radiation protection and biology radiation physics and...
TRANSCRIPT
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Radiation Protection and
Biology
Dawn Couch Moore, M.M.Sc., RT(R)
Assistant Professor
Emory University
Medical Imaging Program
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A.R.R.T.
• Safety
– Radiation Physics and Radiobiology
• Biological Aspects of Radiation 11
– Radiation Protection 31
• Minimizing Patient Exposure
• Personnel Protection
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Objective
• To obtain optimum diagnostic information or
therapeutic effects with minimum exposure of
the patient.
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Technologist’s Responsibility
• Recognize your duty to protect.
– The patient
– The public
– Health professionals
• Utilize all mechanisms to reduce dose to the patient without compromising image quality.
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First Triad of Radiation
Protection
• Justification with net
benefit
• Dose Limits
• Optimization (ALARA)
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Second Triad of Radiation
Protection
• “Cardinal Principles”
• Time: Exposure is directly proportional to time.
• Distance: Exposure is inversely proportional to the distance squared.
• Shielding: Exposure is reduced by the placement of barriers between the source and the exposed individual.
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Classification of Radiation
• Corpuscular
• Electromagnetic
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Corpuscular Radiation
• “Particulate”
• Any subatomic particle in
motion.
• Types
– Alpha
– Beta
– Neutrons
– High Speed Electrons
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Electromagnetic Radiation
• An electric and magnetic
field, perpendicular to
each other, but traveling
through space in the
same direction.
• Types
– X-rays
– Gamma radiation
Carlton & Adler, 2006
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Electromagnetic Radiation
• Energy (photon)/ No mass
• Travel at the speed of light
• Highly penetrating
• Ionizing
(x and gamma)
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Electromagnetic Radiation
• In general, x-rays have:
– High energy
– High frequency
– Short Wavelength
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Sources of Radiation Exposure
• Environmental (Background)
– Cosmic
– Terrestrial
– Internal
• Artificial (Man-made)
– Medical (Diagnostic and Therapeutic)
– Nuclear Industry
– Consumer Products
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Sources of Radiation Exposure
1987
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Annual Dose U.S. Population
1987
• Environmental Radiation: 2.95 mSv*
• Artificial Radiation: 0.65 mSv
• Total: 3.60 mSv
* 1.98 mSv from Radon gas
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Sources of Radiation Exposure
2009
NCRP Report 160, 2009
Annual Dose U.S. Population
2009
• Environmental Radiation: 3.10 mSv
• Artificial Radiation: 3.10 mSv*
• Total: 6.20 mSv
* 1.5 mSv from CT
NCRP Report 160, 2009
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Interactions of Radiation with
Matter
• Coherent Scattering
– Unmodified, Classical, Thomson, Rayleigh
• Photoelectric Effect
– True absorption
• Compton Effect
– Modified Scattering
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Coherent Scattering
• Occurs at low energy
levels (< 30 kVp)
• Photon- intermediate
shell electron interaction
• Non-ionizing process
Carlton & Adler, 2006
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Photoelectric Effect
• Photon-inner shell
electron interaction
• Probability depends on:
– “Z” of absorber
– Photon energy
– Mass density of
absorber
Carlton & Adler, 2006
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Compton Effect
• Photon-outer shell interaction
• Scattering with partial
absorption
• Probability depends on:
– Photon energy
– Mass density
Carlton & Adler, 2006
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Prevalent Photon Interactions
Carlton & Adler, 2006
Tissue Penetration
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Radiation Quantities and Units
• Exposure (X): C/kg
• Air Kerma Graya
• Absorbed Dose (D): Gray
• Equivalent Dose (EqD): Sievert
• Effective Dose (EfD): Sievert
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Exposure
• Unit: C/kg
• Measures ionizations
produced in air
• Applicable to x and
gamma radiation at
energies < 3 MeV
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Exposure
• 1 R =2.58 x 10-4 C/kg
• Used in the calibration of
x-ray equipment
Air Kerma
