Basic Physics Flashcards
In decreasing order (highest to lowest) - rank the different imaging modalities for highest spatial resolution to lowest.
Screen film (0.08mm) Fluoroscopy (0.125) Digital radiography (0.17) CT/US (0.3) MRI (1) PET (5) Planar Nuc Med (2.5-5) SPECT (7)
What is an isotope - give an example
Nuclides with the same atomic number (protons) (Z)
I-131 and I-125
Z=53, neutrons are different (78 vs 72)
What is an isobar - give an example
Nuclides with the same atomic mass (A) (# of protons and neutrons)
Mo-99 and Tc-99
What is an isotone - give an example
Nuclides with the same # of neutrons (A-Z)
53-I-131 and 54-Xe-132
N = 78
What is an isomer - give an example
A and Z are the same, but different energy states.
Tc-99 vs Tc-99m - difference in energy is 140keV
There are two equations that are discussed when talking about EM radiation characteristics (wave vs particle) what are these equations and units.
Wave: c= wavelength x frequency.
Speed of light = m/s
Particle: E = 1.24 / wavelength
E = keV
What is ionizing vs non-ionizing radiation?
Ionizing - particles that carry enough energy to cause displacement of orbital electrons (>200nm)
Non-ionizing - particles that cannot displace electrons (
Give the worlds briefest summary on Einstein’s most famous equation and how this relates to radiology
E = mc2
Mass and energy are conserved, either separately or into one another.
Important in radiology because positrons will undergo annihilation in which everything is converted to energy.
How many electrons are in each orbital shell?
2n^2
K = 2
L = 8 etc
What is the orbital binding energy.
What is orbital binding energy for Tungsten K-shell and L-shell
energy required to remove electron from the atom
Tungsten - Z = 54.
k = 69.5keV, L = 11keV, M=2.5
What is characteristic radiation vs auger electron formation?
Characteristic radiation - results in displacement of electron resulting in cascade of electrons, and release of xray photon characteristic of the differences between energy levels (Tungsten M–> K - 67keV photon released)
Auger electron - electron in outer shell is displaced, but energy is transferred to an orbital electron rather than an x-ray photon (typically in the same shell as original electron)
Tungsten M–>K - 64.5keV
What are wavelengths for visible light?
red = 700 orange = 625 yellow = 575 green = 550 blue = 470 violet = 440
What is the difference between inelastic and elastic scattering?
Inelastic - loss of kinetic energy - example: bremsstrahlung
Elastic - total KE of colliding particles is unchanged - Photoelectric effect
What is the probability of Bremsstrahlung occurring?
Z^2/m^2
Proportional to square of atomic number of the absorber
indirectly related to the square fo the mass of the incident particle (more likely to occur with electrons than alpha particles)
What are 4 ways that xrays and gamma rays can interact with matter?
Rayleigh (coherent/classical scatter)
Compton
Photoelectric effect
Pair production
What is rayleigh scatter - and when is it likely to occur?
AKA: coherent/classical scatter Low energies (15-30keV) interacts with the whole atom (not individual electrons) and raises the total energy of atom. When de-excitation occurs - a photon of the same energy is emitted in a different direction.
NOT ionizing
Describe Compton scatter
Inelastic scatter
An xray photon interacts with outer shel electron - and electron and photon are ejected with decreased KE.
Why is Compton scatter bad vs PE?
Scattered photon can be absorbed by film.
PE - 100% of photon is absorbed or energy is transferred to electron.
When is Compton scatter more likely to occur?
High energy xray photons. Higher energy beams - will transfer more KE to electron, vice versa.
Increased electron density (# of electrons/gram x density) - will be relatively similar for all elements EXCEPT for hydrogen (no neutrons contributing to weight) - interacts with lower atomic Z
INDEPENDENT of atomic number (Z)
Describe the PE effect
Xray photon interacts with orbital electron - transfers all of KE to the photoelectron.
Electron is ejected resulting in ion pair.
