Mammography Physics
Jerry Allison, Ph.D.
Department of Radiology
Medical College of Georgia
Georgia Regents University
Augusta, GA
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)
5. Technical Details of Digital Breast Tomosynthesis
(DBT)
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).
Radiation Risk/Benefit Issues
• Radiation is a carcinogen (ionizing radiation, xradiation, 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
Risk/Benefit
©NCRP 2006 (Report 149)
Breast Tissue Composition
©1992 RSNA
The Challenge in Mammography
©1987 IOP
Publishing
kV Dependence
• An increase in kV:
• Reduces subject contrast
• Reduces exposure time
• Reduced motion artifact
©1993 RSNA
X-ray Spectra in Mammography
• 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 Sapire HD
W/Rh, W/Ag for Planmed
W/Al for Philips
X-ray spectra are variable
Compression (Redistribution?)
Scatter
Geometric blurring
Superposition
Increases the proportion of
the X-ray beam that is used
to image a breast
Motion
Beam hardening
Dose
©1994 Williams & Wilkins
Scattered Radiation
Control
• Only 40-75% of the possible contrast
is imaged in mammography unless
scatter is controlled.
• Mammography grids transmit 60-70%
of primary X-rays and absorb 75-85%
of scattered X-rays.
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.
Scattered Radiation Control
• 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
HTC Grid
http://www.hologic.com/oem/pdf/W-BI-HTC_HTC%20GRID_09-04.pdf
HTC Grid
http://www.hologic.com/oem/pdf/W-BI-HTC_HTC%20GRID_09-04.pdf
Magnification
•Increased effective resolution by
the magnification factor.
•Magnification factor: x1.5 – x2.0
•Effective resolution describes the
enlargement of the X-ray pattern
relative to the unsharpness of the
image receptor.
©1994 Williams & Wilkins
Magnification
• Spot compression paddles
http://www.americanmammographics.com/mammopads.htm
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)
Focal Spot and Screen-Film MTF
©1994 Williams & Wilkins
Dose

FDA Dose limit
– 3 mGy (w/grid)
 Mean
glandular dose
 Single view
 4.5cm compressed breast
 Average composition
Physical Principles of Full Field
Digital Mammography (FFDM)
• FFDM Technologies
– Direct detectors
– Indirect detectors
– Computed radiography (CR)
– Slit scanning technology
• FFDM Image Characteristics
– MTF
– DQE
– Dynamic range
FDA Approved CR, FFDM and DBT Units
• As of November 12, 2014
• 14 Vendors
• 31 Models
• 6 CR
• 25 FFDM
• 2 DBT
• Not all vendors still exist
• Not all models actually for sale
http://www.fda.gov/RadiationEmittingProducts/MammographyQualityStandardsActandProgram/FacilityCertificationandInspectio
n/ucm114148.htm
Certification statistics
October 1, 2014
• Total certified facilities / Total accredited units
• 8,734 / 13,827
• Certified facilities with FFDM units /
Accredited FFDM units
• 8,268 / 13,231
http://www.fda.gov/RadiationEmittingProducts/MammographyQualityStandardsActandProgram/FacilityScorec
ard/ucm113858.htm
FFDM Technologies
“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
Does pixel size matter?
• As pixel size decreases:
– Spatial resolution improves
– Noise increases
– Signal-to-noise decreases
• Yet another set of imaging tradeoffs
Detector Technology Overview
Independent (“Indirect”) Conversion:
Dependent (“Direct”) Conversion:
CsI Converter + aSi Substrate Sensor
Matrix
aSe Converter + aSi Substrate Sensor
Matrix
X-Ray Photons
X-ray
X-ray
Selenium
K-edge
Fluoresence
CsI
Light
Electrons
Photodiode
Photodiode
Blocking
Layer
Electrons
Read Out Electronics
Electrode
Digital
Data
Capacitor
Electrons
Read Out Electronics
Digital
Data
Courtesy: Jill Spear, GE Women’s Healthcare
2,600+ Volts
X-Ray Photons
Electrode
Dielectric
Fuji CR Digital Mammography
•
•
•
•
•
ClearView-CSM
Reads image plate from both sides
~50 micron resolution
~10 lp/mm
For CR, the film-screen cassette is
replaced with a photostimulable
phosphor plate cassette (Low $)
• Mammography CR units also offered
by Carestream, Agfa, Konica, iCRco
©Kanal, K, Digital Mammography Update: Design and Characteristics of
Current Systems, 2009 AAPM Annual Meeting
Slit Scanning Technology
• Philips MicroDose
• 325 installed worldwide (July 2013)
• 32 installed USA (May 2014)
Slit Scanning Technology
• Slit Scanning
Technology
(multi-slit)
• X-ray generates electron-hole pairs
creating a short electrical signal
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
Philips MicroDose
•
•
•
•
•
Multi-slit scanning
Pre & post collimation
Photon counting
50 micron pixels
Silicon strip detectors (tapered toward
focal spot)
• Mean glandular dose ~50% of other FFDM
approaches
Philips Micro Dose
•
•
•
•
3-15 sec exposures
2 Mhz digitization rate per channel (15 bit)
Detectors “ready” every 2msec
~5000 electrons per pulse (noise: ~200
electrons RMS)
• Can sort photon events into high energy
and low energy (spectral imaging) for
quantitative breast density measurements
FFDM Image Characteristics
• MTF
• DQE
• Dynamic Range
Modulation Transfer Function (MTF):
• Detector’s ability to transfer
modulations in the pattern of photons
that enter the detector to modulations
in the detector output (the image)
MTF comparison
•
•
•
•
a-Se detector
Screen-film
CsI detector
CR
www.hologic.com/data/W
-BI-CR_11-06.pdf
Detective Quantum Efficiency (DQE)
• DQE is the standard for image quality
in FFDM
Ratio of SNR (signal-to-noise ratio) at
the detector output to SNR at the
detector input
Who has the best DQE?
• It– spatial
depends:
frequency (lp/mm)
– kV
– Target
– Filter
– breast phantom used
– EXPOSURE!!!!!
DQE
http://www.medical.siemens.com/
DQE (Detective Quantum Efficiency)
1.0
CsI
0.9
at 8.5 mR
at 0.5 mR
A-Se (Yorker)
100 µm pitch
0.8
µm pitch / 250 µm Se
0.7
8.5 mR
0.5 mR
0.6
DQE
70
at
at
0.5
0.4
0.3
0.2
0.1
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Spatial Frequency (lp/mm)
The significant advantage in the electronic noise factor allows the CsI-based detector to maintain its high DQE even at ultra low exposure
levels (0.5 mR).
(From Performance of Advanced a-Si / CsI-based Flat Panel X-ray Detectors for Mammography, Medical Imaging 2003: Physics of
Medical Imaging, M. J. Yaffe, L. E. Antonuk, Editors, Proceedings of SPIE Vol. 5030 (2003) © 2003 SPIE · 1605-7422/03)
Courtesy: Jill Spear, GE Women’s Healthcare
Dynamic range
Figure 3. Limitations of SFM in imaging a breast composed of a wide range of tissues
Mahesh M Radiographics 2004;24:1747-1760
©2004 by Radiological Society of North America
Figure 2. Typical response curves for SFM and digital mammography
Mahesh M Radiographics 2004;24:1747-1760
©2004 by Radiological Society of North America
Detector response
S/F
FFDM
~50mAs
~100mAs
~200mAs
©2004 by Radiological Society of North America, Mahesh M Radiographics 2004;24:1747-1760
Breast Dose in FFDM
• Systems display breast dose with image
– Mean Glandular Dose < 300mGy
– 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”

