Physics Flashcards
What is the maximum number of electrons in an orbital shell?
** 2(n)2**
1st shell (K) = 2, 2nd shell (L) = 8, 3rd shell (M) = 18, etc
What is the binding energy of an electron?
When/where is it the highest?
The amount of energy it takes to remove it from an atom.
Electrons in the innermost (K) shell have the highest binding energy, because electric force is inversely proportional to the square fo the distance between two charged particles. Binding energy also increases with higher atomic number (Z) because increased + charge in the nucleus means stronger attractive force on the electrons.
What are the processes of ionization and excitation?
Ionization: enough energy is imparted on an electron for it to overcome its binding energy, and it becomes free. The atom becomes an ion that carries 1 unit of + charge.
Excitation: the imparted energy isn’t large enough to free the electron, and it jumps to a more outer shell. This creates a vacancy in an inner shell and the atom carries excess energy, is in its “excited” state.
What two things can happen when an electron in an outer shell jumps to an inner shell to fill a vacancy in an excited atom?
This releases energy equal to the difference in binding energies between the 2 shells.
The energy can either:
1) Be released as an x-ray photon, called a characteristic x-ray. More likely to happen in heavy atoms.
2) Be imparted onto another orbital electron and free that electron, called the Auger electron. More likely to happen in light atoms.
What happens when a nucleus transitions between energy states (ie between excited state and ground state)?
It releases a discrete amount of energy, may be released by emitting gamma photons.
Ex) when Tc99m (a relatively stable excited state, called a metastable state) returns to its ground state, it emits gamma photons, the majority of which carry 140.5 kEv.
What are:
Isotopes?
Isobars?
Isotones?
Isomers?
Nuclides that have the:
Isotopes: same Z (atomic #, protons) but different N (neutrons). (125I, 127I, 131I- same element).
Iosbars: same A (mass #) (131I, 131Xe, 131Cs- different element).
Isotones: same N but different Z.
Isomers: same composition (N & Z) but different energies (99Tc, 99mTc).
If a nucleus has an unstable N/Z ratio, how can it achieve a stable ratio?
How does a nucleus release excess energy?
It can convert a proton to a neutron or vice versa, and release a charged particle in the process (beta or alpha decay), or the nucleus can be broken into 2 lighter nuclei that have more stable N/Z ratios (nuclear fission).
Excess energy is released in gamma particles.
By what process is 99mTc created?
How does it decay in the patient?
It is created by Beta- decay from 99Mo in a generator. Beta- decay is when a nucleus is too neutron rich, so a neutron converts into a proton, electron, and antineutrino (V)… the electron and antineutrino are emitted from the nucleus immediately. A (atomic mass) stays approx the same (technically slightly lighter) but Z (atomic number, protons) goes up by 1.
99mTc further decays to its ground state 99Tc by emitting gamma photons in the patient.
What are the SI and traditional units of radioactivity? How are they related?
SI = becquerel (Bq). Defined as one decay per second.
Traditional = Curie (Ci). 1 Ci = 3.7 x 1010 decay per second.
So 1 mCi = 37 MBq.
What do the SI units Tesla (T), Gray (Gy), Sievert (Sv), and Becquerel (Bq) measure?
Tesla - magnetic field strength.
Gray- absorbed dose.
Sievert- equivalent/effective dose.
Becquerel- radioactive decay activity.
How do you calculate absorbed dose for a given mass of tissue?
In Gray. 1 Gray = 1j/kg.
So if a 10kg mass of tissue absorbes 40j, the dose is 40/10 or 4 Gray.
What are the equivalent and effective dose, and what is their unit?
Equivalent dose: takes into effect that not all types of radiation have the same effect on living organisms. Heavy particles (alpha particles and neutrons) are more damaging than electrons, gamma rays, and x-rays. So equivalent dose = Wr x dose. Wr is a radiation weighting factor, and is always 1 for x/gamma-rays and electrons). Used for occupational dose (badge readings).
Effective dose: each organ is assigned a weighting factor. Effective dose = SUM(Wt x organ dose).
The unit for both is Sievert.
Which organs have the highest/lowest tissue weighting factors for effective dose?
Highest tissue weighting factor (0.12)- red marrow, lung, colon, stomach, breast.
Lowest (0.01)- salivary glands, bone surface, brain, skin.
