Nuclear Medicine Exam 1

What is a radiopharmaceutical? What is another name for this?

A compound radiolabeled with gamma-ray or positron-emitting radionuclides which is injected into the body; Radiotracer, or tracer

What are the two broad classes of nuclear medicine imaging?

SPECT (Single Photon Emission Computed Tomography) and PET (Positron Emission Tomography)

What is detected in SPECT?

Single photons from radionuclides which decay by gamma-ray emission

What is detected in PET?

Emitted positrons interact with electrons and produce two high-energy photons. These photons are detected by the imaging camera.

What is the major benefit of nuclear medicine over other imaging modalities?

The ability to obtain sensitive measurements of biologic information

Name five examples of specific biological processes that nuclear medicine can detect

1. Tissue perfusion

2. Glucose metabolism

3. Somatostatin receptor status of tumors

4. Density of dopamine receptors in the brain

5. Gene expression

Which two people discovered radioactivity?

Marie Curie and Henri Becquerel

Who discovered X-rays?

Wilhelm Roentgen

Who developed the principles of the tracer approach?

Georg de Hevesy

Who invented the forerunner of all modern nuclear medicine single-photon imaging systems?

Hal Anger

What is the most widely used radionuclide in nuclear medicine?

Technetium-99m

Where does the unified atomic mass unit come from?

It is 1/12 the mass of an unbound Carbon-12 atom at rest in its ground state

What are the two specific forms of radiation important in nuclear medicine?

Particulate and Electromagnetic

Put these in order from least to greatest photon energies: Visible Light, X-rays, UV, Gamma Rays, Infrared, Radio, Microwave

1. Radio

2. Microwave

3. Infrared

4. Visible

5. UV

6. X-Rays

7. Gamma Rays

For a shell with quantum number n, what are the maximum number of electrons allowed?

2n²

Order the shells of an atom from most tightly bound to least tightly bound: L, N, K, M

1. K

2. L

3. M

4. N

What is the energy required to move an electron from the K-shell to the L-shell?

Binding Energy of K-shell minus Binding Energy of L-shell

Define the binding energy of a shell

The energy required to completely remove an electron from the shell

Binding energy is greatest for the (innermost/outermost) shell and (increases/decreases) with Z

Innermost; Increases

What are two ways in which energy is released when an electron moves from an outer shell to fill a vacancy in an inner shell?

Characteristic X-rays and Auger electrons

How is the kinetic energy of an Auger electron determined?

It is equal to the difference between the binding energy of the shell containing the vacancy and the sum of the two shells having vacancies at the end

What is fluorescent yield and what does it depend on?

The probability a vacancy will yield characteristic x-rays instead of Auger electrons; it depends on the specific element and subshell. Heavy elements are more likely to emit x-rays and light elements are more likely to emit Auger electrons.

What do a, b, and c mean in e_abc notation mean for Auger electrons?

a is the shell with the original vacancy, b is the shell from which the electron dropped, and c is the shell from which the Auger electron was emitted

What is a metastable state?

An unstable arrangement of nucleons that has a relatively long lifetime before transforming into another state

What is the dividing line between excited and metastable states, approximately?

10^-12 seconds

What are isomers?

Two nuclides that differ from one another in that one is a metastable state of the other

What is the binding energy of an atom?

The minimum amount of energy required to overcome the binding forces holding the atom together to separate it into its individual components

Nuclear processes that result in the release of energy always (increase/decrease) the binding energy of the nucleus

Increase

Where does the line of stability approximately correspond with for light and heavy elements?

Light: N ~ Z

Heavy: N ~ 1.5 Z

In general, there is a tendency towards instability in atomic systems composed of (small/large) numbers of identical particles confined in a (small/large) volume

Large; small

Unstable nuclides lying above the line of stability are (proton/neutron)-deficient and those lying below the line are (proton/neutron)-deficient

Proton; neutron

Define Transition Energy (Q) and describe how this energy is converted in a radioactive process

The total amount mass-energy converted during a radioactive process. Most of this energy is imparted as kinetic energy to emitted particles or converted to photons, with a small amount given as recoil energy to the nucleus.

True or false: Radioactivity of the nucleus affects the chemical state of the atom

False; Radioactivity primarily involves the nucleus and chemical reactions primarily involve the outermost orbitals of the atom

What is the “isotope effect”?

