Mammography - Physics Flashcards
What are the equipment features of a Mammography unit?
- Angled tube head
- C-arm design
- Fixed focus - detector distance (FDD)
- Compression Device
- Fixed Field Size
- Grids
- AEC
- X-Ray Tube
What is the Optimal FDD of a Mammography unit?
65 - 66cm
What is the standard compression force of the compression device?
100N - 150N
What is the maximum compression force of the compression device?
200N
What type of examination is done in Mammography?
Breast Only
When do we use Grids?
Typical Mammography except for Magnification where air gap is adopted.
What are the types of AEC for mammography unit?
SFM - Photo-timer, placed behind the casette
DM - Detector acts as the AEC.
What are the features of a Mammography X-Ray tube?
- Metal tube housing
- Grounded Mo, Rh anode
- Anode angle with tube tiled to 24 degrees to minimize Anode Heel effect
- Vertical Axis of rotation
- Mo or Rh filters for spectral shaping
Why is single track X-Ray Tube preferred?
Single track tube can deliver high current exposure needed for the largest breasts at an acceptable exposure time - reducing motion artifact.
What is operating voltage for cathode and filament circuit?
24 - 40 kVp
What is the focal spot size for Mammography?
0.3mm
0.1mm for Magnification Mammography
What is the purpose of having a small focal spot in Mammography?
- Minimize geometric blurring
- Maintains spatial resolution
Why is low kVp preferred in Mammography?
- To minimize Compton Scattering
- Maximise Photoelectric effect
- Enhance differential absorption by various breast tissue
- Reduce radiation dose to breast as Glandular tissues are highly radiosensitive
What is the material of the X-Ray tube window?
Beryllium
What is the target-filter & kVp range recommended for fatty breast tissue of up to 4cm thickness?
Mo/Mo
24 - 26 kVp
What is the target-filter & kVp range recommended for Glandular breast tissue of 5 - 7cm thickness?
Mo/Rh
27 - 31 kVp
What is the target-filter & kVp range recommended for dense breast tissue of >7cm thickness?
Rh/Rh
27 - 31 kVp
What is Half-Value Layer?
The thickness of any given material where 50% of the incident energy has been attuenated.
What is the Approximate HVL in breast tissue?
1 to 2cm - strongly dependent on tissue composition.
What is the generator power rating?
3 kW
What is the voltage supplied to the X-Ray tube?
22 - 40 kVp
Why is Breast compression necessary?
- Reduces overlapping anatomy
- Decreases tissue thickness of breast
- less scatter
- less geometric blurring
- more contrast of the anatomic structures
- Better visualization of tissues near the chest wall
What does spot compression entails?
- Better compression over a small area
- Decreases superimposition of overlying tissues.
What are typical interspace materials for the Grid?
Carbon Fibre/wood/paper
Al will attenuate too many of the low energy x-ray used in mammography
What is the the optimal ambient light intensity?
<20 Lux
What is the minimum MP of a Diagnostic Display Monitor?
5MP - to view fine microcalcifications
What is the optimal luminance of a mammography viewbox
3000 cd/m square
What are the advantages of a Digital Mammography?
- Lower dose
- Wider dynamic range
- Higher Contrast Resolution
- Better Signal Noise Ratio SNR
- Reduced False Positives
- Increased Positive Predictive Value PPV
- Image Manipulative Tools - fewer repeats, minimize patient’s exposure.
- Eliminate need to leave exam room to process films
- Eliminates processing issues
- Faster interventional procedures
- Faster patient throughput - cost cutting
