Mammography Physics provides an overview of mammography physics concepts including: 1) Why special attention is paid to mammography physics given breast cancer rates and false positives/negatives. 2) Radiation risk/benefit issues and dose limits for screening mammography. 3) Physical principles of full field digital mammography (FFDM) including detector types, computed radiography, and slit scanning technology. 4) Technical details of digital breast tomosynthesis (DBT) such as acquisition parameters, reconstruction, and dose for the Hologic Selenia Dimensions system.

1. Mammography Physics
Jerry Allison, Ph.D.
Department of Radiology and Imaging
Medical College of Georgia
Augusta University
Augusta, GA

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).

4. Radiation Risk/Benefit Issues
• Radiation is a carcinogen (ionizing radiation, x-
radiation, radiation: National Toxicology Program 2004)
• "No woman has been shown to have developed breast
cancer as a result of mammography, not even from
multiple studies performed over many years with doses
higher than the current dose (~250 mRad)... However
the possibility of such risk has been raised because of
excessive incidence of breast cancer in women exposed
to much higher doses (100-2000 Rad: Japanese A-bomb
survivors, TB patients having chest fluoro and
postpartum mastitis patients treated w/radiation
therapy).” ©1992 RSNA
• Radiation treatment of Hodgkin lymphoma associated
with radiation induced breast cancer

5. ©NCRP 2006 (Report 149)
Risk/Benefit

6. ©1992 RSNA

7. ©1987 IOP
Publishing
The Challenge in Mammography

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

9. X-ray spectra are variable

10. Compression (Redistribution?)
©1994 Williams & Wilkins
Scatter
Geometric blurring
Superposition
Increases the proportion of
the X-ray beam that is used
to image a breast
Motion
Beam hardening
Dose

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

13. HTC Grid
http://www.hologic.com/oem/pdf/W-BI-HTC_HTC%20GRID_09-04.pdf

14. HTC Grid
http://www.hologic.com/oem/pdf/W-BI-HTC_HTC%20GRID_09-04.pdf

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.

16. ©1994 Williams & Wilkins

17. Magnification
• Spot compression paddles
http://www.americanmammographics.com/mammopads.htm

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)

19. ©1994 Williams & Wilkins
Focal Spot and MTF

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

23. “INDIRECT” Detectors (GE)
• Scintillating phosphor (CsI columns) on an array of amorphous silicon
photodiodes using thin-film transistor (TFT) flat panel technology (GE)
– ~100 micron pixels, ~5 lp/mm
“DIRECT” Detectors (Siemens, Hologic, Giotto, Planmed, Fuji)
• Amorphous selenium (direct conversion)
• (TFT) flat panel technology
• ~70-85 micron pixels , ~7 lp/mm
• Direct optical switching technology (Fuji Aspire HD))
• ~50 micron pixels , ~10 lp/mm
Computed radiography (Fuji, Carestream, Agfa, Konica, iCRco)
– ~50 micron pixels, ~10 lp/mm
– ~100 micron pixels, ~5 lp/mm
Slit scanning technology (Philips)
– ~50 micron pixels, ~10 lp/mm
FFDM Technologies

24. Does pixel size matter?
• As pixel size decreases:
– Spatial resolution improves
– Noise increases
– Signal-to-noise decreases
• Yet another set of imaging tradeoffs

25. Independent (“Indirect”) Conversion:
CsI Converter + aSi Substrate Sensor
Matrix
Blocking
Layer
CsI
X-Ray Photons
Light
Photodiode Photodiode
Electrons
Read Out Electronics
X-ray
Digital
Data
2,600+
Volts
Electrode
Dielectric
Digital
Data
Electrons
X-Ray Photons
Selenium
K-edge
Fluoresence
Electrons
Read Out Electronics
X-ray
Electrode
Capacitor
Dependent (“Direct”) Conversion:
aSe Converter + aSi Substrate Sensor
Matrix
Detector Technology Overview
Courtesy: Jill Spear, GE Women’s Healthcare

26. Fuji CR Digital Mammography
• Deleted (obsolete)

27. Slit Scanning Technology
• Philips MicroDose
• 650 installed worldwide (June 2015)
• 35 installed USA (June 2015)

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”

32. http://www.fda.gov/Radiation-
EmittingProducts/MammographyQualityStandardsActandProgram/FacilityCertificationandInspectio
n/ucm114148.htm
• FDAApproved DBT Units
• Hologic Selenia Dimensions Digital Breast
Tomosynthesis (DBT) System on 2/11/11
• GE SenoClaire Digital Breast Tomosynthesis
(DBT) System on 8/26/14
• Siemens Mammomat Inspiration with
Tomosynthesis Option (DBT) System on 4/21/15
• Fujifilm ASPIRE Cristalle on 1/10/17
• GE Senpgraphe Pristina on 3/3/2017
Technical Details of Digital Breast
Tomosynthesis (DBT)

33. Breast tomosynthesis
Hologic Selenia Dimensions
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf

34. Cone Beam Breast CT
 University of Rochester
 300 views
 10 seconds
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf

35. Breast tomosynthesis
©www.hologic.com/data/W-BI-001_EmergTech_08-06.pdf

36. Breast tomosynthesis
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf

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

40. Characteristics: Hologic DBT Breast Tomo
• Data acquisition (tomo)
– 15 discrete views (exposures)
– Limited arc (± 7.5 degrees)
– 4 sec
• Anode
– Tungsten

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 ?

45. References
– ©NCRP 2006
NCRP Report 149, “A Guide to Mammography and Other
Breast Imaging Procedures” National Council on Radiation
Protection and Measurements, 2004
– ©1994 Williams & Wilkins
Bushberg, JT, Seibert, JA, Leidholdt, EM Jr., Boone, JM, ”The
Essential Physics of Medical Imaging” Williams & Wilkins,
Baltimore, Maryland, 1994
– ©1993 RSNA
Haus, AG, Yaffe, MJ, Eds., “Syllabus: A Categorical Course
in Physics Technical Aspects of Breast Imaging”, 2nd
Edition, RSNA, 1993
– ©1992 RSNA
Haus, AG, Yaffe, MJ, Eds., “Syllabus: A Categorical Course
in Physics Technical Aspects of Breast Imaging”, RSNA,
1992
– ©1987 IOP Publishing
Johns, PC, Yaffe, MJ, “X-Ray characterisation 675-695
of normal and neoplastic breast tissues”, Phys Med Biol, 1987,
32,