Radiographic Image and Technique Flashcards
X-ray interacts with … and this allows recording
matter
What’s inside an x-ray?
- oil for cooling
- transformers
- tube
Define ‘beam quality’
- related to the energy and penetration of the beam
- KV setting (KV affects contrast)
Define ‘beam quantity’
- related to the intensity of the photons exiting the tube
- mA/current affects quality
KV impacts … and …
If there’s a high KV, image will be …
- contrast and skin dose
- dark
What is the usual KVp?
Why?
- 70
- reduces skin dose
What happens when you increase current and time?
- more electrons
- more x-ray photons
- no change in energy
- darker film
- increases dose
- aim is to increase the kVp, decrease mAs
- payoff between dose and contrast
Define ‘image contrast’
- difference in density between 2 adjacent structures
Define ‘image resolution’
also called clarity
- how small a structure can be resolved
What equipment looks at image contrast?
- aluminium step wedge
- assess develooper processor/film contrast
Image contrast is affected by …
- Kv
- developer temperature
- use of intensifying screens
- film fogging
- scatter
Explain scatter
- also known as secondary radiation
- scatter degrades the image if it lands on receptor or film
- only primary beam not deflected - contributes to image
- scatter goes in all directions - forward scatter may degrade the image
What may look like scatter that isn’t scatter?
- streak artefacts on CTs
How can you minimise scatter?
- use collimators
- and aiming devices
- DPT or CBCT machine
How to reduce scatter reaching staff?
- double distance
- 1/4 the radiation dose
What is the control area in radiation protection?
- inverse square law deems a controlled area
- controlled area is anywhere in the primary beam and 1.5m from x-ray source
3 ways x-ray may interact with matter
- pass through
- absorption
- scattering
Explain absorption of x-rays
- photon is destroyed
- it’s entire energy is transferred to target
Explain scattering of x-rays
- photon is deflected
- transfers portion of its energy to the target - source of noise/fog
What does absorption + scatter make?
attenuation of beam
What forces do atoms have that mean they don’t collapse?
- nuclear attractive force
- centrifugal force
5 interactions with matter
- coherent scattering
- comptom effect
- photoelectric effect
- pair production
- photodisintegration
Photoelectric absorption increases with …
Z
- atomic number
Compton scattering increases with …
- increasing KV
- photon energy
What is the photoelectric effect?
- in the object
- x-ray photon targets K-shell electrons in the atoms
- x-ray photon transfers all energy to electron and energetic K shell electron is ejected
- leaves vacancy in K shell
- L shell electron fills vacancy in K shell
- characteristic photon emitted
- photon energy - difference in K and L shell electron energies
Explain Compton scattering
- photon hits free electron
- photon has reduced energy and transfers energy to an energetic electron
- photons having small deflections retains most incident energy
- photons will scatter many times losing energy each time until energy is absorbed
Features of dental film
- waterproof and lightproof packet
- lead foil (with pattern)
- black paper
- film
- embossed dot
How does air/soft tissue look on an x-ray?
- lots of x-rays penetrate
- expose many silver halide crystals
How does bone look under x-ray?
- less x-rays penetrate than air/tissue
- fewer crystals exposed
How does amalgam/gold look under x-ray?
- few if any x-rays can penetrate
- silver halide crystals not exposed
What is the developing stage of film scientifically?
- exposure centres in crystals are converted into black metallic silver
How to stop having bulky sensors?
- use CCD-based sensors
How does a phosphor plate work?
- conduction band with electron traps below it
- x-rays are absorbed and you get a valence band
Explain photo-stimulated luminescence
- conduction band and red laser light at electron tags below
- becomes blue light - get grouped electron tags
