breast physics

Question 1

Why is the XR tube for mammo oriented w/the cathode at the chest wall side?

Answer 1

• To mitigate heel effect & provide a more uniform photon flux at the detector.

Question 2

Why is a small focal spot used in mag mammo?

Answer 2

• 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.

Question 3

• What focal spot size is used for a standard mammo (CC/MLO)?
• Mag?

Answer 3

• 0.3mm
• Mag = 0.1mm

Question 4

What target anode is used in mammo & why?

Answer 4

• 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.

Question 5

Compare mammo to general XR re:

1. Tube current
2. XR beam energy
3. Most common anode
4. Exposure time
5. Receptor air kerma
6. Window material
7. Focal spot size
8. Grid ratio
9. Optic density
10. View box brightness
11. Processing time

Answer 5

Question 6

Why are grids used in mammo, in general?

Answer 6

• To increase contrast.
  
  • They do not compromise spatial resolution, but they increase dose.

Question 7

In a standard mammo image, why is the glandular tissue brighter than the fatty?

Answer 7

• 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.

Question 8

Consider this breast image; what was the source of radiation (and keV) used to generate this image?

Answer 8

• This is molecular breast imaging (MBI).
• Tc-99m sestamibi was used & emits 140 keV gamma rays.

Question 9

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?

Answer 9

• 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

Question 10

For a typical mammo study, what is the average dose to the breast?

Answer 10

• 3 mGy (1.5 mGy per view, i.e., CC & MLO).

Question 11

Compare “contact” mammo to mag mammo re:

1. Breast contact
2. Grid status
3. Focal spot size
4. Paddle size
5. Tube current
6. Exposure time

Answer 11

Question 12

• For contact mammo shown here, where is the antiscatter grid located?

Answer 12

• Between the breast & image receptor.
  
  • The anti-scatter grid prevents much of the scatter from the breast from reaching the image receptor.

Question 13

Which XR target/filter combo is best to image a thicker (7cm) dense breast?

Answer 13

• Tungsten target / rhodium filter.
• This provides a higher effective energy than does Rho/Rho.

Question 14

What compression force is used in mammo compression?

Answer 14

• 25-45lbs.

Question 15

What are the max mAs for the 2 focal spots used in mammo?

Answer 15

• Direct contact mammo: FS = 0.3mm; mA = 100
• Mag mammo: FS = 0.1mm; mA = 50

Question 16

Which kind of XR tube window is used in mammo & why?

Answer 16

• Beryllium, instead of glass.
• Beryllium doesn’t attenuate the energies used in mammo.

Question 17

What is the dominant photon interaction for creating soft tissue contrast in mammo?

Answer 17

• PE effect.
  
  • As is the case of most diagnostic XR studies.

Question 18

What is the relationship of breast compression, noise, and contrast?

Answer 18

• Breast compression reduces breast thickness, which reduces scatter, which reduces noise, which improves contrast.

Question 19

In mammo, what cm of soft tissue attenuates half the XR beam?

Answer 19

• ~1cm.
  
  • In conventional XR (80 kV), ~3cm of soft tissue attenuates 1/2 the XR beam.

Question 20

Why are grids not used in magnification mammo?

Answer 20

• 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.

Question 21

Should thyroid shielding be used in mammo?

Answer 21

• NO, NEVER!
• Since 2002, it has been shown that the thyroid dose is insignificant.
• Also, they can result in inadequate or repeat studies.

Question 22

What is the normal spectrum of molybdenum?

Answer 22

• High peak of characteristic x-rays at 18 keV.

Question 23

What is the recommended source to image distance for mammo?

Answer 23

• 50-70 cm.

Question 24

What is the biggest advantage of compression in mammo?

Answer 24

• Dose & scatter reduction.

Question 25

In digital mammo, what is the XR tube voltage range?

Answer 25

• 25-35 kVp

Question 26

Mammo grid:

1. Why is a smaller grid ratio used?
2. What is the grid ratio?
3. Mammo vs. XR grid ratio?
4. How does grid affect dose?
5. Bucky factor for mammo vs. XR?

Answer 26

1. B/c the breast is compressed & you are using a lower kVP, both which reduce scatter, so a smaller grid ratio is used.
2. Grid ratio = H / width b/w grids.
3. Mammo GR=4-5; XR GR = 6-16.
4. Grid increases dose as the power is increased else you will underexpose.
5. Mammo bucky = 2; XR bucky = 5.

Question 27

In mammo, the cathode side of the tube should be placed where?

Answer 27

• By the chest wall. (Cathode / chest)

Question 28

• In what 3 ways is mammo different from XR & why?

Answer 28

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.