Day 2 p. 1
RADIOLOGY AN
DIAGNOSTIC IMAGING
Dr hab. Zbigniew Serafin, MD, PhD
[email protected]
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.
Radiation Biology
Alpha particles

large and positively charged

tend to cause ionizations and lose energy over a very short distance

are composed of two protons and two neutrons (i.e. a naked helium
nucleus)

large size & relatively high charge prevent deep penetration of
matter (blocked by dead skin or paper)

chronic exposure to inhaled alpha particles is a lung cancer risk

are important in the uranium decay series, of which radon is a
product
Radiation Biology
Neutrons

uncharged particles

may carry significant kinetic energy ("Fast Neutrons")

may collide with a nuclear proton, causing its ejection

produce biologically-important ionizations and excitations due to
such collisions

are often produced as part of fussion reactions
Radiation Biology
Beta Particles

are smaller and less energetic than alpha particles

have a negative charge

are created when a neutron transmutates into a proton

are emitted during decay of iodine-131, phosphorus-32, carbon-14,
and strontium-90
Radiation Biology
Gamma rays and X-rays (photons)

represent pure electromagnetic energy

progress at the speed of light

having no mass or charge, are neither attracted to nor repulsed by
charged particles

gamma-rays originate from the nucleus, usually carries higher
energies than X-rays

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 → positivelycharged ion)
ii.
excitations induced by electromagnetic radiation from many
sources, including radiology
Photons interact with subatomic structures in one of the following three
ways:
 Photoelectric absorption
 Compton Scatter
 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
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:

an average energy for a given path length traveled or

an average path length for a given deposited energy.
The standard unit of measure is keV/um.
Radiation Biology
Ionizations lead to chemical changes:

Free radical production

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).
Radiation Biology
interference with DNA
structure and function
chromosome breakage
gene mutation
effect on cell
multiplication
cell cycle influence
division delay
tissue effects
(reduced growth,
abortive colonies,
degeneration)
interphase death
Radiation Biology
Radiation Biology
Dq – quasi-threshold dose
or sub-lethal dose (SLD)
 most radiosensitive phases:
G2-phase and mitosis (M-phase)
 least radiosensitive phase:
latter part of S-phase (synthesis of
DNA)
Radiation Biology
Law of Bergonie’ and Tribondeau

The radiosensitivity of cell is directly proportional to their
reproductive activity and inversely proportional to their degree of
differentiation.

Cells most active in reproducing themselves and cells not fully
mature will be most harmed by radiation.

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
Radioresistant cells
Radiosensitive cells

bone

germinal cells

liver

lymphoid tissues

kidney

basal cells

cartilage

hematopoietic tissues

muscle

epithelium of the GI tract

nervous tissue
Radiation Biology

children could be expected to be more radiosensitive than adults

fetuses more radiosensitive than children and embryos especially in
the first weeks of pregnancy when organs are forming
Radiosensitivit
y
Cell type
Low
muscle cells, nerve cells
Intermediate
osteoblasts, endothelial cells, fibroblasts,
spermatids
High
spermatogonia, lymphocytes, stem cells,
intestinal mucosa cells and erythroblast
Radiation Biology
Deterministic effects of radiation:

are predictable, are occurring with dose-dependent severity,

generally do not occur below a certain threshold value,

are generally associated with intermediate to high radiation
exposure (orders of magnitude above most doses used in
diagnostic radiology)

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

human LD50 is estimated at 3.25 Gy
Radiation Biology
Whole body irradiation – prodromal syndrome


associated with exposures as low as 1 Gy, nearly universal above 2
Gy
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

has a latent period of 2-6 hours

Sx/Si: sense of fatigue, headache, confusion, depression, vomiting,
diarrhea at higher doses

recovery after 2-3 days (may be shorter for very mild cases)

more common in women than men, children and elderly at higher
risk.
Radiation Biology
Whole body irradiation – hematopoietic syndrome


associated with exposures of at least 3 Gy
mechanism – loss of pluripotent stem cells from hematopoietic
tissues

has a latency period 2-4 weeks

Si/Sx: pancytopenia, leading to infection and hemorrhage

survival: 50% spontaneous recovery at exposure of 3.5 Gy. 180
days required to regain maximum function

death 1-2 months post-exposure from infection; anemia is not a
cause of death
Radiation Biology
Whole body irradiation – GI syndrome


