7.1 intro to nuclear medicine Flashcards
what do we use in nuclear medicine?
radiopharmaceuticals - radioactive substances/agents used to diagnose certain medical problems or treat certain diseases
what is the main difference in the source of radiation of nuclear imaging compared to other types of imaging?
- in nuclear imaging, the source of radiation is the patient
- rather than the X-ray tube used in other types of imaging
describe this mechanism
the patient breathes in/or is injected with the radiopharmaceutical, and the gamma camera detects the photons from the patient
what are the modes of radioactive decay?
(1) alpha decay
(2) beta decay
(3) gamma decay
what are the two purposes of nuclear medicine?
nuclear medicine can be used for BOTH diagnostic and therapeutic procedures that use radioisotopes
in nuclear medicine, (1) what do we use for therapeutic purposes and (2) what do we use for diagnostic/imaging purposes?
(1) beta-emitters
(2) gamma radiation
why is it necessary for the radioisotope to emit gamma rays in diagnostic work?
- gamma radiation has greater penetrability so it can exit the patient & reach the gamma camera
- it can penetrate tissue and be detected outside the body
why is it more desirable for the radioisotope to emit β particles for therapeutic work?
- β particles have lower penetrability, so they can pass through shorter distances inside the patient
explain why we would use β-emitters for treatments such as tumor cleaning?
- because they have a short range in tissue
- can deliver a high radiation dose to the location of the radioisotope (localize the radiation)
what is used for thyroid gland treatments?
radioiodine (i131)
who discovered natural radioactivity and when?
- Henry Becquerel
- 1896
radioactivity is characterized by an unstable nucleus; what is an unstable nucleus?
a nucleus in which the number of protons and neutrons are NOT equal
what are radioactive nuclides?
- a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable
- since they’re unstable, they try to reach more stable nuclear configurations
- either occur in nature or are man-made
what is the process that allows radioactive nuclides to reach more stable nuclear configurations?
radioactive decay
radioactive decay processes are divided into six main categories:
(1) alpha (α) decay
(2) beta (β) decay
(3) gamma (γ) decay and internal conversion (IC)
(4) spontaneous fission (SF)
(5) proton emission (PE) decay
(6) neutron emission (NE) decay
what are nuclear transformations usually accompanied by?
emission of energetic particles
what are the particles released in the various decay modes:
(1) α decay
(2) β- decay
(3) β+ decay
(4) γ decay
(5) internal conversion (IC)
(6) neutron emission (NE) decay
(7) spontaneous fission (SF)
(8) proton emission (PE) decay
SOS
(1) α particles (helium particles)
(2) electrons or antineutrinos
(3) positrons or neutrinos
(4) γ-rays
(5) atomic orbital electrons
(6) neutrons in spontaneous fission
(7) heavier nuclei
(8) protons
what are neutrinos and antineutrinos?
subatomic particles with no electric charge
why do we use them?
- because they carry energies and angular momentum during the process of the decay
- in all transformations, the energy, atomic and mass numbers, and the angular momentum are conserved
- so these subatomic particles remove some energy from the system to reach this configuration
what is an alpha particle?
- high-energy helium nuclei consisting of 2 protons and 2 neutrons
- atomic number: 2
- mass number: 4
what is a beta particle?
- high-energy electron
- charge: -1
what is a positron?
- particles with the same mass as an electron but with 1 unit of positive charge
- charge: +1
what is a proton?
- nuclei of hydrogen atoms
- atomic number: 1
- mass number: 1
what is a neutron?
- particles with a mass approximately equal to that of a proton but with no change
- no protons, so atomic number: 0
- mass number: 1
what is a gamma ray?
very high-energy electromagnetic radiation
in each nuclear transformation, several physical quantities must be conserved:
- total energy
- momentum
- charge
- atomic number
- atomic mass number (number of nucleons)
how is nuclear stability achieved in elements with a low atomic number (Z)?
when the number of neutrons (N) is approximately equal to the number of protons (Z)
what happens as the atomic number Z increases?
- N/Z increases from 1 to about 1.5
- N/Z: ratio of # of neutrons/# of protons
compare a light nucleus (low atomic number) to a nucleus with high atomic number
- light nucleus = more stable
- high atomic number nucleus = more UNSTABLE
what are the 3 possible cases of N/Z in radioactive nuclei
- N/Z is too high for nuclear stability, nucleus is neutron-rich
- N/Z is extremely high
- N/Z is too low for nuclear stability, nucleus is proton-rich
what happens when a nucleus has N/Z too high for nuclear stability?