The chemical properties of an atom may be changed by radioactivity because chemical properties depend on mass, so a radioactive nuclide may be chemically different from its stable isotope.

Describe beta-minus emission

A neutron in the nucleus is transformed into a proton and an electron. The electron and a neutrino are ejected from the nucleus and carry away the energy released as kinetic energy. The electron is called a beta-minus particle.

What does the transition energy Q represent in a beta-minus decay reaction?

The maximum possible beta-minus energy

What is the difference between beta-minus decay and (β^-, γ) decay?

In beta-minus decay, the parent radionuclide decays into a stable daughter. In (β^-, γ) decay, the parent decays into a daughter in an excited state and promptly decays to a more stable arrangement by emitting a gamma ray. The daughter may decay to another excited state (possibly multiple times) and must keep releasing gamma rays until it reaches a stable state.

The sum of all participant’s energies in any decay event is always equal to ___

The transition energy

Define an isomeric transition

A parent radionuclide decays to a daughter in a long-lived metastable/isomeric state. An isomeric transition is when this daughter decays by emission of a gamma ray.

What is internal conversion?

An alternative to decay by gamma ray emission (especially common in metastable decay): the nucleus decays by transferring energy to an orbital electron which is then ejected instead of a gamma ray

Conversion electrons commonly originate from an (inner/outer) shell

Inner

What is electron capture decay?

Sometimes called “inverse beta-minus decay,” electron capture occurs when an orbital electron is captured by the nucleus and combined with a proton to form a neutron and a neutrino. The neutrino is emitted and carries away some transition energy, and the remaining energy appears in the form of characteristic X-rays and Auger electrons.

Beta-minus decay, beta-plus decay, and electron capture are all ___ decays

isobaric

What is positron emission decay?

A proton in the nucleus is transformed into a neutron and a positively charged electron. The positron and a neutrino are ejected from the nucleus.

Why does positron-emission decay require additional energy to occur?

Because a neutron is heavier than a proton and antimatter is created

What happens to an emitted positron after it is ejected from the nucleus?

The positron and an electron combine in an annihilation reaction in which their masses are converted to energy in the form of two 0.511 MeV annihilation photons which leave in exact opposite directions

What is the minimum transition energy required for positron decay to occur and why?

1.022 MeV; A positron is ejected from the nucleus and the daughter atom has an excess electron it must release since the atomic number decreased by 1. The rest-mass energy of these two particles is 0.511+0.511=1.022 MeV

What two types of decay have the same effect on the parent nucleus and thus are simply alternative means for reaching the same endpoint?

Positron emission and electron capture; both are isobaric decay modes that decrease atomic number by 1

Positron emission is more common for (light/heavy) elements and electron capture is more common for (light/heavy) elements

Light; Heavy. In heavier atoms, inner-orbital electrons are closer to the nucleus and thus are more likely to be captured.

By what mode would a light, neutron-deficient atom most likely decay?

Positron emission

What is the decay constant?

The fraction of atoms in a radionuclide sample undergoing a radioactive decay per unit time

What is a branching decay constant?

An individual decay constant for a radionuclide that can undergo radiation by multiple possible decay modes

What is the branching ratio B.R.?

The fraction of nuclei decaying by a specific decay mode (λ_i/λ)

What are image-frame decay corrections?

Corrections of data acquired in a series of image frames made for the decay that occurs during measurements over a period of time not short in comparison with half-life

What is a “carrier”?

A stable isotope present with a radionuclide of interest (e.g. a given ¹³¹I sample contains ¹²⁷I; ¹²⁷I is stable so it is the carrier)

What is specific activity?

The ratio of radioisotope activity to total mass of the element present

What is the Bateman equation used for?

Determining the activity of the daughter in a sample containing a mixture of parent-daughter radionuclides

What is secular equilibrium?

When the decay of the parent is so long that the decrease of parent activity is negligible during the observation period [T_d « T_p (approx infinity)]

What is transient equilibrium?

It occurs when the parent half-life is longer than the daughter half-life but is not “infinite” [e.g. ⁹⁹Mo (T_1/2=66 hr) → ^99mTc (T_1/2=6 hr)]. Importantly, the decay constant of the parent cannot be assumed to be approximately 0. After some time, the daughter reaches a maximum and then follows the decay of the parent, and this is when transient equilibrium occurs

What situation will lead to no equilibrium between the parent and daughter?

