Mammography Physics Flashcards
Be aware that fibrous tissue and cancer have similar attenuation coefficients
What is the difference between breast tissue composition in older and younger breasts
Larger/older breast = higher fat content
Younger breast = tends to be fibro glandular
How does high glandularity of breast tissue affect the sensitivity and specificity
High glandularity limits the sensitivity and specificity of
What is high glandularity
Lots of fibrous tissue
What does sensitivity and specificity mean in terms of breast tissue
Sensitivity - ability to identify cancer
Specificity - ability to identify normal/benign properties
What are the 3 types of tissue found in the breast
Glandular
Fibrous
Adipose tissue
Compare the variation of attenuation coefficient with kV of carcinoma, fibrous tissue, fatty tissue
Carcinoma has the highest linear attenuation coefficient at a certain photon energy, then fibrous tissue then fatty tissue at the same photon energy
What type of energy is needed for mammography and why
Low energies are required to provide adequate contrast in image
Where would the automatic exposure chamber be found on the mammography machine
Positioned in area of typical high density breast tissue
What effect does mammography utilise
Anode-heel effect
What are the 3 different filter materials used in mammography
Molybdenum (k-edge 20kev)
Rhodium (k-edge 23kev)
Tungsten (W) (k-edge 69kev)
What material is the window of the mammography made of
Beryllium
Know that mammography uses shorter SID than xray
What is the size of the fine focal spot of mammography
0.1-0.3mm
Find a diagram of a mammography and label the different parts
- angulation indicator scale
- tube head
- face guard
- compression device
- cassette holder
- AEC chamber
- automatic release foot pedal
- up and down movement controls
Describe what the anode heel effect is
- anode is angled
- photons that hit the anode end have further to travel to exit anode hence form lower energy X-rays
- photons at anode angle/cathode end have less material to travel through to exit anode hence form higher energy xrays
- overall this forms a spectrum of xray energies increasing in energy as it travels up the angle of the anode
Explain how the attenuation of xrays work in mammography / how patient absorbs dose
- xray bream travels through tissue, low energy xray give their energy more easily than high energy xray
- absorption of energy of xray gives the dose to the patient
Why is the patient dose in mammography greater than regular xray
Low energy xrays need to be used for better contrast hence more xrays are required to get the image which increase dose
What concept/ principle brings about image formation in mammography
Different tissues attenuate xrays by different amounts providing our image
Be aware that at low energies, the difference between tissues is greater. So low energies give more contrast.
Hence, you must balance between dose and contrast
What is the purpose of anode rotation in mammography
Heat dissipation
Know that in mammography we can have filter combination/targets, give an example
Molybdenum and rhodium
What is the purpose of filter combination in mammography
for the spectral shaping of the beam for a given target (anode), which affects image contrast and radiation dose
- e.g W/Rh combination filter is used to produce higher energy xray beam
What is k-edge
the abrupt increase in the photoelectric absorption of x-ray photons observed at an energy level just beyond the binding energy of the k-shell electrons of the absorbing atom.
What 4 things are factors contributing to image quality in mammography
- contrast
- resolution/blur
- noise
- dose
What are some things that affect contrast in terms of imaging quality
- energy
- target/filter combination
- scatter
- windowing/levelling
- digital post processing
- display monitors
- ambient viewing conditions
How is scatter reduced in 2 ways in mammography
Grid
Compression
What are 3 types of scatter and how does scatter affect image quality
- unchanged
- absorbed
- scattered
- reduced contrast
What 6 things can affect image quality resolution
- focal spot size
- magnification
- compression
- movement
- digital processing
- pixel pitch
How does focal spot size affect image quality
Large focal spot causes blurred image due to penumbra
Hence small focal spot used in mammography
Why is an even smaller focal spot needed if there is magnification
Magnification creates larger image of small details, hence you need an even smaller focal spot as blurring increased
What is the typical thickness of a compressor breast
2-11 cm
What are 2 main types of abnormalities that can be found in the breast
- low/high contrast masses
- calcification
What equation calculates the force applied to breast
F = ma
How does compression of breast support mammography in terms of dose and contrast
As compression give lower breast thickness, you don’t need as penetrating a beam.
