What is energy fluence?
Ratio of radiant energy passing through a sphere
What is KERMA stand for?
Kinetic Energy Released in Matter
What are the parts of KERMA?
*Radiative KERMA (KR)-Energy transfer via radiative photons (bremsstrahlung (g))
*Collisional KERMA (Kc)Collisions b/t photons and charged particles. Results in e- which dissipate energy from collisions, leading to ionization & excitation.
Define KERMA
Kinetic energy released in the medium per unit mass.
What is radiation equilibrium?
Radiation equilibrium- When the volume of radiation (photons/electrons) entering and leaving a system are the same in number, energy, and direction.
What are the units of KERMA?
1Gy=1 J/kg
What are units of exposure?
Roentgen (R); Coulomb per kilogram (C/kg)
How do you convert R to C/kg?
1R=2.48x10-4 C/kg of air
Define Exposure
“a measure of ionization produced in air by photons. (X=dQ/dm; Q-charge produced in mass of air (m)) (Charge/ Mass of air)
What is Charged Particle Equilibrium used for?
Measure exposure-Ion Chamber
Free air Ion Chambers
*Open system to outside air
*Big and Bulky/ delicate, not used in clinics
*Standard @ National labs to calibrate secondary/field equipment (ex thimble chamber)
*Low energy (equipment ex superficial/orthovoltage x-ray)
What is Charged Particle Equilibrium?
Charged Particle Equilibrium-When the number, energy, and direction of charged particles are the same entering and leaving a measurement volume. (absorbed dose equal to collisional kerma)
Purpose of Guard Electrons (Free Air Ion Chamber)
*Helps insulate collecting electrode
*reduce charge leakage to the collecting electrode
*Defines the active chamber volume
Free Air Ion Chamber Corrections
i. Air Density: Temperature, pressure, and Humidity
ii. Ion recombination- charge created but not collected
iii. Air attenuation of photons
iv. Secondary chambers calibrated against fee air chambers use exposure calibration factors
Optimal Chamber Characteristcs
*consistency in sensitivity
*non-dependent with direction of incident radiation
*minimal stem leakage
*minimal ion recombination
*Calibrated against primary standard
What are the three electrodes in a well-guarded farmer chamber?
ii. Central electrode (collector)
iii. Thimble wall
iv. Guard electrode
USED IN THE CLINIC
What is the role of the Guard electrode in a Farmer Chamber?
defines the collecting volume and prevents current leakage
What does the Central electrode (aka collecting electrode) do?
Delivers current to the charge measuring device
(Bias Voltage 300 V)
Why is a bias needed in an ion chamber?
What is the voltage?
Need an electric field b/t electrodes to pull the ions produced from radiation towards the electrodes. w/o the ions would recombine before reaching the electrodes. Thus reduced detection. Allows for measurement of the ions as a current.
300 V
What is the stem effect?
How to minimize?
When radiating of the stem can cause arcing or false reading.
Minimize with fully guarded chamber and rectangular phantoms with stem rotated 90o
AAPM stands for?
Purpose?
American Association of Physicists in Medicine
Advance patient care through education, research, and maintaining professional standards. (Task Groups)
What does ADCL stand for?
What is it?
Accredited dosimetry calibration laboratory
Lab that test medical physics equipment. Sets standards.
absorbed dose definition
Energy of ionizing radiation absorbed per unit mass (body).
What are the unites for Absorbed dose?
Traditional -Rad
SI- Gy
100 rad= 1Gy
What is dose equivalent?
biologic effect of radiation that is dependent on dose and type of radiation
Same Dose High LET (alpha) vs Low LET (gamma)
1 Sv= 1 J/kg
1rem= 10^-2 Sv
Dose Equivalent Equation
H=D x Q
H-Dose Equivalent
D-dose
Q-Quality factor
What happens with a deeper buildup region?
greater skin sparing
What is relative biologic effectiveness?
Comparing biologic damage to a standard 250kVp.
The ability of a specific type of radiation to produce biologic damage.
CPE stands for?
Charged Particle Equilibrium
What is fmed factor?
Converts exposure to dose
What are some fmed limitations?
relative to exposure (photons only)
up to 3MV
What is Aeq?
???
Describe Cavity Therory
Relates a measured dose in a cavity (ion chamber) to dose at same point in a medium w/out the cavity device.
