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88 Cards in this Set
- Front
- Back
Definition of electomagnetic radiation
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The propagation of energy thru space as oscillating electromagnetic fields.
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Physical characteristics of EM radiation
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No mass
No charge Travels a speed of light |
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Types of EM radiation
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radio waves
infrared visible light UV light x-rays γ-rays |
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Differences between types of EM radiation
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All same speed.
Differ in wavelength/frequency. x-rays/γ-rays=shortest wavelengths, radio waves=longest wavelength |
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Speed of light
(defined by wavelength) |
c= wavelength/frequency
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Energy as related to wavelength
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E α 1/wavelength
as wavelength increases, energy decreases |
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Origin of x-rays
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from outside the nucleus by interactions between high speed particles
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Origin of γ-rays
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from inside the nucleus of spontaneously decaying atoms
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Parts of basic x-ray tube
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Filament
Target Glass tube Anode Cathode |
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Filament
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thin coiled wire that serves as the source of electrons
made of Tungsten Part of the Cathode assembly |
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Thermionic Emission
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Forcing a low voltage current through the filament (against its resistance) generates heat.
Heat results in the release of free electrons. |
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Focusing cup
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Negatively charged cup surrounging the filament, prevents generated electrons from dispersing
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Voltage
(potential difference) |
Negative cathode-positive anode
Pulls and accelerates electrons away from filament, towards target |
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Voltage used for diagnostic procedures
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40-140 kVp
Kilovolts peak |
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Voltage used for therapeutic procedures
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1-4 MV (megavolt)
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Target
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Part of the anode.
Contains the focal spot |
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Focal Spot
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Area of the target where the actual interactions that produce the x-rays occurs
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Housing
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the glass/pyrex tube that holds the vacuum.
Surrounded by a lead shield. |
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Purpose of the vacuum
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Prevents electrons from interacting with atoms in the air.
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Window
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a small port in the lead shielding that allows the "useful beam" to escape
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Types of interactions
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Transition
Bremsstrahlung |
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Transition interactions
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Characteristic
x-rays in a very specific energy range |
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Brehsstrahlung interactions
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General
X-rays in a broad range of energies |
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Efficiency of x-ray production
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very inefficient
99% of incoming electron energy is converted to heat, only 1% is converted to useable x-rays |
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Effective focal spot
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The apparent size of the focal spot from patients point of view
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Effect of increasing the size of focal spot
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greater output capability,
greater heat dissipation |
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Effect of decreasing the size of the focal spot
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greater detail of image,
done by making angle of target steeper |
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Two type of anodes
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Stationary
Rotating |
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Uses of stationary anode
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Portable diagnostic equipment
Therapy (when auxillary cooling is incorporated) |
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Limitations of stationary anode
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Lower output, due to limited ability to dissipate heat
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Uses of rotating anode
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Fixed diagnostics
Special procedures Low portability/portability-when greater output in required |
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Value of a rotating anode
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Greater output capability, due to increased ability to dissipate heat
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Filaments of rotating anode systems
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most have two filaments
one for large focal spot, the other for small focal spot |
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Quantity of x-rays
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= # x-rays in the beam
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Quality of x-rays
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= energy of x-rays in the beam
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Characteristics of x-ray quality
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Penetrability-higher quality=higher energy=more penetrability
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Control of x-ray quality
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Controlled by the kVp of the instrument
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Control of x-ray quantity
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Controlled by kVp-energy of electrons in the beam=amount of interactions
and mA-current in milliamps=rate of electron emissions |
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mAs
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Current * exposure time
influences the quantity of electrons in the beam |
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kVp
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controls tube voltage
influences the quality (mainly) & the quantity |
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Target loading chart
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Chart used for older equiment to ensure you don't damage the tube by using settings that are too high
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x-ray filters
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a sheet of aluminum/copper placed between the x-ray tube and the collimator
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Purpose of x-ray filters
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Lower radiation exposure of the patient since low energy x-rays would otherwise be absorbed by the patient
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Function of x-ray filters
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To selectively remove low energy x-rays from the primary beam
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Collimator
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adjusted the size of the x-ray field
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Function of the collimator
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limit the x-ray field to only the areas of interest.
Enhance image quality by absorbing scatter radiation |
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Proper collimation
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Should have a white border around x-ray field.
Limits the radiation exposure to patient & holder |
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Function of Grids
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Reduce the amount of scatter radiation striking the x-ray film.
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Compton Scattering
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Most important form of scatter radiation.