Technical Details of Digital Breast
Tomosynthesis (DBT)
• FDA Approved 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
http://www.fda.gov/RadiationEmittingProducts/MammographyQualityStandardsActandProgram/FacilityCertificationandInspectio
n/ucm114148.htm
Breast tomosynthesis
Hologic Selenia Dimensions
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf
Breast tomosynthesis
GE SenoClaire
http://www3.gehealthcare.com/en/products/categories/mammogr
aphy/senoclaire_3d
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf
Cone Beam Breast CT



University of Rochester
300 views
10 seconds
Breast tomosynthesis
©www.hologic.com/data/W-BI-001_EmergTech_08-06.pdf
Breast tomosynthesis
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf
Breast tomosynthesis
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf
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 may be very important
• A grid is not used in breast tomosynthesis, which
reduces dose (x2 – x3)
Characteristics: Hologic DBT Breast Tomo
• 2D: one conventional FFDM image
• 3D Tomo: 15 views over 15 degrees that are used to
reconstruct 1mm tomographic slices
• Combo: acquisition of both 2D and 3D tomo (still <
3 mGy)
• Can acquire 3D tomo in CC, MLO or any arbitrary
angle
Characteristics: Hologic DBT Breast Tomo
• Data acquisition (tomo)
– 15 discrete views (exposures)
– Limited arc (15 degrees)
– 4 sec
• SID
– 70 cm
• Detector
– Stationary
– Similar to Hologic Selenia
• Anode
– Tungsten
Characteristics: Hologic DBT Breast Tomo
• Filters
– Rh: for 2D only
– Ag: for 2D only
– Al: for 3D tomo only
• Density control
– None
• No grid during tomo
• No MAGnification in tomo
Characteristics: Hologic DBT Breast Tomo
• System resolution
– > 3 lp/mm (45 degrees)
• Tomo phantom criteria
– 4 fibers
– 3 speck groups
– 3 masses
– Can scroll up/down through 3D
stack in assessing phantom
scores
Characteristics: Hologic DBT Breast Tomo
• Pixel binning
– In 3D tomo mode, pixels are “binned” into groups
of 2x2 pixels (140 micron pitch)
• 3D tomo collimation
– 18 x 29 cm exclusively
• 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)
Characteristics: Hologic DBT Breast Tomo
• Auto AEC positioning
• Based on intensity of 2 cells
chosen from an array of 70 cells
(5 x 14 with each cell occupying
1 sq.cm.)
Hologic DBT MGD
• 2D:
1.2 mGy
• 3D Tomo:
• Combo*:
1.45 mGy
2.65 mGy
*Combo: 2D and 3D tomo of the same
breast view (e.g. MLO)
Characteristics: GE DBT Breast Tomo
• 3D Tomo:
• 9 views
• Step and shoot (versus continuous motion)
• No detector binning
• Tomo grid is used
• Iterative reconstruction (versus filtered
backprojection)
• 3D dose same as 2D dose
Characteristics: GE DBT Breast Tomo
• V-Preview 3: a 2D image generated from the raw DBT
projection data that helps the user get an overview of the entire
stack, before examining the DBT planes
• http://www3.gehealthcare.com/en/products/categories/mammography/seno
claire_3d#tabs/tab0A5E89E4B6F442DE962349399E6B384D
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,