Electron cascade then occurs - and can result in characteristic xray or Auger electron
What are absorption edges? (K-edge)
probability of PE effect occurring decreases with increasing energy xray photons. EXCEPT for at the absorption edges of matter.
Absorption edge = K-shell orbital binding energy.
Example: Tungsten (Z=74), K-shell 69.5keV. Photoelectric effect is less at 69 than at 70keV
What are absorption edges (K-edge) for Tungsten and Iodine?
Tungsten (74) K = 69.5keV
Iodine (53) K = 33.1keV
What is probability of PE effect occurring?
Z^3/E^3
Higher atomic number
Less likely to occurring with increasing energy of the incident photon(except at the absorption edge value of that element)
What is the linear attenuation coefficient?
fraction of photons that are removed from the beam per unit thickness (generally in 1/cm)
If you increase the energy of a beam - the linear attenuation coefficient will decrease (higher penetration)
What is exponential equation to calculate the linear attenuation coefficient?
N = No x e^(-ux) N = photons transmitted No = incident photons u = LAC x = thickness of tissue
What is LAC proportional to?
Give basic example.
Density. LAC will be greater with increasing density.
ubone > uwater > uice > uwater vapor
Give units of density, electrons per mass, and electron density
Density = g/cm3
Electrons per mass = e/g
Electron density = e/cm3
How do water, ice and water vapor differ in density, electrons per mass and electron density
Water density = 1 g/cm3. Water vapor and ice will be lower than this
Electrons/g will be exactly the same.
Electron density = e/cm3 will be less for ice and water vapor.
What is the mass attenuation coefficient?
Overcomes the dependency on density of LAC by normalizing (dividing out density)
LAC/p
What is the half value layer - and how does this relate to the linear attenuation coefficient?
Thickness of tissue required to attenuate the beam by 50%
HVL = 0.693/p
How do you calculate the % photon beam transmitted through X HVL’s?
(1/2)^X
2 HVL - 0.5 x 0.5 = 0.25 = 25% transmitted
What are factors that affect attenuation?
energy
composition of matter: density, atomic number, electrons per gram
If you increase the energy of an xray photon beam - what happens to the LAC?
It will decrease.
Greater penetration with increasing energy of xray photons.
How is atomic number and electrons per gram related?
Higher atomic number - lower electrons per gram.
Hydrogen has the highest electrons per gram - because no neutrons present in the nuclei.
What is the fluorescent yield?
Likelihood that characteristic x-ray will be produced
1-fluorescent yield = likelihood that Auger electron is produced.
What is the maximum number of electrons per orbital shell?
2n^2
K = 2
L = 8
M = 18
Define contrast.
Examples?
Differences in grayscale values in the image
A uniformly gray image has no contrast
Image with sharp transitions between dark gray and light gray (very few shades of gray) has high contrast
What determines contrast of an image for each modality?
Rads: tissue composition (atomic number, density)
Nuclear imaging: ability to concentrate radioactive material
MR: proton density and relaxation phenomena
US: Acoustic impedence between tissues.
Define molecular excitation
Transfer of some of an incidents particles energy to electrons in the absorbing material - promoting them to orbits further from the nucleus (higher energy level)
Energy level does not exceed orbital binding energy
Electron will return to lower energy level with emission of EM radiation or Auger electron
Define molecular ionization
Incident particles energy exceeds electrons binding energy an electron will be ejected from the atom
An ion pair will be created (positively charged atom, and negatively charged electron)
What is an ion pair?
A positively charged atom and a negatively charged electron
(can measure some forms of radiation by number of ion pairs produced - alpha particles produce more ion pairs than protons or neutrons)
Rank the charged particulates in order from highest energy to lowest.
Alpha (3727MeV)
Neutron (940 MeV)
Proton (938 MeV)
Electron/negatron/positron (0.511MeV)
How do neutrons interact with matter?
How does this differ from all other particulate radiation?
