Mammography Flashcards
Why is mammography technically demanding, requiring dedicated equipment?
There is little difference in the properties and, therefore, contrast of different tissues within the breast. Detecting cancers requires the distinguishing of small (e.g. ~ 0.1 mm microcalcifications), low contrast objects.
How is the geometry of a mammography examination different to that use in general radiography?
- Geometry is generally fixed for mammography examinations (i.e. on breast on the support table), except for magnification modes (i.e. breast on magnification platform).
- The geometry is such to make use of the anode-heel effect (reduced intensity towards the nipple edge).
- The position of the focal spot relative to the image receptor is different. It resides almost directly above the chest wall edge of the detector with collimation applied to cut off the beam at the chest wall edge (at what would have been the centre of the beam). This provides an asymmetric diverging beam to ensure no missed tissue at the chest wall edge and also no exposure to organs at risk (e.g. heart).
What are the differences between a mammography x-ray tube and a general radiography x-ray tube?
- A smaller focal spot is required to detect smaller objects (0.3 mm or 0.15 mm for magnification compared to 1-1.2 mm for general radiography).
- Low tube voltage (25-35 kVp) to maximise contrast and, therefore, the ability to detect low contrast objects.
- W, Rh or Mo (less common) target materials are used compare to just W in general radiography.
- Greater range of filtration materials (Mo, Rh, Ag, Al) compared to general radiography (Al, Cu).
- Position of focal spot relative to the detector means the anode-heel effect can be used to reduce intensity towards the nipple edge where there is less tissue.
What does target/filter combination selection depend on in mammography?
- Unit manufacturer/model.
- Characteristics of breast being imaged.
What is the function of the filter in mammography?
- Removes low energy photons to reduce skin dose. This also increases the mean energy of the beam and, therefore, reduces contrast.
- To tailor the photon energy spectrum using the material K-edge (i.e. variations in the probability of a K-shell photoelectric interaction, and therefore photon attenuation, across the photon energy range).
How is the target/filter combination selected to tailor the photon energy spectrum?
- The target is selected to produce characteristic lines at specific energies to match the breast being imaged. For one of the most common target materials (W), characteristic lines reside outside of the tube’s energy range to remove their effect.
- The filter is typically selected to have a K-edge just below the target’s characteristic meaning they are transmitted well. Above the K-edge, significant attenuation of Bremsstrahlung photons is apparent, thus improving image contrast. Variations of target/filter combination can vary the above effects to match the breast being imaged.
Why is compression applied in mammography?
- Immobilises the breast, preventing motion artefacts.
- Reduces dose to the breast - a thinner volume of requires lower exposure factors and less scatter dose will also be apparent.
- Improves image quality due the reduction in scatter and reduction in geometric unsharpness (due to objects being further from the image receptor).
What will happen to the collimation on most mammography systems when a compression paddle is attached?
Collimation will automatically change to the size and shape of the paddle.
What is pre-compression?
Some systems will allow a small amount of initial compression to hold the breast in place and allow for final positioning by the operator before full compression is applied.
How are the non-uniformities of the breast accounted for in compression on most mammography systems?
Flexible or titling paddles allow for more uniform compression.
How are smaller breasts positioned more easily on some systems?
Left-right shifting paddles allow for easier positioning of smaller breasts. Collimation will track the shift in paddle position.
How is compression optimised in some mammography systems?
The compressibility of breast tissue is analysed to determine the optimum compression force as a trade-off between patient discomfort, dose and image quality.
What safety systems are in place for compression in mammography?
- Auto compression release post-exposure.
- Limits to the levels of automatic and manual compression that can be applied.
- Powered gantry movement inhibited when compression is applied.
Why is the AEC so important in mammography? When is the AEC not used?
- Due to the large variability in breast composition and thickness, determination of the exposure factors required is very difficult.
- AEC is not used for implants due to the differences in attenuation properties when compared to breast tissue.
Where is the AEC located on a mammography unit? How does it work?
- The AEC is integrated into the detector.
- Certain areas of the detector are used to monitor air kerma and terminate the exposure when a limit has been reached. These areas can either be of fixed location or can be determined by the system depending on the breast being imaged.
- For basic AECs, the operator selects kV, target and filter and the AEC selects mAs.
- Operator selection of target and filter and AEC selection of kV and mAs is also possible on some systems. kV is selected based on compressed breast thickness or determined from measured dose rate during a low dose test shot.
- Fully automatic AEC systems (most common) control selection of kV, mAs, target and filter based on a test shot or on the compressed breast thickness.
