Atomic and Nuclear Physics

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• Introduction
• Atomic and Nuclear Physics
• Radioactivity
• Interaction of photons and matter
• Photon Detectors
• Radiopharmaceuticals
• Radiation Safety
• Study Types
• Gamma Cameras

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Introduction

• Nuclear Medicine combines Physics and Medicine in
  a very strong way.
• 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 detection.

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Electromagnetic Waves

• 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
• 3.0e03 1.0e05 6.6e-11 LF,
  MF
• 3.0e00 1.0e08 6.6e-08
  VHF,UHF,FM
• 3.0e-03 1.0e11 6.6e-05
  m-wave,radar
• 3.0e-06 1.0e14 6.6e-02
  IR,Light, UV
• 3.0e-09 1.0e17 6.6e01 UV,
  X-ray
• 3.0e-12 1.0e20 6.6e04
  X-ray,gamma
• 3.0e-15 1.0e23 6.6e07
  gamma

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Atomic Physics
Nucleus
Electrons

• The Atom can be divided into the nucleus and the
  electron envelope.
• The electrons generate all chemistry (and
  biology) and are in large responsible for
  interaction between radiation and matter.
• All radiation detectors are mainly based on the
  interaction between radiation and the electron
  envelope.

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Atomic Physics

• 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.

Free Electrons
4-5 keV
L
33.2 keV
Binding Energy
K
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Atomic Emissions

• 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 two ways
• Characteristic X-ray photons EBK-BL
• Auger electrons EBK-2BL

Auger Electron
K-shell Vacancy
Nucleus
K
Characteristic X-ray
L
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• Ionization
• Exitation

kev
800
400
300
200
100
400 kev gamma
Ground state
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The Nucleus

• 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 AZxN.
• AX is sufficient to define a nucleus.
• For example Iodine 131
• 13153I78 131I

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The Nucleus

• 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 in
  atoms.
• The nucleus can be on
• Excited state Unstable, decays promptly to
  ground state
• Ground state Stable
• Metastable state Unstable decays slowly
  (lifetime gt 10-12 sec) to ground state
• Metastable nuclei are very important in nuclear
  medicine
• 99mTc is a metastable nucleus.

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Isotopes

• 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 238U
  and 235U, 36Cl and 35Cl.
• All isotopes of an element have the same
  chemical characteristics.
• Isotopes have different nuclear properties, i.e..
  some are ground state, some are in excited
  states etc..

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Radioactivity

• Unstable isotopes will try to reach the ground
  state by emitting radiation
• 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
• neutrons
• alpha rays change the number of protons and
  neutrons by 2
• AZXN A-4Z-2YN-2
• beta- rays turn a neutron into a proton AZXN
  AZ1YN-1
• 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.

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Radionuclides

• Natural
• Exist in an unstable state in nature ( Z gt 82 )
• Artificial
• Produced by bombarding stable nuclides with
  high-energy particles.

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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

Ionization
Excitation
EM Radiation

Nucleus
K
L
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Interaction of Photons and Matter

• Photons interact with matter through 3
  processes
• Photoelectric effect
• Compton Scattering
• Pair production

Electron
Nucleus

Photoelectric
Photon
K
L
Electron
Compton
Photon
Anti-Electron
Photon
Pair Production
gt 1.02 Mev !
Electron
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Radiation Detectors

• Detectors are devices that translate radiation
  into recognizable signals
• The signals can be electric, light or even visual
  
• Ideal detector is
• Fast
• Precise
• Linear in Energy
• Efficient
• 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

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Scintillation Detectors

• Scintillators produce light in presence of
  radiation
• Scintillators have to be
• Efficient
• Generate light proportional to Energy
• Transparent (Low Absorption)
• Fast
• 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.

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Scintillators

• NaI(Tl) scintillation

radiation
holes
electrons
Free
Drift to impurity center and Ionize it
Holes and electrons recombine producing excited
atoms
Light radiation
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Amp
Pre - Amp
Signal
Anode
High Voltage Supply
Dynodes
Dynodes
Electrons
Light
Photocathode
NaI(Tl) Crystal
Collimator
Gamma Ray
Collimator
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Summary

• Emission of radiation
• Absorption and Detection of Radiation
• Scintillation Detectors
• Transformation of Scintillator Light into
  electric pulses
• We have all the ingredients needed to start with
  Nuclear Medicine devices

Gamma Camera
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Radiopharmaceuticals

• 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 molecules
  distribution in the body.
• Purpose
• To follow their absorption, distribution,
  metabolism, and excretion through the use of
  detection device.

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Physical Properties

• Gamma or x-ray emission with an energy between 60
  and 400 kev
• Physical half - life between 1 hour and 1 year.
• Almost ideal agent
• Tc-99m 140 kev 6.02 hour half life.

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Radiopharmaceuticals

• Ideal properties
• Readily available.
• Easy to prepare.
• Short half - life.
• Pure gamma emitter.
• Localization in only the tissue or organ desired.
• No significant radiation exposure to critical
  organs.

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Common Radionuclides

• Nuclide Half-Life Application
• 67Ga 78 hr Tumor/infection imaging
• 201Tl 73 hr Myocardial imaging
• 131I 8 days Thyroid imaging therapy
• 99mTc 6 hr Nuclide for majority of
  radionuclide imaging
• 123I 13 hr Thyroid imaging
• 133Xe 5.2 days Ventilation imaging
• 111In 68 hr Labeling white blood cells,
  antibodies

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Radiation Safety

• Radiation Exposure
• Intensity of ionizing radiation,
• The number of ions produced when radiation passes
  through a specific volume of air at a standard
  temperature and pressure.
• Exposure is measured in roentgens (R)
• Radiation Absorbed Dose
• Measured the amount of energy that is deposited
  per gram of substance.
• 1 Rad1 erg/gram tissue
• 1Gy100 Rads

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Radiation Safety

• Radiation Dose Equivalent
• accounts for the quality of radiation.
• Quality Factor
• is a measurement of relative biologic damage
  caused by a specific type and energy of
  radiation.
• 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.

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Radiation Protection

• 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 Achievable.
• To reduce radiation exposure
• Time
• Distance
• Shielding

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Study Types

• Static Imaging
• Dynamic Imaging
• Whole Body Scan
• Gated Imaging
• SPECT Imaging
• PET Imaging
• TET Imaging

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APEX 409
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