Radiation Chemistry. Radionuclides The radioactive component of a radiopharmaceutical is referred to as a radionuclide. Nuclides are identified as atoms.

Technetium-99m generator

Radiation Chemistry2

Radionuclides The radioactive component of a radiopharmaceutical is referred to as a radionuclide.

Nuclides are identified as atoms having a specific number of protons and neutrons in the nucleus.

A nuclide is typically identified by the chemical symbol of the element with a mass number (mass number = #n+#p in nucleus) shown as a superscript, indicating the sum of protons and neutrons (e.g., iodine-

131 is indicated as I-131 ).

When the atom is radioactive, it is called a radionuclide.

Radioactive decay involves the spontaneous transformation of one element into another. The only way that this can happen is by changing the number of protons in the nucleus (an element is defined by its number of protons). There are a number of ways that this can happen and when it does, the atom is forever changed. There is no going back -- the process is irreversible.

Radioactive decay

Three Types of Radioactive DecayThere are three main types of radiation:Alpha radiationBeta radiationGamma radiation

Alpha Decay

The reason alpha decay occurs is because the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a Helium nucleus is emitted.

Beta (β) decay

Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron.

Beta decay involves a class of particles called ‘leptons’ whichinclude electrons (e-), positrons (e+).In β decay, there are 2 processes:

n → p + β- (β- decay)

p → n + β+ (β+ decay)

Gamma Decay

Gamma decay occurs because the nucleus is at too high an energy. The nucleus falls down to a lower energy state and, in the process, emits a high energy photon known as a gamma particle.

Radionuclides

Radionuclides undergo spontaneous radioactive decay accompanied by the release of energy.

X gamma X

X beta Y ( N e- P)

The halflife is the amount of time it takes for half of the atoms in a sample to decay. The halflife for a given isotope is always the same . it doesn't depend on how many atoms you have or on how long they've been sitting around.

For example, the halflife of technetium-99m is 6 hours. Let's say you start with, 16 mCi of Tc-99m. Wait 6 hours, and you'll have 8 mCi left; the rest will have been decayed. Another 6 hours go by, and you're left with 4 mCi ,another 6 hours more, and you have 2 mCi.

Half-lives of radionuclieds

a. The physical half-life (Tp) of a radiopharmaceutical is the amount of time necessary for the radioactive atoms to decay to one half their original number. Each radionuclide is characterized by a specific half-life that is a physical constant.

b. The biological half-life (Tb) of a radiopharmaceutical is the amount of time required for the body to metabolize or eliminate one half of the administered dose of any substance through biological processes.

c. The effective half-life (Te) of a radiopharmaceutical is the time required for an administered radiopharmaceutical dose to be reduced by one half as a result of both physical decay and biological mechanisms. It is defined as:

Te = Tp * Tb /Tp + Tb

where Te is the effective half-life, Tp is the physical half-life, and Tb is the biological half-life.

Half-lives of radionuclieds

A technetium-99m generator, or colloquially a technetium cow or moly cow, is a device used to extract the of technetium-99m (Tc-99m) isotope from a source of decaying molybdenum-99 (Mo-99)

Tc-99m, the most commonly used radionuclide in diagnostic imaging today is produced by the radioactive decay of molybdenum-99 ( Mo-99)

Mo-99 Tc-99m Tc-99

Tc-99m is obtained via commercially supplied, sterile, pyrogen-free generator systems.

Tc-99m decay Tc-99

Mo-99 decay Tc-99m

Mo-99 has a half-life of 66 hours ,and can be easily transported over long distances (from nuclear reactor) to hospitals where its decay product technetium-99m (with a half-life of only 6 hours, and so is inconvenient for transport) is extracted and used for a variety of nuclear medicine diagnostic procedures, where its short half-life is very useful.

A generator is a device used to separate a short half–life radionuclide from the longer-lived parent nuclide while retaining the parent to produce more of the daughter nuclide. In this way, short half–life nuclides can be made available continuously

The half-life of the parent nuclide (Mo-99) is much longer than that of the daughter nuclide (Tc-99m)

Inlet -- salineoutlet --- Tc-99m pertechnetate

The most frequently used method to obtaine Mo-99 requires a uranium target with high enriched uranium-235 (up to 90% of Ur-235) The target is irradiated with neutrons to form Mo-99 as a fission product.

Molybdenum-99 is then separated from other fission products in a hot cell.

Ur-235 + n fission in reactor Mo-99 + othersA large percentage of the 99mTc generated by a 99Mo/99mTc generator is used in the first 3 parent half lives, or approximately one week. Hence, clinical nuclear medicine units purchase at least one such generator per week

Mo-99 in the form of molybdate, is adsorbed onto acid alumina foam When the Mo-99 decays it forms Tc-99m pertechnetate Tc O4

-, which is less tightly bound to the alumina (unlike Mo-99, it is free) .

Pulling normal saline solution through the column of immobilized Mo-99 elutes the soluble Tc-99m, resulting in a saline solution containing the Tc-99m as the pertechnetate.

The solution of sodium pertechnetate may then be added to the organ-specific pharmaceutical to be used, or sodium pertechnetate can be used directly without pharmaceutical tagging as a pharmaceutical by itself .

PROPERTIES OF MOLYBDENUM-99 (Mo-99)

Half-Life : 66 hoursPrincipal Modes of Decay :Beta and Gamma

PROPERTIES OF TECHNETIUM-99m (Tc-99m)

Half-Life : 6 hoursPrincipal Modes of Decay : GammaEnergy : 140Kev

Tc-99m SODIUM PERTECHNETATE

Sodium Tc-99m pertechnetate (Tc-99m O4- ) is an eluted from the generator in 0.9% sodium chloride (NaCl ) solution

It is an isotonic, sterile non-pyrogenic, diagnostic radiopharmaceutical suitable for IV injection, oral administration, and direct instillation.

Physical properties

a. The solution should be clear, colourless, and free of visible foreign material . The pH is 4.5-7.0b. Tc - 99m O4- is a radiopharmaceutical by itself , and it may be used to radiolabel all other Tc-99m radiopharmaceuticals

Bio distribution

a. Tc-99mO4- is handled by the body in a fashion similar to

I-131 that is, it is taken up and released but not organified by the thyroid.

b. After IV administration, Tc-99mO4- concentrates in the thyroid gland, salivary gland, and stomach, but remains in the circulation long enough for first -pass blood-pool studies, and organ perfusion studies.

c. It is primarily excreted by the kidneys

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Half-Life Calculations

1-You have 400 mCi of a radioisotope with a half-life of 5 minutes. How much will be left after 10 minutes?

2-A radioisotope has a half-life of 1 hour. If you began with a 100 mCi sample of the element at noon, how much remains at 2 pm?

Answers to Half-Life Calculations

Half-Life Calculation #1 100 mCi

Half-Life Calculation #2 50 mCi

Thank you and Good Luck

Prof. Dr. Omar Shebl Zahra