Part 2 Flashcards
What is the purpose of a grid?
To reduce the amount of scatter getting to the image.
Where is the grid located?
Between the patient and the film.
What would the image look like without a grid?
Reduced contrast and the edges would not be as well defined.
What do you think governs the interspace material used to construct the grid?
The energy of the x-ray that we need to use. If we are using low energy photons (e.g. mammography) then aluminium interspace material would absorb the useful x-rays as well as the scattered ones.
What is the advantage to using focused grids rather than linear parallel grids in terms of film density?
Film density is more uniform as we move from the centre of the film to the edge of the film. This is due to the fact that the septa in the grid are aligned with the primary rays of the x-ray beam.
Explain the construction of a grid that has a “grid ratio” of 10:1 ?
This means that the septa are 10 times longer than the width of the interspace material.
Explain why we need a variety of grid ratios if high grid ratios absorb more scatter?
Although high grid ratios eliminate a lot of the scatter they result in an increased dose to the patient. In some cases such as mammography increasing the dose too much is not an option and therefore we need to use grids that have a lower grid ratio.
What affect will increasing the grid frequency have on the amount of scatter absorbed by the grid?
Changing grid frequency does not affect the amount of scatter absorbed if all other parameters are unchanged.
A patient is given an x-ray at 100kVp and 100mA and 0.6s.1. If a second image were taken using a grid giving the system a bucky factor of 4 what would be the new exposure?2. What would happen to the entrance dose of the patient after the grid is introduced?
- Old exposure = 60 mAs New exposure = 60 x 4 - 240 mAs. 2. It would also increase by a factor of 4.
Describe the different ways of positioning “linear parallel” and “focused” grids incorrectly and what the resulting image would look like?
Linear grid: angled (or tilted) with respect to the central ray. rotated 90°. Focussed grid: angled (or tilted) with respect to the central ray. at wrong FFD (or wrong focus). up-side-down. off-centre. rotated 90º.
Which of the following when changed (while all other parameters remain constant) will alter the amount of scatter reduction?
grid ratio and bucky factor.
Which of the following situations would cause grid cut-off with a linear grid?
Tilted.
A system has a “bucky factor” of 3. By what factor will the patient dose change if the grid is used compared to if it is not used?
Increase by a factor of 3.
Which of the following pieces of equipment would not be needed to carry out quality assurance tests on an automatic film processing unit?
A penetrometer.
Which of the following best describes single-sided films?
Higher resolution than double sided but suffer from ‘halation’.
“Contrast” “Spatial Resolution” and ‘Modulation Transfer Function’ are concepts used to describe the quality of an image. Explain in detail what is meant by each one.
Contrast is the grey/density difference between two adjacent areas on an image [must say “adjacent” for the full mark]. Spatial Resolution is the ability to distinguish separate objects as separate on the image. Modulation Transfer Function measures the performance of an imaging system at different spatial frequencies (different sized objects/gaps).
Discuss what SNR measures and which image quality parameter it is linked to.
SNR - How high the signal is in relation to the noise. Linked to contrast resolution.
Provide one method of reducing quantum mottle and a consequence of this.
Increase mA or Increase exposure time. Increased dose loading patient motion blurring - but has to be relevant to the first part of the answer.
Contrast and latitude are two characteristics of a film used in radiology. Discuss what is meant by each of these and how they are related.
Contrast is how the film translates exposure into a shade of grey. Latitude is how wide the range of exposures are that will still produce the best contrast with that film. Higher the contrast the narrower the latitude.
Discuss what property of a film/screen system governs the spatial resolution of the image and why.
Size of the phosphor crystals as these provide the light to the film crystals and one larger phosphor crystal can expose a number of smaller film crystals and resolution would be reduced.
Discuss in detail the cause of “Crossover” and “Halation” how this will appear in the image and how this is prevented.
Crossover: caused by light travelling through the base of the film to the opposite emulsion layer (1). Appears as geometric unsharpness in the image (1). Can be prevented by using a coloured subbing layer. This absorbs only this light and allows the rest to pass through. Halation: caused when light reflects from the back of the film and returns to the emulsion layer (1). Appears as a lighter image of the object surrounding the original image (1). Can be prevented by using an absorption layer on the back of the film (1). This absorbs all light that reaches it (1).
Discuss in detail the difference between fluorescence and phosphorescence as well as which of these is preferred in a film/screen system and why.
Fluorescence is immediate De-excitation (1). Phosphorescence is delayed (1). Fluorescence is preferred (1) due to improved efficiency of the system (1).
Discuss the operation of a ‘grid tube’ and an advantage and disadvantage if using one.
Operation (2) - The grid tube has a strong bias (voltage) on the focusing cup that confines the electron beam from the filament more than in a standard tube. Advantage (1) - can obtain a much finer focal spot while using a larger filament. Disadvantage (1) - focuses all of the electron energy into a much smaller area on the target which increases the chance if wear on the target.
