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Basic physics of nuclear medicine
Basic physics of nuclear
medicine
Nuclear structure
• Atomic number (Z): the number of protons in
a nucleus; defines the position of an
element in the periodic table.
• Mass number (A) is the number of nucleons
in a nucleus
Binding Energy
• The stability of the nucleus is explained by
the presence of strong binding force
(nuclear force) that outcomes the repulsive
forces of protons
• Nuclear force is equal among all nucleons
and exists only in the nucleus having no
influence outside the nucleus
Nuclear nomenclature
Nuclide: an atomic species with a definite
number of protons and neutrons
Radionuclide: unstable nuclide that
decays by emission of particles or by
electromagnetic radiation
Nuclear nomenclature
Isotope: nuclides having same atomic
number but different mass number.
Example: 116C, 12
6C, 136C
Isotones: nuclides having same number of
neutrons but different number of protons,
example: 13455Cs, 133
54Xe,13253I
Nuclear nomenclature
Isobars: nuclides with the same number of
nucleons; that is the same mass number but
different combination of neutrons and protons.
Example: 82Y, 82Sr, 82Rb, 82Kr.
Isomers: nuclides with the same number of
protons and neutrons but different energy states
(99Tc and 99mTc); the excited state of a nuclide is
called the isomeric state; when the isomeric state
is long lived it is called a metastable state and
denoted with “m”
Radioactivity
• There are about 2,450 known isotopes of the elements
in the Periodic Table
• The unstable isotopes lie above or below the Nuclear
Stability Curve
• These unstable isotopes attempt to reach the stability
curve by splitting into fragments (fission) or by emitting
particles and/or energy (radiation)
Radioactivity
When people like Henri Becquerel and Marie Curie were
working initially on these strange emanations from
certain natural materials it was thought that the
radiations were somehow related to another
phenomenon which also was not well understood at the
time - that of radio communication. It seems reasonable
on this basis to appreciate that some people considered
that the two phenomena were somehow related and
hence that the materials which emitted radiation were
termed radio-active
Radioactive decay
Beta minus decay
Certain nuclei which have an excess of neutrons
may attempt to reach stability by converting a
neutron into a proton with the emission of an
electron. The electron is called a beta minus particle.
Radioactive decay
Beta minus decay
Certain nuclei which have an excess of neutrons
may attempt to reach stability by converting a
neutron into a proton with the emission of an
electron. The electron is called a beta minus particle.
Radioactive decay
Beta plus decay
When the number of protons in a nucleus is too
large for the nucleus to be stable it may attempt to
reach stability by converting a proton into a neutron
with the emission of a positively-charged electron
Radioactive decay
Beta plus decay
When the number of protons in a nucleus is too
large for the nucleus to be stable it may attempt to
reach stability by converting a proton into a neutron
with the emission of a positively-charged electron
Radioactive decay
Electron Capture
An inner orbiting electron is attracted into an
unstable nucleus where it combines with a proton to
form a neutron, the vacant site left in the K-shell is
filled by an electron from an outer shell. The filling of
the vacancy is associated with the emission of X ray
Radioactive decay
Electron Capture
An inner orbiting electron is attracted into an
unstable nucleus where it combines with a proton to
form a neutron, the vacant site left in the K-shell is
filled by an electron from an outer shell. The filling of
the vacancy is associated with the emission of X ray
Gamma Rays
• The energies of γ-rays emitted from a radioactive source are always distinct. For example:
• 99mTc (Technetium 99m) emits γ-rays which have an energy of 140 keV.
• 51Cr (Chromium-51) emits γ-rays which have an energy of 320 keV.
• The effects described here are also of relevance to the interaction of X-rays with matter since as we have noted before X-rays and γ-rays are essentially the same entities.
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Radioactive decay
Gamma Decay
Gamma decay involves the emission of energy
from an unstable nucleus in the form of
electromagnetic radiation
Radioactive decay
Gamma Decay
X rays and gamma rays are high energy
electromagnetic rays and are therefore
virtually the same.
Radioactive decay
Gamma Decay
The difference between them is not what they
consist of but where they come from.
Radioactive decay
Gamma Decay
In general we can say that if the radiation
emerges from a nucleus it is called a gamma-
ray and if it emerges from outside the nucleus
it is called an X-ray
Radioactive decay
There are two common forms of gamma decay
(a) Isomeric Transition
(b) Internal Conversion
Radioactive decay
There are two common forms of gamma decay
(a) Isomeric Transition
A nucleus in an excited state may reach its ground or unexcited state
by the emission of a gamma-ray
99mTc →99Tc + γ
(b) Internal Conversion
Here the excess energy of an excited nucleus is given to an atomic
electron, e.g. a K-shell electron.
