Fundamentals of Radiation and Radioactive Decay

Radiation

Energy emitted from a source that is transmitted through space or a material medium.

Ionizing Radiation

Four types of ionizing radiation: alpha, beta, positron and gamma.

Radioactive Decay

Atom with an unstable nucleus emits radiation to produce a more stable nucleus.

Alpha Decay

Particulate radiation composed of 2 protons and 2 neutrons, very large mass, very energetic, range in tissue is less than a tenth of a mm, destructive, NOT useful for diagnostic purposes (good for therapeutic).

Beta-minus Decay

A type of radioactive decay where a beta particle (electron) is emitted.

Positron Decay (Beta-plus Decay)

A type of radioactive decay where a positron is emitted.

Gamma Decay

Non-particulate radiation; no mass or charge, emission as packets or bundles of energy = photons, ability to penetrate large thicknesses of material, overall damage is relatively low because it travels fast, ideal for diagnostic purposes.

Radioactivity Units

Units for measurement of Radioactivity: disintegrations per second (dps), Becquerel (Bq) - often used as MBq or GBq, Curies (Ci) - often used as mCi.

Becquerel (Bq)

A unit of radioactivity equal to one disintegration per second.

Curie (Ci)

A unit of radioactivity equal to 37 billion disintegrations per second (37 GBq).

Half-life

Half-life (t1/2) = time during which one half of the radioactive atoms emit their characteristic radiation.

Decay Constant (λ)

A constant that describes the probability of decay of a radioactive isotope.

Radioactive Decay Equation

D = λN, where D is the disintegration rate, λ is the decay constant, and N is the number of atoms.

Decay Equation

A(t) = A0 e^(-λt), where A(t) is the total activity at time t, A0 is the initial activity, λ is the decay constant, and t is the elapsed time.

Mo-99 Half-life

Half-life = 65.9 hrs, Decay constant = 0.105/hr.

Tc-99m Half-life

Half-life = 6.01 hrs, Decay Constant = 0.1153/hr.

Decay Calculation for Mo-99

If you start with 100mCi of Mo-99, after 66 hours, 50mCi will be available.

Decay Calculation for Tc-99m (10 hours)

If you start with 100mCi of Tc-99m, after 10 hours, 31.57mCi will be available.

Decay Calculation for Tc-99m (10 hours)

If you start with 100mCi of Tc-99m, after 10 hours, 50.1mCi will be available.

Decay Calculation for Tc-99m (10 hours)

If you start with 100mCi of Tc-99m, after 10 hours, 65.9mCi will be available.

Decay Calculation for Tc-99m (10 hours)

If you start with 100mCi of Tc-99m, after 10 hours, 100mCi will be available.

Mo-99 Half-life

65.9 hrs

Tc-99m Half-life

6.01 hrs

Decay constant of Mo-99

0.1153/hr

Decay constant of Tc-99m

0.1153/hr

Decay Factor

Rate at which the variable's value diminishes through time

Decay factors usage

Decay factors can be used in combination to achieve a particular elapsed time

Decay factors calculation

Decay factors assist in calculating radioactive decay without the use of a scientific calculator

Naturally Occurring Radionuclides

All elements can have radioactive isotopes, which decay if enough neutrons are added.

Artificially Produced Radionuclides

Produced in a particle accelerator by bombarding a stable nucleus with particles.

Unique half-life

Each radioactive isotope has a unique half-life and unique decay constant.

Long half-lives of naturally occurring radionuclides

Most naturally occurring radionuclides have half-lives of millions to billions of years.

Tc-99m significance

Tc-99m is widely used in nuclear medicine studies across the globe.

Pharmacy technician dosage calculation

To calibrate 30mCi of Tc-99m at 0930 from 0230, draw 67.242mCi.

Decay constant formula

λ = ln(2) / t1/2

A(t) formula

A(t) = A0 e^(-λt)

Current time for dosage calculation

The current time is 0230.

Calibration time for dosage

The calibration time is 0930.

Options for dosage calculation

A. 67.2 mCi, B. 35.2 mCi, C. 60.0 mCi, D. 100mCi

Decay constant options

A. 0.1153/hr, B. 0.0105/hr, C. 0.0095/hr, D. 0.0862/hr

Naturally Occurring Radioactivity

Occurs in nature as members of a decay chain.

Long half-life parent radionuclide

Decays to start a series of radionuclides that decays until a stable end-product is produced.

