Projection Imaging Technology Flashcards
What determines whether you can see an object in an image? (x3)
- Contrast
- Noise
- size
How do you define radiation contrast?
WHat does it depend on?
* Incoming radiation = I0
- Radiation leaving patient soft tissue (u1) = I1 = Io e(-u1 t)
- Radiation leaving patient soft tissue (u1) & bone (u2) = I2 = Io e-u1(t-x) e-u<span>2</span>(x)
- Define contrast as (I1-I2)/Io
Contrast depends on the thiclness of th eobject and the difference in linear attenuation coefficent of th eobject and the background. Property of object and spectum used u=u(E).
At what energies is contrast between bone and soft tissue the best?
Disadvantage?
Difference in attenuation coefficents for bone and soft tissue both fall with energy, largest difference between the two (greatest contrast) occurs at low energies. But that comes with a dose penalty.
What effect does scatter have on image?
What can you use to reduce it?
What is no noise a symptom of?
reduced contrast, difference in intensities is the same, but the denomenator increases due to more noise.
reduce it with an anti-scatter grid which only permits x-rays that are travelling in perpendicular direction to pass, move the gird about to make sure it isn’t in the image afterwards.
Noise is normal, presnet in all images, too make = non-diognostic, too little = overexposure
3 types of noise
- Quantum
- Electronic
- Fixed pattern
Describe quantum noise
- Number of photons in beam is governed by random processes, subject to quantum fluctuation, Poission distributed
- Standard deviation is measure of noise, sigma = root N
- SNR = N/sigma = root N
Describe electronic and fixed pattern noise
Electronic
- Can be significant at low doses
fixed pattern i
- pixel to pixel variation
- should be low due to flat-fielding
Not really sure what this means. Quantum should dominate at clinical range.
Spatial resolution:
Definition
Factors that affect it
Spatial res describes the ability of the system to resolve fine detail. Higher res = finer detal recorded.
Affected by:
- Detector characteristics (pixel size, spread of signal, focussing of e)
- Focal spot size
- Magnification
Fixed Radiography system components:
Bucky
Where is it in room?
What is in it?
Sits between exit side of patient and image receptor. Houses grid (that moves) and AEC sensors, holds cassette or DR detectors.
Automatic Exposure Control
Purpose
How does it work?
Purpose
- Ensure DD recieves right amount of radiation
- Removes guesswork from exposure settings
- Copes with varying body thickness and tube kV
How does it work?
- 3 of them, behind grid, in front of detector, transparrent to x-rays
- Ionisation chambers to monitor air kerma and terminate exposure when predicted limit is reached
Some stuff here on equipment needed for different projection techniques, mammo, dental, flouro, mobile, etc. ignored these.
4 types of image receptors & Use
- Glass plates - obsolete
- Film - dental
- Screen-film systems - obsolete
- DR detectors and CR
Advantages (4) and disadvantages (3) of analogue imaging.
Advantages:
- Proven to work
- High res
- Affordable
Disadvantages:
- contrast comprimise
- Limited dynamic range
- No post-processing
- Instability of film processing
Advantages (6) and disadvantages (2) of analogue imaging.
Advantages
- Better contrast
- larger DR
- improved dose efficency
- image processing
- PACS
Disadvantages
- Lower reoslution
- Start-up cost
Film-Screen radiography:
How does it work?
- Photons captured by flourescent screens give off light
- Light recorded on double emulsion film
- Chemically processed for image
- Non-linear response requires precise exposure
- BECOMING OBSELETE
