breast physics Flashcards
Why is the XR tube for mammo oriented w/the cathode at the chest wall side?
- To mitigate heel effect & provide a more uniform photon flux at the detector.
Why is a small focal spot used in mag mammo?
- Focal spot blurring increases as objects are moved further away from the Ix receptor.
- In mag mammo, the breast is further from the Ix receptor, so the blurring could be significant.
- So a small focal spot is used to reduce blurring & maintain high spatial resolution.
- What focal spot size is used for a standard mammo (CC/MLO)?
- Mag?
- 0.3mm
- Mag = 0.1mm
What target anode is used in mammo & why?
- Moly or rhodium anode (general rads uses tungsten).
- Mammo needs characteristic energies way lower than that of tungsten (ideally b/w 16-23 keV), so a low atomic # target is used.
- Both have low Z so increased PE effect.
Compare mammo to general XR re:
- Tube current
- XR beam energy
- Most common anode
- Exposure time
- Receptor air kerma
- Window material
- Focal spot size
- Grid ratio
- Optic density
- View box brightness
- Processing time
Why are grids used in mammo, in general?
- To increase contrast.
- They do not compromise spatial resolution, but they increase dose.
In a standard mammo image, why is the glandular tissue brighter than the fatty?
- B/c the glandular tissue has a higher linear attenuation coefficient than fat, through the diagnostic energy range.
- So fewer XR photons make it through the glandular regions of the breast.
- In mammo/XR, tissues that are more attenuating (glandular tissue, bone), are displayed w/brighter values.
Consider this breast image; what was the source of radiation (and keV) used to generate this image?
- This is molecular breast imaging (MBI).
- Tc-99m sestamibi was used & emits 140 keV gamma rays.
In this set-up, the distance from the small focal spot to the Ix receptor is 66cm.
The breast is on a platform that is 26cm above the Ix receptor.
How much bigger will the breast in this mag image compared to breast that is imaged in contact mode?
- Magnification factor = the ratio of the source to image receptor distance (SID) / to the source to object distance (SOD):
Mag = SID / SOD
Mag = 66cm / 40cm
Mag = 1.65
For a typical mammo study, what is the average dose to the breast?
- 3 mGy (1.5 mGy per view, i.e., CC & MLO).
Compare “contact” mammo to mag mammo re:
- Breast contact
- Grid status
- Focal spot size
- Paddle size
- Tube current
- Exposure time
- For contact mammo shown here, where is the antiscatter grid located?
- Between the breast & image receptor.
- The anti-scatter grid prevents much of the scatter from the breast from reaching the image receptor.
Which XR target/filter combo is best to image a thicker (7cm) dense breast?
- Tungsten target / rhodium filter.
- This provides a higher effective energy than does Rho/Rho.
What compression force is used in mammo compression?
- 25-45lbs.
What are the max mAs for the 2 focal spots used in mammo?
- Direct contact mammo: FS = 0.3mm; mA = 100
- Mag mammo: FS = 0.1mm; mA = 50
Which kind of XR tube window is used in mammo & why?
- Beryllium, instead of glass.
- Beryllium doesn’t attenuate the energies used in mammo.
What is the dominant photon interaction for creating soft tissue contrast in mammo?
- PE effect.
- As is the case of most diagnostic XR studies.
What is the relationship of breast compression, noise, and contrast?
- Breast compression reduces breast thickness, which reduces scatter, which reduces noise, which improves contrast.
In mammo, what cm of soft tissue attenuates half the XR beam?
- ~1cm.
- In conventional XR (80 kV), ~3cm of soft tissue attenuates 1/2 the XR beam.
Why are grids not used in magnification mammo?
- Adding a grid would increase dose w/o significantly reducing scatter.
- The air gap already gets rid of much of the scatter from the breast.
Should thyroid shielding be used in mammo?
- NO, NEVER!
- Since 2002, it has been shown that the thyroid dose is insignificant.
- Also, they can result in inadequate or repeat studies.
What is the normal spectrum of molybdenum?
- High peak of characteristic x-rays at 18 keV.
What is the recommended source to image distance for mammo?
- 50-70 cm.
What is the biggest advantage of compression in mammo?
- Dose & scatter reduction.
In digital mammo, what is the XR tube voltage range?
- 25-35 kVp
Mammo grid:
- Why is a smaller grid ratio used?
- What is the grid ratio?
- Mammo vs. XR grid ratio?
- How does grid affect dose?
- Bucky factor for mammo vs. XR?
- B/c the breast is compressed & you are using a lower kVP, both which reduce scatter, so a smaller grid ratio is used.
- Grid ratio = H / width b/w grids.
- Mammo GR=4-5; XR GR = 6-16.
- Grid increases dose as the power is increased else you will underexpose.
- Mammo bucky = 2; XR bucky = 5.
In mammo, the cathode side of the tube should be placed where?
- By the chest wall. (Cathode / chest)
- In what 3 ways is mammo different from XR & why?
B/c the difference b/w regular breast tissue & breast cancer is very small, mammo needs:
- Lower energy: 16-23 keV (lower voltage to get this, 25-30 kVP).
- Nearly mono-energetic beam.
- Increased spatial resolution: to see microcalcs.