The rest are in the middle. Gonads 0.08.
What is Candela? Lux?
Candela- unit of luminous intensity. Luminance = luminous intensity/square meter. Brightness of monitors is measured in candelas. Typical PACS brightness = 500 cd/m2. Mammo min brightness = 3000 cd/m2.
Lux- the unit for illuminance (light falling on a surface, which decreases contrast and interferes with ability to use monitors). Illumination of monitors should be below 15 lux.
What is the Bragg peak and what does it tell us about where the most dose is delivered for diagnostic x-rays?
A plot of specific ionization of a charged particle vs distance traveled. There is a sharp increase in ionizing potential near the end of a particle’s range.
In diagnostic x-rays, thie maximal dose is delivered at the skin surface.
What is linear energy transfer?
Defines the energy deposited per unit path length traveled by a charged particle (energy/distance - eV/cm). Proportional to particle charge, inversely proportional to particle velocity.
High LET = large amount of energy deposited in a small distance. Alpha particles have high LET, and thus are more damaging to tissue.
Low LET = energy deposition more sparsely deposited in a material. Beta particles and electrons have low LET.
What is Bremsstrahlung x-ray emission?
Occurs when energetic particles interact with nuclear electric fields. The electrostatic forces cause the charged particle to slow down and change direction, resulting in a loss of kinetic energy. An x-ray is released with energy equal to the amount of energy lost by the charged particle.
Where do the gamma ray photons detected in PET scans come from?
Positron annihilations.
When positrons (e+, emitted from the nucleus of certain radioactive atoms with too many protons relative to neutrons) come to rest, they join with a free electron, forming positronium. This complex breaks down, known as positron annihilation. This creates two oppositely directed gamma ray photons with energy of 511 keV each.
What is the basic structure of a traditional x-ray tube?
A cathode generates electrons by heating a thin filament to very high temperatures. Electrons are accelerated across a small gap to the anode (target) by high voltage applied between the anode and cathode. Charged particle interactions occur in the anode and produce the x-ray.
In diagnostic radiography, fluoro, and CT, the target (anode) is typically made of tungsten (K shell binding energy of -69.5 keV).
In a tungsten target x-ray tube, what is the primary electron-target interaction that occurs?
What is the primary x-ray generating interaction?
Excitation- this produces large amounts of heat, which must be dissipated to prevent damage to the tube.
Ionization accounts for only 0.1-0.15% of the electron-target interactions. This results in the characteristic x-ray emission (-69.5 keV).
The primary x-ray generating interaction is Bremsstrahlung (radiative losses), producing 85-90% of the beam. The energy of these polyenergetic x-rays can range from 0 keV to the maximum accelerating voltage (kVp) applied between the cathode and anode.
What is the kVp in an xray tube?
The maximum accelerating voltage (kVp) applied between the cathode and anode.
What are the 3 principle photon (packet of electromagnetic energy) interactions that can occur in clinical imaging?
1) Coherent (classical) scattering- an incoming photon excites an entire atom, and a photon of the same energy is released in a different direction. More likely to occur at lower energies (mammo). Photons that reach the image receptor result in a loss of contrast in the recorded image. No absorbed dose.
2) Compton scattering- incoming photon interacts with a loosely bound outer shell or free electron, transfers some of its energy to the electron which is ejected from the atom (compton electron), and the incoming photon changes direction. This is the dominant effect above 25-30 keV, results in degradation of image quality unless measures are taken to remove scattered photons using a grid or air gap. Predominant cause of occupational exposure.
3) Photoelectric absorption- all incoming photon energy is transferred to an inner shell electron, which is ejected from the atom. This effect produces the differential contrast observed in images, and is the most desired interaction. The probability of this effect is proportional to the atomic number cubed of the absorbing material, so tissue with high atomic number (bone) is more likely to absorb x-rays than tissue with low atomic number (fat). Inversely proportional to energy cubed of the incident photon, so less likely with higher energy beam. This is the primary contributor to patient dose.
Why are barium and iodine good contrast agents?
Because they have strong photoelectric absorption at diagnostic x-ray energies. They both readily absorb x-ray photons in the 35-50 keV range. When an x-ray tube is operated at 65-90 kVp, there are many x-rays produced in the 35-50 keV range. Iodine and barium strongly absorb this radiation, producing high contrast.
