PET exam Flashcards
Types of disintegration/transformations (decay)
(1) alpha decay
(2) beta decay
(3) electron capture
(4) isomeric transition
Positron Emission
beta transformation:
emission of positively charged electron (positron) from nucleus of radioactive atom
Types Beta Decay
1 Positron Emission
2 Simple B Decay
Types of Positron Decay
1 Simple - daughter is at ground state after decay
» Only ionizing radiation is positron emission
2 Daughter is at excited state after the decay
» Prompt emission of gamma ray after positron emission
Gamma Ray
photon of electromagnetic radiation from excited nucleus
y ray
Photon
the smallest entity of energy emitted in the form of electromagnetic radiation and is sometimes referred to as a quantum of energy
Important examples of positron emitters with simple positron decay used in the PET laboratories are…
carbon-11 (11C), oxygen-15 (15O) and fluorine-18 (18F)
Important examples of positron emitters with decay leaving an excited daughter
copper-60 (60Cu)
gallium-66 (66Ga)
iodine-124 (124I)
Disintegrations of a particular radionuclide generally result in…
the emission of one or more distinctive types of ionizing radiation
unit of energy commonly used to denote particle and photon energies in atomic physics
electron volt (eV)
kiloelectron volt = 1 keV = 1,000 eV, and
megaelectron volt = 1 MeV = 1,000,000 eV
What is the average positron energy for a particular positron-emitting radionuclide?
about 40% of its maximum energy
Positron interactions with matter
- dislodging electrons from atoms to form ions (ionization)
- exciting atomic electrons to higher energy levels (excitation)
- causing vibrations of molecules (excitation)
- breaking molecular bonds (excitation), and
- producing electromagnetic radiation subsequent to a sudden change of direction in the positron’s path (bremstrahlung)
>> (1) ionization, (2) excitation and (3) bremstrahlung
What is the level of energy emitted by a positron from a radionuclide
continuum of kinetic energies…
from essentially zero kinetic energy to a maximum energy that is uniquely characteristic for the radionuclide
> > less than 10% of the emitted positrons from a given radionuclide will travel as far as one-half of the range before dissipating their kinetic energies
Annihilation
After an emitted positron has dissipated its kinetic energy, it very quickly combines with an ordinary electron and the two particles undergo mutual annihilation.
- rest mass of each particle is converted to electromagnetic energy
- The two photons are emitted in opposite directions to conserve momentum
- The simultaneous emission of the two 511 keV photons in opposite directions serves as the foundation of PET detection
Another name for radioactive decay
Disintegration rate
What does the disintegration rate tell us
Used to indicate the “radioactivity” of a sample and it is termed the activity
Traditional unit of activity in radioactive decay
curie (Ci)
curie (Ci)
the amount of radioactive material having a disintegration rate of
3.70x1010 (37 billion) disintegrations per second
> > millicurie = mCi = 0.001 Ci,
microcurie = uCi = 0.000001 Ci
What does activity denote?
only the rate of disintegration;
provides no information concerning the kind of radiation emitted
Half-life
Time necessary for one-half of the radioactive atoms of a sample to
decay
Half-Value Layer (HVL)
Amount of material (often expressed as the thickness) required to reduce the transmitted photon radiation to one-half of the incident value
> > Often used to express the attenuation/absorption properties of a material for photons
Dose equivalent
The expression of biological damage caused by “ionizing radiation”
> > Often referred to as “dose”
Unit = rem
Ionizing radiation examples
Gamma rays
x-rays
beta particles
What is the naturally occurring “background” radiation level
0.3 rem per year in the U.S.
(300 millirem)
What levels of radiation are known to cause harm (considered large)?
“acute” doses of greater than 10 rem received over a short period of time
“chronic” doses incurred over a prolonged period, e.g., years, of greater than one hundred rem
Known radiological effects on humans
elevated risk of cancer, cataracts, genetic effects, etc
Opinions concerning the harm of relatively low radiation dose
- No threshold model
» This concept says unit dose risk factors, e.g., % chance of harm per rem that are numerically the same as those that apply to much higher doses
» Used by state & federal regulatory agencies - Thresholds exist for most harmful effects but those are greater than the doses received by occupational radiation workers
- Hormesis
» Low doses of ionizing radiation are actually beneficial
Which model must we abide by concerning radiation dose levels?
The view of the regulatory agencies is based on the assumption that any amount of radiation exposure is potentially harmful
- Who has authority of the use of the cyclotron-produced radioactive materials employed in the PET laboratories?
- Who additionally provides recommendations concerning dose limits for radiation workers?
- Who was it, historically?
- The NRC
- NCRP - The National Council on Radiation Protection and Measurements
- The state of Missouri
The three classic methods for controlling external radiation exposure are:
(a) minimize time spent in a radiation field
(b) maximize distance from the source of radiation
(c) utilize shielding between you and the source
Why does the relatively large annual radiation doses incurred by PET personnel occur?
(a) the emitted radiations are abundant and generally penetrating and
(b) large activities are often handled to compensate for the rapid decay occurring during the studies in which they’re used
Why are doses due to internal contamination are not a primary concern in your work
Because of the short half-lives of the radionuclides that we use
ALl
Maximum annual limits of intake specified by federal government
» represents the activity of a given radionuclide that, if ingested or inhaled by a person will result in the more limiting of
(a) 5 rem of effective whole body dose
or
(b) 50 rem to the “critical” organ
Where is the “Emergency Procedures Involving Radioactive Material Contamination” notice posted?
In each PET laboratory
What does the “Emergency Procedures Involving Radioactive Material Contamination” notice tell you?
The instructions address the clean up of a spill and personnel
decontamination
Also, each notice explains how to contact Radiation Safety for
assistance and who to notify
Pneumatic lines
Used to transport very short-lived radiopharmaceuticals
What happens if the carrier of the pneumatic lines is unable to arrive at their destination?
The cyclotron personnel should be immediately informed
What happens if the wrong radiopharmaceutical is sent from the cyclotron?
Promptly notify the cyclotron (314-362-2261, 314-362-8393)
& Sally Schwarz (314-362-8426, pager # 314-490-3081)
What happens when a patient will not remain motionless during PET imaging?
a family member or other accompanying individual should be asked to restrain the patient; it
should not be done by PET personnel
body dosimeters
“optically stimulated luminescence”
(OSL)
» “Luxel” dosimeters
» very sensitive to ionizing radiation
» The minimum reported dose is one millirem (mrem)
» A measured dose less than one mrem is reported as “minimal” (M)
The reported doses for each body dosimeter are
(1) the deep dose (the calculated dose at a tissue depth of 1.0 cm)
(2) the shallow (or skin) dose
(3) the lens dose.
ring dosimeters
“thermoluminescent” dosimeters
(TLD)
The doses for ring dosimeters are reported as shallow doses down to a minimum of 10mrem
Ring doses less than 10 mrem are reported as “minimal “ (M).
(ALARA)
maintain personnel radiation doses as low as reasonably achievable
- all reported external doses are reviewed for unusually high levels by a member of the Radiation Safety Staff
- reports the investigations to a special subcommittee of our institutional Radiation Safety Committee
relationship between distance and radiation
The exposure level varies inversely with the square of the distance from the source (the relationship applies for distances large compared to the physical dimensions of the source)
> > 4-fold reduction of the dose rate
Shielding
pigs or lead containers used to transport cyclotron-produced materials should be sufficiently thick to reduce external levels to an acceptable value
» In addition, it is helpful to use a Lucite sleeve over a syringe, a so-called “beta shield”, to absorb energetic positrons
