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Title: Radiation%20and%20the%20Atom


1
Radiation and the Atom
  • Robert Metzger, Ph.D.

2
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.

3
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.

4
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.

5
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.

6
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.

7
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).

8
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!

9
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.

10
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.

11
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!

12
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

13
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.

14
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.

15
Non-Ionizing Radiations
  • MR Imaging (FM Region)
  • Ultrasound.
  • Microwave Diathermy.
  • Lasers used for various treatments.
  • Visible Light to read images.

16
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

17
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.
18
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.
19
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

20
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

21
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).

22
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.
23
Electronic Structure Electron Binding Energy
Eb ? Z2

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.22.
24
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.
25
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.
26
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.
27
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

28
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

29
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).

30
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.

31
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.

32
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

33
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.

34
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

35
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.

36
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.

37
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
38
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.

39
Radiation Units
  • Radiation Exposure
  • Roentgen 2.58 x 10-4 Coulombs/kg air.
  • Charge liberated in air.
  • Defined only for gamma and x-ray

40
Radiation Units
  • Radiation Absorbed Dose
  • rad 0.01 Joules/kg
  • Energy Absorbed per gram.
  • Defined for all radiations all tissue

41
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.

42
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

43
Radiation Units
  • Relative Biological Effectiveness
  • Varies with End Point and Tissue Type
  • Quality Factor is Average RBE

44
Quality Factors
45
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
46
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.

47
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

48
Annual Dose Equivalent360 mrem
49
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.
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