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,