How does kVp affect differential absorption and contrast?
Patient dose?
As kVp increases, differential absorption will decrease, and contrast decreases. Higher kVp = reduced imaged contrast.
Higher kVp can be used in places where there is already good contrast (chest), but low kVp must be used when there isn’t much contrast (breast) in order to maximize differential absorption and increase contrast.
Higher kVp = less absorption in the patient = lower dose.
What is the linear attenuation coefficient (u)?
The fraction of incident photon that will be attenuated per unit thickness of material. Strongly energy dependent (decreases as energy increases due to the reduced likelihood of photoelectric absorption).
What is half value layer?
Typical HVL in soft tissue?
The thickness of material required to reduce the intensity of an x-ray beam to half its original. HVL increases as photon energy increases.
For diagnostic xrays, typical HVL in soft tissue is 3-4 cm. In mammo, about 1 cm.
What is beam hardening?
Occurs with polyenergetic x-ray beams. The preferrential attenuation of lower energy photons, which increases the average energy of the beam.
What is the heel effect?
Attenuation and beam hardening that occurs within the xray tube, in the direction of the angled target towards the anode.
Can compensate by placing denser portions of the body on the cathode side.
According to NCRP 160, what is the average background and medical radiation dose for the average US citizen?
Medical exposure and background exposure both = 3 mGy/yr.
What is the law of Bergonie and Tribondeau?
States that the radiosensitivity of cells is directly proportional to their reproductive activity and inversely proportional to their degree of differentiation.
Exception- lymphocytes and oocytes.
What are the 3 types of acute radiation syndrome, and at what whole body doses do they occur?
1) Bone marrow syndrome (>2 Gy)- decreased peripheral blood cell counts.
2) GI syndrome (>8 Gy)- N/V, decreased appetite. Feel better after about 1 week, then symptoms return plus fever and diarrhea. Death in 2nd week.
3) CNS Syndroem (>20 Gy)- immediate symptoms, can have unconsciousness in minutes at high doses.
What is the lethal dose (LD) 50/60?
The dose of radiation that would kill 50% of a population in 60 days. It is somewhere between 3.5 and 7 Gy depending on level of care.
What skin dose could lead to early transient erythema?
Severe erythema?
Dry desquamation?
Dermal necrosis?
Secondary Ulceration?
When would these effects occur?
Early transient erythema- onset in hours- dose = 2 Gy.
Severe erythema- onset 1.5 wks- does = 6 Gy.
Dry desquamation- onset 4 wks- dose = 14 Gy.
Dermal necrosis- onset 10 wks- dose = 18 Gy.
Secondary ulceration- onset > 6 wks = 24 Gy.
In males and females, what radiation dose would lead to temporary sterility, and how long does the effect last?
Dose for permanent sterility?
Temporary: Males- 2 Gy (12 months). Females- 1.5 Gy (1-3 years).
Permanent: Males- 5 Gy. Females- 4 Gy.
What is the difference between stochastic and non-stochastic radiation effects and what are the other names?
Stochastic (probabilistic)- more likely to occur with increasing dose. Severity is independent of the dose (ie cancer).
Non-stochastic (deterministic)- dose threshold below which the effect will not occur, severity increases with dose (ie cataracts, sterility)
How is radiation risk divided in utero, and what effects can be seen?
What is the increased risk of childhood cancer from radiation?
Preimplantation (day 0-9)- effect tends to be all or nothing. Threshold 50-100 mGy pre-implantation.
Organogenesis (weeks 2-8)- highest risk of malformations, most commonly CNS.
Fetal Growth (week 8-term)- no sig risk of death. Can see CNS defects, sense organ defects.
Fetal risks are only considered significantly increased at doses above 0.15 Gy.
Excess risk of childhood cancer from in utero radiation is about 6% per Gy.
What are the effective radiation dose rate limits for restricted and unrestricted areas (regarding nuclear medicine)?
For controlled area (regarding radiation shielding)?
Unrestricted areas: max 0.02 mSv/hour, 1 mSv/year (cumulative).
Restricted areas: 1.0 mSv/hour, 50 mSv/year.
Controlled areas: 0.1 mSv/wk or 5 mSv/yr.
What are the annual radiation dose limits for:
Adult occupation workers?
