10/15/2012
Day 2 p. 1
RADIOLOGY AN
DIAGNOSTIC IMAGING
Dr. Zbigniew Serafin, MD, PhD
[e-mail]
Radiation Biology and Radiation Protection
mainly based on:
C. Scott Pease, MD, Allen R. Goode, MS, J. Kevin McGraw, MD, Don Baker, PhD, John Jackson, MA, Spencer B. Gay, MD: Basic Radiobiology.
Radiation Biology
The core of an atom exists precariously: massive repulsive
electromagnetic forces between closely-assembled protons in the
nucleus must be counterbalanced.
"Stability" thus reflects the balance of power between strong nuclear
force, weak nuclear force, and electromagnetic force. The nuclear
binding energy quantifies the energy necessary to maintain coherence.
Isotopes are atoms with the same atomic number (proton count) but
different atomic masses (number of neutrons). Heavier elements are
more likely to have binding energies insufficient to maintain a stable
nuclear configuration. Such radioisotopes may undergo decay by
emission of energetic quanta.
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Radiation Biology
Alpha particles
¨ð large and positively charged
¨ð tend to cause ionizations and lose energy over a very short distance
¨ð are composed of two protons and two neutrons (i.e. a naked helium nucleus)
¨ð large size & relatively high charge prevent deep penetration of matter
(blocked by dead skin or paper)
¨ð chronic exposure to inhaled alpha particles is a lung cancer risk
¨ð are important in the uranium decay series, of which radon is a product
Radiation Biology
Neutrons
¨ð uncharged particles
¨ð may carry significant kinetic energy ("Fast Neutrons")
¨ð may collide with a nuclear proton, causing its ejection
¨ð produce biologically-important ionizations and excitations due to such
collisions
¨ð are often produced as part of fussion reactions
Radiation Biology
Beta Particles
¨ð are smaller and less energetic than alpha particles
¨ð have a negative charge
¨ð are created when a neutron transmutates into a proton
¨ð are emitted during decay of iodine-131, phosphorus-32, carbon-14, and
strontium-90
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Radiation Biology
Gamma rays and X-rays (photons)
¨ð represent pure electromagnetic energy
¨ð progress at the speed of light
¨ð having no mass or charge, are neither attracted to nor repulsed by charged
particles
¨ð gamma-rays originate from the nucleus, usually carries higher energies than
X-rays
¨ð X-rays originate from electron clouds
Radiation Biology
NOTE: regardless of the type of energy carrier or the specific type of energy-
matter interaction, biologic hazard ultimately results from:
i. atomic ionizations (loss of one or more electrons positively-charged ion)
ii. excitations induced by electromagnetic radiation from many sources,
including radiology
Photons interact with subatomic structures in one of the following three ways:
¨ð Photoelectric absorption
¨ð Compton Scatter
¨ð Pair production
The particular type of interaction reflects probability statistics based on both
the energy of the photon and the atomic number of the traversed atom. For
most tissues of the body, average atomic number does not vary greatly 
though cortical bone has the highest effective atomic number.
Radiation Biology
Linear Energy Transfer  the amount of energy transferred to the matter in
the form of ionizations and excitations. LET indicates the potential for
biologically important damage from radiation.
LET can be thought of in two ways:
¨ð an average energy for a given path length traveled or
¨ð an average path length for a given deposited energy.
The standard unit of measure is keV/um.
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Radiation Biology
Ionizations lead to chemical changes:
¨ð Free radical production
¨ð Broken bonds, importantly double-strand DNA breaks
Since the intracellular environment is essentially aqueous, water is the most
likely molecule encountered by radioactive energies. Radiolysis of water may
produce H·, OH·.
Damage caused by such free radicals represents the INDIRECT action of
ionizing radiation. Most biological effects of low LET radiation can be
attributed to free radicals.
Less commonly, nucleoproteins or DNA may be ionized directly by charged
particles, but not electromagnetic radiations.
Radiation Biology
ionizing radiations
free radicals ionizations
(indirect effect) (direct effect)
changes in configuration
of DNA macromolecules
interference with DNA
structure or replication
Radiation Biology
Cell death is operationally defined as loss of function, such as reproductive
capacity for stem cells or synthesis of some specific product (enzyme, hormone).
Apoptosis is the process of programmed cell death  biochemical pathways
within a cell leading to its own organized dismantling.
When DNA is damaged and not successfully repaired, the cell may die  cell
death may occur immediately (interphase death) or during its attempt to
divide (mitotic death) or after a few cell divisions (abortive colonies).