associated with exposures of at least 7 to10 Gy
mechanism – loss of stem cells from intestinal crypts, leading to
eventual loss of GI mucosa

has a latency period 3-5 days

Si/Sx: diarrhea and vomiting leading to profound dehydration

survival: none; death in 1-2 weeks post-exposure
Radiation Biology
Whole body irradiation – cerebrovascular syndrome


associated with catastrophically high acute exposures ≈ 100 Gy
mechanism – severe damage to CNS, cardiovascular and
respiratory systems

latency period of minutes to hours

Si/Sx: ataxia, disorientation, hypotension, shock and respiratory
distress

survival: none; dath within one day
Radiation Biology
Stochastic effects of radiation

probability that an effect will occur is related to exposed dose

severity of effect is unrelated to exposed dose – “all or nothing”

involve a degree of randomness

usually do not recognize a threshold dose
•
hereditary / genetic effects
•
carcinogenesis
Radiation Biology
Stochastic effects of radiation
Genetically Significant Dose (GSD).

the gonadal dose equivalent received by persons of reproductive
potential also taking into account the expected number of children
for that population

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

most analyses utilize the cohort of Japanese atomic bombing
survivors for extrapolating low-dose exposure risk

statistical noise prevents direct assessment of human risk for
exposures below 50 mSv

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)
Radiation Biology
(D) – is it possible ?
Radiation Biology
In utero exposure

Radiation risks to the fetus:
•
Fetal demise
•
Congenital malformation
•
CNS/cognitive effects
•
Carcinogenesis
•
Intrauterine growth retardation

risk = first trimester > second > third

dose of 0.1 Gy during the period of major organogenesis gives
significant risk of congenital malformation
Radiation Biology
attenuation

the removal of photons from a beam of x- rays as it passes through
matter

is caused by both absorption and scattering of the primary photons

Linear Attenuation Coefficient – fraction of photons removed from a
monoenergetic beam of x-rays per unit thickness of material (cm-1)

however, as the thickness increases, the relationship is not linear

LAC normalized to unit density is called the Mass Attenuation
Coefficient
31
Radiation Biology
attenuation
C.F. Wolbarst. Physics of Radiology, pp. 108, 110.
32
Radiation Biology
radiation units

KERMA

Absorbed Dose

Exposure

Dose

Equivalent Dose

Effective Dose
Radiation Biology
KERMA = Kinetic Energy Released in MAtter

= kinetic energy transferred to charged particles by indirectly
ionizing radiation, per mass matter

SI units are 1 Gy = 1 J/kg (traditional 1 rad = 0.01 Gy)
Absorbed Dose

= amount of energy deposited by ionizing radiation per unit mass of
material

SI units are 1 Gy = 1 J/kg (traditional 1 rad = 0.01 Gy)

used to calculate organ dose
Radiation Biology
Exposure

= amount of electrical charge (ionization) produced by ionizing
radiation per mass of air

SI units are C/kg (traditional R = 2.58x10-4 C/kg)

used to compare assessment of equipment performance
Radiation Biology
Dose

= Exposure × conversion factor

SI units are C/kg

Exposure is nearly proportional
to dose in soft tissue over the
diagnostic radiology range

for bone, the conversion factor
approaches 4
C.F. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p.55.
36
Radiation Biology
Equivalent Dose

= Dose ∙ wR; weighs the „quality” of radiation

SI units are Sv (traditional rem 1 rem = 10 mSv)

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 xrays and gamma rays and thus are given different radiation
weighting factors (wR)
•
X-rays/gamma rays/electrons: LET ≈ 2 keV/μm; wR = 1
•
protons (< 2MeV): LET ≈ 20 keV/μm; wR = 5-10
•
neutrons (E dep.): LET ≈ 4-20 keV/μm; wR = 5-20
•
alpha Particle: LET ≈ 40 keV/μm; wR = 20
Radiation Biology
Effective Dose

= a measure of radiation- and organ-specific damage in humans

takes into account different radiosensitiveness of tissues (tissue
weighting factors – wT)

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.
38
Radiation Biology
EXERCISE

indentify the sources of background radiation, and describe the
magnitude of each source

indentify the sources of medical radiation, and describe the
magnitude of each source

what is estimated average annual total exposure to radiation
(mSV)?
39
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.
40
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
41
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.
42
Radiation Biology
EXERCISE

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
43
Radiation Biology
EXERCISE

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
44