Mammography & screening Flashcards
Core issues of mammo
1) Low tissue contrast
2) Subtle distortions in breast architecture
3) looking for small object - micro calcs ~0.1mm
Mammo geometry
1) FIxed geometry - except in mag
2) Uses anode heel effect
3) Small focal spot
Mammo x-ray tube
- Small focal spot
- Low kV (25-35kVp)
- Mo, Rh or W annode
- Mo, Rh Ag or Al filter
- Anode heel effect reduces intensity at nipple edge.
Mostly W target now
Impact of filtration
- Removes low E photons
- reduces skin dose
- increases mean energy of beam
- K edge filters generally used for mammo
Effect of k-edge filter
- Filter is relatively transparent below the k-edge
- > char x-rays transmitted
- Signif atten of bremstrah above k-edge
- increases contrast
Benefits of compression
- Immobilises breast
- Reduces the dose
- Improves image quality
- Reduces scatter within the breast - thinner tissue
Compression systems
Pre-compression : Position before full compression
Tilting paddles: conform better.
Compressibility detection available
Shifting paddle: L-R adjustment
Define AEC
Controls amount of radiation at the detector
Ensures correct exposure regardless of thickness / composition
Not used with implants
Must work with vary low rad level due to attenuation
AEC types
Basic: mAs
kV selection: dose rate det select on test shot
Adjust based on dose rate
Full auto: filt/voltage based on test shot
target/ filt/volt based on breast thickness
All based on test shot
Mammo detector type
Mainly a-Se and CsI with a-Si
Advantages and limitations of DR mammo
• Better contrast performance • Larger dynamic range • Improved dose efficiency • Imaging processing • PACS • Lower resolution • Start-up costs • Image processing
Advantages and limitations of film mammo
Analogue • Contrast-latitude compromise • Limited dynamic range • No post processing • Stability of film processing • Proven technology • High resolution • Affordable
DR spectrum
Harder more pen beam
reduction in contrast but can inc depth dose to compensate
Film would get too dark, lower patient dose while maintaining im quality
Phillips sys
Collimator defs many thin beams which scan accross
No need for a grid
Photon counting det features
Electronic noise effectively eliminated
• Secondary carrier noise eliminated
• Digitisation noise eliminated
• DQE potentially improved due to good detection
efficiency coupled with less detector noise
• Response has to be very quick
Measurement of mammo dose
Mean glandular dose - at-risk dose
ESD doesn’t take into account changes at depth
Measurement of breast dose
• Simulated with PMMA blocks - QA • Dose surveys • Calculate entrance surface air kerma • Derive MGD using HVL and composition dependent conversion factors
NDRL
for 50-60mm thick breast NDRL is < 3.5mGy
Define stereo-tactic mammo
Add-on for standard mammo or dedicated
Uses two images recorded at different angles, combined to give depth information - via ref point matching
For placing guide wires
Digital breast tomosynthesis
- X-ray tube moves in an arc about a pivot point
- Breast and detector (in general) do not move
- Acquire a series of low dose projections
- Computer reconstructs data into slices at different depths in the breast
removes effect of overlying anatomy
large datasets
Breast cancer incidence
1 in 8 women will develop breast cancer
Patient Screen requirements
Originally aged 50-64 every 3 years
2005 - 50-70 every 3 years, 2 views
2016 - 43-73 every 3 years
Issues with younger women
less common
harder to image
increased risk of induction from screening
Protocol for older women
- Routine invites stop at 70
- Can still attend though
Risk vs benefit
Risk : 1 induced per 14000 screened 3 times over 10 years (aged 50-70)
Benefit : 1 saved for every 400 over 10 years
IRMER refferal
Women invited (referred) for screening if they meet the referral criteria in the Cancer Reform Strategy document. Invitation letter is signed by director of screening centre (or lead radiologist). No individual signed request.
Irmer just / auth
• Radiologist, breast clinician or breast screening radiographers
are practitioners
• Identity of woman and eligibility confirmed before authorising
exposure.
• Consider previous screening history
• Record kept that woman meets local and national criteria
• Record confirming that examination has been justified,
authorised and by whom.
Exemptions to standard protocol
Women who have recently been screened • Women who attend without invitation • Women over current age range • Partial examinations • Technical recalls and repeats