• Kinetic energy released in a unit mass (kg) of air
• Unit: J/kgair / Graya
• Expresses radiation concentration at a specific
point
– Patient’s body surface
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Air Kerma
• DAP: Dose Area Product (mGy-cm2)
– Sum total of air kerma over the exposed area
– Energy entering the patient’s body surface
– DAP = Air kerma x surface area
= (0.05 Gy)(80 cm2)
= 4 Gy-cm2
= 4000 mGy-cm2
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Absorbed Dose
• Unit: Gray
• Measures energy
deposition in matter
• Applicable to all types of
radiation
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Equivalent Dose
• Unit: Sievert
• Unit of biological effects
• EqD = D x WR
(absorbed dose x radiation weighting factor)
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Equivalent Dose
• Radiation Weighting (Quality) Factors
– X-ray, Beta, Gamma 1
– Neutrons < 10 keV 5
– Neutrons > 100 keV – 2Mev 20
– Alpha particles 20
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Effective Dose
• Unit: Sievert
• Unit of biological effects
• EfD = D x WR x WT
D: absorbed dose
WR: radiation weighting factor
WT: tissue weighting factor
Effective Dose
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Minimizing Patient Exposure
1. Exposure Reduction
1. Communication
2. Patient Positioning
2. Exposure Factors
3. Shielding
4. Beam Restriction
5. Filtration
6. Image Receptor Type
7. Grids
8. Fluoroscopy
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1. Exposure Reduction
• Patient Positioning
(PA vs AP)
– Scoliosis examinations
– Skull examinations
– Lumbar Spine
examinations
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2. Exposure Factors
• High kVp / low mAs
techniques are best for
patient protection.
• Choose a kVp level which
is “optimum”.
• Optimum kVp gives
adequate penetration and
an acceptable level of
scatter production.
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3. Shielding
Shielding should be used if:
1. the reproductive organs are within or near the primary beam.
2. the objectives of the exam are not compromised.
3. the patient has reasonable reproductive potential.
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Types of Shields
• Flat Contact Shields
• Shaped/ Contour Contact
Shields
• Shadow Shields
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4. Beam Restriction
• Apertures
• Cones and cylinders
• Collimators
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Beam Restriction
• Increased beam
restriction results in:
– Decreased patient
dose
– Improved image
quality
• Contrast
– Less scatter
production
Carlton & Adler, 2006
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5. Filtration
• Reduces exposure to the
patient’s skin and superficial
body tissues.
• Absorbs low energy, long
wavelength photons.
• Increases average beam
energy
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Filtration
• Two types:
– inherent
– added
• Total Filtration:
– 2.5 mm Al (>70 kVp)
– 1.5 mm Al (50-70 kVp)
– 0.5 mm Al (< 50 kVp)Carlton & Adler, 2006
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6. Image Receptor Type
• More efficient image receptors
require less exposure.
• Digital Receptor DQE
– PSP 30%
– Flat panel 67%
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7. Grids
• Grids are an image quality
device
– Absorb scatter radiation
– Improve image contrast
• Increase patient dose
– Higher grid ratio; higher
patient dose
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8. Fluoroscopy
• Pulsed fluoro
– Duty factor
– Decreases dose
• Fluoro Time
– Increased time; increased
dose
– Cumulative timer
• High kVp Technique
Fluoroscopy
• Receptor positioning
• Magnification mode
• Last image hold
• Dose documentation
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Personnel Protection
• Cardinal Principles
–Time
–Distance
–Shielding
• Sources of Exposure
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Sources of Radiation Exposure
• Medical Diagnostic Exposure
– Primary Beam
– Secondary Beam
– Leakage
– Stray Radiation (leakage + scatter)
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Protective Barriers
• Primary
– 1.5 mm (1/16th inch) Pb. eq.
– 7 feet high
• Secondary
– 0.75 mm (1/32nd inch) Pb. eq.
– Extend to ceiling
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Equipment Recommendations
NCRP Report # 102• Aprons/Thyroid Shields
– 0.50 mm Pb equivalent
• Gloves
– 0.25 mm Pb equivalent
• Glasses
– 0.35 mm Pb equivalent
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Equipment Recommendations
NCRP Report # 102
• Mobile Equipment
– Exposure Cord:
• 6 feet (180 cm).
