BME 560
Medical Imaging: X-ray, CT, and
Nuclear Methods
Radiation Physics Part 1
Today
• Forms of Radiation
• Particulate Radiation
– Electron interactions with matter
• EM Radiation
• Nuclear Transitions
• Decay
What is Radiation?
• Radiation: Energy in the form of traveling
waves or subatomic particles moving through
space
– May or may not have mass
– May or may not have charge
Types of Radiation
• Ionizing: capable of producing an ion pair
from interaction with an atom
– Usually with energy > 13.6 eV
• Non-ionizing: incapable of producing an ion
pair
• Ion pairs are reactive and may cause biological
damage
Radiation
Forms of Radiation
• Particulate: particle-based, has mass
– Protons, neutrons, electrons, positrons, alphas
• Electromagnetic: no mass
– Gamma rays, X-rays, Ultraviolet
• Examples:
– X-ray tubes
– PET radionuclides
Ionization and Excitation
Ejected electron (Ee)
N
Radiation
(E)
K L M
• Both transfer energy to an orbital electron
– Let Eb be the electron binding energy
– Ionization: Energy is enough to remove electron from orbit
– Excitation: Energy is enough to transition electron to higher
orbital
– In both cases, the hole will be filled and energy released.
Particulate Radiation
• Alpha particle: He nucleus (2 protons, 2
neutrons)
• Beta particle: Electron (b-) or positron (b+)
• Proton
• Neutron
Particulate Radiation
• Properties:
– Alpha: very high mass, highly charged, very
highly ionizing, very low penetration
– Beta: low mass, charged, highly ionizing, low
penetration
– Proton: high mass, charged, highly ionizing, low
penetration
– Neutron: high mass, no charge, less ionizing,
unusual penetration properties
• In all cases, penetration depends on energy.
Particulate Radiation
• Medical uses:
– Alpha: Radiotherapy
– Beta: X-ray production, PET imaging,
Radiotherapy
– Proton: Radiotherapy
– Neutron: Radiotherapy
• So we are primarily interested in electrons and
positrons for imaging.
Energetic Electron Energy Transfer
• How electrons lose energy to a medium:
– Collisional transfer: Interaction between particle
and orbital electrons
• High-energy: Ionization
• Low-energy: Heat
• Energetic electron loses kinetic energy
– Radiative Transfer: Energy transfer results in
production of X-ray
• Characteristic X-rays
• Bremsstrahlung radiation
Energetic electron energy transfer
Energy loss rate vs. electron energy in water and lead
207
82
diagnostic
therapy
Pb
Characteristic X-rays
• Electron moves from higher energy level to a
lower energy level.
• Since the orbital energy levels are well defined
for individual elements, each element has its
own characteristic transitions.
– These are very specific energy levels.
• Usually, it is the transitions into the K shell
that are important.
Characteristic X-rays
Bremsstrahlung
• “Braking radiation”
• The effect of charge-charge interaction between an
energetic electron and an atomic nucleus
• The electron slows and releases energy in the form of
an X-ray photon.
• Bremsstrahlung X-rays may have any energy up to
the energy of the incident electrons.
“White” bremsstrahlung x-rays
Filtered x-rays
Bremsstrahlung production
Efficiency = 9 x 10-10 Z (atomic number)V(voltage)
(This is an approximation.)
Efficiency: the ratio of the Bremsstrahlung x-ray energy to the incident
electron energy.
The remaining portion of the electron energy (1 - Efficiency) is converted into
heat in the x-ray target. Anode heating is a major issue in x-ray tubes.
Exercise:
Calculate the efficiency for x-ray production for 100keV electron beams on
tungsten (Z = 74).

Electron energy, mass and velocity
E  mc
m

m0
1  /c
Kinetic energy (KE)
2
2
2
(c : speedof
light)
KE  E  E 0
Exercises:
What is the rest energy of an electron? (mass = 9.11 x 10-28 g)
What is the rest energy of a proton? (mass = 1.67 x 10-24 g)

