Radiation%20and%20the%20Atom

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Radiation and the Atom

• Robert Metzger, Ph.D.

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Course Objectives

• Prepare for the Physics portion of the ABR
  Boards.
• Understand image formation and image quality for
  all standard imaging techniques.
• Assess patient dose and risks.
• Accrue the 200 hours required by NRC for
  licensing.

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Course Outline

• Approximately 100 hours of lecture, lab, and
  problem solving from July 2005 to June, 2006.
• Another 50 200 hours is acquired by reading
  Bushberg and Huda, and solving problems from
  Huda.
• The final 50 hours are from preparation for the
  boards using old exams. This occurs from July
  2006 to exam time in September.

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Course Structure

• Each Thursday we will meet for approximately 2
  hours. The first 1 to 1.5 hours will be lecture
  and the remainder will be taken up with problem
  solving and labs. When labs occur at other times
  (e.g. equipment surveys, image quality
  evaluations), the class time will be shortened
  accordingly.

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Course Structure

• As the class develops, these slide programs and
  worked problems will be posted on the web. If
  you miss a class you can obtain all of the
  information from the web (www.radsafe.com).
• Quarterly practice tests will be offered over the
  materials covered the previous quarter, with
  questions drawn from previous board exam
  questions.

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Textbooks

• Bushberg, J, Seibert, A, Leidholdt, E., Boone M.,
  The Essential Physics of Medical Imaging, 2nd
  Ed., Lippincott Williams, Wilkins, 2002.
• Huda, W., Slone, R., Review of Radiologic
  Physics, 2nd Ed., Lippincott Williams, Wilkins,
  2003.

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Example Problems

• RAPHEX Exams 2001 2004. 2001 to 2003 exams will
  be used for example problems while the 2004 exam
  will beused for the quarterly practice exams.
• Nickoloff, E., Radiology Review, Radiologic
  Physics, Elseveir Saunders, 2005.
• RSNA website (www.rsna.org).

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Fast Moving Technology

• Radiology is moving faster than the textbooks can
  keep up. Academics create the board exam
  questions and include areas not covered in the
  textbooks (e.g. radiobiology, fluoro safety,
  multislice CT, digital mammo, etc, etc, etc.
• RSNA has excellent tutorials in areas where there
  is insufficient treatment in the textbooks. Read
  them!

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ABR

• ABR has indicated that they are going to toughen
  up the physics requirements for Radiology
  Residents.
• They have already added questions in areas
  previously uncovered and indicate (see handout)
  that they intend to increase the passing
  percentage for the test.

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NRC/ARRA

• The NRC has established training requirements for
  Radiologist that read Nuclear Medicine or are
  Radiation Safety Officers (see 10 CFR 35).
• 200 hours of physics and a preceptor statement
  are required.
• Preceptors are hard to obtain the further you are
  from your residency. Many older radiologists are
  not able to be licensed as they never had their
  preceptor filled out and were listed on a
  license.

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NRC/ARRA

• One of the objectives of this course is to
  complete the 200 required hours of physics
  training and fill out the preceptor statement
  immediately.
• Try to get listed on the Radioactive Materials
  license the first place you work.
• Once licensed, the preceptor is no longer
  required. You are good for life!

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Basic Physics Review

• Describe the basic characteristics of
  electromagnetic (EM) radiation and how they are
  mathematically related
• Describe how atomic electronic structure
  determines the characteristics of emitted EM
  radiation
• Describe the various ways x-rays can interact
  with and are attenuated in matter Describe the
  energy dependence of these interactions
• Describe and calculate the various quantitative
  parameters used to characterize x-ray attenuation
  
• Differentiate between radiographic exposure
  absorbed dose and equivalent dose as well as use
  the correct radiological units

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Radiation

• Propagation of Energy Through Space or Matter
• Particulate Radiation Electrons, Protons, Alpha
  Particles, Beta Particles, Neutrons, etc.
• Electromagnetic Radiation. No Particle With
  Mass. See electromagnetic spectrum.
• Acoustic Radiation Ultrasound (reviewed later.

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Ionizing Radiation

• Radiation with sufficient energy to ionize human
  tissue. That is, it must impart enough energy
  to clip an electron off a water molecule and
  produce an ion pair (free electron positively
  charged nucleus. Requires about 10 to 20 eV/ion
  pair.
• Radiations that do not impart enough energy are
  called non-ionizing radiations. Both are used in
  medical imaging.

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Non-Ionizing Radiations

• MR Imaging (FM Region)
• Ultrasound.
• Microwave Diathermy.
• Lasers used for various treatments.
• Visible Light to read images.