Declared pregnant employees?
Minor occupation workers?
Members of the public?
Adult workers: 50 mSv total body (5 rem), 150 mSv to eye (15 rem), 500 mSv extremity or single organ (50 rem).
Pregnant worker: 5 mSv to fetus during entire gestation (divided as 0.5 mSv/month).
Minor worker: 5 mSv total body, 15 mSv to eye, 50 mSv extremity or single organ.
Public: 1 mSv (5 mSv in “special circumstances”).
What is the threshold dose for the formation of radiation-induced cataracts?
How are they different from regular cataracts?
Start at 0.5 Gy.
They form in the posterior pole of the lens, vs senile cataracts which start anteriorly.
What is the MQSA limit on dose per image for mammogram of a 4.3 cm compressed breast with 50/50 fat/glandular tissue ratio?
3 mGy.
What skin dose is considered a sentinel event by the joint commission?
15 Gy.
What does the nuclear regulatory commission consider a reportable “medical event?”
How do they have to be reported?
Deviation from normal procedure that results in greater than 20% increase in radiation to a patient, administration of the wrong radiopharmaceutical, the wrong route, or to the wrong patient.
Additionally, effective dose must be > 50 mSv (or a dose equivalent to an organ or the skin > 500 mSv).
Medical events must be reported to the NRC (or agreement state, plus ordering MD and patient) verbally within 1 day of the event being discovered. A written report to the NRC and ordering physician due in 15 days.
At what dose to a fetus or nursing infant does it have to be reported to the NRC?
Greater than 50 mSv for both.
What is the excess relative risk of developing cancer per Sv of radiation exposure for the entire population?
For adults only?
Entire population: excess RR of cancer = 5.5-6% per Sv.
Adults only = 4.1-4.8% per Sv.
Peds more like 10-15%.
What produces the largest amount of x-rays in an x-ray tube?
Bremsstrahlung radiation- electron transversing the target giving up variable amounts of energy due to attracting forces from the target nuclei. Max energy the prodcued photos can have is equal to the kinetic energy of the accelerated electron (determine by voltage, kVp). Produces a spectrum of energies, majority of the photons produced are as heat (99.9%).
The characteristic spikes in the spectrum below are from characteristic radiation, released when ionization occurs.
Name the artifact.
Cause?
How to fix it?
Susceptibility Artifact.
Occurs at locations where magnetic susceptibility varies rapidly with location in tissue- the change perturbs the local magnetic field.
Remedy: try not to use GRE sequences (worst), increase the bandwidth (tradeoff = decreased SNR), increase the matrix, decrease slice thickness, decrease field strength.
Name the artifact.
Cause?
How to fix it?
Aliasing (wrap-around).
Tissue outside of the FOV is recorded artifactually within the FOV. Occurs along the phase-encoding direction.
Remedy: orient the thinnest anatomy along the phase encoding direction, enlarge FOV, exclude all anatomy from the top few slices (3D), use sat bands to null tissue outside the FOV.
Name the artifact.
Cause?
How to fix it?
Ghosting (motion artifact).
Due to patient or physiologic motion, tissue is not where it is expected based on the gradient. (Nyquist ghosting = with EPI, large burden on gradient performance and imperfections result in a ghost that is shifted by 1/2 the FOV)
Remedy: Use flow compensation or apply sat bands, switch frequency and phase encoding direction, ask patient to be still, gating, (Nyquist- apply eddy-current correction).
Name the artifact.
Cause?
How to fix it?
Truncation artifact.
Caused by partially sampled K-space. The Fourier-based image formation process is most inaccurate aruond sharp boundaries that abruptly change intensity. Results in Gibbs “ringing…” multiple alternating bright and dark lines parallel to the boundary.
Remedy: increase matrix, apply pre- or post-reconstruction image filtration.
Name the artifact.
Cause?
How to fix it?
Due to noise spikes in k-space.
Cause- erroneous detection of large signals at one or more times during readout. Scanner gives too much contribution frmo one or more sinusoidal pattern.
Remedy: check for vibrating metal or loose wires.
Name the artifact.
Cause?
How to fix it?
Zipper artifact.
Caused by detection of extraneous RF signals during readout. Line runs in the phase-encoding direction.
Remedy: close scan room door, check equiptment in room for RF emissions, check for leak in RF shielding.