Neutrons have NO charge - so cannot cause excitation or ionization with orbital electrons
CAN interact with atomic nuclei (mostly water) and cause excitation or ionization with atomic nuclei
Can be retained (conversion to different isotope), some become radioactive
What is specific ionization and how does it relate to alpha particles?
Specific ionization - # of ion pairs produced per unit length of charged particles path
Q^2 / v^2 - increased with charge, inversely related to velocity
What is the Bragg peak?
Has to do with specific ionization
When an alpha particle loses velocity - the ion pairs/mm increase to a peak
Q^2/v^2
What is difference between elastic and inelastic radiation?
Elastic - total KE of colliding particles is unchanged - Incident particle interacts with orbital electrons - requires very little energy, and minimal KE is expended
Inelastic - loss of KE - incident particle interacts with K-shell electron - requires much more KE to removed that electron (energy will be conserved via release of x-ray but the important thing with these definition is that its the loss of KE)
What is bremsstrahlung?
Braking, inelastic radiation
Electron is deflected by positively charged nucleus with a loss of KE. This energy is converted to an x-ray
What is probability of Bremsstrahlung occurring?
Z^2 / m^2
Proportional to atomic number
Inversely proportional to mass of incident particle (alpha and neutrons do not do this)
Describe ionization and free radical formation and how this may cause biologic effects?
Free radical - atom that carries unpaired electron int he outer shell
Electron –> produces an ion radical (H2O+ + e-)
H2O+ is both an ion radical and free radical
H2O+ will react with additional H2O to produce hydroxyl radical (OH that has 9 electrons, 1 of which is unapired) which is highly reactive to DNA
What is frequency of x-rays on EM spectrum?
10^17-10^19 Hz
What is frequency of gamma rays on EM spectrum?
> 10^19 Hz
What is wavelength of the visible spectrum?
750-440nm
Convert nm to m
1nm = 10-9m 1m = 10^9nm
What is wavelength of red?
620–750 nm
What is wavelength of violet?
380-450nm
What is wavelength of yellow?
570–590 nm
What is wavelength of orange?
590–620 nm
What is wavelength of green?
495–570 nm
What is wavelength of blue?
450–495 nm
It’d be easiest to remember wavelengths of colors if you remember at least 2 - what are the wavelengths of red and green? Violet?
Red - 620-750nm
Green - 495-570nm
Violet - 380-450nm
What is the atomic number?
What are atoms with the same # of protons called?
# of protons Isotopes
What is the atomic mass?
What is it called when two atoms have the same atomic mass?
The number of protons and neutrons
Isobar
(b for both protons and neutrons)
How do you calculate the neutron number?
What is it called when two atoms have the same neutron number?
A - Z = neutrons
Isotones (n for neutron)
How much KE does the incident photon retain in Compton scatter?
Depends on the energy of the photon
Low energy (diagnostic radiology) - scatter photon retains most of its energy
Higher energy - more is given away to electron
For a given angle of deflection, will a high energy or low energy retain more KE in Compton scatter?
A low energy photon
As energy of the photon increases - a greater proportion of KE will be imparted onto the scattered electron
What is probability of Compton scatter to occur?
Higher energies
Higher electron/gram (Hydrogenous materials - mostly soft tissues)
INDEPENDENT of atomic number
Why is PE better then Compton?
In what aspect is it worse?
PE allows for differentiation between tissue types - as the photon will preferentially be absorbed at higher Z’s (PE ~ Z^3 and due to absorption edge), as long as the energy of the beam is low
PE does increase dose to the patient
Describe pair production
Can only occur when ergies of x-rays or gamma rays exceed 1.02 MeV
High energy photons interact with the nucleus –> photon energy is transformed into a negatron and positron. These particles will lose their KE
When the positron comes to rest - will itneract with neighboring electron and create two 0.511 annihilation photons
Describe photodisintegration
High energy photon (>7-15MeV = greater than nuclear binding energy) causes ejection of portions of nucleus (protons, neutrons, alpha particles)
What things will increase the amount of scatter?
kVp, field size, patient thickness