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Characteristics of Waves

• Amplitude intensity of the wave
• Wavelength (l) distance between identical points
  on adjacent cycles m, nm (1 nm 10-9 m)
• Period (t) time required to complete one cycle
  (l) of a wave sec
• Frequency (n) number of periods

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Electromagnetic Radiation
Radiation consists of the transport of energy
through space as a combination of an electric
and magnetic field, both of which vary
sinusoidally as a function of space and time. The
fields are normally orthogonal.
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.19.
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Electromagnetic Waves

• Wave characteristics used to explain
  interference and diffraction phenomena c m/sec
  l m n 1/sec
• As c is essentially constant, then n 1/ l
  (inversely proportional)
• Wavelength often measured in nanometers (nm
  10-9 m) or Angstroms (Å 10-10 m, not an SI
  unit)
• Frequency measured in Hertz (Hz) 1 Hz 1/sec or
  sec-1

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.18.
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Particle Characteristics

• Particle characteristics when interacting with
  matter, high Electromagnetic radiation act as
  quanta of energy called photons E Joule hn
  hc/l, where h Plancks constant (6.62x10-34
  Joule-sec 4.13x10-18 keV-sec)
• When E expressed in keV and l in nm E
  keV 1.24/l nm 12.4/l Å

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An X-Ray

• Consider an X-ray with a frequency (n) of 1018
  cps.
• c m/sec l m n 1/sec
• l3 x 108 /1018 3 x 10-8 cm
• Convenient to think of it as a photon or a bullet
  with no mass.
• E keV 1.24/l nm
• Energy is 4.13 keV

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Particulate Radiation

• Corpuscular radiations are comprised of moving
  particles of matter and the energy of which is
  based on the mass and velocity of the particles
• Simplified Einstein mass-energy relationship E
  mc2
• Kinetic energy (KE) ½ mv2 (for
  non-relativistic velocities)

• The most significant particulate radiations of
  interest are
• Alpha particles a2
• Electrons e-
• Positron ß
• Negatrons ß-
• Protons p
• Neutrons n0
• Interactions with matter are collisional in
  nature and are governed by the conservation of
  energy (E) and momentum (p mv).

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Electronic Structure Electron Orbits

• Pauli exclusion principle
• No two electrons can have the same energy
• ? 2n2 electrons per shell
• quantum numbers
• n principal q.n. which e- shell
• l azimuthal angular momentum q.n. (l 0, 1,
  ... , n-1)
• ml magnetic q.n. orientation of the e-
  magnetic moment in a magnetic field (ml -l,
  -l1, ..., 0, ... l-1, l)
• ms spin q.n. direction of the e- spin (ms ½
  or -½)

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.21.
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Electronic Structure Electron Binding Energy
Eb ? Z2

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.22.
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Radiation from Electron Transitions

• Characteristic X-rays
• Auger Electrons and Fluorescent Yield (wK)
  (characteristic x-rays/total)
• Preference for Auger e- for low Z

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.23.
c.f. Sorenson, et al. Physics in Nuclear
Medicine, 1st ed., p.8.
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Excitation, Ionization and Radiative Losses

• Energetic charged particles interact via
  electrical forces
• Lose KE through excitation, ionization and
  radiative losses
• Excitation imparted E lt Eb ? emits ?? or Auger
  e- (de-excitation)
• Ionization imparted E gt Eb ? sometimes e- with
  enough KE to produce further ionizations
  (secondary ionizations)
• Such e- are called delta rays
• Approx. 70 of e- E deposition leads to
  non-ionizing excitation

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.32.
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Charged Particle Tracks

• e- follow tortuous paths through matter as the
  result of multiple Coulombic scattering processes
  
• An a2, due to its higher mass follows a more
  linear trajectory
• Path length actual distance the particle
  travels in matter
• Range effective linear penetration depth of the
  particle in matter
• Range path length

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.34.
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The Atomic Nucleus

• Covered in Nuclear Medicine course (August 2005)
• Composition of the Nucleus
• Protons and Neutron
• Number of protons Z
• Number of neutrons N
• Mass number A Z N
• Chemical symbol X
• Isotopes same Z, but different A
• Notation AZXN, but AX uniquely defines an
  isotope (also written as X-A) as X ? Z and N A
  - Z
• For example 131I or I-131

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Linear Energy Transfer (LET)

• Amount of energy deposited per unit length
  (eV/cm)
• LET ? q2/KE
• Describes the energy deposition density which
  largely determines the biologic consequence of
  radiation exposure
• High LET radiation a2, p, and other heavy ions
  
• Low LET radiation
• Electrons (e-, ß- and ß)
• Gamma X-Rays
• High LET gtgt damaging than low LET radiation

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Atomic Nucleus

• Consists of Protons and Neutrons held together by
  the exchange of pions amongst the nucleons
  (strong forces).
• The number of protons is the Atomic Number (Z).
• The total number of protons and neutrons is the
  mass number (A).