The ejected electron is called the conversion electron.
This is followed by the emission of characteristic X ray or by
emission of an orbital electron (Auger electron)
Interaction of Radiation with Matter
Interaction of Radiation with Matter
Photoelectric effect
Gamma-ray collides with an orbital electron of an atom
of the material through which it is passing it can
transfer all its energy to the electron. Gamma-ray energy
is totally absorbed in the process.
Interaction of Radiation with Matter
Photoelectric effect
Occurs primarily at low energy range
Its occurrence increases with increasing atomic number
of the absorbing crystal
Photoelectric Effect• When a γ-ray collides with an orbital electron of an
atom of the material through which it is passing it can transfer all its energy to the electron and thus cease to exist.
• On the basis of the Principle of Conservation of Energy we can deduce that the electron will leave the atom with a kinetic energy given by:
kinetic energy = energy of the γ-ray - orbital binding energy
• The resulting electron is called a photoelectron.
• The following phenomena are of importance:• An ion results when the photoelectron leaves the atom.
• The γ-ray energy is totally absorbed in the process.
• X-ray emission can occur when the vacancy left by thephotoelectron is filled by an electron from an outer shellof the atom (electron capture).
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Interaction of Radiation with Matter
Compton Effect
Gamma-ray transfers only part of its energy to a
valance electron which is essentially free
Compton Effect (Scattering)
• Here a γ-ray transfers only part of its energy
to a valance electron which is almost free.
• The electron leaves the atom and may act
like a β-particle
• The γ-ray deflects off in a different direction
to that with which it approached the atom.
• This deflected or scattered γ-ray can
undergo further Compton scatterings within
the material.
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Attenuation of Gamma-Rays
• The photoelectric and the Compton effects
give rise to both absorption and scattering
of the radiation beam.
• The overall effect is referred to as
attenuation of γ-rays.
• Remember: γ-rays and X-rays are
essentially the same physical entities.
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Specific Gamma Ray
Constant (G)• It is defined as the exposure rate per unit
activity at a certain distance from a source.
• SI units:
C∙kg-1∙s-1∙Bq-1 (at 1 m)
• Traditional units:
R∙h-1∙mCi-1 (at 1 cm)
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Specific Gamma Ray Constant G(mSv∙h-1∙GBq-1 at 1 m)
γ-Ray ConstantNuclide
0.004241Am
0.012201Tl
0.01657Co
0.01799mTc
0.04199Mo
0.057131I
0.084111In
0.087137Cs
0.36060Co 31
Specific Gamma Ray Constant and Dose
• Given that an object at distance (d) m
away from the source, and that the source
activity is (A) Bq, one can compute the
dose (D) in Sv/h as follows:
2d
AGD
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Specific Gamma Ray Constant and Dose
• Given that an object at distance (d) m
away from the source, and that the source
activity is (A) Bq, one can compute the
dose (D) in Sv/h as follows:
If you know that Gamma Ray Constant of 99mTc
= 0.017, and its activity = 1.7 x 10-5 curies,
calculate its dose in Sv/h at 1 m from the
source.
2d
AGD
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Nuclear Medicine Scans• In a nuclear medicine scan, a radiopharmaceutical is
administered to the patient, and an imaging instrument that detects radiation is used to show biochemical changes in the body.
• Nuclear medicine imaging, in contrast to imaging techniques that mainly show anatomy (e.g., conventional ultrasound, computed tomography [CT], or magnetic resonance imaging [MRI])*, can provide important quantitative functional information about normal tissues or disease conditions in living subjects.
* Exceptionally with the emergence of advanced (functional) MRI methods the pure anatomical role of these traditional imaging techniques is slowly reaching an end.
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• Because human senses cannot sense radiation, instruments that detect radiation are essential tools.
• After a nuclear disaster detecting radiation becomes particularly invaluable, as high levels of radiation can become hazardous to life.
• Regular monitoring while usingradioactive substances is critical to thesafety of personnel.
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• Detection of radioactivity is necessary
to ascertain their
• presence and
• Intensity
• Detection indirect (based on the effects
of radioactivity)
• Darkening of photographic plates
• Ionization of atoms
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• Next Lecturer
• Ch 7 & 8.
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