Decay chain

All the parent isotopes have extremely long half-lives and undergo multi-step decay series with a wide variety half-lives in the decay chain.

Examples of naturally occurring radioactivity

Lantern mantles (thorium nitrate), granite countertops (veins can contain naturally occurring radioactive elements), vaseline glass (uranium), fiestaware plates (uranium oxide glaze).

Artificially Produced Radionuclides

All radioisotopes for medical imaging and therapy purposes are artificially produced.

Nuclear transmutation

When we artificially create isotopes we perform a nuclear transmutation.

Unstable nucleus

Convert a stable nucleus into an unstable nucleus.

Energy requirement for instability

Force something into the nucleus to create instability -> requires a lot of energy.

Cyclotrons

Particle accelerators used to create artificially produced radionuclides.

Nuclear reactors

Used in the process of artificially producing radionuclides.

Desirable characteristics of radioisotopes in medicine

A. Half-life B. Energy type C. Chemistry D. All of the above.

Energy type in radioisotopes

Decays with relatively low energy (if possible) and can be detected by a camera easily. Usually, gamma or beta.

Alpha and Beta particles

May be useful for therapy due to the effective damage to abnormal cells.

Chemistry in radioisotopes

Radioisotopes that can easily attach to a drug to image different areas/organs of interest or for use as therapy.

Half-life in radiopharmaceuticals

Radiopharmaceuticals should have a relatively short effective half-life - long enough to examine metabolic processes, but short enough to minimize exposure.

Availability of radiopharmaceuticals

The radiopharmaceutical should be easily produced and readily available to nuclear medicine facilities.

Tc-99m

Has the most desirable properties and is most commonly used in nuclear medicine.

Gamma radiation

Versatile chemistry

Half-life of In-111

2.83 days

Uses of In-111

MAb labeling, WBC labeling, cisternography

Source of In-111

Cyclotron produced

Chemical form of Tc-99m

Sodium Pertechnetate

Half-life of Tc-99m

6 hours

Uses of Tc-99m

Kit preparation, MUGA, GI Bleed, Thyroid

Source of Tc-99m

Generator produced (Mo-99 generator -> reactor produced)

Chemical form of Tl-201

Thallous Chloride

Half-life of Tl-201

73.1 hours

Uses of Tl-201

Myocardial Perfusion

Source of Tl-201

Cyclotron produced

Chemical form of Ga-68

Gallium chloride

Half-life of Ga-68

68 minutes

Uses of Ga-68

Tumor and infection imaging

Source of Ga-68

Generator produced (Ge-68 generator -> reactor produced)

Chemical form of I-123

Sodium iodide

Half-life of I-123

13 hours

Uses of I-123

Thyroid uptake and imaging

Source of I-123

Cyclotron produced

Chemical form of I-131

Sodium iodide

Half-life of I-131

8 days

Uses of I-131

Thyroid uptake and therapy/ablation

Source of I-131

Reactor produced

Chemical form of Xe-133

Xenon (gas)

Half-life of Xe-133

5.25 days

Uses of Xe-133

Pulmonary perfusion

Source of Xe-133

Reactor produced

Half-life of F-18

110 minutes

Uses of F-18

Complexed with tracers for cancer imaging

Source of F-18

Cyclotron produced

Mo-99/Tc-99m Generator Overview

Generators are shipped to nuclear pharmacies

Elution of sodium pertechnetate

Resulting solution is called sodium pertechnetate (NaTcO4)

Specific activity of molybdate solution

1000Ci/g

Small amounts of alumina

Small elution volume and higher concentrations

Small column size

Easier shielding

Tc-99m and Mo-99

Both Tc-99m (TcO4-) and Mo-99 (MoO42-) are anions with Mo-99 having the strongest fixation

Small amounts of radionuclidic impurities

Chemistry of Mo-99/Tc-99m Generators

Effective half-life

After Transient Equilibrium is reached, the daughter isotope appears to decay with the half-life of the parent isotope

Tc-99m generator decrease rate

The amount of Tc-99m on a generator decreases about 20% each day

Maximum build up time

Maximum build up on each generator occurs ~24 hours after each elution

Tc-99m Sodium Pertechnetate

When Tc-99m is eluted off of the generator it exists as [Tc+7O4] -, oxidation state of +7

Imaging agents oxidation states

Most imaging agents contain Tc-99m in the +5, +3, +1 or +7 oxidation state