Physics of radiography and radiation safety Flashcards
What are the 4 things you can alter to affect the image that is produced?
- Kv
- mA
- Time
- Film Focal Distance
What is KVp?
Peak voltage across the cathode and the anode
What does increasing KVp do?
- More photons
- Increased energy
- More penetrating
What does increasing mA do?
- Increased heat in the cathode filament
- Increased number of electrons
- Increased number of protons
What is FFD?
Film Focal Distance
distance between the focal spot on the anode and the detector under the patient
What is absorption?
- Removed energy is transferred to the patient
- Increases with increasing atomic number
- Creates contrast between tissues
What is Scatter?
- Removed energy is emitted away from the patient
- Gets worse with increasing KV
- Hazard to radiographer
- Causes loss of contrast due to fogging
What causes film blackening?
Transmission and interaction with film-screen detector
What is the function of the emulsion part of the image?
- Produces image
- Silver bromide questions
What is a latent image?
- Not visible to the naked eye
- Must undergo chemical processing
What is the advantage of an intensifying screen/ casette?
- Reduced dose
- Shorter exposure time
- Less motion blurring
- Less scatter
What is the disadvantage of an intensifying screen/ casette?
Lost resolution
How does computed radiography work?
- Casette contains a storage phosphate image plate containing photosimulable crystals
- X-ray energy is absorbed and temporarily stored during an exposure to create a latent image
- Cassette put into the processor where the image plate is removed and scanned by a laser which sets the stored energy as visible light
- Photodiodes capture the light emitted and converts it to a digital signal
- Image plate is exposed to bright white light to delete the latent image and allow re-use
What is direct radiography?
- Flat-Panel detectors convert x-rays into electrical charge
- either direct or indirect converting systems
- Detectors either sit underneath the x-ray table or on the tabletop and can be used with grids
- Signal from the detector to the computer can be wired or wireless
What are the advantages of digital radiography?
- Greater tolerance to sub-optimal exposure factors
- Images can be manipulated
- Images can be shared
- Quicker
- Easy storage
- No replacement film costs
What are the disadvantages of digital radiography?
- High initial set up cost and ongoing maintenance
- Overexpenses can be overlooked
- Interpretation can be limited if the computer monitors are not of adequate quality
What does deterministic mean in terms of radiography?
- threshold levels for effects exist
- Severity is proportional to the dose received
What does stochastic mean in terms of radiography?
- No threshold level of radiation exists
- Probability is proportional to the dose received
What does hereditary mean in terms of radiography?
Stochastic effects which occur in the offspring of the exposed
What are the three principles of radiation protection?
- Radiation should only be undertaken if there is a definite clinical justification
- Any exposure to personnel should be kept to a minimum
- No legal dose limit should be exceeded
How would you limit occupational exposure?
- Rotate staff
- Record any exposure involving staff
- Exposure chart
- Electronic charge
How would you use a personel dosimeter?
- One badge per employee
- Worn on trunk underneath the protective clothing
- Stored outside the controlled area
- Store away from heat and sunlight
- Retain records for two years
What is the function of emulsion in x-ray film?
- Produces an image
- made from silver bromide crystals
What do the photon + Silver Bromide crystal produce together?
Silver atom is deposited
What are the three different X-ray interactions with tissue?
- Deterministic
- Stochastic
- Hereditary
How do you create a controlled area for radiation?
- Defined by a physical boundary
- Walls must be shielded
- Warning light and sign
