ENTC 4390 MEDICAL IMAGING

1
ENTC 4390MEDICAL IMAGING

• STRUCTURE OF MATTER

2

• Since the late 1920s it has been understood that
  electrons in an atom do not behave exactly like
  tiny moons orbiting a planet-like nucleus.
• Their behavior Is described more accurately if,
  instead of defining them as point particles in
  orbits with specific velocities and positions,
  they are defined as entities whose behavior is
  described by wave functions.

3

• While a wave function itself is not directly
  observable, calculations may be performed with
  this function to predict the location of the
  electron.
• In contrast to the calculations of classical
  mechanics in which properties such as force,
  mass, acceleration, and so on, are entered into
  equations to yield a definite answer for a
  quantity such as position in space, quantum
  mechanical calculations yield probabilities.

4

• At a particular location in space, for example,
  the square of the amplitude of a particles wave
  function yields the probability that the particle
  will appear at that location.
• However, it is important to emphasize that the
  probability of finding the electron at other
  locations, even in the middle of the nucleus, is
  not zero.
• This particular result explains a certain form of
  radioactive decay in which a nucleus captures an
  electron.
• This event is not explainable by classical
  mechanics, but can be explained with quantum
  mechanics.

5
Atomic Theory

• An atom consists of a nucleus of protons and
  neutrons surrounded by a group of orbiting
  electrons.
• Electrons have a negative charge, protons have a
  positive charge.
• In its normal state, each atom has an equal
  number of electrons and protons.

6
(No Transcript)
7
Atomic Theory

• Electrons orbit the nucleus in discrete orbits
  called shells.
• These shells are designated by letters K, L, M,
  N, etc.
• Only certain numbers of electrons can exist
  within any given shell.

8
Atomic Theory

• The outermost shell of an atom is called the
  valence shell.
• The electrons in this shell are called valence
  electrons.
• No element can have more than eight valence
  electrons.
• The number of valence electrons affects its
  electrical properties.

9

• The binding energy of an electron (Eb) is defined
  as the energy required to completely separate the
  electron from the atom.
• When energy is measured in the macroscopic world
  of everyday experience, units such as joules and
  kilowatt-hours are used.
• In the microscopic world, the electron-volt is a
  more convenient unit of energy
• One electron volt is the kinetic energy imparted
  to an electron accelerated across a potential
  difference (i.e., voltage) of 1 Volt.

10

• The electron volt can be convened to other units
  of energy
• 1eV 1.6 x 109J
• 1.6 x 1012 erg
• 4.4)lt 1026 kW-hr
• Nott 103eV 1 keV
• lO6eV lMc\

11

• The electron volt describes potential as well as
  kintnic energy. The binding energy of
• an electron in an atom is a form oF potential
  energy

12

• An electron in an inner shell of an atom is
  attracted to the nucleus by a force greater than
  that exerted by the nucleus on an electron
  farther away.
• An electron may be moved from one shell to
  another shell that is farther from the nucleus
  only if energy is supplied by an external source.
  
• Binding energy is negative (i.e., written with a
  minus sign) because it represents an amount of
  energy that must be supplied to remove an
  electron from an atom. The energy that must be
  imparted to an atom to move an electron from an
  inner shell to an outer shell is equal to the
  arithmetic difference in binding energy between
  the two shells.

13

• For example, the binding energy is ?13.5 eV for
  an electron in the K shell of hydrogen and is ?
  3.4 eV for an electron in the L shell.
• The energy required to move an electron from the
  K to the L shell in hydrogen is (?3.4 eV) ?
  (?13.5 eV) 10.1 eV

14

• Electrons in inner shells of high-Z atoms are
  near a nucleus with high positive charge.
• These electrons are bound to the nucleus with a
  force much greater than that exerted upon the
  solitary electron in hydrogen.
• All of the electrons within a particular electron
  shell do not have exactly the same binding energy
  
• Differences in binding energy among the electrons
  in a particular shell are described by the
  orbital, magnetic, and spin quantum numbers,

15

• The combinations of these quantum numbers allowed
  by quantum mechanics provide
• three subshells (LI to LIII) for the L shell and
• five subshells (MI to Mv) for the M shell
• the M subshells occur only if a magnetic field is
  present.
• Energy differences between the subshells are
  small when compared with differences between
  shells.
• These differences are important in radiology
  however, because they explain certain properties
  of the emission spectra of x-ray tubes.

16

• Various processes can cause an electron to be
  ejected from an electron shell.
• When an electron is removed from a shell, a
  vacancy or hole is left in the shell
• (i.e., a quantum address is left vacant.
• An electron may move from one shell to another to
  fill the vacancy
• This movement, termed an electron transition,
  involves a

17
Conductors

• Materials that have large numbers of free
  electrons are called conductors.
• Metals are generally good conductors because they
  have few loosely bound valence electrons.
• Silver, gold, copper, and aluminum are excellent
  conductors.

