2. Educational Objectives
Our educational objectives are to understand:
1. Why pay special attention to mammography physics?
2. Radiation Risk/Benefit Issues
3. Physical principles of mammography
4. Physical principles of full field digital mammography
(FFDM): 2D
5. Technical Details of Digital Breast Tomosynthesis
(DBT): 3D
3. Why pay special attention
to mammography physics?
• Approximately 1 of 8 women will
develop breast cancer over a lifetime.
• 10-30% of women who have breast
cancer have negative mammograms.
• ~80% of masses biopsied are not
malignant (fibroadenomas, small
papillomas, proliferating dysplasia).
8. • X-ray spectral distribution is determined by:
– kV
– target/filter combination
– Mo/Mo, Mo/Rh, Rh/Rh for GE
– Mo/Mo, Mo/Rh, W/Rh for Siemens
– Mo/Mo, Mo/Rh or W/Rh, W/Ag for Hologic
– W/Rh, W/Ag, W/Al for Hologic DBT Tomo
– W/Rh for Giotto
– W/Rh for Fuji Saphire HD
– W/Rh, W/Ag for Planmed
– W/Al for Philips
X-ray Spectra in Mammography
11. Scattered Radiation Control
• Linear Grids
– Grid ratio (height of lamina/distance between
laminae): 4:1 or 5:1 w/ 30-40 lines/cm.
– Conventional grids are 8:1 to 12:1 (up to 43
lines/cm).
– Breast dose is increased by grids (Bucky Factor:
x2 to x3) w/40% improvement in contrast.
– Laminae are focused to the focal spot to prevent
grid cut off.
12. • High Transmission Cellular (HTC) Grids
– Focused
– Increased 2D absorption of scattered radiation
– Increase contrast
– Must move the grid a very precise distance
during exposure regardless of exposure duration
– Essentially same grid ratio and dose as
conventional linear grids
Scattered Radiation Control
15. Magnification Mammography
• Magnification factor: x1.5 – x2.0
• Increases the size of the projected
anatomical structures compared
with the noise of the image
• Valuable for visualization of
calcifications and spiculations.
18. Magnification
• Reduction of effective image noise (less
quantum noise, more photons per object
area)
• Air gap between breast and image
receptor reduces scattered radiation
without attenuating primary photons or
increasing radiation dose (no grid!)
• Small focal spot: 0.1 - 0.15mm (low
mA, long exposure times)
• Increased dose (x2-x3)
20. Dose Limits
FDA Dose limit for screening
mammograms
– 3 mGy (w/grid)
Mean glandular dose
Single view
4.5cm compressed breast
Average composition
21. Physical Principles of Full Field
Digital Mammography (FFDM): 2D
• FFDM Technologies
– Direct detectors
– Indirect detectors
– Computed radiography (CR)
– Slit scanning technology
22. Certification statistics
August 6, 2018
http://www.fda.gov/Radiation-
EmittingProducts/MammographyQualityStandardsActandProgram/FacilityScorec
ard/ucm113858.htm
• Total certified facilities / Total accredited units
• 8,688 / 19,242
• Certified facilities with FFDM units /
Accredited FFDM units
• 8,622 / 12,817
• Certified facilities with DBT units /
Accredited DBT units
• 4,526 / 6,341
28. Slit Scanning Technology
• Slit Scanning
Technology
(multi-slit)
http://incenter.medical.philips.com/doclib/enc/fetch/2000/4504/577242/577260/593280/593
431/8477093/Photon_Counting_White_Paper.pdf%3fnodeid%3d8477094%26vernum%3d1
• X-ray generates electron-hole pairs
creating a short electrical signal
29. Philips MicroDose
• Multi-slit scanning (~ 26 slits)
• Pre & post collimation
• Photon counting
• 50 micron pixels
• Silicon strip detectors (tapered toward
focal spot)
• Mean glandular dose ~50% of other FFDM
approaches
30. Philips Micro Dose
• 3-15 sec exposures
• Can sort photon events into high energy
and low energy (spectral imaging) for
quantitative breast density measurements
31. Breast Dose in FFDM
• Systems display breast dose with image
– Mean Glandular Dose < 3 mGy
– Dose recorded in DICOM image header
Entrance skin exposure and/or mean glandular dose
Vendors use different dose calculation algorithms
• Dance
• Wu & Barnes
• U.S. Method
• As of the 3.4.2 software upgrade, Hologic “follows
the latest EUREF adopted method if the system is set
up to use EUREF dose calculation”
37. DQE in Breast Tomosynthesis
• Mean glandular dose (MGD) for tomosynthesis
is expected to be the same as for projection
mammography (< 300 mRad)
• Since breast tomosynthesis requires several
exposures (e.g.15), low exposure DQE
performance of digital detectors used in breast
tomosynthesis is important
• A grid might not be used in breast
tomosynthesis, which reduces dose (x2 – x3)
38. Characteristics: DBT Breast Tomo
• Tiling of very large breasts (more than one view to
cover very large breasts) may not work since tissue
outside of FOV can cause artifacts
39. Characteristics: Hologic DBT Breast Tomo
Modes/ views:
• 2D: one conventional FFDM image
• 3D Tomo: 15 views used to reconstruct
tomographic slices
• Combo: acquisition of both 2D and 3D tomo (still
< 3 mGy)
• Synthetic view: reconstruction of a pseudo
projection mammogram from a stack of
tomographic images
41. Characteristics: Hologic DBT Breast Tomo
• Filters
– Rh: for 2D only
– Ag: for 2D only (thick/dense breasts)
– Al: for 3D tomo only
• Density control
– None
• No grid during tomo
• No MAGnification in tomo
42. Characteristics: Hologic DBT Breast Tomo
• Pixel binning
– In 3D tomo mode, pixels are “binned” into groups
of 2x2 pixels (140 micron pitch)
• Reconstruction
– 1 mm thick
– Number of tomo images: (compressed breast
thickness/ 1mm => 40 – 80)
• Interpretation
– 1mm tomographic slices
– 15 individual projection views (good for motion
detection)
– May also have a conventional 2D view and/or
synthetic view
43. Hologic DBT MGD
• 2D: 1.2 mGy
• 3D Tomo: 1.45 mGy
• Combo*: 2.65 mGy
*Combo: 2D and 3D tomo of the same
breast view (e.g. MLO)
44. Characteristics: DBT Breast Tomo
DBT System
General
Electric
Essential
Hologic
Selenia
Dimensions
Siemens
Mammomat
Inspiration
Type of geometry Full-field Full-field Full-field
Detector type
Energy
integrating
Energy
integrating
Energy
integrating
Detector material CsI-Si a-Se a-Se
Detector element size
(μm) 100 70 85
Focal plane pixel size 100 95-117 85
X-ray tube motion
Step-and
shoot Continuous Continuous
Target Mo/Rh W W
Filter
Mo: 30μm
Rh: 25 μm Al: 700 μm Rh: 50 μm
Angular range 25 15 50
Number of projection
images 9 15 25
Source to detector
distance (mm) 660 700 655
Distance between
detector and centre of
rotation (mm) 40 0 47
Reconstruction
algoorithm Iterative
Filtered back
projection? Analytic
Grid used for tomo yes no
no (scatter
correction
software)
Detector binning for
tomo no yes ?
