Download Radiation Physics, X-ray safety and protection

Başar Sarikaya, M.D.
Associate Professor of Radiology
Yeditepe University
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Radiation: is the transfer of energy in the form
of particles or waves.
Energy: the ability to do work (Force·Distance)
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Energy: the ability to do work (Force·Distance)
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Electromagnetic radiation (EM
radiation or EMR) is one of the fundamental
phenomena of electromagnetism, behaving as
waves propagating through space, and also
as photon particles traveling through space,
carrying radiant energy.
The types of electromagnetic radiation are
broadly classified into the following classes:
I.
Gamma radiation
II.
X-ray radiation
III. Ultraviolet radiation
IV. Visible radiation
V. Infrared radiation
VI. Terahertz radiation
VII. Microwave radiation
VIII. Radio waves
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Radiation with sufficiently
high energy can ionize atoms. This occurs
when an electron is stripped (or "knocked out")
from an electron shell of the atom, which leaves
the atom with a net positive charge. Because
living cells and, more importantly, the DNA in
those cells can be damaged by this ionization, it
can result in an increased chance of cancer.
photons and particles with energies above
about 10 electron volts (eV) are ionizing.
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Alpha particles, beta particles, cosmic
rays, gamma rays, and X-ray radiation, all
carry enough energy to ionize atoms. In
addition, free neutrons are also ionizing since
their interactions with matter are inevitably
more energetic than this threshold.
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X-radiation (composed of X-rays) is a form
of electromagnetic radiation. Most X-rays have
a wavelength in the range of 0.01 to
10 nanometers, corresponding to frequencies in
the range 30 petahertz to
30 exahertz (3×1016 Hz to 3×1019 Hz) and
energies in the range 100 eV to 100 keV.
Wilhelm Conrad Roentgen
(27 March 1845 – 10 February 1923)
Nobel Prize in Physics 1901
December 22, 1895
Nouvelle Iconographie de la Salpetrière", a medical journal. (1896)
1874-1901
1897
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X-rays are produced when high velocity
electrons are decelerated (slowed or stopped)
or by a nucleus of an atom especially by high
atomic number material, such as the tungsten
target (anode) in a X-ray tube.
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An electrically heated filament (cathode) within
the X-ray tube generates electrons that are
accelerated from the filament to the tungsten target
by the application of a high voltage to the tube.
The energy gained by the electron is equal to the
potential difference (voltage) between the anode
and cathode. This electron energy is typically
expressed in kilovolts (kV).
The accelerated electron interacts with the target
(anode) nucleus. As the electric field of the
electron interacts with nucleus, the electron
releases energy in the form of X-rays. This method
of of x-ray production is called bremsstrahlung or
braking radiation
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The quantity of electron flow (current) in the
X-ray tube is described in units
of milliamperes (mA). The rate of X-ray
production is directly proportional to the X-ray
tube current. Higher mA values indicate more
electrons are striking the tungsten target,
thereby producing more X-rays.
The voltage (kVp) primarily determines the
maximum X-ray energy produced but also
influences the number of X-rays produced.
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No interaction: X-ray passes completely through
tissue and into the image recording device.
Producing an image
Complete absorption: X-ray energy is completely
absorbed by the tissue. This produces radiation
dose to the patient.
Partial absorption with scatter: Scattering involves
a partial transfer of energy to tissue, with the
resulting scattered X-ray having less energy and a
different trajectory. This interaction does not
provide any useful information (degrades image
quality) and is the primary source of radiation
exposure to staff.
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The probability of X-ray interaction is a function of
tissue electron density, tissue thickness, and X-ray
energy (kVp). Electron dense material like bone and
contrast dye attenuates more X-rays from the X-ray
beam than less dense material (muscle, fat, air). The
differential rate of interaction provides the contrast
that forms the image.
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As electron density increases, the interaction
with X-rays substantially increases. Higher
atomic number materials have increased
electron density.
Bone, which is substantially comprised of
calcium, produces more attenuation, than
tissue, which is comprised of carbon, hydrogen
and oxygen (all of which have a lower electron
density or atomic number than calcium). Thus,
the image of bone and soft tissue has contrast,
or difference, between bone and soft tissue.
