Mammography 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