Radiation Chemistry
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., iodine131 is indicated as I-131 ).
When the atom is radioactive, it is called a radionuclide.
Radioactive decay
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.
Three Types of Radioactive Decay
There are three main types of radiation:
Alpha radiation
Beta radiation
Gamma 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’ which
include 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
accompanied by the release of energy.
X
gamma
X
X
beta
Y
( N e-
P)
radioactive
decay
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:
Half-lives of radionuclieds
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.
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)
Tc-99m is obtained via commercially supplied, sterile, pyrogen-free
generator systems.
Mo-99
decay
Tc-99m
Tc-99m
decay
Tc-99
Mo-99
Tc-99m
Tc-99
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
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.
The half-life of the parent nuclide (Mo-99) is much longer than that of the
daughter nuclide (Tc-99m)
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 + others
A 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 Tc99m as the pertechnetate.
The solution of sodium pertechnetate may then be added to the organspecific 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 hours
Principal Modes of Decay :Beta and Gamma
PROPERTIES OF TECHNETIUM-99m (Tc-99m)
Half-Life : 6 hours
Principal Modes of Decay : Gamma
Energy : 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 4.5-7.0
b. 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
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?
30
Answers to Half-Life Calculations
Half-Life Calculation #1
100 mCi
Half-Life Calculation #2
50 mCi
Prof. Dr. Omar Shebl Zahra