Name the artifact.
Cause?
How to fix it?
Fat saturation failure.
The hydrogen nuclei in water and fat have different resonant frequencies, so a special RF pulse with a narrow band of frequencies centered on the resonant frequency of fat can be applied to “saturate” fat molecules. If the resonant frequency of fat doesn’t match this pulse, fat sat will fail. Can result from poor uniformity of B0, anatomy distant from isocenter.
Remedy: apply shimming within the region of interest, move anatomy as close as possible to isocenter. In hand, hold fingers together.
Name the artifact.
Cause?
How to fix it?
Dielectric Effects.
Caused by variation in tissue conductivity. Worse with increasing field strength, because the wavelength of the MRI RF excitation pulse becomes comparable in size or smaller than the torso width of large patients (shorter wavelength RF in 3T than 1.5). This can cause a strong decrease in signal near the center of the patients.
Remedy: use dielectric pads (fluid filled pads that increase signal near the patient), engage multi-channel transmission if the scanner is capable.
Name the artifact?
Cause?
Remedy?
Type 1 chemical shift.
Caused by misregistration of water and fat. Fat is shifted along the FREQUENCY encoding direction.
Remedy: increase the bandwidth.
Name the artifact?
Cause?
Remedy?
Type 2 chemical shift (India ink).
Boundaries of tissues with a large difference in fat/water are outlined in black.
Remedy: Lengthen TE (>30 msec) and apply shimming. For a short TE, change TE so fat and water are in phase.
What is the name of this artifact?
Cause?
Slice (section) thickness artifact.
Because 3D structures in the slice of an ultrasound image are displayed in 2D. Limited elevational resolution by the slice thickness.
What is this artifact?
What causes it?
Speckle artifact.
The result of a return sound beam being intercepted by an incoming sound beam. The net effect is cancelling out of the signal, giving a grainy appearance.
What is the name of this artifact?
What causes it?
Reverberation artifact.
The result of two parallel strong reflectors, the beam reflects back and forth between the two. The echo that returns to the transducer after a single reflection will be displayed in the proper location. The sequential echoes will take longer to return to the transducer, and the
ultrasound processor will erroneously place the
delayed echoes at an increased distance from the
transducer.
What is the name of this artifact?
What causes it?
Mirror image.
The duplication of an image on the opposite side of a specular reflector. The primary beam encounters a highly
reflective interface. The reflected echoes then
encounter the “back side” of a structure and
are reflected back toward the reflective interface
before being reflected to the transducer for detection.
RLQ pain in an immunocompromized patient. Diagnosis?
Who?
Imaging features?
Typhlitis (neutropenic colitis)- a necrotising inflammatory condition which typically involves the cecum. It can sometimes extend into the ascending colon or terminal ileum.
Most commonly occurs in the setting of immunocompromise, chemotherapy and steroid therapy.
Thickening of the cecum as well as fat stranding, pneumatosis intestinalis, +/- ileus or obstruction. There may be intramural areas of low attenuation which may represent hemorrhage or edema.
Treatment- IV antibiotics, nasogastric suctioning, and bowel rest. Surgical resection may be necessary for bleeding or bowel infarction.
What is the typical grid ratio for:
General x-ray?
Fluoro?
Mammo?
What is comet tail artifact in US?
Comet tail artifact is a form of reverberation. The two reflective interfaces (and thus sequential echoes) are closely spaced. On the display, the sequential echoes may be so close together that individual signals are not perceivable. In addition, the later echoes may have decreased amplitude secondary to attenuation, displayed as decreased width.
Which US artifact is this figure depicting?
What causes it?
Ring-down artifact.
The transmitted ultrasound energy causes resonant vibrations within fluid trapped between a tetrahedron of air bubbles. These vibrations create a continuous sound wave that is transmitted back to the transducer. This phenomenon is displayed as a line or series of parallel bands extending posterior to a gas collection.
What is the name/cause of this artifact?
Speed displacement artifact.
When sound travels through tissue at a speed significantly different than the assumed 1540 m/s (speed of sound in soft tissue), and the echo gets placed at the wrong location. A common place for this to happen is because of focal fat in the liver.
(Speed of sound in air is much slower 330 m/s, speed in fat is slightly slower 1450 m/s, and speed in bone is faster 4080 m/s)
