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)
Physical Principles of Mammography
•
•
•
•
•
•
Breast Composition
kV Dependence
X-ray Spectra
Compression
Scattered Radiation Control
Magnification
Breast Tissue Composition
©1992 RSNA
The Challenge in Mammography
©1987 IOP
Publishing
kV Dependence
• Reduction in beam energy will
always improve contrast at the
expense of increased dose
©1993 RSNA
kV Dependence
• Mammo kV has a modest impact
on contrast and is used principally
to:
– Adjust exposure time
– Minimize degradation of image quality
due to motion
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
Uniform breast thickness ->
high contrast screen-film
Motion
Beam hardening
Dose
©1994 Williams & Wilkins
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
Certification statistics
March 1, 2012
• Total certified facilities / Total accredited units
• 8,670 / 12,800 (7/1/2013)
• Certified facilities with FFDM units /
Accredited FFDM units
• 7,827 / 11,705 (7/1/2013)
• FDA Approved FFDM Units 7/10/2013
• 26 models by 12 Vendors
http://www.fda.gov/RadiationEmittingProducts/MammographyQualityStandardsActandProgram/
FFDM Technologies
“INDIRECT” Detectors
• 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
• Amorphous selenium (direct conversion)
• (TFT) flat panel technology (Siemens, Hologic, Giotto, Planmed)
• ~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 and Konica)
– ~50 micron pixels, ~10 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
CsI detector structure
©2000 RSNA, Wang, J, Blackbyrn, TJ, RadioGraphics 2000; 20:1471–1477
Computed Radiography (CR)
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 and Konica
©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)
• 10 installed USA (+5 awaiting
installation)
• More when relevant
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
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
Who has the best DQE?
• It– spatial
depends:
frequency (lp/mm)
– kV
– Target
– Filter
– breast phantom used
– EXPOSURE!!!!!
DQE for CR
www.hologic.com/data/W-BI-CR_11-06.pdf
We will revisit the importance of
DQE at low exposures when breast
tomosynthesis is introduced.
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
• FDA approved for:
– Screening
– Diagnostic
– Breast biopsy
http://www.fda.gov/RadiationEmittingProducts/MammographyQualityStandardsActandProgram/FacilityCertificationandInspectio
n/ucm114148.htm
Breast tomosynthesis
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf
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
Hologic DBT Dose
• 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: 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
• 2D imaging mode: one conventional FFDM image
• 3D Tomo mode: 15 views over 15 degrees that are
used to reconstruct 1mm tomographic skices
• Combo mode: for a given breast view (e.g.MLO),
acquisition of both a conventional 2D image and 3D
tomographic slices that are co-registered (still < 3
mGy)
• Can acquire tomo images CC, MLO or any arbitrary
angle
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.)
Characteristics: Hologic DBT Breast Tomo
• Clinical trials
– 2D vs. 3D vs. Combo
– 3D and 3D Combo both increase area under
ROC curve
– 3D and 3D Combo both reduce recall rate
– Overall performance
• 3D Combo > 3D Tomo > 2D
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,