OMP Radiation Physics
image spatial resolution formula (lp/mm)
(lp/mm) = 1/(2* voxel sixe (mm/voxel))
Energy of Auger Electron
difference between electron shell vacancy energies minus Auger electron binding energy
Photodisintegration
direct interaction with the nucleus of the atom, causing a state of excitement within the nucleus, followed by the emission of a neutron
a harder energy spectrum implies a ____ HVL
greater
at the surface, KERMA is ___ than absorbed dose
greater
a photon spectrum with less low energy components means what?
harder spectrum, more filtration, higher HVL
thermionic emisson
heated element (cathode) generates free electrons
Triplet production
high energy photon passes near an orbital electron ejecting the electron and forming an additional electron and positron
when would a larger xray focal spot be desired
high mAs needed to help dissipate heat
what type of EM radiation can be blocked by a wire mesh
microwaves
mg-Radium equivalent
the mass of radium that will produce the same exposure rate as that from the radioactive source of interest at the same distance
why do neutrons have a high radiation weighing factor
they transfer large amounts of energy to protons which have a high LET
how does detector crystal thickness relate to spatial resolution
thicker detector --> less spatial resolution (because of increased scintillation photon scatter)
isotope
Atoms of the same element that have different numbers of neutrons
mg-Radium equivalent equation
YmgRaeq = X(mCi) * (Gamma(x) / Gamma(Ra-226))
in a 180 degree Compton scatter, the photon retains how much energy
0.255 MeV
HVL formula
0.693/mu
Effective half life equation
1 / effective = 1/physical + 1/biological
minimum pair production energy
1.022 MeV
Co-60 mean energy
1.25 MeV
mean energy of a clinical kV photon beam
1/2-2/3 Emax
mean energy of MV beam
1/3 Emax
at 10 MeV, which effect dominates
10 MeV
photodisintegration energy threshold
10 MeV
Co-60 Exposure Rate Constant
13.07 [r.cm2/mCi.hr]
Ra-226 half life
1600 years
minimum pair production energy with an electron
2.04 MeV
PET axial resolution
3-4 mm
therapy proton beam energy range
30-250 MeV
Ir-192 Exposure Rate Constant
4.69 [R.cm2/mCi.hr]
Ra-226 exposure rate constant
8.25 [R.cm2/mCi.hr]
Well counters have approximately what geometric efficiency?
93%
Bremsstrahlung efficiency
9e-10 * Z * E
1 mg Ra eq in mCi
= 0.98 mCi
Bragg-Gray Cavity Theory Assumptions
Assumptions: a. Energy of secondary electrons created inside the cavity is deposited locally (large cavity) b. Secondary electron spectrum is unchanged by cavity presence (small cavity) c. all electrons causing ionization in the cavity arise from the phantom material
the term pristine peak refers to
Bragg peak
what interaction is used to create clinical kV photon beams
Bremsstrauhlung
which theory extends the Spencer-Attix theory to large cavities
Burlin Cavity Theory
at 1 MeV, which effect dominates
Compton
R90 (cm) for an electron beam
E(MeV) / 3.2
Rp (cm) for electron beam
E(MeV)/2
Rt (cm) for electron beam
E(MeV)/3 (80-90% isodose line)
mean energy of electron beam at depth, d
E(d) = E0[ 1 - (d/Rp)]
isomeric transition
Gamma decay- excited nucleus releases energy in the form of the gamma ray. Z, N, and A all stay the same. Most common is 99mTc --> 99Tc + gamma
SI unit of KERMA
Gy
which radionuclide loses 1% of its activity per day
Ir-192
which electron shell has the highest binding energy
K
transmittance from multiple films
N*T
optical density of N films, each with transmittance T
OD = -log10(N*T)
Power, Current, Voltage relationship
P [W] = I [A] X V [V]
how to calculate PDD of rectangular field
PDD(x,y) = sqrt[PDD(x,x) * PDD(y,y)]
first daughter product of Ra-226
Radon
absorption edge
Represents an abrupt increase in attenuation due to the photon energy being equal or greater than the binding energy of each electron shell
cut-off energy and CSDA are introduced in the
Spencer-Attix Theory
which type of charged particle equilibrium exists in reality
TCPE (not CPE)
TERMA vs. KERMA
TERMA includes energy losses due to scattering and KERMA does not
effective energy
The monoenergetic equivalent of an x-ray beam that would give the same HVL
transformer law
Vs/Vp = Ns/Np
what causes sharp peaks in xray spectra
absorption edges where an inner shell (usually k) electron is ejected and char. xrays are emitted as outer shell electrons descend to the vacancy
primary decay mode of Ra-226
alpha
in a kV beam generator, the target acts as the
anode
isotone
atoms that have the same number of neutrons but different numbers of protons
components of PDD curve
attenuation, inverse square law, build-up
primary mode of decay Ir-192
beta-
primary mode of decay for Co-60
beta-
primary mode of decay for Cs-137
beta-
where is KERMA greater than dose
build-up region
Where does thermionic emission occur?
cathode
for equal peak voltage, which type of xray generator produces the highest effective voltage
constant potential
geometric efficiency formula
eg = A/(4*pi*r^2)
what causes build-up region
electrons liberated upstream do not deposit sufficient energy to make up for locally liberated electrons depositing energy downstream
Fluence
energy delivered per unit area, measured in J/cm2
2 isomers vary in
energy state
for constant potential, how is the effective voltage related to the peak voltage
equal
metastable state
excited state of the nuclei of an isotope that lasts long enough after a or B emission for the isotope to be separated from the parent isotope
Exposure rate constant
exposure rate in air due to a small unit source of activity at a standard distance. Also called the gamma-factor
what causes the low intensity region at low energies on an xray spectrum curve
filtration and self-attenuation in the target
in an xray tube, electrons travel from where to where
from the cathode to the anode
increased width of CT collimator detector ____ spatial resolution
improves
increasing photon beam energy will ____ the penumbra
increase
how does increasing xray tube focal spot effect penumbra
increases
in Compton scattering, as photon energy increases, the efficiency of the transfer of energy to the electron ____
increases
how does ultrasound wave velocity relate to frequency
independent
photoelectric effect occurs with which electrons
inner shell
term for transition a daughter nuclide undergoes in changing from an excited to a ground state
isomeric transition
KERMA is ____ than absorbed dose at dmax
less
after dmax, KERMA is ___ than absorbed dose
less
Nyquist frequency
limit on the highest spatial frequency that can be accurately imaged. Equal to 1/2 the sampling frequency
pair production Z dependence
linear
for phased potential generators, how does the effective voltage related to the peak potential
lower
in reality, when is collisional KERMA equal to absorbed dose
never
Compton dependence on Z
no dependence, depends on electron density
k-edges are caused by
photoelectric effect
competing modes of decay to increase n/p ratio
positron decay and electron capture
what causes PET resolution
positrons travel a few mm before being annihilated and the gamma rays do not travel perfectly 180 degrees from each other
Flux
rate of energy flow through an area per unit time J/cm^2*s
purpose of dual light pipe design for scintillators
remove background and Cherenkov signal
Spencer-Attix theory works for ___
small cavities
(larger/smaller) focal spot increases CT geometric efficeny
smaller
well counters use what type of detector
sodium iodide crystal scintillator