18
Insulators

• Materials that do not conduct because their
  valence shells are full or almost full are called
  insulators.
• Glass, porcelain, plastic, and rubber are good
  insulators.
• If high enough voltage is applied, an insulator
  will break down and conduct.

19
Semiconductors

• Semiconductors have half-filled valence shells
  and are neither good conductors nor good
  insulators.
• Silicon and germanium are good semiconductors.
• They are used to make transistors, diodes, and
  integrated circuits.

20
Electrical Charge

• Objects become charged when they have an excess
  or deficiency of electrons.
• An example is static electricity.
• The unit of charge is the coulomb.
• 1 coulomb 6.24 1024 electrons.

21
ENTC 4390

• THE NUCLEUS

22
Nucleons

• A nucleus consists of two types of particles,
  referred to collectively as nucleons.
• The positive charge and roughly half the mass of
  the nucleus are contributed by protons.
• The second type of nucleon is the neutron.

23
Protons

• Each proton possesses a positive charge of 1.6 x
  10-19 coulombs.
• equal to in magnitude and opposite in sign to the
  charge of an electron.
• The number of protons in nucleus is the atomic
  number of the atom.
• The mass of a proton is 1.6734 x 10-27 kg.

24
Neutrons

• Neutrons are uncharged particles with a mass of
  1.6747 x 10-27 kg.
• Outside the nucleus, neutrons are unstable and
  divide into protons, electrons, and
  antineurtrinos.
• The number of neutrons in in a nucleus is the
  neutron number N for the nucleus.

25

• The mass number
• A Z N
• The standard form used to denote the composition
  of a specific nucleus is
• where X is the chemical symbol.

26
Isotopes

• Isotopes are atoms that possess the same number
  of protons but a varying number of neutrons.
• Isotopes of hydrogen are
• 1Hprotium
• 2Hdeuterium
• 3Htritium

27
ENTC 4390

• NUCLEAR FISSION FUSION

28
Nuclear Power

• Nuclear power may be produced in two ways
• Nuclear fission involves the splitting of an atom
  into two fragments, particles, and the release of
  energy
• Nuclear fusion involves the combination of two
  nuclei into a single, more massive nuclei, plus
  energy
• Stars are powered by nuclear fusion

29
Nuclear Fusion

• Nuclear Fusion has been used since the early
  1950s in Hydrogen bombs
• These are the most powerful type of nuclear
  weapon
• We have not yet devised a method of utilizing the
  power of nuclear fusion in the laboratory, nor in
  any commercial reactor
• Therefore, we will not further consider fusion in
  this course

30
Nuclear Fission

• Fission induced by neutron bombardment and capture

31
Fission Diagram

• When a heavy nucleus undergoes fission, a variety
  of fragment pairs may be formed, depending on the
  distribution of neutrons and protons between the
  fragments

32
Fission Yield

• This leads to probability distribution of both
  mass and nuclear charge for the fragments
• The probability of formation of a particular
  fragment is called its fission yield and is
  expressed as the percentage of fissions leading
  to it

33
Fission Products

• A fission product is any of the lighter atomic
  nuclei formed by splitting heavier nuclei
  (nuclear fission), including both the primary
  nuclei directly produced (fission fragments) and
  the nuclei subsequently generated by their
  radioactive decay

34
Fission Fragment Decay

• Fission fragments are highly unstable because of
  their abnormally large number of neutrons
  compared with protons
• Consequently, they undergo successive radioactive
  decays by emitting neutrons, by converting
  neutrons into protons, antineutrinos, and ejected
  electrons (beta decay), and by radiating energy
  (gamma decay)

35
Fission of 235U

• One of the many known fission reactions of
  uranium-235 induced by absorbing a neutron
  results, for example, in two extremely unstable
  fission fragments, a barium and a krypton nucleus
• These fragments almost instantaneously release
  three neutrons between themselves, becoming
  barium-144 and krypton-89

36
Barium Decay

• By repeated beta decay, the barium-144 in turn is
  converted step by step to other fission products
• Lanthanum-144
• Cerium-144
• Praseodymium-144
• Eventually relatively stable neodymium-144

37
Krypton

• krypton-89 is similarly transformed by repeated
  beta decay to
• Rubidium-89
• Strontium-89
• To stable yttrium-89

38
Fission Product Identification

• Fission products are identified by their chemical
  properties and by their radioactive properties,
  such as their half-lives and the kinds of
  particles they emit
• The multiple decays mean fission products are
  highly radioactive and therefore quite dangerous

39
Why are Fission Products Radioactive?

• To maintain stability, the neutron-to-proton
  (n/p) ratio in nuclei must increase with
  increasing proton number
• The ratio remains at unity up to the element
  calcium, with 20 protons
• It then gradually increases until it reaches a
  value of about 1.5 for the heaviest elements

40
(No Transcript)