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Atomic Structure

• Nuclei with the same number of protons, but
  different number of neutrons are said to be
  isotopes.
• Isotopes of an element have the same electron
  structure (same No. of Protons)and therefore the
  same chemical properties.
• Isotopes of an element may have very different
  nuclear properties.
• Stability is governed by N and Z ratios.

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Radioactivity

• Process by which an unstable nucleus decays by
  one or more discreet steps until a stable state
  is reached.
• May occur in one step (e.g. 3H)
• Or may require many steps. 238U heads a chain of
  14 isotopes before stopping at stable 206Pb.

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Characteristics of RadiationAlpha Particles

• Essentially a helium nucleus (huge on an atomic
  scale)
• Low penetrating power
• Range in air lt 7 cm
• Range in tissue lt 40 ?m
• One cell diameter is 10 ?, so the alpha particle
  traverses less than 4 cell diameters.
• Linear Energy Transfer High

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Radiation CharacteristicsBeta Particles

• Electrons ejected from the nucleus of the
  radioactive atom.
• Range in air lt 1 meter
• Range in tissue lt 1 cm
• LET Low
• They can be an external skin hazard and an
  internal hazard.

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Radiation CharacteristicsGamma and X-Rays

• Electromagnetic (particle with no mass)
• Gammas originate in the nucleus.
• X-Rays are generated in the electron cloud
  through Bremmstrahlung.
• Once formed, they are identical.
• Range in air meters to kilometers.
• LET Low

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Radiation CharacteristicsNeutrons

• Only one neutron emitting isotope Cf-252
• Range in air meters to kilometers.
• Range in tissue centimeters.
• High LET note short range in tissue.
• Best shielding is hydrogenous materials such as
  polyethylene, wax, paraffin, and water.

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Neutrons

• Nuclear weapons run on neutrons.
• Fissile materials (e.g. Pu-239, Uranium enriched
  in U-235) fission when struck by a neutron.
  Fission produces two smaller nuclei, 1 to 3
  additional neutrons, and a release of a
  significant amount of energy.

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Radiation Characteristics
Type Alpha Beta Gamma Neutron
External No lt1 m Yes Yes
Internal Yes Yes Yes Cf-252
Skin No Yes Yes Cf-252
Pathway Yes Yes Yes Cf-252
LET High Low Low High
Shield Paper Plastic Lead Water
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Radiation Units

• Activity Curie 3.7 x 1010 radioactive
  disintegrations per second or about 1 gram
  radium.
• Becquerel- one disintegration per second. This
  is the SI Unit. Required on all shipping
  documents. The Ci quantity can be listed in
  parentheses.

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Radiation Units

• Radiation Exposure
• Roentgen 2.58 x 10-4 Coulombs/kg air.
• Charge liberated in air.
• Defined only for gamma and x-ray

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Radiation Units

• Radiation Absorbed Dose
• rad 0.01 Joules/kg
• Energy Absorbed per gram.
• Defined for all radiations all tissue

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Radiation Units

• Dose Equivalent
• rem Dose (rads) x Quality Factor
• Quality Factor
• Unitless Number
• Adjusts for higher risk associated with densely
  ionizing radiations.
• Quality Factor 1 for gamma and x-rays.
• Therefore rad rem for photons.

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Radiation Units

• Quality Factor Related to the Relative
  Biological Effectiveness (RBE)
• RBE Energy Required to Produce an Effect
  Energy Required to Produce Same Effect with
  200 keV X-Rays

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Radiation Units

• Relative Biological Effectiveness
• Varies with End Point and Tissue Type
• Quality Factor is Average RBE

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Quality Factors
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SI Radiation Units
SI Historical
Exposure C / kg air Roentgen ( R )
Absorbed Dose Gray ( Gy) 100 rad rad
Dose Equivalent Sievert (Sv) 100 rem Dose (Gy) x QF rem
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Units Concepts

• Exposure
• Roentgen
• Charge liberated in air
• Dose
• Rad or Gray
• Energy absorbed per kilogram
• Dose Equivalent
• rem or Sievert
• Dose modified to reflect increased risk from
  densely ionizing radiations.
• The risk based unit. Standards are written in
  dose equivalent.

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Common Doses

• Chest Film - 40 mrem/study
• Mammogram - 240 mrem/study
• KUB - 800 mR ESE/film
• Dental Bitewing 100 - 400 mR/film
• Fluoroscopy - 3 R/min ESE
• CT - 3 -5 rads per study
• MRI - None - not ionizing radiation

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Annual Dose Equivalent360 mrem
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Annual Dose - Other 1

• Occupational Dose 0.3
• Fallout - lt0.3
• Nuclear Fuel Cycle 0.1
• Miscellaneous 0.1
• Natural Sources Account for 82 of total annual
  dose with only 18 coming from man made sources.