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Transcript of Mammography Physics Jerry Allison, Ph.D. Department of Radiology Medical College of Georgia Georgia...
Mammography PhysicsMammography Physics
Jerry Allison, Ph.D.
Department of Radiology
Medical College of Georgia
Georgia Regents University
Augusta, GA
Educational ObjectivesEducational Objectives
Our educational objectives are to understand:1. Why pay special attention to mammography physics?2. Radiation Risk/Benefit Issues3. Physical principles of mammography4. Physical principles of full field digital mammography
(FFDM)5. Technical Details of Digital Breast Tomosynthesis
(DBT)
Why pay special attention Why pay special attention to mammography physics?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 IssuesRadiation 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
©NCRP 2006 (Report 149)
Risk/BenefitRisk/Benefit
©1992 RSNA
Breast Tissue CompositionBreast Tissue Composition
©1987 IOP Publishing
The Challenge in MammographyThe Challenge in Mammography
kV DependencekV Dependence
©1993 RSNA
• 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 in MammographyX-ray Spectra in Mammography
X-ray spectra are variableX-ray spectra are variable
Compression (Redistribution?)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
Scattered Radiation Scattered Radiation ControlControl
• 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 ControlScattered 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.
• 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 ControlScattered Radiation Control
HTC GridHTC Grid
http://www.hologic.com/oem/pdf/W-BI-HTC_HTC%20GRID_09-04.pdf
HTC GridHTC Grid
http://www.hologic.com/oem/pdf/W-BI-HTC_HTC%20GRID_09-04.pdf
MagnificationMagnification
•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
MagnificationMagnification• Spot compression paddles
http://www.americanmammographics.com/mammopads.htm
MagnificationMagnification• 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)
©1994 Williams & Wilkins
Focal Spot and Screen-Film MTFFocal Spot and Screen-Film MTF
DoseDose
FDA Dose limit– 3 mGy (w/grid)
Mean glandular dose Single view 4.5cm compressed breast Average composition
Physical Principles of Full Field Physical Principles of Full Field Digital Mammography (FFDM)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 UnitsFDA Approved CR, FFDM and DBT Units
http://www.fda.gov/Radiation-EmittingProducts/MammographyQualityStandardsActandProgram/FacilityCertificationandInspection/ucm114148.htm
• 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
Certification statisticsCertification statisticsOctober 1, 2014October 1, 2014
http://www.fda.gov/Radiation-EmittingProducts/MammographyQualityStandardsActandProgram/FacilityScorecard/ucm113858.htm
• Total certified facilities / Total accredited units• 8,734 / 13,827• Certified facilities with FFDM units /
Accredited FFDM units• 8,268 / 13,231
“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 TechnologiesFFDM Technologies
Does pixel size matter?Does pixel size matter?
• As pixel size decreases:– Spatial resolution improves– Noise increases– Signal-to-noise decreases
• Yet another set of imaging tradeoffs
Independent (“Indirect”) Conversion:CsI Converter + aSi Substrate Sensor
Matrix
BlockingLayer
CsI
X-Ray Photons
Light
Photodiode Photodiode
Electrons Read Out Electronics
X-ray
DigitalData
2,60
0+
Vo
lts
ElectrodeDielectric
DigitalData
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
Fuji CR Digital MammographyFuji 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 TechnologySlit Scanning Technology
• Philips MicroDose• 325 installed worldwide (July 2013) • 32 installed USA (May 2014)
Slit Scanning TechnologySlit Scanning Technology
• Slit Scanning Technology (multi-slit)
http://incenter.medical.philips.com/doclib/enc/fetch/2000/4504/577242/577260/593280/593431/8477093/Photon_Counting_White_Paper.pdf
%3fnodeid%3d8477094%26vernum%3d1
• X-ray generates electron-hole pairs creating a short electrical signal
Philips MicroDosePhilips 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 DosePhilips 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 CharacteristicsFFDM Image Characteristics
• MTF• DQE• Dynamic Range
Modulation Transfer Function (MTF): 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 comparisonMTF comparison
• a-Se detector
• Screen-film
• CsI detector
• CRwww.hologic.com/
data/W-BI-CR_11-06.pdf
Detective Quantum Efficiency (DQE)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?Who has the best DQE?
• It depends:– spatial frequency (lp/mm)– kV– Target– Filter– breast phantom used– EXPOSURE!!!!!
DQEDQE
http://www.medical.siemens.com/
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)
0.1
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Spatial Frequency (lp/mm)
DQ
E
A-Se (Yorker) 70
µm pitch / 250 µm Se at 8.5 mR at 0.5 mR
CsI 100 µm pitch
at 8.5 mR at 0.5 mR
DQE (Detective Quantum Efficiency)
Courtesy: Jill Spear, GE Women’s Healthcare
Dynamic rangeDynamic 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 responseDetector response
S/F
FFDM
~200mAs~100mAs~50mAs©2004 by Radiological Society of North America, Mahesh M Radiographics 2004;24:1747-1760
Breast Dose in FFDMBreast 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”
http://www.fda.gov/Radiation-EmittingProducts/MammographyQualityStandardsActandProgram/FacilityCertificationandInspection/ucm114148.htm
• 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
Technical Details of Digital Breast Technical Details of Digital Breast Tomosynthesis (DBT)Tomosynthesis (DBT)
Breast tomosynthesisBreast tomosynthesisHologic Selenia Dimensions Hologic Selenia Dimensions
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf
Breast tomosynthesisBreast tomosynthesisGE SenoClaireGE SenoClaire
http://www3.gehealthcare.com/en/products/categories/mammography/senoclaire_3d
Cone Beam Breast CTCone Beam Breast CT
University of Rochester 300 views 10 seconds
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf
Breast tomosynthesisBreast tomosynthesis
©www.hologic.com/data/W-BI-001_EmergTech_08-06.pdf
Breast tomosynthesisBreast tomosynthesis
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf
Breast tomosynthesisBreast tomosynthesis
http://www.hologic.com/data/WP-00007_Tomo_08-08.pdf
DQE in Breast TomosynthesisDQE 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 TomoCharacteristics: 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 TomoCharacteristics: 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 TomoCharacteristics: 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 TomoCharacteristics: 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 TomoCharacteristics: 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 TomoCharacteristics: 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 MGDHologic 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)
Characteristics: GE DBT Breast TomoCharacteristics: 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 TomoCharacteristics: GE DBT Breast Tomo
• http://www3.gehealthcare.com/en/products/categories/mammography/senoclaire_3d#tabs/tab0A5E89E4B6F442DE962349399E6B384D
• 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
ReferencesReferences– ©NCRP 2006
NCRP Report 149, “A Guide to Mammography and Other Breast Imaging Procedures” National Council on Radiation Protection and Measurements, 2004
– ©1994 Williams & WilkinsBushberg, JT, Seibert, JA, Leidholdt, EM Jr., Boone, JM, ”The
Essential Physics of Medical Imaging” Williams & Wilkins, Baltimore, Maryland, 1994
– ©1993 RSNAHaus, 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 PublishingJohns, PC, Yaffe, MJ, “X-Ray characterisation 675-695of normal and neoplastic breast tissues”, Phys Med Biol, 1987, 32,