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Mammography: Physics of Imaging

Robert G. Gould, Sc.D. Professor and Vice Chair

Department of Radiology and Biomedical Imaging University of California

San Francisco, California

Mammographic Imaging: Uniqueness

•  Low subject contrast • High spatial resolution

requirements • Complicated structures

– Priors critical • Dedicated imaging equipment • Use compression

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Factors Affecting Subject Contrast

•  X-ray energy: contrast decreases with increasing energy

•  Scattered radiation – Function of tissue volume irradiated

•  For small objects (e.g. Ca specks), image blurring

Tissue Characteristics

Glandular tissue Fat Calcium

Atomic number ~ 7.4 5.92 20

Density, gm/cm3 ~ 1.0 0.91 1.55

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Mammography: Subject Contrast

KeV Tissue µ, cm-1

Ca µ, cm-1 Δµ C, %*

20 0.793 20.15 19.36 8.4 25 0.513 10.08 9.57 4.2 30 0.373 6.185 5.18 2.5 50 0.227 1.547 1.32 0.57 80 0.183 0.566 0.383 0.17

100 0.170 0.397 0.237 0.1

*For 100 µm Ca particle w/o scatter! Cs (%) = Log I1/I2 = 0.434 Δµt x 100!

µCa t

Grid

calcium speck

I2 I1

Cs ~ Log I1-Log I2 ~ Δµt

Compression paddle

X-rays

µtissue

Mammographic vs. Standard Radiographic Equipment

•  Lower x-ray energy •  < 35 KVp vs > 40 KVp

•  Smaller focal spot •  0.1-0.3 mm vs 0.6-1.2 mm

•  Shorter source - receptor distance •  60-70 cm vs 100 cm

•  Apply compression

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Mammography X-ray Sources

•  Anode materials – Molybdenum (Z=42) – Rhodium (Z=45)

•  X-ray production efficiency –  ~ 40% less than in tungsten

•  Nominal focal spot sizes –  Large 0.3 mm –  Small 0.1 mm

•  Typical exposure times ~ >1 sec

Element Atomic Number

Melting Point, Co

Relative X-ray

Emission

W 74 3407 1.0

Mo 42 2617 0.57

Rh 45 1963 0.61

Characteristic Radiation

•  Produced at specific energies – Equal to differences in

binding energies of orbital electron shells

•  For standard x-ray source, typically < 15% of output

•  In mammography, can exceed 25% of output

Energetic free electron

e-

e-

(Kinetic energy > K shell binding energy)

(hν = K shell BE - M shell BE) Characteristic x-ray

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Characteristic Radiation

•  Use to increase contrast – No Bremsstrahlung low energy

‘penalty’

•  Increase output relative to Bremsstrahlung

–  Filter with same material – Materials relatively translucent

to their characteristic emissions

Energy, KeV

Molybdenum 17.4 19.6

Rhodium 20.2 22.7

Molybdenum X-Ray Spectrum

25000"

20000"

15000"

10000"

5000"

0"0" 5" 10" 15" 20" 25" 30"

Photon energy, KeV"

Num

ber o

f pho

tons"

0.3 mm polycarbonate"

0.3 mm polycarbonate + 0.17 mm Mo"

10

100

1000

10000

100000

µ (cm-1)

K edge Mo"19.9 KeV"

Filter !

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10" 20" 30"Photon Energy, KeV"

Rel

ativ

e In

tens

ity"

10" 20" 30"Photon Energy, KeV"

Rel

ativ

e In

tens

ity"

Entrance spectrum"

Exit"spectrum"

Increasing breast

thickness/density"

Mammography X-Ray Beam

As breast thickness/density increases, characteristic radiation has lesser effect on image!

Mammographic Tubes

•  Benefit of characteristic radiation in image formation greatest for thin or fatty breasts

– Bremsstrahlung component above the K-edge becomes increasingly dominate in exit beam with increasing breast thickness

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Compression

•  Reduces anatomical motion

•  Reduces dose •  Reduces exposure

latitude •  Spreads out detail

–  Less superimposition

•  Decreases distortion Detector!

X-rays! X-rays!

Compression!paddle!

Scatter in Mammography

•  Exit surface to detector ~ 2-3 cm

–  Little ‘air gap’ effect

•  Scatter/primary ratio ~ 0.5 –  If contrast is 5% w/o scatter,

contrast reduced to 3.4% with 0.5 S/P

–  33% contrast reduction

•  Use ~5:1 grid –  Increases dose by 2-3x (Bucky

factor) –  Improves contrast by ~ 40%

Contrast  of  100  µm  Ca  speck,  %  No  

Sca5er  Sca5er  to  Primary  Ra:o  

KeV 0.1   0.2   0.5   1  20 8.40 7.64 7.00 5.60 4.20 25 4.15 3.78 3.46 2.77 2.08 30 2.25 2.04 1.87 1.50 1.12 50 0.57 0.52 0.48 0.38 0.29 80 0.17 0.15 0.14 0.11 0.08

100 0.10 0.09 0.09 0.07 0.05

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Hologic“HTC” Grid

•  ‘High-Transmission Cellular’ •  Focused, 2 dimensional grid •  Air interspacing •  ~ 4-10% improvement in contrast

compared to 5:1 linear grid

After J. Gray"

Mammography Dose Levels

•  Screening exam – 2 views per breast – Peak (skin) dose 8-10 mGy/view – By law in the USA, glandular dose < 3 mGy/view

•  Based on the ACR Phantom = “average compressed breast of 50% adipose and 50% glandular tissue”

•  4.2 cm thick (3.5 cm Lucite + 0.7 cm Paraffin)

– Function of KVp, anode material, filtration (HVL)

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Digital Imaging in Mammography

•  Improved diagnostic capability? – True in dense breasts

•  Use of digital processing – Spatially adaptive – Feature enhancing – CAD

•  Include in PACS w/o digitizing film – Better image management

•  Dose reduction?

