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.
What factors in the film can improve spatial resolution?
fine grain structure to give good resolution, but the resolution can be improved further by using single screens and single-sided emulsion films. In general radiography, double-sided emulsion film is used and there are two screens in the cassette. This decreases the dose needed for a particular blackening of the film, but also makes the resolution poorer because of parallax and crossover unsharpness
back screen is used so most interactions take place at the edge of the screen nearest to the film and the light does not spread out as much, giving less unsharpness i.e. better spatial resolution.
What drawbacks does using single sided films have?
By using single-sided film the emulsion is in direct contact with the screen and the resolution is therefore as good as possible. However, use of a single screen and single-sided emulsion means than a higher dose is required than if doubled-sided film with two screens were used.
How are the film characteristics different between mammography and general?
mammography film has a small latitude, i.e. the range of exposure needed to cover the greyscale is reduced, so that a small change in exposure gives an enhanced change in blackening of the film. This means that mammography film has a very steep linear part of the sensitometric curve
What is often the best compromise of target, filter and tube energy for mammography?
To get acceptable image quality for acceptable dose, we need to juggle the target filter and kV used in mammography. A good compromise turns out to be MoMo at 28 kV, and this is used extensively in mammography.
How does thicker breasts effect dose and contrast in mammography?
the doses are much higher than for average breasts as there is a greater thickness of material for the radiation to penetrate and so more is absorbed. Also loss of reciprocity in the film means even larger doses are needed to achieve the right film density.
Image quality will be worse as there is much more scatter as well as increased beam hardening within the breast, which lowers contrast; also the geometric unsharpness will be greater. The long exposure times needed to get suitable film densities at standard factors (i.e. 28 kV MoMo) can also lead to problems with movement blur further degrading the image.
How is dose and contrast mitigated for in thicker breasts?
it is advantageous to decrease the dose and exposure time. By varying the kV you can reduce the dose and exposure time. However, by changing the target filter combination, a greater dose saving can be made for the same loss in contrast, or a lower loss in contrast can be achieved for the same dose decrease.
Are these typical features of x-ray equipment used for mammography?
A. Tube voltage of 30 kV to 40 kV
B. Molybdenum target
C. Stationary anode
D. No grid for magnified views
E. Focal spot size 0.8 mm
F. The axial ray is directed to the centre of the x-ray field
A. False. The kV is usually 25 kV to 32 kV.
B. True. The target is usually molybdenum, with perhaps a selectable rhodium or tungsten target.
C. False. A rotating anode is used.
D. True. There is an air-gap for magnified views.
E. False. Focal spot size is usually about 0.3 mm.
F. False. The tube is angled so that the axial ray is at the chest wall edge; this helps to produce films with a more uniform density.
Are these reasons why low energy x-ray photons are used in mammography?
A. To reduce radiation dose to the breast
B. To increase the probability of photoelectric interactions in the breast
C. To avoid the need for an anti-scatter grid
D. To improve contrast between different breast structures
E. Because the screens used in mammography are specifically designed to interact with low energy x-rays
A. False. The aim is to improve contrast.
B. True. This is done by using low energy photons to improve the ratio of photoelectric events to Compton events.
C. False. Low kV will tend to increase breast dose, and it will not avoid the need for a grid.
D. True.
E. False. Even if screens were designed specifically to interact at low photon energies, this would be false. The screens are chosen to work with the x-rays, not the other way round.
Can a rhodium target tube can be used in mammography in the following ways?
A. To produce characteristic radiation at higher energies than a molybdenum target
B. In conjunction with a molybdenum filter
C. In conjunction with a rhodium filter
D. To give a higher mean photon energy than a molybdenum target
E. To give a higher peak photon energy than a molybdenum target
A. True. Rhodium has a higher atomic number than molybdenum and therefore its characteristic radiation occurs at higher energies.
B. False. If used with a molybdenum filter, the filter’s k-edge would result in the characteristic photons being strongly absorbed, so a rhodium filter is used.
C. True.
D. True.
E. False. The peak photon energy is determined by the tube potential (kV), and the target material does not affect this.
Regarding compression of the breast done in mammography:
A. Is principally to reduce movement unsharpness
B. Should not exceed a force of 200 N
C. Is only needed for magnified views
D. Reduces breast dose
E. Reduces scattered radiation reaching the image receptor
A. False. The reasons for using compression are to reduce the breast thickness, thereby reducing both dose and scatter. Any benefit in terms of reducing patient movement is coincidental rather than intentional.
