Mammography: Physics of Imaging

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Nigel Nichols
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1 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 Page 1

2 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 ~ Density, gm/cm 3 ~ Page 2

3 Mammography: Subject Contrast t calcium speck µ Ca X-rays Compression paddle µ tissue KeV Tissue µ, cm -1 Ca µ, cm -1 Δµ C, %* Grid I 2 I 1 C s ~ Log I 1 -Log I 2 ~ Δµt *For 100 µm Ca particle w/o scatter! C s (%) = Log I 1 /I 2 = Δµt x 100! Mammographic vs. Standard Radiographic Equipment Lower x-ray energy < 35 KVp vs > 40 KVp Smaller focal spot mm vs mm Shorter source - receptor distance cm vs 100 cm Apply compression Page 3

4 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, C o Relative X-ray Emission W Mo Rh 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 Energetic free electron (Kinetic energy > K shell binding energy) Characteristic x-ray (hν = K shell BE - M shell BE) e - e - In mammography, can exceed 25% of output Page 4

5 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 Rhodium " Molybdenum X-Ray Spectrum Filter! 0.3 mm polycarbonate" Number of photons" 20000" 15000" 10000" 0.3 mm polycarbonate mm Mo" K edge Mo" 19.9 KeV" µ (cm -1 ) " 100 0" 0" 5" 10" 15" 20" 25" 30" 10 Photon energy, KeV" Page 5

6 Mammography X-Ray Beam Relative Intensity" Entrance spectrum" Relative Intensity" Exit" spectrum" Increasing breast thickness/ density" 10" 20" 30" Photon Energy, KeV" 10" 20" 30" Photon Energy, KeV" 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 Page 6

7 Compression Reduces anatomical motion Reduces dose Reduces exposure latitude Spreads out detail Less superimposition X-rays! X-rays! Compression! paddle! Decreases distortion Detector! 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 to Primary Ra:o KeV Sca5er Page 7

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.

8 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 cm Paraffin) Function of KVp, anode material, filtration (HVL) Page 8

9 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 Page 9

10 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 Page 10

7 cm 100 µm pixels 1914 x 2294 (9 MB) or 2394 x 3062 (14 MB) image size 14 bit dynamic range

11 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 Page 11

Photon counting, multi-slit scanning system Scan time: 3-15 sec Direct detector of

(37 MB) image size 12 bit dynamic range Sectra MicroDose Image Interpretation All

using display for images from other vendors Critical features: Zoom and pan Ability

12 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 Sectra MicroDose 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 Page 12

13 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 Page 13

14 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 singleset 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 Page 14

15 Digital Tomosynthesis exposures! 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 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? Page 15

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.

16 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 Page 16

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

17 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 Page 17

18 Conclusions Digital mammography is currently the gold standard for breast imaging Not a dose saving technique Remains an active area of development Tomographic acquisitions Page 18

Breast Tomosynthesis. Bob Liu, Ph.D. Department of Radiology Massachusetts General Hospital And Harvard Medical School

Breast Tomosynthesis. Bob Liu, Ph.D. Department of Radiology Massachusetts General Hospital And Harvard Medical School Breast Tomosynthesis Bob Liu, Ph.D. Department of Radiology Massachusetts General Hospital And Harvard Medical School Outline Physics aspects of breast tomosynthesis Quality control of breast tomosynthesis