(1) it has an excess number of neutrons and is called NEUTRON-RICH
(2) it decays through conversion of a neutron into a proton and emits an electron and anti-neutrino
what happens when a nucleus has N/Z extremely high for nuclear stability?
a direct emission of a neutron is possible
what happens when a nucleus has N/Z that is too low for nuclear stability?
(1) it has an excess number of protons and is called PROTON-RICH
(2) it decays through conversion of a proton into a neutron and emits a positron and a neutrino
‘nucleus decays through conversion of a neutron into a proton, and emits an electron and anti-neutrino’
what is this process known as?
β- decay
what is β- decay in practice?
we have more neutrons, so they’re converted into protons in order to achieve stability (equal protons & neutrons)
‘nucleus decays through conversion of a proton into a neutron, and emits a positron and neutrino’
what is this process known as?
β+ decay
what is another process we can emit a neutrino?
- a process known as electron capture
- this occurs when the nucleus may capture an orbital electron
- then transform a proton into a neutron and finally, emit a neutrino
what process is electron capture the inverse of?
- electron capture is the inverse of β- (minus) decay
- β- decay: we have the emission of an electron
- electron capture: the nucleus captures an electron from the orbital, and emits a neutrino
what was the first mode of radioactive decay detected?
alpha (α) decay
what is alpha decay characterized by?
a nuclear transformation
what is an alpha particle?
a helium-4 nucleus that has a VERY stable configuration
describe alpha decay
- we have a parent nucleus, which is unstable
- it attains a more stable nuclear configuration (daughter D) through the ejection of an α-particle (Helium)
- because the atomic and mass numbers must be conserved, the daughter nucleus has to have a mass number lower by 4 and an atomic number lower by 2
- therefore, in α-decay the number of protons & neutrons is conserved by producing a helium nucleus
what happens when an α-particle is emitted by the radioactive parent (Z, A) nucleus?
- the atomic number Z of the parent decreases by 2
- the mass number A decreases by 4
- it sheds two orbital electrons from its outermost shell
describe what happens to an emitted α-particle
- the energetic α-article slows down through the absorber
- this allows it to capture 2 electrons from its surroundings to become a neutral He atom again
why can α-particles be dangerous?
- since they are relatively heavy & slow particles, they can only travel short distances before losing their energy and being absorbed
- this can be dangerous if absorbed by the human body
α-particles have what range (1) in air and (2) in tissue
(1) air: about 1cm - 10cm
(2) tissue: about 10^-3 cm - 10^-2cm
what is the most important example of radioactive decay in α-decay?
the decay of radium-226 into radon-222
what occurs in β decay modes of radioactive decay?
- the atomic number Z of the parent nuclide changes by one unit (+1)
- the mass number A remains constant
what are other characteristics of β-decay processes?
- the number of nucleons (protons & neutrons) and total charge are both conserved in the process
- the daughter D can be referred to an isobar of the parent P
what 3 processes fall into the category of β-decay?
(1) β minus decay - emission of an electron of negative charge (therefore, β-)
(2) β plus decay - emission of a positron of positive charge
(3) electron capture
what occurs in β minus decay?
- # of neutrons > # of protons
- neutron is converted into a proton by the emission of an electron and an anti-neutrino
- mass number: same
- atomic number: +1
what occurs in β plus decay?
- # of protons > # of neutrons
- proton is converted into a neutron by emission of neutrino and positron
- mass number: same
- atomic number: -1
what occurs in electron capture?
nucleus captures an electron from the orbital and converts it into a neutrino
β decay of a parent nucleus sometimes does not lead directly to the ground state of the daughter nucleus, this leads to?
an unstable or metastable excited state of the daughter nucleus
what is the condition under which β decay requires to take place?
β decay can only occur when the binding energy of the daughter nucleus exceeds the binding energy of the parent nucleus
several radionuclides decaying by β(-) decay are used in medicine for:
- external beam radiotherapy
- brachytherapy
what is brachytherapy?