B. True. Automatic compression is limited to 200 N, but greater force can be applied manually, though it should not be necessary to do this, and it would probably cause pain. Immobilisation could be achieved with much smaller compression force.
C. False. Compression is used for both magnified and normal views.
D. True. Reducing dose is one of the reasons for using compression.
E. True. Reducing scatter is one of the reasons for using compression.
Why is an angled tube head used in mammography?
In mammography we want the highest intensity part of the beam to coincide with the thickest part of the breast, i.e. the chest wall edge. To achieve this, the tube is positioned with the focus above the chest wall and the anode-cathode axis running from the nipple to the chest wall
What is the FFD used in mammography and why?
65 or 66 cm.
mammography equipment needs a long FFD to keep the patient dose low.
The further the film is from the source of the x-rays, the lower the radiation dose rate hitting the film. If the dose rate is too low then loss of reciprocity can occur in the film so it’s important to keep the radiation dose rate to the film high. Also the exposure times needed for a suitable dose at the film will be long, which can lead to movement blur on the images. Therefore short FFDs are wanted.
Optimal FFD is therefore a compromise between the low patient dose and high dose rates at the film.
What is the maximum force for compression in mammography?
The maximum force applied should be no greater than 200 N (equivalent to approximately 20 kg weight). Standard compression forces are normally between 100 and 150 N.
Why is the compression plate ooften angled?
As the breast is not the same thickness across the whole area, flat compression plates will often mean that the nipple edge is not properly compressed and this can lead to blurring on the images
Why are the doses for magnification views in mammography higher than standard?
For magnification views, the breast support table is above the film, giving magnification factors of around 1.8. In this case, there is a large air gap between the breast and the film and this works as a scatter reduction method and no grid is used. As the air gap is used instead of a grid, the increased dose necessary with grid use is not needed. This is good, as the FFD does not vary with magnification views but, by raising the breast above the film, the focus to skin distance is much shorter and therefore, the doses for magnification views are higher (inverse square law).
What are the main reasons for compressing the breast in mammography?
The reasons for using compression are to:
Reduce the breast thickness, thereby reducing both dose and scatter
Spread the breast tissues, reducing the number of tissues superimposed on any one part of the image and aiding the detection of abnormalities
Any benefit in terms of reducing patient movement is coincidental rather than intentional.
Why is an AEC detector so important in mammography?
It is important in mammography for the correct radiation dose to reach the film. We require a limited range of film density for a wide range of breasts. Breast tissue varies considerably, although in general smaller breasts tend to be more dense and large breasts tend to be more fatty, there are large variations across the population. For this reason it is difficult to manually select the correct exposure factors. Hence the use of AEC devices which automatically terminate the x-ray exposure when a predetermined level has been reached.
Where is the AEC placed in mammography?
In general radiography, the AEC detector is normally placed between the patient and the film. However, if this were the case in mammography, the AEC device would be visible on the images and for this reason, the AEC detector is placed behind the cassette.
Why can beam hardening cause issues with the AEC?
If an ion chamber is used as a detector, the energy deposited is measured. In mammography, this can be a problem since when the radiation beam passes though the breast, the lower energy x-rays are absorbed, so the spectrum of the radiation leaving the breast is different to that entering it. This is called beam hardening. Denser breasts and thicker breasts cause more beam hardening than thinner or less dense breasts. The amount of interactions with the screen in the cassette also depends on the spectrum of the radiation entering it. More of a harder beam will pass through the cassette and reach the AEC chamber than for the beam spectrum for a standard breast. In this case more photons will reach the detector also those photons will have higher average energies than standard. In this case the AEC detector will terminate the exposure sooner than for a standard breast and the film will be pale.
Regarding AEC on a mammography set:
A. Uses a detector that is positioned below the cassette
B. Is affected by beam hardening within the breast
C. Terminates the exposure after a fixed dose to the detector
D. Can be placed in different positions relative to the chest wall
E. Should increase the density for exposures done at higher kV
A. True. The AEC detector has to be below the cassette to avoid it appearing on the image.
B. True. Beam hardening increases the transmission of radiation through the cassette.
C. False. Due to beam hardening, the amount of radiation dose reaching the detector varies with breast thickness and composition. If the AEC worked by detecting a fixed dose, film density would vary with breast variations.
