גרעינית רפואה ברפואה פיסיקה ישומי
Atomic and Nuclear Physics
Interaction of photons and matter
Nuclear Medicine combines Physics and Medicine in a very
Nuclear Medicine uses non-invasive methods to image the
physiology of human body by detecting the radiation emitted
by radiopharmaceuticals inside the body.
Understanding how radiation is detected is important in order to
use optimally Nuclear Medicine detectors.
In the following hour we hope to describe in sufficient detail the
basics of the phenomena of the emission of radiation and its
Characterized by wavelength.
Wavelength related to frequency and energy:
1 ev = 1.6 x 10e-19 joules
1 kev = 1000ev ; 1 Mev = 1000000 ev
wavelength frequency Energy Comments
[m] [Hz] [eV]
3.0e+03 1.0e+05 6.6e-11 LF, MF
3.0e+00 1.0e+08 6.6e-08 VHF,UHF,FM
3.0e-03 1.0e+11 6.6e-05 m-wave,radar
3.0e-06 1.0e+14 6.6e-02 IR,Light, UV
3.0e-09 1.0e+17 6.6e+01 UV, X-ray
3.0e-12 1.0e+20 6.6e+04 X-ray,gamma
3.0e-15 1.0e+23 6.6e+07 gamma
The Atom can be divided into the nucleus and the electron
The electrons generate all chemistry (and biology) and are
in large responsible for interaction between radiation and
All radiation detectors are mainly based on the interaction
between radiation and the electron envelope.
The electrons are arranged in “layers” (or “shells”) each
with its binding energy.
(For each layer n there 2n-1 sublayers , and 2
electrons sit in each sublayer) .
The innermost layer has an index of 1 and is called the
K layer. The next layer is called L layer and so on.
The electrons tend to fill the atomic layers from the bottom up, i.e.
if a K shell electron is kicked out , an outer shell electron will move
to take its place, releasing energy. This energy can be released in
– Characteristic X-ray photons E=BK-BL
– Auger electrons E=BK-2BL
The nucleus is comprised of protons and neutrons:
– Z = number of protons
– N = number of neutrons
– A = Z + N , the total mass of the Nucleus
Z defines the number of electrons in an atom, therefore defining
which element it is.
The atom is noted by A
X is sufficient to define a nucleus.
For example Iodine 131:
The nucleus is kept together by the strong nuclear force, which is
active at short distances.
The protons and the neutrons in the nucleus can be arranged in
energy shells, not much different from the arrangement of electrons
The nucleus can be on:
# Excited state: Unstable, decays promptly to ground state
# Ground state: Stable
# Metastable state: Unstable decays slowly (lifetime > 10-12
Metastable nuclei are very important in nuclear medicine:
Tc is a metastable nucleus.
Isotopes are nuclei with the same number of protons ,
but different number of neutrons.
Many elements exist in nature with different number of
neutrons in the nucleus. Examples:238
U and 235
All isotopes of an element have the same chemical
Isotopes have different nuclear properties, i.e.. some are
ground state, some are in excited states etc..
Unstable isotopes will try to reach the ground state by emitting
There are 4 main types of radiation:
– alpha rays: nuclei of helium, 2 protons and 2 neutrons
– beta rays: electrons
– gamma rays: electromagnetic radiation of short wavelength
alpha rays: change the number of protons and neutrons by 2:
rays: turn a neutron into a proton: A
gamma rays: keep the same nuclear numbers, just the state change.
neutron emission is usually associated with fission, when a heavy
nucleus break into lighter parts.
Exist in an unstable state in
nature ( Z > 82 )
Produced by bombarding
stable nuclides with high-
Interaction of Radiation and Matter
Gamma and Beta rays interact with the electrons in the Atom
Alpha rays interact both with electrons and nuclei
Neutrons interact only with the nucleus
Atoms are either Ionized or excited by radiation
Interaction of Photons and Matter
Photons interact with matter through 3 processes:
– Photoelectric effect
– Compton Scattering
– Pair production
> 1.02 Mev !
Detectors are devices that translate radiation into recognizable
The signals can be electric, light or even visual
Ideal detector is:
– Linear in Energy
All radiation detectors work by the principle that radiation deposits
energy in matter.
Atoms are ionized and free negative charges (electrons) and
positive charges (cations or holes) are created.
3 types of detectors:
– Gas detectors : Geiger - Muller counter
– Solid State detectors
– Scintillation detectors
Scintillators produce light in presence of radiation
∗ Scintillators have to be:
– Generate light proportional to Energy
– Transparent (Low Absorption)
∗ Among the Scintillation detectors, NaI(Tl) is the most popular in NM.
NaI(Tl) - Thallium-activated sodium iodide crystal. The purpose of
thallium impurities to create activator centers to trap electrons
“kicked out” by gamma rays.
Drift to impurity center and
Holes and electrons recombine
producing excited atoms
Gamma Ray CollimatorCollimator
Pre - Amp
Emission of radiation
Absorption and Detection of Radiation
Transformation of Scintillator Light into electric pulses
We have all the ingredients needed to start with Nuclear Medicine
Radiopharmaceuticals are radioactive agents or drugs
used for diagnostic or therapeutic procedures.
Consist of two parts:
1. A radioactive substance to provide the signal.
2. A ligand that determines the molecule’s distribution in
To follow their absorption, distribution, metabolism, and
excretion through the use of detection device.
Gamma or x-ray emission with an energy between 60 and
Physical half - life between 1 hour and 1 year.
Almost ideal agent:
Tc-99m: 140 kev & 6.02 hour half life.
Easy to prepare.
Short half - life.
Pure gamma emitter.
Localization in only the tissue or organ desired.
No significant radiation exposure to critical organs.
Nuclide Half-Life Application
Ga 78 hr Tumor/infection imaging
Tl 73 hr Myocardial imaging
I 8 days Thyroid imaging & therapy
Tc 6 hr Nuclide for majority of
I 13 hr Thyroid imaging
Xe 5.2 days Ventilation imaging
In 68 hr Labeling white blood
Intensity of ionizing radiation,
The number of ions produced when
radiation passes through a specific
volume of air at a standard temperature
Exposure is measured in roentgens (R)
Radiation Absorbed Dose:
Measured the amount of energy that is
deposited per gram of substance.
1 Rad=1 erg/gram tissue
Radiation Dose Equivalent
accounts for the quality of
is a measurement of relative
biologic damage caused by a
specific type and energy of
1 Rem (Roentgen equivalent man)
= 1 Rad * Q.F (quality factor).
x-rays, gamma rays and beta
particles are assigned a quality
factor of 1.
Natural Exposure ( 300mRem/yr)
150 - 250 mRem/yr (W. Body)
Radiation worker - 5000 mRem/yr
1 chest X rays - 80 - 150 mRem
C.T - 1500 - 2000 mRem
W.B Bone Scan - 750 mRem,
ALARA - As Low As Reasonably
To reduce radiation exposure:
Whole Body Scan