- can reduce energy of beam meaning contrast increases
What 3 benefits does compression of breast provide
- reduces breast thickness variation
- reduces latitude required
- reduces variation in optical density (OD) across image
What are the 5 ways breast compression can improve image quality or patient dose
- reduced dose
- improved contrast ( less scatter, softer spectrum)
- improved resolution/blurring
- reduced range of tissue attenuation
- improved visualisation (less overlap of structures)
What are 2 main ways of reducing movement blur
- minimise exposure time
- compression
What are the 4 areas/measurement of quality control as part of the quality assurance program
- dosimetry
- image quality tests
- equipment movement
- safety features
What does DID stand for in mammography
Digital image display
What is threshold contrast detection
a commonly used tool to assess image quality of imaging systems.
Find image and know what it looks like
What is digital breast tomosynthesis
- series of 2D images created in planes angled to the detector
- reconstructed tomohynthesis planes can be synthesised into a 2D image
What is the angular range of digital breast tomosynthesis
Varying angular range up to 50 degrees
Why does DBT have improved sensitivity and speciality compare my to regular mammography
Removes overlapping tissues in the reconstructed planes
What is digital breast tomosynthesis particularly useful for scanning
Useful for imaging dense tissue
DBT is not used in routine screening only assessment
What is the difference between step and shoot and continuous shooting in digital breast tomosynthesis
A CSSM acquisition provides more counts than the corresponding SSM due to extra data collected as the detector moved from one view to the next
Be aware that DBT issues anti scatter grid and regular does not
What 4 things can be measured to analyse image quality in DBT
- geometric distortion
- artefact spread
- CNR
- MGD
What is geometric distortion
unwanted “warping” of the image that distorts the spatial relationship among objects in the image
What is artefact spread
when there is an area with an assemblage or numerous artifacts spread on the surface
What is MGD
Mean glandular dose
- the average radiation absorbed dose to the breast glandular tissue in mammography examinations.
What are 6 issues/disadvantages associated with digital breast tomosynthesis
- longer exposure time
- larger datasets
- increase QC workload
- additional dose
- artefacts
- additional reporting time
What is contrast enhanced mammography
- using iodine contrast agent
- low and high energy images in succession
What is contrast enhances mammography image formed
- post processing creates image showing iodine accumulation and removes breast structure
Why is iodine used as contrast in contrast enhances mammography
- iodine accumulation seen on scan is a potential indicator of cancer
What is the relation of contrast to noise ratio and iodine concentration
- CNR increases linearly with iodine concentration
What is the mean glandular dose
Dosimetry model for a standard breast
The standard percentage of glandular tissue in the central region of breast is ?
29%
What is the remedial level of dose value compared to the standard breast
2.5mGy
What is the correlation between mean glandular dose and breast thickness
Increase in breast thickness directly causes Increase in mean glandular dose (mGy)
What is NDRL
- national diagnostic reference level
What is the purpose of national DRLs for screening mammography
Early identification of breast cancer
What is dose audit/ its purpose
dose audit is to audit the minimum dose needed and make sure it hasn’t increased due to any changes in the manufacturing process.
Breast tissue is relatively radio sensitive
What does SQAS, RQAS, NQAT, PHE stand for
Screening quality assurance service
Regional QA centre
National QA team
Public health england
Clear establishment of remedial and suspension levels of performance as part of quality assurance
What is NHSBSP
National breast screening program
- established in 1988 and routinely invites women aged 50-70 for breast screening every 3 years
What is the average dose given to patient for scan of 2 breast
4mGy
What is the assumed mean glandular dose per mammogram
1 mGy
What are 2 risks not relating to dose with mammography/screenjng
Overdiagnosis - detecting cancers that would not have a detrimental effect on life
False positives/negatives
3 main areas of artefacts on mammography
- patient related
- equipment
- digital post processing
Know what an under or over exposed breast looks like on mammogram
What are 3 specific artefacts that can be seen on mammogram image + describe what they are
- grid lines (grid lines superimposed on image)
- anti perspirant artefact (faint radiopaque densities in axilla region due to deoderang)
- image processing artefact (attempts to find edge of breast resulting in loss of edge artefact)
Familiarise yourself with what the mammography artefacts look like, grid lines, anti persoirant, loss of edge
Low kVp permits utilisation of photoelectric absorption and maximises radiographic contrast between fibro glandular tissue and cancer
Quality assurance is essential to maintaining standards in the breast screening programme, particularly focusing on image quality and dose performance measures