D. True. The detector can be moved to several positions so that different breast sizes can be accommodated.
E. False. The AEC is supposed to produce the same density for a wide variety of exposure factors, so it should not increase density at higher kV.
How are parameters optimised in mammography?
Currently available sets all have some sort of optimisation for target, filter and kV.
These come in two main types:
Thickness only
Thickness and composition
How does thickness only compensation work?
the thickness of the compressed breast is determined from the position of the compression paddle and then the target, filter and kV are suggested for that breast. MoMo will be used at kVs such as 25 kV for the thinnest breasts and MoRh or even WRh for very thick breasts with kVs up to around 32 kV.
How does thickness and composition compensation work?
Starting factors for the exposure are determined on the compressed breast thickness. After this, some manufacturers’ equipment does a pre-exposure to determine whether the breast is as dense as expected for this thickness, by looking at the dose rate and beam hardening. If it is more or less dense, the factors can then be altered. The kV can be adjusted, as can the filter, and if available, the target material. For example, for a thick breast that is more dense than expected, the starting factors may be MoRh 29 kV but will change after the pre-exposure to RhRh 30 kV.
Other manufacturers look at the dose rate reaching the AEC detector during the first part of the exposure and adjust the kV up or down if necessary. The filter cannot be adjusted on systems that do not use a pre-exposure.
How does stereo localisation work in mammography?
The stereotactic device is attached to the normal x-ray set and two images of the area of the breast containing the lesion are taken from different angles on the same film .
Parallax between the images is then used to determine the position of the lesion in 3D. Parallax is the apparent change in position of an object, caused by a change in observation point. The method of determining the position can be worked out using geometry, but the systems do this for you.
Once the position has been determined, a needle can be positioned correctly in the x and y directions before being lowered into the breast to the correct depth.
What sort of screen can be used for sterolocalisation?
Instead of film being used as a detector, a direct digital detector is used. It is called a small field digital detector as the field of view is very limited - up to around 40 cm2.
When the film/screen is used as the detector (analogue system), the film must be taken to a processing area and developed before returning the film to the room and finding the required position. This can take many minutes. Small field digital equipment has been developed primarily to speed this process.
What are the benefits of a dedicated stereotactic unit?
There are several dedicated stereotactic units, rather than those already described that fit to the general mammography x-ray set. With these, the patient lies on the horizontal tabletop in a prone position, with the breast positioned through an aperture in the table. The design is said to:
Improve patient comfort
Allow for easier positioning
Reduce the stress levels for the patient because they cannot see the procedure in progress
The x-ray tube is mounted below the table top, with a horizontal beam being used. The x-ray beam area is smaller than for a conventional mammography and, often, a digital detector is used rather than film.
What is direct digital mammography?
one-stage process of image production. In film and CR, the latent image is formed (stage 1) and then read/processed (stage 2). In direct digital radiography (DR), no latent image is formed. The signal is directly detected and used to form the image immediately. The detector is built into the x-ray set with the electronics incorporated into the system and no cassette handling is required. There are currently three types of direct digital detector:
Charge coupled device (CCD) detectors
Amorphous silicon detectors
Amorphous selenium detectors
Once the digital images (CR or DR) have been processed, they can either be printed on to laser film (hardcopy display) or displayed on a monitor for reporting (softcopy).
Why should all mammography reporting be done on a dedicated mammography reporting screen?
This is because acquisition workstation monitors are not normally of high enough specification to allow reporting. In mammography in particular, the specification and especially the resolution of the monitors is crucial to seeing the anomalies we are looking to detect. In general, reporting monitors used for mammography will be 5 MPixel monitors.
What are the advantages of digital mammography?
Wider dynamic range
Manipulation of image
No film processor - chemical hazard
Computer aided diagnosis (CAD)
Links in with PACS systems
Storage of images
Time, and immediate display with DR
Images visible immediately - fewer technical recalls
Lower doses if alternate target filters to MoMo can give acceptable image quality
The images may be transferred immediately to other sites allowing remote reporting by radiologists
Which of the following are typical features of an x-ray set used for mammography?
A. The anode does not rotate
B. Multiple target and filters
C. The tube voltage is typically 30-35 kV
D. The focal spot is above the centre of the image receptor
E. The nominal focal spot size is typically 0.8 mm
A. Incorrect. If the anode did not rotate the anode would not cope with heating.
B. Correct.
C. Incorrect. The tube voltage is typically 25-32 kV.
D. Incorrect. The focal spot is above the chest wall edge of the image receptor; this helps to produce films with a more uniform density.