EM Radiation
• Electromagnetic radiation is considered in the
form of photons.
– No charge, no mass
– The wavelengths of interest are much smaller than
the scales of typical interactions, so we are safe to
treat this radiation as “particles” rather than
“waves”.
EM Radiation
• X-rays versus gamma-rays (or gamma
particles)
– Physically, they are the same thing.
– X-rays: Produced by energetic electrons striking a
material
• Man-made
• Secondary effects
– Gamma-rays: Produced by radioactive decay of
materials
Radiation
Electromagnetic (EM) radiation
• Energy (keV): 1 eV = 1.602 x 10-19 J
• Frequency (Hz): E = hf (Hz)
Planck Constant: h = 6.625 x 10 -27 erg-second = 4.136 x 10-15 eV-s
• Wavelength (): E (keV)= 1.24 /(nm)
(Physical Principles of Medical Imaging by Perry Sprawls)
X-ray production process in imaging and therapy
systems involves both particulate and EM types of
radiation
1) Free electron production (temporal res. control)
Electrons are “pulled” out of filament (thermionic or field emission).
1) Electron acceleration
Electron energy under voltage (E = eV)
3) Electron bombards anode to produce
Bremsstrahlung x-rays
(spatial res. Control- focal spot size)
Once x-rays are generated, they are
shaped and controlled to suit the need
of the specific application.
EM Radiation
• Interactions with matter
–
–
–
–
Rayleigh scatter
Photoelectric effect
Compton scatter
Pair production; positron annihilation
• More next time
Radioisotopes
• Gamma photons are given off as unstable or
metastable isotopes try to go to a more stable
atomic state.
• The nuclear transitions are well defined for a
given isotope.
– Specific energies are emitted
– Sometimes, there are multiple transition paths;
thus, multiple energies.
Atomic Structure
• Atomic structure
– Nucleus (Z protons and N neutrons)
– Z orbital electrons
• Atomic number: Z (element)
• Mass number: A = Z+ N
(total number of nucleons)
• Symbols
element
A
Z
Atomic number
X
XA
mass number
Example :
12
6
C
C 12
Radionuclide
Definition: Certain natural and man-made atoms with unstable
nucleus that can undergo spontaneous breakup or decay and, in the
process, emit alpha, beta, or gamma radiation.
Naturally occurring radionuclides (U-238, Ra-226, Rn-222)
Man-made radionuclides (isotope)
– All radionuclides for diagnostics and therapy are man-made.
Therapy: I-125, Ir-192, Cs-137, Co-60, etc.
Diagnostic: Ga-67, Tc-99, I-131, C-11, O-15, etc.
Nucleus instability and decay
H He Li Be B C N O F Ne Na Mg Al Si P S
• A nucleus with excess energy
is at an excited state. It will
release the energy and go to
the ground (energy) stable
state eventually.
• N/P (number of neutrons over
protons) ratio is a good
indicator of nucleus stability.
For low (<15) Z atoms, the
stable N/P ratio is 1.0 and it
increases to ~1.5 for high Z
atoms.
N/P =1
Nuclear transitions
There are several ways an unstable nucleus can decay:
• Isotopic transition (mother and daughter nuclei have the same
number of protons) – Lose a neutron
• Isobaric transition (different number of protons but same
number of nucleons (N+P)) – Exchange proton and neutron
• Isomeric transition (different energy levels) – Lose energy from
nucleus
• Isotonic transition (have the same number of neutrons) – Lose a
proton
Isotopes, isobars, isomers, and isotones.
Examples of isobars, isotones, and isotopes
Products of radioactive decay:
• Beta particle (electron)
Isobaric transition
• Positron
Isobaric transition
Isotones
• Electron capture
Isobaric transition
• Gamma ray
Isomeric transition
• Alpha particle
Secondary decay products:
• Characteristic x-rays
• Auger electrons
Examples
Nuclear Transitions in Medicine
• Isobaric (exchange neutron and proton)
– b- emission (high N/P ratio)
• Neutron > proton + b- + neutrino
– b+ emission (low N/P ratio)
• Proton > neutron + b+ + neutrino
– Electron capture (low N/P ratio)
• Proton + K-shell e- > neutron + gamma
• Isomeric (lose energy from nucleus)
– Gamma emission
– Internal conversion
• Atom ionizes itself; b- emitted
Composite diagram of nuclear transitions
Low N/P
energy
High N/P
Electron
capture
X-rays
Auger
Gamma
K shell
Internal
conversion
electron
Stable N/P
Z, atomic number
Activity (A)
A measure of the rate a radioactive nucleus decays.
Activity is the change of total number of original
radioactive nuclei (N) in a given time (dt):
A  N
t
Question:
If two different gamma-ray emitting radionuclides have
the same radioactivity, do they give the same photon
flux? (Flux = photons/area/time)

Activity Units: curie (Ci) and becquerel (Bq)
Ci = 3.7 x 1010 dps
Bq = 1 dps
dps: disintegrations per second
Curie is a very large quantity of radioactivity.
Commonly used activity level is ~ 100 mCi for
therapy and ~ mCi for diagnostics.
The SI unit is becquerel and 1mCi = 37M Bq.
Decay constant 
N t  No e
 t
N t /t  N t
N t
At

/t 
Nt

Nt
Decay constant is the fraction of radionuclei decayed per time.

Fmin --> 0.63% per min
Iday --> 1.17% per day
Half life T1/2
• T1/2: time for 50% of the original radionuclides to decay.
N tT 
1/ 2
N0
2
N0
 N 0e T
2
1/ 2
ln 2  T1/ 2

T1/ 2 
ln 2

Nremaining = Noriginal/2n
(n: number of half lives elapsed)
Radioactive Decay
Activity remaining for 24 hours for common SPECT (99m-Tc) and PET (18-F) isotopes
1
Relative activity remaining
0.8
99m-Tc (6.01 hours)
18-F (1.83 hours)
0.6
0.4
0.2
0
0
4
8
12
Time (hours)
16
20
24
Measure of Radiation
• Radiation dose
Definition: ionization energy absorbed per unit mass.
Dose (Gy) = Energy (J)/ Mass (kg)
• Radiation Exposure
Definition: ionization charge collected per unit mass of air.
Exposure (R) = charge (Q)/Mass (kg)|air
Roughly speaking: Dose (cGy) ~ Exposure (R)