– Operators should wear
leaded apron.
– Operators should stand at
a right angle to the
scattering object.
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Equipment Recommendations
NCRP Report # 102
• Radiographic Units
– Tube Housing:
• < 100 mR/hr at 1 meter from source.
– SID:
• +/- 2% of indicated SID.
– Collimator:
• +/- 2% of SID.
– Filtration:
• 2.5 mm Al (> 70 kVp).
Carlton & Adler, 2006
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Equipment Recommendations
NCRP Report # 102• Fluoroscopic Units
– Source-to Table-top Distance:
• Should not be less than 15 inches (38 cm);
• Shall not be less than 12 inches (30 cm).
– Primary Protective Barrier:
• Must be interlocked with x-ray tube.
• 2 mm Pb equivalent
– Exposure Switch:
• Dead-Man type.
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Equipment Recommendations
NCRP Report #102• Fluoroscopic Units
– Protective Drape: Minimum 0.25 mm lead equivalent.
– Bucky Slot Cover: Minimum 0.25 mm lead equivalent.
– Cumulative Timer: Produces an audible signal or interrupts beam at 5 minutesfluoro time.
– Exposure Rate: Must not exceed 100 mGya/ min (10 R/min).
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Dosimeters
• Film Badge
• Pocket Dosimeters
• TLD
• OSL Dosimeters
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Film Badge
Three parts:– lightweight plastic film
holder
– metal filters
– film packet
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Advantages/Disadvantages
• Advantages– Constitutes a permanent,
legal record of personnel exposure
– Economical
– Distinguish all types of low level radiation and their energies.
• Disadvantages
– Temperature and humidity
extremes cause fogging
– Radiation exposure cannot
be determined on the day
of occurrence.
– Has a limited range of
sensitivity (>10 mR).
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Pocket Dosimeter
• Resembles a fountain pen
• Contains a thimble ionization
chamber
• Two types
– The self-reading type
– The non-reading type
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Function
• Contains two electrodes, one
positive (the central electrode)
and one negative (the outer
electrode)
• The quantity of charge
determines the position of the
shadow along the scale
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Advantages/Disadvantages
• Advantages
– Immediate readout for
radiation workers
– Compact, easy to carry,
and easy to use
– Accurate and sensitive,
great for procedures that
are relatively short in
duration
• Disadvantages
– Expensive
– If not read every day, could
give an inaccurate reading
– Discharge if subjected to
mechanical shock
– No permanent legal record
of exposure
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Thermoluminescent dosimeter
(TLD)
• Lithium Fluoride Crystals
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Function
• When irradiated, electrons in e absorb energy and are “Excited”.
• When heated, the electrons return to their original position and emit light.
• The intensity of light is proportional to the amount of radiation.
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Advantages/Disadvantages
• Advantages– The LiF crystals interact as
human tissue does.
– Read as low as 5 mR
– Not affected by temperature and humidity
– May be worn up to 3 months/ reusable.
• Disadvantages
– Can be read only
once.
– Calibrated dosimeters
must be prepared and
read with each group
of TLD’s as they are
processed
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Optically Stimulated
Luminescent Dosimeter (OSL)
• A lightweight plastic
holder
• Aluminum oxide detector
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Function
– Radiation exposure is detected
by the Aluminum oxide detector.
– The dosimeter is “read” using a laser.
– When laser light strikes the dosimeter, the sensing material becomes luminescent in proportion to the amount of radiation exposure received.
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Advantages/Disadvantages
• Advantages– Read as low as 1 mR
– Temperature and humidity changes do not affect the OSL
– Can undergo reanalysis
• Disadvantages
– No known
disadvantages
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Effective Dose Limits
NCRP Report # 116
• a.k.a. Dose Limits.
• Based on the lifetime risk of somatic or
genetic injury.
