Lecture 1 + 2: DR Basics Flashcards
What do we require to generate x-rays? (4 marks)
- Source of electrons.
- Means of accelerating the electrons (PD).
- A target to fire the electrons at.
- A means of cooling the target as most of the energy goes to heat.
What are the processes that generate x-rays at diagnostic energies and draw a typical spectrum. (6 marks)
- Characteristic x-rays: produced when an inner shell electron is ejected by an incoming electron interacting with the (electrostatic interaction). An electron from an outer shell jumps down to the lower energy state, emitting a characteristic photon. Only gives off finite energies dependent on the nature of the atom.
- Bremstrahlung: german for braking radiation, occurs when the electron interact with the nucleus and are decelerated, as well as changing direction, releasing a continuous spectrum of photons, depending on the degree of deceleration which occurs.
- Spectrum should be drawn and show which is which…
Give doses for chest x-ray and barium swallow. (2 marks)
- Chest PA = 0.02 mSv
- Barium swallow = 1.5 mSv
What percentage of doses in the UK are from medical exposures and what is the total average annual dose? (2 marks)
~ 15%, 0.4 mSv from medical exposures, 2.7 mSv total.
Sketch an x-ray tube. (8 marks)
Sketch to include:
- Lead shielding to limit leakage to 1 micro Sv per hour at 1 m over 100cm^2, as per MDGN
- Cathode with a focussing cup, possibly two for different exposures.
- Anode with Cu stem to take away the heat.
- Rotor and a stator.
- High T cables for PD between anode and cathode.
- Surrounding oil for cooling.
- Berylium window to allow x-rays to escape.
- Sealed in a glass vacuum to make the generation more efficient.
What are the three main image quality parameters in DR which determine what is visible in an image? (3 marks)
Contrast, noise and res.
Explain the PE effect and how does this vary with Z and E? (4 marks)
- Photon interacts with a bound inner shell electron, ejecting it provided the photon has enough energy (above the work function)
- Gives rise to contrast in an image, useful in imaging.
- Probability proportional to Z^3 / E^3
- Dominates at low energies
What is contrast and how does the linear attenuation coefficient, u, vary with beam energy? (6 marks)
- Contrast is differences in intensity, or grey-scale values, within an image.
- C = (I2 - I1) / I2
- I(x) = I(0) exp (-ux)
- Exponetially decreases for both bone and soft tissue, it is higher in bone due to increased Z and density.
- Differences are greater at lower energies, hence more contrast at lower keV settings
What is compton scatter and how does it affect images in the diagnostic energy range? (5 marks)
- Compton scatter is the scattering of electrons from free, or loosely bound, electrons.
- The photons lose energy and change direction as a result.
- This is not useful in DR as the trajectory of the photons has been changed which introduces a source or noise in the image (unwanted signal)
- This reduces the SNR
- Compton scatter is roughly linear with energy over the diagnostic range, but then decreases (1/E) above 100 keV
- It is proportional to the number of free (ish) electrons
- Can use an anti-scatter grid to reduce this, strips of lead which move during the acquisition
What is noise and what are the three sources? (6 marks)
- Noise = unwanted signal
- Sources = electronic noise, fixed pattern due to pixel to pixel variations, quantum noise (N)^0.5 should dominate
- If there is no noise in an image then there is likely an overexposure
What is meant by the term resolution and what three things can affect it? (4 marks)
- Resolution = ability to resolve objects apart, can be determined in line pairs per mm or FWHM of line source measurements.
- Affected by focal spot size, pixel size and spread of signal in the detector, and magnification
How does an AEC work and what is it’s purpose? (5 marks)
- AEC is essentially an ionisation chamber located in front of the detector.
- It works by measuring the amount of ions produced in air, the exposure X, which is collected by polarising plates.
- Therefore it measures the amount of air kerma.
- It works as an exposure control device which terminates the exposure once a set amount of charge has been collected.
- This takes the guess work out of performing exposures and automatically extends the exposure time for larger patients.
- Housed inside the bucky which also contains a grid and detector.
Give pro’s and con’s of digital versus analogue systems? (8 marks)
Pro’s of analogue: high resolution of film, possibly cheap but out-dated so maybe not available.
Cons of analogue: wet processing is required, no connection to PACS and image processing is more limited. Lower contrast and dynamic range compared to digital.
Pro’s of digital: connected to PACS, image processing is available, no wet processing required. Better contrast and dynamic range (number of intensity levels which can be displayed)
Cons of digital: lower res and cost.
How does screen-film work? (3 marks)
- Uses silver halide embedded in emulsion for the film
- This is sandwiched between two fluorescent screens which give off light when irradiated.
- Processing is then required to develop the film and produce the image.
What types of digital detectors are there? (7 marks)
- Computed Radiography: PSP plate contained within a cassette. Requires reading out by laser light.
- Direct digital (amorphrous selenium) -> x-ray to electron to signal
- Indirect digital (amorphrous silicon) -> x-ray to visible photon to electron to signal