- a type of radiotherapy where we insert the radioactive source inside the patient
- ex: in prostate cancer, the radioactive source is placed near the prostate so the the radiation can kill the tumors
a parent nuclide decays into an excited daughter nuclide through?
β(-) decay
describe the decay of a parent nuclide into an excited daughter nuclide
- the nuclide decays either instantaneously or through a metastable decay process into its ground state
- by doing so it emits the excitation energy in the form of γ-ray photons
- these photons are then used for radiotherapy
which radionuclide is practically suitable for use in radiotherapy?
cobalt 60
what is β(+) decay characterized by?
the production of positrons
what are radionuclides undergoing β(+) decay called? what are they used for?
- called positron emitters
- they are used in medicine for functional imagine with positron emission tomography (PET)
what is an example of a β(+) decay?
the decay of nitrogen-13 into carbon-13 with a half-life of 10 min
what is nitrogen-13 labelled ammonia, which is injected intravenously, mainly used in?
- cardiac imaging for diagnosis of coronary artery disease and myocardial infarction
- liver imaging
- brain imaging
what is one desirable characteristic of a radiopharmaceutical? give an example
- having a short half-life
- ex: fluorine-18 decays into oxygen-18 with a half-life of 110 min, which protects the patient as it is short
- a drug with a half-life of 5 years means the patient will be radioactive & receive radiation for 5 years
what is a sugar compound that can be injected intravenously into a patient for use in PET functional imaging?
fluorodeoxyglucose (FDG) labeled with radionuclide fluorine-18
what are some functions of the FDG PET scan?
- can detect malignant disease
- can distinguish benign from malignant disease
- can be used for staging of malignant disease
- can be used for monitoring response to therapy of malignant disease
how does gamma decay differ from both alpha and beta decay?
- gamma decay only occurs after alpha and beta decay in a sample
- gamma decay doesn’t change the mass or any other properties of the atom, they are simply high energy photons, electromagnetic radiation
what do we need to have in order for gamma decay to take place?
- an EXCITED PARENT NUCLEUS through β decay
- which de-excites through the emission of gamma photons
- gamma photons are electromagnetic radiation
- so excess of energy on this excited nucleus is released as E.M. radiation in order for the daughter nuclei to reach a stable configuration
what are the two processes in which a daughter nucleus reaches its ground state?
(1) emit the excitation energy in the form of a γ photon in a decay process called γ-decay
(2) transfer the excitation energy to one of its associated atomic orbital electrons in a process called internal conversion (IC)
compare X-rays to γ-rays
- characteristic X-rays are produced when we have a vacancy, and an electron from a higher orbital drops down to fill that vacancy & the excess in energy is released in form of radiation
- gamma radiation is when we have an excited nucleus (this is why gamma energies are higher in energy compared to X-rays because this nucleus reaches a stable configuration by the release of gamma photons)
why do we refer to the emitted γ-rays as if they were produced by the parent nucleus?
because in most radioactive α or β decays the daughter nucleus de-excitation occurs instantaneously
give an example of γ-rays being referred to as from the parent nucleus (gamma decay)?
- the cobalt-60 β decays into nickel-60 after the release of 2 γ photons
- the γ-rays following the β decay originate from nuclear de-excitations of nickel-60 (daughter nucleus)
- we refer to these γ-rays as the cobalt-60 γ-rays for convenience
describe the 99.88% path of cobalt-60 decay into stable nickel-60
- first part of the transformation we have the Co-60 which decays into the EXCITED nickel by β decay, and we have the emission of 1 electron (so β(-) decay)
- then the excited nickel reaches the stable configuration by the release of 1 γ photon and then another γ photon
describe the other 0.12% path of cobalt-60 decay into stable nickel-60
here we have the excitation of Co-60 into Ni-60 by β(-) emission where we have the emission of 1 electron followed by the emission of only one γ photon
what do BOTH paths have in common?
(1) to first excite the nucleus by β emission
(2) nucleus reaches stability by release of γ photons
in some α or β decays, the excited daughter nucleus does not immediately decay into its ground state; what is this state referred to as?
- the excited state of the daughter is then referred to as a metastable state
- a nucleus in a metastable state is identified with a letter “m” next to the mass number
what is the process of de-excitation of a metastable daughter nucleus called?
an isomeric transition
what is the term isomer used for?
designation of nuclei that have the same atomic number Z and same atomic mass number A but differ in energy states
γ decay stands for nuclear de-excitation by? it only implies emission of what?