E. Incorrect. The nominal focal spot size is typically 0.3 mm; the spatial resolution from a 0.8 mm focal spot would not be good enough.
Which of these are a normal part of mammographic x-ray equipment?
A. Compression device
B. Stationary grid
C. FFD of 65 cm
D. Automatic exposure control
E. Variable field sizes
A. Correct.
B. Incorrect. A stationary grid would be visible on the images, therefore, a moving grid is used.
C. Correct.
D. Correct.
E. Incorrect. Mammographic equipment has fixed field sizes, normally 18 × 24 cm and 24 × 30 cm.
Quality assurance (QA) is defined as:
The planned and systematic actions necessary to provide adequate confidence that a product or service will satisfy given requirements for quality.
Who mandates QA in mammography?
Quality assurance is a legal requirement under the Ionising Radiation (Medical Exposure) Regulations 2017
Why is QA in mammography extra important?
in mammography the system is working close to the best achievable performance of equipment, so a slight drop in the performance can mean that the system is no longer giving acceptable image quality.
In the Breast Screening Programme, we often x-ray women without immediately processing the films. This can mean that if there is a problem, up to fifty women may have had at least four exposures that are undiagnostic and need to be repeated.
What material is used for ruotine QA testing in mammography?
For routine testing, polymethyl methacrylate (PMMA) (perspex) is generally used. The relationship between breast tissue and PMMA varies depending on breast composition and thickness at the energies used in mammography, but for most breast thicknesses encountered clinically, the amount of PMMA is not that different to actual equivalent breast thickness. For testing, 4 cm of PMMA is equivalent to about 4.5 cm breast tissue.
Who offers guidance for what QA tests to do?
The Institute of Physics and Engineering in Medicine (IPEM) and NHSBSP publish guidance (Fig 1) on the tests and the frequency of the tests.
What QA tests should be done daily by the radiographers in mammography?
AEC consistency
Film processor sensitometry
Small field digital tests
Cassette cleaning
Inspection of breast support table and associated equipment
What factors are measured in processor sensitometry?
Speed index (SI). The density of a specific step
Contrast index (CI). The difference between two steps, to give an indication of the slope of the curve
Base + fog. A measure of the density of unexposed film
D-Max. Maximum density of the film
What QA tests should be done weekly by the radiographers in mammography?
Automatic exposure control consistency with thickness variation
Image quality
Stereotactic localising device
How is image quality tested in mammography?
This is carried out using a test object with details that allow compliance with the standards to be shown. the Leeds TOR(MAS) phantom contains a range of details (Fig 1) within a test plate (Fig 2). With the test plate placed on a 4 cm PMMA, an exposure should be made at 28 kV MoMo (molybdenum target and molybdenum filter) under AEC control to give the target density. The processed film should then be scored.
What are the most important QA tests in mammography?
AEC: If the AEC is not performing correctly, either in terms of target density or variation with perspex thickness, then the density of all or some films may be too high or low for the images to be diagnostic. Also if the AEC performance is not consistent, then the x-ray set is not usable
Image quality: If the image quality falls much below the required standard, the images may not be diagnostic
Sensitometry: If the performance of the processor is below standards, then the images will be poor no matter what the performance of the x-ray set
What quantity best identifies risk to breast tissue in mammography?
The tissue within the breast which is most sensitive to radiation is the glandular tissue, so the quantity which best indicates the risk from mammography is the mean absorbed dose to the glandular tissue within the breast . This is often called the mean glandular dose (MGD). Being an absorbed dose, it is measured in gray, or more commonly in milligray.
This is based on air kerma with some corrections for breast composition, thickness and the spectra used
What is the national DRL for mammography?
The national diagnostic reference level for mammography is 3.5 mGy. This applies to the average dose for an oblique view for a sample of women with 50 to 60 mm compressed breast thickness.
What is defined in IPEM as as ‘Standard’ breast?
5.3 cm compressed thickness
Outer 0.5 cm is adipose tissue
29% glandularity in the central region
The limit for dose to the standard breast is 2.5 mGy at clinical settings.
How many lp/mm is espected in mammography?
12
What is the smallest OD difference that can be detected by eye?
0.05 although this depends on the person. Very small differences can be seen when two films are side by side, and an experienced viewer will be able to detect changes of as little as 0.05 in film density. This will be greater for less experienced viewers.
