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