Imaging with X-rays Flashcards
(31 cards)
Describe the structure of double emulsion film
Foam
Intensifying screen
Film - Emulsion
Plastic
Emulsion
Intensifying screen
Foam
Describe the process of latent image formation using double emulsion film and the processing
- Incident x-ray
- X-ray converted to light (fluorescence)
- Light + Ag+ (from AgHalide in film) -> inert Ag deposited on plastic
Latent image - dark where the Ag has been deposited - Remove film
- Process in aqueous solution
- Reducing agent in solution reduces Ag to catalyse further nearby Ag
- Film place in oxidising fixer which dissolves surround AgHalide
What are the advantages of digital imaging over film?
Reusable
No processing - quicker, less skill required, immediate
Post-imaging modifications
Easier storage
What factors need to balanced when determining focal spot size?
Spatial resolution and exposure time
Smaller spot - better resolution but slower exposure time to allow for heat dissipation
Larger spot - poorer resolution but faster exposure time
What factors affect spatial resolution?
Focal spot size
Detector size
Pixel size
For CT
Projection number
Pitch
Slice thickness
What are the three primary ways in which gamma-rays interact with matter?
- Transmission
- Absorption (photoelectric effect)
- Scatter (compton and rayleigh)
Pair production
At which energy does Compton scatter start to dominate?
~50 keV
At what energy does pair production start?
> 1.022 MeV
What is the active material in computed radiography?
Photostimulable phosphor (PSP) e.g. barium fluorohalide
Briefly describe the key points in the process of image acquisition and readout in Computed Radiography imaging.
- Incoming x-ray excites electron
- Electron moves to the conduction band
- Electron drops back to valence band emitting light
- Some electrons drop to trapping sites - impurities - where they will stay for a while, for later reading
The electrons are trapped until read either by light or heat. For CR this is done with a laser readout - fine focussed red light (∼600-680nm) in a raster fashion. Emission of blue light (∼300-500nm) can be collected and viewed
What happens if you change the thickness of phosphor in a CR detector?
If you Increased the phosphor thickness
Increased efficiency (more to trap the photons)
You would get more light spread, leading to a worse resolution
What are the advantages of CR?
- Similar work-style to film: image cassette -> develop
- Digital imaging: improved contrast, image processing
What are the disadvantages to CR?
- Poorer resolution than film - pixel/matrix size, laser scattering, laser beam diameter, size of phosphor grains
- Readout times around 20-30 seconds
Two detectors are compared in terms of their Modulation Transfer Function (MTF).
Detector A has a MTF50 of 3 cycles/mm whereas Detector B has a MTF50 of 5 cycles/mm.
Which detector would be beneficial to use for mammographic procedures and why?
Detector B - more cycles per mm meaning it has a bettter spatial resolution meaning it can detect finer details in the image. For mammography, you need to detect small, low contrast structures (microcalcifications) which are often thenths of a mm
What is MTF50?
The spatial frequency at which the detector’s MTF drops to 50% of maximum
In a planar X-ray acquisition of a patient’s image, how does the post-patient radiation
spectrum incident upon the detector differ from the radiation spectrum incident upon the
patient?
The post-patient beam is harder due to preferential absorption of lower energy photons due to increased probability of the photoelectric effect at lower photon energies and hence lower energy photons are absorbed, leaving higher energy photons in the beam, creating a harder (higher energy) beam.
The intensity has also reduced due to the attenuation
There is also scatter within the patient which reduces contrast and adds a radiation protection risk.
What is an XR grid?
Physical collimation which only allows XRs from the direction perpendicular to the detector, usually made out of lead
How does the use of a grid improve image quality?
Reduces the amount of scatter that is accepted to the detector, reducing noise due to scatter and improving contrast
What are disadvantages of grids?
Reduces the sensitivity and amount of photons collected (removing useful radiation) which may mean the dose needs to be increased to get the same number of photons reaching the detector and hence constant image quality
What factors reduce scatter in XR imaging?
- Decrease kV
- Collimate the beam
- Reduce patient thickness
- Introduce an air gap
- Anti-scatter grid
How does reducing the kV affect the amount of scatter produced in XR imaging?
More photons are absorbed (lower energies), but this means that to get enough photons at the detector, the mAs must increase but alongside this (less significantly) there is a decrease in the forward scatter so the mean energy of scatter is lower and less leaves the patient
However, decreasing kV increases contrast (bonus)
How does collimating the beam reduce scatter in XR imaging?
Collimating the beam gives a smaller field, meaning there are less photons in the body and hence less scatter is created
How does introducing an air gap reduce scatter in XR imaging?
Scattered photons at the edge of the beam leaving the patient don’t reach the detector
What is the effect on image quality when introducing an air gap in XR imaging?
Magnification
