Mammography Equipment Flashcards
Why are doses kept to ALARA when imaging breast tissue?
Because it is sensitive to ionising radiation and is monitored carefully
What are the specific equipment designed to do when imaging breast tissue?
They are designed to give excellent resolution and contrast to allow subtle changes in breast density (and tissue atomic number) to be detected
What is mammogram?
A mammogram is an x-ray of the breast used as a screening tool to detect early breast cancer in women with no symptoms at all, it is also used to detect and diagnose breast disease in women with symptoms such as lump, pain, skin dimpling or nipple discharge
What are the components of a standard equipment for mammography?
Lower energy x-rays: 24-30 KVP
Smaller focal spot size: 0.1-0.3 mm vs 0.6-1.2 mm
Shorter source - receptor distance: 60-70 cm vs 100 cm SID
Manual/Automatic anode/filter, Exposure factor & Gantry height/ Angulation selection control
Compression plate
AED
What is done to fully allow the dose administered is ALARA?
Reject analysis, individual staff performance and recall rates, equipment QA, maintenance and testing all carefully monitored, supervised and adhere to with regular national statistic reporting, additional training, QC and service as required
What are the equipment specifications?
Anode materials: Molybdenum, Rhodium (Tungsten)
Filter materials: Molybdenum, Rhodium (Aluminium)
AEC device with posterior, middle & anterior Chambers (+/- lateral & medial chambers)
Quick/easy cassette release from lateral or medial aspect of the bucky (+/- anti ‘double-exposure’ feature)
Hard-wearing, detachable, cleanable bucky
Plastic compression paddle with AEC guide: capable of ~20N (20-25) pressure
Plastic face guard & patient handles
What does the dose/ contract depend on?
Breast composition
Equipment use
Patient positioning
What are factors to be considered in breast composition?
Atomic number
Size
Shape
Thickness
Density of the tissue
What are the aspects of patient positioning ?
Counselling
Preparation
Technique
Compression
What should be considered when equipment is being used?
Anode material/ filter selection
Photon energy
Spectrum of the applied x-ray Beam
What are the photons energy limits in mammography?
The standard exposures using clinical practise range from 24 to 30 keV
Above 31 keV, the ranges of attenuation of the two tissues types overlap
How do you attenuate differences atomic number?
By relying light on photoelectric attenuation of X-rays
Why does x-ray photons need to be very small?
The photoelectric absorption is inversely proportional to x-ray energy cubed.
If they are to experience much photoelectrical attenuation in the low atomic number of soft tissues of the breast to x-ray energy cubes energy, the xray photons need to be small but not to the point that they are absorbed in the skin as it will increase the radiation dose
Inversely proportional - as the photoelectric absorption increases the x-ray energy decreases
What is a bremsstrahlung process?
It provides a continuous spectrum of x-ray production
What is the relationship between the photon energy and the attenuation of the beam?
If the photon energy is too high, too low a proportion of the beam is attenuated and too little absorbed by the breast tissue reducing the radiation dose and reducing image contrast with too much beem penetration
Conversely, photon energy levels must not be so low that too high a proportion of the beam is attenuated and too much is absorbed by the breast tissue increasing radiation dose and reduces image contrast with too little beam penetration
What is the process of optimising contrast?
To optimise image contrast we must optimise the photon energy absorption to attenuation ratio
This is achieved by the application of an x-ray beam with a relatively low and narrow photon energy spectrum
What is a Focus Spot?
Very small, to reduce your geometric sharpness and maximise images resolution
Low tube KVp values means small focal spot can be used (low tube heating)
Additionally, the node angle may be tilted, in order to obtain a larger effective focal spot (line focus principle)
What is the line locus principle?
Increase target angle means actual focal spot size is maintained, but the effective focal spot size is increased this allows use of a very small focal spot which can achieve the required image coverage
What is used for the filtration of the beam and how?
Beryllium is used in the ports to allow low energy X-rays to exit.
The beam filtration is essential to eliminate the low energy x-rays that would add to patient dose and scatter
Filters are made of the same target material, this eliminates energies lower or higher than required for producing a nearly perfect beam
What is the use of compression?
It allows immobilization, prevents blur/ unsharpness
Reduces and evens tissue thickness (less exposure required to penetrate)
Separation of densities such as superimposition of breast tissue lesions would reduce scatter which in turn would reduce the ionising radiation dose
Why is a grid used?
The use of a grid greatly improves radgraphic quality but it does slightly increase the patient dose
It’s used for scatter control which is important as it reduces image quality
Scatter increases for larger and more dense breast with higher KV settings
Why is automatic exposure control (AEC) used?
Provides consistent image receptor exposure for various thickness and densities of breast tissue, it is virtually impossible for radiographer to estimate the density /composition of breast tissue therefore an AEC is required as it is an integral part of a mammo equipment
Why is magnification used?
To enhance an uncertain area or look at microclassification in more details
Magnification allows increase resolution, reduction in scatter, and improved visibility of detail
What is the use of a large SID and why is the anode end of the tube positioned toward the nipple?
A large SID is used to further reduce your geometric unsharpness
Positioning of the anode end of the tube towards the nipple permits a slightly larger x-ray intensity towards the chest wall, using the anode heel effect
What is the equipment construction in order?
X-ray tube housing
X-ray tube
Anode
Beryllium window
Filter
Extension cone
Compression paddle
Grid
Cassette
Variable position AEC
What is FFDM?
Full field digital mammography - digital imaging
Solid state detectors
What is the difference between a mammography unit and a general x-ray unit? - open ended question upon discussion
Mammography requires a much lower KV than conventional radiography common clinical practice uses 24-28 KVP
Mammo also utilises low mA settings which are necessary because of the smaller size anode small focal spot sizes is used
The anode configuration in a mammo x-ray tube produces a prominent heel effect due to the combination of the source to image receptor distance (SID) and the use of an narrow target angle- more concentration at densest area
The effective target angle, measured from the central point, in a mammo tube is 22-24 to achieve a narrower angle and maintain coverage the x-ray tube must be tilted
Anode target is made of molybdenum because it uses the useful photon energy range
Mammo has a relatively short SID of 60 - 76 cms
Beryllium filter in port to remove very low energy photons, and reduce skin dose
Built-in compression device