(1) emission of γ-ray photons
(2) internal conversion
- only implies emission of γ photons
the γ decay process can be represented as:
- check ppt
- star - means daughter nucleus is excited
- Q - symbol indicating heat or energy
- nucleus de-excites through the release of a γ photon
what is the process of internal conversion (IC)?
nuclear de-excitation in which the de-excitation energy is transferred from the parent nucleus almost in full to an orbital electron of the same atom
describe spontaneous fission
- type of radioactive decay found in only very heavy chemical elements (nuclei with large atomic mass numbers A)
- occurs when an unstable nucleus spontaneously & randomly splits into smaller parts
- this simultaneously emit 2-4 neutrons
- accompanied by the release of a significant amount of energy
what does ALL fission release?
neutrons
spontaneous fission follows the same process neutron-induced nuclear fission; what is the only difference between them?
SF is not self-sustaining
where does SF only occur?
in thorium, protactinium, uranium, and transuranic elements (Z>92)
spontaneous fission is a competing process to?
α decay
the higher the atomic mass number above uranium, what happens to (1) spontaneous fission and (2) half-life
(1) the more prominent the spontaneous fission
(2) the shorter the half-life
what is SF a limiting factor of?
how high in atomic number Z and mass number A one can go in producing new elements
describe proton emission
- also known as proton radioactivity
- occurs when a proton is ejected from the nucleus
- proton-rich nuclides normally approach stability through β(+) or α decay, so in extreme cases of a large proton excess, a nucleus may move toward stability through emission of one or two protons
proton emission is a competing process to?
β(+) and α decay
what effect does proton emission have on atomic number and mass number?
both atomic number Z and mass number A decrease by 1
what happens when a proton is ejected from a radionuclide P?
- the parent nucleus P sheds an orbital electron from its outermost shell to become a neutral daughter atom
- the energetic proton slows down in moving through the absorber medium and captures an electron from its surrounding to become a neutral hydrogen atom
what are isotones?
atoms that have the same neutron number, but different proton number
why are the parent P and daughter D nuclides in proton emission considered isotones?
because the number of neutrons does not change in proton emission decay (we only have the release of a proton)
for lighter, very proton-rich nuclides (A≈50), what is more likely to occur when they (1) have an odd number of protons Z and (2) have an even number of protons Z
(1) odd number of protons - proton emission decay is likely
(2) even number of protons - a simultaneous two-proton emission is likely
describe neutron emission
- mode of decay where one or more neutrons are ejected from the nucleus
- the atomic number Z remains the same
- the mass number A decreases by 1
regarding atomic number Z, neutron emission is the inverse of?
spontaneous fission
in neutron emission, the parent and daughter nuclei are thus?
isotopes
(of the same nuclear species)
why do we not use spontaneous fission, proton and neutron emissions for imaging?
the half-lives in these types of decay are too short, therefore time is not enough to obtain an image
(1) process competing with β(-) decay, not observed in naturally occurring radionuclides
(2) process competing with/ less common than β(+) and α decays, not observed in naturally occurring radionuclides
(3) process competing with α decay
(1) neutron emission
(2) proton emission
(3) spontaneous fission
what is the type of decay in each of the following:
(1) atomic number of parent nucleus changes by +1, mass number stays constant
(2) atomic number of parent nucleus decreases by 2, mass number decreases by 4
(3) atomic and mass numbers are not affected
(4) both atomic and mass numbers decrease by 1
(5) atomic number remains the same, mass number decreases by 1
(6) atomic number decreases, mass number decreases
(1) beta decay
(2) alpha decay
(3) gamma decay
(4) proton emission
(5) neutron emission
(6) spontaneous fission
what does an ideal radiopharmaceutical need?
- to have a short physical half-life
- to be eliminated from the body with an effective half-life approximately equal to the examination time
- to emit pure gamma rays with no subsequent change in the nucleus
- to emit mono-energetic gamma rays
- to have a high activity per unit mass (specific activity)
- to be able to localize largely and quickly at the target site (ex: iodine is captured by the thyroid)
- to decay into a more stable daughter nucleus
- to. easily and effectively be attached to the chemical compound at room temperature
- to be easily produced or found at the hospital site
