Screen/film mammography - X-ray tube

• Xray energy level required for optimal contrast in the breast is 20 keV

  • Higher energy = decreased contrast

  • Lower energy = inadequate penetration of the breast and increased radiation dose

• Molybdenum and Rhodium are used as target materials in the anode

  • Both produce xray radiation at the optimal energy level

  • Mo = 17.9-19.5 keV

  • Rh = 20.2-22.7 keV

    • Provides better penetration in thick/dense breasts

    • Xrays are slightly higher energy

• To produce these characteristic xrays, the xray tube peak voltage must be higher than these values (25-30 kVp)

• Typical xray tube current is 80-100 mA

• Exposure time is about 1 sec

  • Can be longer for dense thick breasts (<=4 sec)

Xray tube filters

• Xray tube window - Beryllium - minimizes xray beam attenuation

• Mo filters are used to remove bremsstrahlung radiation (high energy)

  • 30 nanometers thick

  • Improves contrast

  • Filters out very low energy xrays -> decrease patient dose

• Use Ro filter for a Ro anode

Target-filter recommendations

• Fatty breast up to 4 cm thick

  • Mo target and 30 micron Mo filter

  • 24-26 kVp

• Glandular breast 5-7 cm

  • Mo target and 25 micron Rh filter

  • 27-31 kVp

• Breast thickness >7 cm

  • Rh target and 25 micron Rh filter

FILTERS: DBT

• Filters:

  • RH = for 2D only

  • Silver/Ag (25.5 keV) = for 2D only (thick/dense breasts)

  • Aluminum/Al (1.8 keV) = for 3D tomo only

• Anode is Tungsten (58.0-67.2 keV)

• No grid

• <3 mGy for both 2D and DBT acquisition

  • Synthetic view (C-view) - reconstruction of a mammogram from a stack of tomo images

  • 3D tomo: 1.45 mGy

  • Combo: 2.65 mGy

Grids

• Grids are used to maximize image quality by (1) reducing scatter and (2) improving contrast

  • Scatter increases w/ breast thickness and peak voltage

• Mammography usually uses moving grids

  • Typical ratios are 4:1 or 5:1

• Downside of grids = increase patient dose 3x

Radiation dose

• Radiation is a carcinogen

• No known cases of mammography induced breast CA

• Mantle radiation for Hodgkin's lymphoma has been associated w/ radiation induced breast CA

Breast dose

• Mean glandular dose (MGD) - measure of dose in mammography

  • Can be estimated from the measured entrance skin exposure, xray beam peak voltage and the half-value layer

  • Dependent on breast composition - assumes 50% glandular tissue and 50% adipose tissue

  • Use of grid doubles MGD

  • ACR recommends that the MGD for a 4.2 cm thickn breast be <3 mGy for screen/film w/ grid and digital full field w/ grid.

• Most radiation induced breast CA result from MGD of 1-20 Gy.

• Risk of radiation induced breasts CA is 1 in 10,000. 

  • Risk of a fatal breast CA is 1 in 200-250

• Use 12 for deep things

• Use 17 for most things bc it shows superficial structures