Digital Mammography-Outcomes

•  ACRIN study – Comparison made to screen/film systems – Apparently no light shed on differences

between FFDM systems

•  Greater sensitivity in woman < 50 years old and in woman with dense breasts

•  Equal to screen/film systems otherwise

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Digital Mammographic Systems

•  CR!  Carestream DirectView (computed radiography)!  Fuji FCRm (computed radiography)!

•  Indirect Conversion DR!  General Electric Senographe 2000D, DS, Essential, Senographe

Care!•  Direct Conversion DR!

  Hologic Loard Selenia, Selenia S, Selenia Dimensions, Selenia Encore!

  Siemens Mammomat Novation DR, Mammomat Novation S, Mammomat Inspiration, Mammomat Inspiration Pure!

•  Scanning systems!  Fischer Senoscan!  Sectra MicroDose Mammography L30!

Fuji CR

•  Use ‘conventional’ mammo unit (eg GE DMR)

•  BaFBr phosphor •  Coverage (plate size): 18 x

24 cm or 24 x 30 cm •  50 µm pixels •  3328 x4096 (24 MB) image

size •  14 bit dynamic range

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GE Senographe

•  “Indirect” DR using CsI phosphor •  Mo and Rh anodes, Mo and Rh filters •  Coverage: 19.2 x 23 cm or 24 x 30.7 cm •  100 µm pixels •  1914 x 2294 (9 MB) or 2394 x 3062 (14

MB) image size •  14 bit dynamic range

Hologic Selenia

•  “Direct” DR using amorphous selenium

•  Mo anode, Mo and Rh filters •  Coverage: 24 x 29 cm •  70 µm pixels •  3328 x 4096 (24 MB) image size •  14 bit dynamic range

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Sectra MicroDose •  Photon counting, multi-slit

scanning system –  Scan time: 3-15 sec

•  Direct detector of crystalline silicon

•  W target/Al filter •  Coverage: 24 x 26 cm •  50 µm pixels •  4800 x 5200 (37 MB) image

size •  12 bit dynamic range

Image Interpretation

•  All systems come with a dual, high resolution display system

– 5 megapixels – Obfuscation on using display

for images from other vendors

•  Critical features: –  Zoom and pan –  Ability to annotate and save

annotations to PACS –  Easy comparison with priors

•  CAD software

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Digital Mammo: Image Quality Compared to Screen/film

•  Lower limiting spatial resolution – < 10 lp/mm vs > 12 lp/mm

•  Higher DQE – More pronounced at lower spatial frequencies

(< 2 lp/mm)

•  Density more uniform across the image – Adaptive filtering – Skin line to chest wall

Screen-film vs Digital

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Digital Mammography-Dose Considerations

•  Geometric efficiency (fill factor) < 100% – 50-80% depending on pixel size – Screen-film systems = 100%

•  Quantum noise – Influence on processing algorithms? – Influence on CAD performance?

•  Dose savings remain “potential”

Developments in Mammography

•  Computer-aided diagnosis (CAD) – Widespread for screening mammography – Improved performance compared to ‘single-

set’ of eyes   Relative improvement depends on experience of

mammographer   Little or no improvement over double reading by

experienced mammographers

– Higher false positive rate •  Tomosynthesis •  Dedicated breast CT systems

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Digital Tomosynthesis

•  Acquire a series of low dose projection images at different angles

•  Electronically align and weight images to isolate a plane through the breast

–  Scroll through breast

10-15 exposures!

15-25°!

Digital Tomosynthesis •  Active area of development by

multiple vendors – Hologic Selenia Dimensions system has

FDA approval

•  Can total dose be kept same as for screening mammogram?

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Selenia Dimensions 3D •  FDA approved •  Allows both 3D and 2D images

to be obtained with same compression

–  ~140 µm pixels (3D) –  70 µm pixels (2D)

•  Scans 15° in 3.7 sec •  Obtains 15 projections by

pulsing the x-ray tube – Continuous tube movement

•  W target x-ray tube –  200 mA capable –  Al filter (3D) – Rh or Ag filters (2D)

Selenia Dimensions 3D

•  Dose – 2D: 12 mGy – 3D Tomo: 14.5 mGy – Combo: 26.5 mGy

•  Each projection: 1 mGy – Low dose detector performance critical (DQE)

•  Grid removed for tomo acquisition

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Selenia Dimensions 3D: Reconstruction

•  1 mm thick tomographic slices – Number of slices = compressed breast thickness/1mm – ~ 20 to 80 slices per tomographic image

•  Pixels binned into 2x2 size (140 µm) •  2-5 sec reconstruction time

Breast CT

•  Experimental •  Challenge to minimize dose •  Challenge to achieve

adequate spatial resolution •  Challenge to image near the

chest wall •  Impressive preliminary

results

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Conclusions

•  Digital mammography is currently the gold standard for breast imaging

•  Not a dose saving technique •  Remains an active area of development

– Tomographic acquisitions