Caused by a primary x-ray photon hitting a dislodging a electron in the patient, turning it into a secondary x-ray, which strikes film randomly |
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Effect of scatter radiation
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Produces fog= a decrease in image quality, overall grayness to the film
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Factors affecting amount of scatter radiation
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tissue density
total volume being x-rayed -field size -thickness of patient As each increases scatter increases. kVp |
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Grids
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Plates with alternating lead & aluminum strips. 80-160 strips/inch
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Focused Grids
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Strips are angles to match the angle of the primary beam.
Need to increase mAs when using, b/c not absorbed |
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Grid ratio
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"One of the most important factor when buying."
Ratio=height of strip:distance between them Most 8:1/10:1 |
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Advantages of higher grid ratio
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↑ grid ratio=↑image quality
Removes more scatter |
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Disadvantages of higher grid ratio
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↑grid ratio=↑ absorption of primary beam as well.
Have to use higher mAs. Much more expensive. |
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Bucky tray
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Device in tables that shakes the grid during exposure.
↑ image quality, by blurring gridlines |
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Types of grid cutoff artifacts
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Lateral decentering
Angled Upside-down Cause underexposure of film |
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Lateral decentering
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Most common cutoff
Focused Grid is not centered, so angle of strips doesn't match angle of beam. Causes general underexposure, with apparent gridlines |
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Angled grid artifact
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Caused by grid being tilted, relative to beam/film.
Common in Large animal. Severe underexposure, ↑gridlines seen |
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Upside-down grid artifact
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caused by=== upside-down grid
completely absorbs beam, except in very center |
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X-ray film emulsion
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A gelatin mixture with silver halide crystals
1 layer in ultrasound both sides coated in diagnostics |
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Conversion of silver halide to image
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Crystals absorb energy (x-ray or light), release photoelectrons, which are caught by silver ions-converted to metallic silver during development
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Function of intensifying screens
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intensify x-rays that hit it, allowing for lower radiation use.
Actually responsible for most of the film exposure that occurs |
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Types of intensifying screens
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Fast
Detail |
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How intensifying screens work
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contain light-emitting phosphors in plastic support.
When an x-ray hit it releases a burst of light that exposes the film. |
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Fast screens
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thick phosphor layer
larger phosphor crystals for short exposure, lower detail, used for LA films |
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Detail screens
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Thin phosphor layer
smaller phosphor crystals much greater detail, but need higher mAs. Used for head/extremity films |
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Intensifying screen artifacts
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Screen craze
Screen dirt |
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Different color of screens for color sensitive film
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Blue-sensitive=Calcium tungstate
Green-sensitive=Rare earth |
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Rare Earth screens
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For Green-sensitive screens (orhto film).
far more efficient than Calcium tungstate screens=lower mAs |
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Screen Craze artifacts
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Sharp white dots or irregular lines
superimposed over the radiograph Caused by cracks/scratches in intensifying screen |
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Cassette dirt artifacts
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Caused by objects on the screen.
Appear a sharp white areas on film (shaped like object) Prevent by cleaning darkroom & screens @ least 1x/month |
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Cassettes
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Hold film & intensifying screen.
Must be light-proof-felt strips around outside |
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Cassette components
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Radiolucent material on top (carbon fiber/aluminum)
Lead on back-absorbs backscatter. Foam on inside to keep film/screen together. |
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Artifacts of the cassette
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Light leak artifacts
Film-screen artifacts |
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Light Leak Artifacts
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Caused when light leaks in around edges of cassette.
Exposes the film, causing fuzzy black areas @ edges |
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Film-screen contact artifacts
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Caused when film & intensifying screen aren't touching=decreased image detail
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Affects of radiation on the body
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Main effect is on DNA; causes damage that leads to cell death/ mutation or prevents cell reproduction
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Units of Radiation
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rad
gray |
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rad
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=Radiation absorbed dose
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gray
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(IU)
1 gray=100 rads |
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Radiation dose equivalent
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rad*quality factor=
based on type of radiation, & how much damage it can do |
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Units of radiation does equivalent
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Rem-Radiation equivalent Man
sievert=Sv -(IU) 1mSv = 0.1 rem 1 rem = 10 mSv |
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Yearly limits of Radiation exposure
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maximum of 5 rem/year
(5000 mrem) |
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Sources of radiation exposure
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Background radiation
Man-made sources |
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Sources of Background radiation
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Cosmic radiation
Earth's Crust Interal exposure |
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Man-made sources of radiation
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x-rays, MRI, tv, luminous watches, nuclear fallout
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