mamo

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,