Mammography Flashcards
Below what energy level should we be in mammography?
Above approximately 28 keV there is no significant difference between glandular tissue and carcinoma and even below this energy the difference is very small. Therefore to be able to identify carcinoma we need to be below 28 keV.
At what energy level does the compton effect overtake the photoelectic effect in mammography?
To achieve good subject contrast when imaging soft tissues we need to increase the contribution of the photoelectric effect. For soft tissues, the probability of photoelectric interactions exceeds that of Compton scatter interactions below about 22 keV. For mammography we therefore need a significant proportion of the x-rays below an energy of 22 keV.
Why is tungsten not generally used as an anode target in mammography?
At the low generating potentials required in mammography, this would give a spectrum made up entirely of Bremsstrahlung radiation with a wide range of energies and a low output, which is not ideal.
Which descriptions refer to Bremsstrahlung x-ray production?
A. Electron is removed from inner shell
B. An electron, decelerating, radiates energy as an x-ray photon
C. Any energy is possible, up to a maximum equal to the electron energy
D. X-ray is produced as an electron falls into the vacancy
E. The process gives rise to a continuous spectrum
F. Produces a spectrum with discrete lines corresponding to the differences between the energy levels of the electron shells
A. F
B. T
C. T.
D. F
E. T
F. F
What creates bremsstrahlung radiation?
Electrons interact with the nucleus of an atom and are decelerated i.e. lose kinetic energy. The electron can lose any amount of energy in a single interaction up to all of its energy, or it may interact several times losing some energy at each interaction. Therefore the x-rays produced can have all energies up to the original energy of the electron, so a continuous spectrum is produced.
How are characteristic xrays produced?
The incoming electron ‘knocks’ an electron out of orbit, an electron from an outer shell ‘drops’ down to fill the gap. The excess energy is given of as an x-ray. The amount of energy released i.e. the energy of the x-ray corresponds to the energy difference between shells. Therefore, characteristic x-rays can only have discrete energies. These energies are characteristic of the atom.
What are the characteristic levels for Molybdenum?
Mo has characteristic x-rays at 17.5 and 19.6 keV, so a large proportion of the radiation beam is in the desired energy range.
What material is often used as a filter in mammography and why?
Since we want the spectrum to be as mono-energetic as possible, ideally we would like to filter out all energies other than those of the characteristic x-rays. For this reason a filter is chosen with a K-edge at an energy just above the characteristic energies. Mo has a K-edge of 20.0 keV.
How are kV and tube output related in mammography?
Output at mammographic energies is roughly proportional to kV^3. In general radiography, output is approximately proportional to kV^2.
Why is MoMo not the only target filter combo for mammography?
The reason that different target filter combinations are required is that, for large breasts, the energy spectrum of MoMo does not give energies that are high enough, i.e. the 20–23 keV required.
What materials are used to image larger breasts in mammography?
A rhodium (Rh) filter has similar properties to a Mo filter, but has a K-edge at 23.3 keV. This does not change the energies of the characteristic x-rays, as Mo is still the target material, but increases the amount of x-rays with energies in the range 20 to 23.3 keV. This is an advantage when imaging large breasts.
The Rh target has a spectrum similar in appearance to Mo, but has characteristic x-rays at 20.2 and 22.7 keV. A RhRh combination has a higher beam mean energy than that for MoMo and for MoRh.
In what situation can tungsten be used as a target in mammography?
This does not give a very high output as no characteristic x-rays are present and can lead to problems with long exposure times. However, the reason that W is used as an alternative material is the cost. Rh is extremely expensive compared to W, so although Rh gives better results, W is sometimes chosen as an alternative target material. W can be useful in very large or dense breasts. In practice it is mostly used in breasts with implants or breasts that have been treated with radiotherapy.
There is some evidence that W can be suitable when used with digital mammography sets.
Why is a high spatial resolution needed in mammography?
not only looking for features with low contrast and little variation from the background, i.e. carcinomas, we are also looking for microcalcifications. These have a relatively high radiographic contrast but can be extremely small, typically 0.001 mm2 to several mm2.