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Photoelectric absorption
Compton scattering
the predominant interaction between X-rays and
soft tissue in medical imaging
Rayleigh scattering
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All plain X-ray films
Fluoroscopic Imaging
Mammography
Computed Tomography
Angiograms (including DSA)
No X-rays, therefore no ionizing radiation
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Ultrasonography
Magnetic Resonance Imaging
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Everyone on the planet is exposed to background
radiation, including from internal body sources,
with a worldwide average annual effective dose of
2.4 mSv. Airline crews on long flights experience a
higher level of cosmic radiation and can receive
doses of 4-5 μSv each hour, for instance, so that
one flight may result in the equivalent of a number
of chest X-rays for them and their passengers. The
annual effective doses for aircrew are typically on
average 1–2 mSv for those employed on short-haul
flights and 3–5 mSv for those on long-haul flights
For this procedure:
* Approximate effective radiation dose Comparable to natural background
is:
radiation for:
** Additional lifetime risk of fatal cancer
from examination:
ABDOMINAL REGION:
Computed Tomography (CT)-Abdomen and Pelvis
10 mSv
3 years
Low
Computed Tomography (CT)-Abdomen and Pelvis,
repeated with and without contrast material
20 mSv
7 years
Moderate
Intravenous Pyelogram (IVP)
3 mSv
1 year
Low
1.5 mSv
6 months
Very Low
0.001 mSv
3 hours
Negligible
BONE:
Radiography (X-ray)-Spine
Radiography (X-ray)-Extremity
CENTRAL NERVOUS SYSTEM:
Computed Tomography (CT)-Head
2 mSv
8 months
Very Low
Computed Tomography (CT)-Head, repeated with and
without contrast material
4 mSv
16 months
Low
Computed Tomography (CT)-Spine
6 mSv
2 years
Low
7 mSv
2 years
Low
Computed Tomography (CT)-Chest Low Dose
1.5 mSv
6 months
Very Low
Radiography-Chest
0.1 mSv
10 days
Minimal
0.005 mSv
1 day
Negligible
Coronary Computed Tomography Angiography (CTA)
12 mSv
4 years
Low
Cardiac CT for Calcium Scoring
3 mSv
1 year
Low
0.001 mSv
3 hours
Negligible
25 mSv
8 years
Moderate
0.001 mSv
3 hours
Negligible
0.4 mSv
7 weeks
Very Low
CHEST:
Computed Tomography (CT)-Chest
DENTAL:
Intraoral X-ray
HEART:
MEN'S IMAGING:
Bone Densitometry (DEXA)
NUCLEAR MEDICINE:
Positron Emission Tomography – Computed
Tomography (PET/CT)
WOMEN'S IMAGING:
Bone Densitometry (DEXA)
Mammography
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Dose limits for staff:
annual effective dose limits of 20 mSv for occupationallyexposed people (averaged over 5 years, with an annual limit
of 50 mSv in any single year) and of 1 mSv for the public are
recommended by the ICRP - along with additional limits for
the skin, the hands and feet, and the lens of the eye and for
pregnant workers.
Personal dose monitors are therefore worn by radiation
workers to ensure that doses are below the annual limits
and to assess their radiation safety practices. Annual staff
doses are of the order of 0.25 mSv for radiographers, 0.75
mSv for radiologists and 2.5 mSv for interventionists. It is
important to realize that the dose limits should not be
considered as acceptable levels, but rather as maximum
values which should not be exceeded.
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Three fundamental principles for radiation
protection have been developed by the ICRP
for any exposure to ionizing radiation:
Justification of exposure;
Optimization of protection;
Application of Dose Limits.
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An implication of the principle of optimization
is that all exposures should be kept as low as
reasonably achievable (ALARA). This should
be applied with both economic and societal
factors taken into account which implies that
the level of protection should be the best
available given the circumstances.
Time scale
Effects
Fractions of seconds
Energy absorption
Seconds
Changes in biomolecules
(DNA, membranes)
Biological repair
Minutes
Hours
Days
Weeks
Months
Change of information in cell
Effects
Energy absorption
Years
Changes in biomolecules
(DNA, membranes)
Decades
Biological repair
Generations Change of information in cell
Mutations in a
Germ cell Somatic cell
Leukaemia
or
Cancer
Hereditary
effects
Early
(deterministic only)
Local
Common
Radiation injury of
individual organs:
functional and/or
morphological
changes within
hrs-days-weeks
Acute radiation disease
Acute radiation syndrome
Late
Deterministic Stochastic
Radiation dermatitis
Radiation cataracta
Teratogenic effects
Tumours
Leukaemia
Genetic effects
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Deterministic effects develop due to cell
killing by high dose radiation, appear above a
given threshold dose, which is considerably
higher than doses from natural radiation or
from occupational exposure at normal
operation, the severity of the effect depends
on the dose, at a given high dose the effect is
observed in severe form in all exposed cells,
at higher doses the effect cannot increase.
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Stochastic effects develop due to mutation
effect of low dose radiation, the threshold
dose is not known accurately; it is observed
that cancer of different location appears
above different dose ranges, the severity of
the effect does not depend on the dose, but
the frequency of the appearance of the
(probabilistic) effect in the exposed
population group is dose dependent, (in most
cases) linearly increasing with the dose.
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Carcinogenic effects have been known practically
since the discovery of radioactivity and since the first
case of radiation-induced cancer was described in
1902.
The epidemiological assessment was made from over
575 cancers and leukaemias for the 80,000 survivors
irradiated at Hiroshima and Nagasaki, and about
2,000 cancers of the thyroid in children in the
Chernobyl region.
The actual data does not enable us to show a risk of
cancer at greater than 0,1 Gy by acute irradiation.
Nevertheless, it is considered that risk of cancer and
the relationship dose/risk remains linear for doses
below 0,1 Gy.
Latency period of cancers after exposure