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Radiation Biology
interference with DNA
structure and function
chromosome breakage gene mutation cell cycle influence
effect on cell multiplication
division delay
tissue effects
(reduced growth, abortive
interphase death
colonies, degeneration)
Radiation Biology
Radiation Biology
Dq  quasi-threshold dose
or sub-lethal dose (SLD)
qð most radiosensitive phases:
G2-phase and mitosis (M-phase)
qð least radiosensitive phase:
latter part of S-phase (synthesis of DNA)
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Radiation Biology
Law of Bergonie and Tribondeau
¨ð The radiosensitivity of cell is directly proportional to their reproductive
activity and inversely proportional to their degree of differentiation.
¨ð Cells most active in reproducing themselves and cells not fully mature will
be most harmed by radiation.
¨ð The more mature and specialized in performing functions as cell is, the less
sensitive it is to radiation.
Radiation Biology
Radioresistant cells Radiosensitive cells
Radiation Biology
¨ð children could be expected to be more radiosensitive than adults
¨ð fetuses more radiosensitive than children and embryos especially in the first
weeks of pregnancy when organs are forming
Radiosensitivity Cell type
Low muscle cells, nerve cells
Intermediate osteoblasts, endothelial cells, fibroblasts, spermatids
spermatogonia, lymphocytes, stem cells,
High
intestinal mucosa cells and erythroblast
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Radiation Biology
Deterministic effects of radiation:
¨ð are predictable, are occurring with dose-dependent severity,
¨ð generally do not occur below a certain threshold value,
¨ð are generally associated with intermediate to high radiation exposure
(orders of magnitude above most doses used in diagnostic radiology)
¨ð examples:
" cataracts (single dose of 2-6 Gy)
" transient erythema (2-6 Gy)
" desquamation (> 10 Gy)
" epilation (3-7 Gy)
" sterility (> 6 Gy in males and 4-6 Gy in females)
Radiation Biology
Whole body irradiation
¨ð human LD50 is estimated at 3.25 Gy
Radiation Biology
Whole body irradiation  prodromal syndrome
¨ð associated with exposures as low as 1 Gy, nearly universal above 2 Gy
¨ð mechanism  increased tissue and cell permeability, allowing substances like
serotonin and histamine to enter chemosensitive cells of the GI tract and
activate neural pathways to the vomiting center in the medulla
¨ð has a latent period of 2-6 hours
¨ð Sx/Si: sense of fatigue, headache, confusion, depression, vomiting, diarrhea
at higher doses
¨ð recovery after 2-3 days (may be shorter for very mild cases)
¨ð more common in women than men, children and elderly at higher risk.
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Radiation Biology
Whole body irradiation  hematopoietic syndrome
¨ð associated with exposures of at least 3 Gy
¨ð mechanism  loss of pluripotent stem cells from hematopoietic tissues
¨ð has a latency period 2-4 weeks
¨ð Si/Sx: pancytopenia, leading to infection and hemorrhage
¨ð survival: 50% spontaneous recovery at exposure of 3.5 Gy. 180 days
required to regain maximum function
¨ð death 1-2 months post-exposure from infection; anemia is not a cause of
death
Radiation Biology
Whole body irradiation  GI syndrome
¨ð associated with exposures of at least 7 to10 Gy
¨ð mechanism  loss of stem cells from intestinal crypts, leading to eventual loss
of GI mucosa
¨ð has a latency period 3-5 days
¨ð Si/Sx: diarrhea and vomiting leading to profound dehydration
¨ð survival: none; death in 1-2 weeks post-exposure
Radiation Biology
Whole body irradiation  cerebrovascular syndrome
¨ð associated with catastrophically high acute exposures H" 100 Gy
¨ð mechanism  severe damage to CNS, cardiovascular and respiratory
systems
¨ð latency period of minutes to hours
¨ð Si/Sx: ataxia, disorientation, hypotension, shock and respiratory distress
¨ð survival: none; dath within one day
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Radiation Biology
Stochastic effects of radiation
¨ð probability that an effect will occur is related to exposed dose
¨ð severity of effect is unrelated to exposed dose   all or nothing
¨ð involve a degree of randomness
¨ð usually do not recognize a threshold dose
" hereditary / genetic effects
" carcinogenesis
Radiation Biology
Stochastic effects of radiation
Genetically Significant Dose (GSD).
¨ð the gonadal dose equivalent received by persons of reproductive potential
also taking into account the expected number of children for that
population
¨ð the 1991 estimated GSD in the United States is approximately 0.3 mSv
from  man-made radiation (medical and dental X-rays,
radiopharmaceuticals, commercial nuclear power, miscellaneous
occupational exposure, weapons-testing fallout, consumer products, air
travel)
WHAT CT DOSE IS SAFE?
Radiation Biology
Stochastic effects of radiation
Carcinogenesis
¨ð most analyses utilize the cohort of Japanese atomic bombing survivors for
extrapolating low-dose exposure risk
¨ð statistical noise prevents direct assessment of human risk for exposures
below 50 mSv
¨ð most common neopalsms:
" thyroid cancer (Hiroshima, Chernobyl)
" breast cancer (Hiroshima, Nova Scotia, mammography)
" leukemia (Hiroshima)
" lung cancer (Hiroshima, uranium miners)
" bone cancer (radiotherapy)
" skin cancer (early radiology)
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Radiation Biology
(D)  is it possible ?