When the daughter’s half life is longer than the parent’s

What is the parent material in the Technetium-99m generator?

Molybdenum-99

How do the products of nuclear reactors contribute to nuclear medicine?

The fission products always have an excess of neutrons and undergo further radioactive decay by beta-minus emission. If one of the intermediates has a sufficiently long half life, it can be extracted from the fission products and used as a medical radionuclide

What is neutron activation? What are the two types?

When a neutron strikes a target nucleus which is then converted into a radioactive product nucleus as a result; (n, γ) and (n, p)

What does the notation (n, γ) represent?

A target nucleus captures a neutron and emits a gamma ray

How do charged particle accelerators form medically useful radionuclides?

The accelerators accelerate electrically charged particles to very high energies. When directed onto a target material, these particles may cause nuclear reactions that result int the formation of radionuclides similar to neutron activation in a reactor. The high energies are necessary for the particles to overcome the coulombic forces surround the nucleus.

Describe how cyclotrons work

An ion source released charged particles into the gap between the dees. An AC voltage is applied across the gap which accelerates the particle into the dee. When the particle rounds back to the gap, it is perfectly in sync with the maximum voltage in the opposite direction. The particles gain energy as they are accelerated in this way.

What is the main reason a synchrotron would be used instead of a cyclotron?

Higher particle energies can be achieved

What radioactive decay reaction would you expect from an atom that lies beneath the line of stability?

EC or positron emission

When neutrons are added to a nucleus, the products of neutron activation generally lie (above/below) the line of stability and therefore tend to decay by __

above; beta-minus emission

What is the difference between a negatron and an electron?

A negatron is an electron that is formed inside a nucleus

What is a radionuclide generator? What is the most important generator for radionuclide imaging?

A parent and daughter radionuclide pair contained in an apparatus that permits separation and extraction of the daughter from the parent; Molybdenum-99-Technetium-99m

Why is Technetium-99m so useful for nuclear medicine?

It emits gamma rays very favorable for a gamma ray camera, it has a reasonable half-life. it delivers a relatively low radiation dose per emitted gamma ray, and it can be used to label a wide variety of imaging agents

What is the activation cross-section?

The probability a particle will interact with a target nucleus; the effective “target area” presented by a target nucleus

What is activation rate?

The speed at which stable nuclei are transformed into radioactive isotopes through particle bombardment; the number of nuclei activated per unit beam area per second

What is saturation activity?

The buildup and decay of product radioactivity is exactly analogous to secular equilibrium wherein the irradiating beam acts as a long-lived parent generating daughter nuclei at a constant rate. The product activity gradually approaches a saturation level at which disintegration rate matches production rate. The saturation activity, then, is the disintegration rate per gram, which is equal to the activation rate per gram.

What is production rate A’?

The rate at which radioactivity is produced during an irradiation, disregarding the simultaneous decay of radioactivity that occurs during irradiation; the slope of the production curve at t = 0

What is a radiopharmaceutical?

When a radionuclide is attached as a label to a compound that has useful biomedical properties

Because nuclear medicine relies on the use of sub-pharmacologic tracer doses that do not perturb the biologic system under study, the mass of the radionuclide compound should be (high/low) and the specific activity should be (high/low)

Low; high

What is radionuclidic purity?

The fraction of the total radioactivity in a sample that is in the form of the desired radionuclide

What is a useful precursor?

Chemical forms that react readily to form a wide range of labeled products

What is radiochemical purity?

The fraction of the radioactivity in the sample that is present in the desired chemical form

What is biologic half-life?

The rate of clearance of the radiopharmaceutical from the body. Together with the physical half-life from the radionuclide, this determines the number of radioactive decays that will be observed from a particular region of tissue as a function of time

What are the two distinct strategies for labelling small molecules with radionuclides?

Direct substitution: A stable atom in the molecule is replaced with a radioactive atom of the same element

Create analogs: involves modifying the original compound; allows chemists to beneficially change the biologic properties of the molecule

What is the purpose of a “cold kit”?

The Mo-Tc generator produces technetium in the form of ^99mTcO^-₄ . The contents of the cold kit is mixed with the technetium reduce it to lower oxidation states