The high contrast spatial resolution of a mammographic system using a film/screen receptor is more than 12 lp/mm (compare this to an image intensifier/TV system of around 2.5 lp/mm).
What 2 aspects allow high spatial resolution in mammogrpahy?
The high resolution is obtained in two main ways, with high resolution film and screens and by use of small focal spot sizes
What are the different focal spot sizes in mammography?
the standard nominal size for a broad focus focal spot is 0.3 mm and a fine focus focal spot is normally 0.1 or 0.15 mm.
How does focal spot size affect the high contrast spatial resolution of a system?
The x-rays are generated within an area of the target, the focal spot, and spread out from that area. if this was a point source, the objects are resolved clearly as separate. This will be the case however close together the objects are. However, what we have in practice is a not a point source . Here the radiation comes from all parts of the source. In this case, the radiation creating the image does not provide a sharp image, but has blurring at the edges. If the objects are too close together they can appear as one or an extra ‘object’ can be created. This is the reason that small focal spots are required in mammography.
Other than focal spot size how else can image quality in mammography be improved?
compression
Antiscatter grids
anode heel effect
How does compression improve image quality in mammography?
By compressing the breast, the thickness is reduced.
This:
Lowers patient radiation dose
Reduces scatter, as the thickness of scattering material has decreased, and thus improves contrast
Spreads the tissues out so that there is less overlaying of features, making identification of problem areas easier
Reduces geometric unsharpness by moving some tissue closer to the image receptor
Reduces movement unsharpness by holding the breast still
What is the typical compression force in mammography?
A typical compression force is 100-150 N.
How do anti-scatter grids improve image quality in mammography?
Anti-scatter grids reduce the amount of scattered radiation reaching the image receptor. In mammography, moving grids are used for all contact (broad focus) images. In general the grids are linear, although more complex honeycomb type grids are available on some x-ray sets.
Are Anti-scatter grids always used in mammography?
For magnification images using fine focus, an air gap technique is used to reduce the amount of scattered radiation reaching the receptor and so a grid is not used.
How does the anode heel effect increase image quality in mammography?
Due to the anode heel effect, the x-ray beam is not uniform in the direction parallel to the anode-cathode axis of the x-ray tube. This is used in mammography. We want the same amount of radiation reaching the film over its entire area. However the breast is not of uniform thickness - it is approximately conical in shape, being thicker nearer the chest wall edge.
The tube is aligned such that the anode-cathode axis is parallel to the chest wall/nipple direction of the mammogram. The tube is also angled so that the maximum output occurs at the chest wall edge of the beam with the output dropping towards the nipple edge of the field. This helps to compensate for the variations in thickness in the breast. Without this effect, it would not be possible to get the correct exposure across the breast, some parts would always be under- or overexposed.
What is reciprocity law failure?
The relationship between optical density and radiation dose is described by the characteristic curve of the film/screen system. Therefore one might expect that if a fixed amount of radiation (dose) is received by the film/screen system the film will always have the same blackening. However, this is only true when the dose rate is not extreme. If the dose rate is either very high or very low, then the film may be less black (lower optical density) than expected for the radiation dose received
How can very low dose rates in mammography effect the film?
the radiation coming out of the breast is at a very low dose rate. X-ray film is made up of a large amount of grains (or crystals), normally of silver halide. The conversion of the silver halide to silver is how the image is formed. For an individual grain to be converted, it needs to receive more than one direct exposure to light from the screen.
If the dose rate is very low it is possible for the time between exposures in one grain to be so long that the grain becomes ‘de-sensitised’ and for the grain to return to its initial state before the next exposure occurs. Therefore, more exposure is needed to convert the crystal to silver than if the dose rate was higher. This leads to underexposed films (films which are not as black as expected for the dose given to them). In mammography, the grain size is very small due to the need for high resolution images. This, therefore, adds to the problem of loss of reciprocity as each grain is less likely to receive the number of exposures required.