Radiation Biology
In utero exposure
¨ð Radiation risks to the fetus:
" Fetal demise
" Congenital malformation
" CNS/cognitive effects
" Carcinogenesis
" Intrauterine growth retardation
¨ð risk = first trimester > second > third
¨ð dose of 0.1 Gy during the period of major organogenesis gives significant
risk of congenital malformation
Radiation Biology
attenuation
¨ð the removal of photons from a beam of x- rays as it passes through matter
¨ð is caused by both absorption and scattering of the primary photons
¨ð Linear Attenuation Coefficient  fraction of photons removed from a
monoenergetic beam of x-rays per unit thickness of material (cm-1)
¨ð however, as the thickness increases, the relationship is not linear
¨ð LAC normalized to unit density is called the Mass Attenuation Coefficient
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Radiation Biology
attenuation
C.F. Wolbarst. Physics of Radiology, pp. 108, 110. 31
Radiation Biology
radiation units
¨ð KERMA
¨ð Absorbed Dose
¨ð Exposure
¨ð Dose
¨ð Equivalent Dose
¨ð Effective Dose
Radiation Biology
KERMA = Kinetic Energy Released in MAtter
¨ð = kinetic energy transferred to charged particles by indirectly ionizing
radiation, per mass matter
¨ð SI units are 1 Gy = 1 J/kg (traditional 1 rad = 0.01 Gy)
Absorbed Dose
¨ð = amount of energy deposited by ionizing radiation per unit mass of
material
¨ð SI units are 1 Gy = 1 J/kg (traditional 1 rad = 0.01 Gy)
¨ð used to calculate organ dose
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Radiation Biology
Exposure
¨ð = amount of electrical charge (ionization) produced by ionizing radiation
per mass of air
¨ð SI units are C/kg (traditional R = 2.58x10-4 C/kg)
¨ð used to compare assessment of equipment performance
Radiation Biology
Dose
¨ð = Exposure × conversion factor
¨ð SI units are C/kg
¨ð Exposure is nearly proportional to
dose in soft tissue over the
diagnostic radiology range
¨ð for bone, the conversion factor
approaches 4
C.F. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p.55.
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Radiation Biology
Equivalent Dose
¨ð = Dose · wR; weighs the  quality of radiation
¨ð SI units are Sv (traditional rem 1 rem = 10 mSv)
¨ð in general,  high LET (Linear Energy Transfer) radiation (e.g., alpha
particles and protons) are much more damaging than  low LET radiation,
which include electrons and ionizing radiation such as x-rays and gamma
rays and thus are given different radiation weighting factors (wR)
" X-rays/gamma rays/electrons: LET H" 2 keV/źm; wR = 1
" protons (< 2MeV): LET H" 20 keV/źm; wR = 5-10
" neutrons (E dep.): LET H" 4-20 keV/źm; wR = 5-20
" alpha Particle: LET H" 40 keV/źm; wR = 20
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Radiation Biology
Effective Dose
¨ð = a measure of radiation- and organ-specific damage in humans
¨ð takes into account different radiosensitiveness of tissues (tissue weighting
factors  wT)
¨ð SI units are Sv (traditional rem 1 rem = 10 mSv)
" first calculate the equivalent dose to each organ: (HT) [Sv]
" Effective Dose (E) = " wT × HT
C.F. Bushberg, et al. The Essential Physics
of Medical Imaging, 2nd ed., p.58.
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Radiation Biology
EXERCISE
¨ð indentify the sources of background radiation, and describe the magnitude
of each source
¨ð indentify the sources of medical radiation, and describe the magnitude of
each source
¨ð what is estimated average annual total exposure to radiation (mSV)?
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Radiation Biology
EXERCISE
c.f. NCRP Press Report.
Medical Radiation Exposures
of the U.S. Population
Greatly Increased Since the
Early 1980s. 3 March 2009. 39
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Radiation Biology
EXERCISE
0.40 mSv/yr
0.28 mSv/yr
0.39 mSv/yr
0.27 mSv/yr
0.14 mSv/yr
0.07 mSv/yr
<0.01 mSv/yr
2.00 mSv/yr
3.00
mSv/yr
3.60 mSv/yr
c.f. NCRP Report #93 40
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Radiation Biology
EXERCISE
c.f. NCRP Press Report.
Medical Radiation Exposures
of the U.S. Population
Greatly Increased Since the
Early 1980s. 3 March 2009. 41
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Radiation Biology
EXERCISE
¨ð discuss ALARA rule (As Low As Reasonably Achievable) and its application
to radiation protection
1) indications for imaging
2) choice of imaging method
3) imaging parameters
4) radiation shielding
5) documentation
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