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Annual Occupational Exposure
• Effective Dose Limits
(stochastic effects):
– 50 mSv
• Cumulative Effective
Dose (CED):
– 10 mSv x age (years)
• Effective Dose Limits
(non-stochastic effects) :
– Lens of eye:
• 150 mSv
– All others:
• 500 mSv
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Annual Educational and Training
Exposure
• Effective dose: 1 mSv
• DE Limit (skin, hands, & feet): 50 mSv
• DE Limit (lens of eye): 15 mSv
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Embryo-fetus Exposure
• Equivalent Dose Limit:
– Month
• 0.5 mSv
– Gestation Period
• 5.0 mSv
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Biological Effects
• Result from energy deposited into the biological
system.
• Mechanisms of interaction:
– Ionization (Photoelectric + Compton)
– Excitation
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Biological Effects
• Somatic Effects
• Genetic Effects
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Somatic Effects
• Effects occurring in the
exposed individual.
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Somatic Effects
• Stochastic
– Mutational
– Non-threshold
– Example:
• Cancer
• Non-stochastic
– aka Deterministic
– Cell killing
– Threshold
– Examples:
• Burns
• Epilation
• Cataracts
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Somatic Effects• Stochastic
– Linear, non-threshold dose response relationship
– Linear-quadratic, non-threshold dose response relationship
• Chronic low-level exposure
• Non-stochastic (Deterministic)
– Non-linear (sigmoidal), threshold dose response relationship
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Genetic Effects
• Effects occurring in
unborn generations.
• Linear, non-threshold
dose response
relationship
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Genetic Effects
• Mutations
– Typically recessive
– Usually harmful
• Chromosome damage
– Breaks
– Gene rearrangement
• Translocation
• Inversion
• Ring
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Genetic Effects
• Genetically Significant Dose
– The average annual gonadal equivalent dose to members of the
population who are of childbearing age
– Estimated at 20 mRad (0.20 mGy)
• Doubling Dose
– The radiation dose that causes the number of spontaneously
occurring mutations in a given population to double.
– Estimated at 100 Rad (1.0 Gy) for low-intensity exposure
Biological Effects
• Short Term (Early) Effects
– Seen soon after exposure
• Long Term (Late or Delayed) Effects
– Seen up to 30 years after exposure
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Short Term Effects
• Acute Radiation Syndromes
– Hematologic
– Gastrointestinal
– Cerbrovascular
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Long Term Effects
• Carcinogenesis
– Radium watch dial
painters
– Uranium miners
– Atomic bomb survivors
– Thoratrast patients
• Cataractogenesis
• Life Span Shortening
• Embryological Effects
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Embryological Effects
• Pre-implantation
• Major organogenesis
• Fetal development
Bushong, 2004
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Factors Influencing
Biological Effects
• Biological Factors
– Related to the Organism
• Physical Factors
– Related to the type and delivery of the radiation
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Biological Factors
• Tissue Sensitivity
• Oxygen Effect (OER)
• Age
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Tissue Radiosensitivity
• Law of Bergonie` and Tribondeau
–Cells which are poorly differentiated and
rapidly dividing are most radiosensitive.
–Cells which are highly differentiated and
slowly dividing are most radioresistant.
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Radiosensitive Tissues
• Blood Cells (Lymphocytes)
• Epithelial Tissues
• Intestinal Crypt Cells
• Reproductive Cells
• Lens of the eye
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Radioresistant Tissues
• Nervous Tissue
• Muscle Tissue
• Liver
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Biological Factors
• Oxygen Effect (OER)
– Oxygen enhances radiation effects
• Age
– The embryonic stage is the most
radiosensitive period in the lifetime of any
organism.
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Physical Factors
• LET: Linear Energy Transfer
– Rate of energy deposition as radiation travels
through matter
• RBE: Relative Biological Effectiveness
– The relative effectiveness of a radiation in
producing a given biological effect when
compared to 250 keV x-rays
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Physical Factors
• Time of dose delivery
– Acute
– Fractionation/
protraction
• Quantity of Radiation
• Quality of Radiation
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Radiation Protection and Biology
• The End!