Performances of different Performances of different digital mammography digital mammography

imaging systems: imaging systems: evaluation and evaluation and

comparisoncomparisonMaria Giuseppina Bisogni

Dipartimento di Fisica “E. Fermi”, Universita’ di Pisa andSezione INFN Pisa

6th International Workshop on Radiation Imaging Detectors, Glasgow 25 - 29th July, 2004

Outline

Introduction• Mammography• Digital mammographic systems

The MEDIPIX pixel detector for digital mammography• The assembly• Si and GaAs Detector development

Imaging performance of the MEDIPIX pixel detector• Experimental set - up• Mammographic Phantom• Beam optimization

Comparison with commercial digital systems

MTF (Modulation Transfer Function) evaluation

Comparison between the linear attenuation coefficient of carcinoma, fat tissue and glandular tissue

The mammography challengeThe mammography challenge

•The problem for an early diagnosis: the X-rays attenuation of a pathological tissue is very similar to that of breast tissue.

•Typical lesions: masses (of the order of a cm and with low contrast) and clusters of microcalcifications (high contrast but small dimensions, of the order of hundreds of m).

Breast cancerGlandular tissueAdipose Tissue

Digital Mammography (DM) Main features

• Linear response with X-ray exposure• Wide dynamic range (104 – 105)

• Mammography of dense breast • Reduced radiation dose

• Exposure determined as a function of the Signal to Noise Ratio (SNR) not of the Optical Density of the film (OD)

• Dose reduction from 20 to 80 %• Image processing• Time required for the examination (texp<1s; Tproc~minutes)

Limits (?)• Spatial resolution

• Film-screen systems 20 lp/mm

• Digital systems 10 lp/mm• Costs

Digital systemsDigital systems Indirect integration detection systems

• the X-rays are absorbed in a phosphor layer and produce light photons which convert into electric charges in a photo detector and then to an electric signal

• Imaging Plates based on photostimulable phosphors• CsI(Tl) – aSi Flat Panels • Phosphor screen +OF taper+ CCD array (slot scanning)

Direct integration detection systems• the X-ray photons directly convert into charges (electron-hole pairs)

and thus to an electric signal in a photoconductor• aSe Flat Panels

Direct photon counting systems• single photons are counted, i.e. the number of photons directly

represent the intensity level in a pixel • Sectra Microdose Mammography (MDM) based on Si detectors • Xcounter based on gas avalanche photodiodes

Indirect integration detection systems• the X-rays are absorbed in a phosphor layer and produce light

photons which convert into electric charges in a photo detector and then to an electric signal

• Imaging Plates based on photostimulable phosphors• CsI(Tl) – aSi Flat Panels • Phosphor screen +OF taper+ CCD array (slot scanning)

Direct integration detection systems• the X-ray photons directly convert into charges (electron-hole pairs)

and thus to an electric signal in a photoconductor• aSe Flat Panels

Direct photon counting systems• single photons are counted, i.e. the number of photons directly

represent the intensity level in a pixel • Sectra Microdose Mammography (MDM) based on Si detectors • Xcounter based on gas avalanche photodiodes

Semiconductor Detector

Read-Out chip

Detector• Si, GaAs• pixel 170 x 170 m2

• channels 64 x 64• area 1.2 cm2

The MEDIPIX1 Pixel detector for DMThe MEDIPIX1 Pixel detector for DM

Input Preamp

LatchedComparator

Pulseshaper

ThresholdAdjust(3 bits)

TestInput

Cfb

Test(1 bit)

Ctest

Rst

AnalogReset

Mask(1 bit)

Shutter

Data

Clk

Clkout

1

0

01

Sel

Sel

Mux

Mux

ShiftReg

To lowerpixel

From upperpixel

Photon Counting Chip (PPC)CERN SACMOS 1 m

FASELEC Zurigo

Pixel 170 x 170 m2

Channels 64 x 64Area 1.7 cm2

Threshold adjust 3-bitPseudo-random counter 15-bits

http://medipix.web.cern.ch/MEDIPIX/http://medipix.web.cern.ch/MEDIPIX/http://medipix.web.cern.ch/MEDIPIX/http://medipix.web.cern.ch/MEDIPIX/

0 100 200 300 400 5000

10

20

30

40

50

60

70

80

90

100

c.c.

e. (

%)

reverse voltage (Volt)

0 100 200 300 400 5001E-3

0,01

0,1

1

10

100

SUM 082 SUM 051 DOWA 077 DOWA 079

curr

ent d

ensi

ty (A

/cm

2 )

reverse voltage (V)

The GaAs pixel detector: material characterizationThe GaAs pixel detector: material characterization

0 5 10 15 20 25 30 35 400

2000

4000

6000

8000

10000

CCE=100 %

GaAs 200 m

VHV

=350 V109

Cd E=22 keVCCE = 90 %C

ou

nts

E(keV)GaAs “bulk”

LEC (Liquid Encapsulated Chochralski),

Semi-insulating (SI) Sumitomo

Resistivity = 108 Ohm cmContacts

Schottky : multilayer Ti/Pd/Au (AMS

Roma)Thickness

200 m

0 200 400 600 800 1000 1200 14000

20

40

60

80

100

with multiguardrings without multiguardrings

elec

tric

po

tent

ial (

Vol

t)

width (m)0 200 400 600 800 1000 1200 1400

0

5000

10000

15000

20000

25000

30000

with multiguardrings without multiguardrings

ele

ctri

c fie

ld (

Vo

lt/cm

)

width (m)

width

The Si pixel detector: geometry simulationThe Si pixel detector: geometry simulation

Simulation performed with the package ISE-TCAD

300 m

Geometry• Pixel 150 m, interpixel 20

m• 64 x 64 pixels• Sensitive area 1.2 cm2

Si detectors• Produced by ITC-IRST (Trento,

Italy)• Thickness 300, 525, 800 m• p+-n junction• Bump bonding by VTT

(Finland)

GaAs detectors• Produced by AMS (Italy)• Thickness 200 m• Schottky contacts• Bump bonding by AMS

150 m

Detection efficiency @ 20 keV• Si 300 m 26 %• Si 525 m 40 %• GaAs 200 m 98 %

The pixel detectorsThe pixel detectorsPixel 150 m x 150 m

guardring

multiguardrings

guardring

500 m

Experimental setup

0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

3500

E(keV)

mammographic spectrum (28 kVp) After 4 cm LuciteSimulated with the Program IPEM Report 78

Source: standard mammographic tube (Instrumentarium Imaging Products Diamond) Molibdenum target

Mo (0.03 mm) and Be (1 mm) filters Distance between the beam focus and the detector = 64 cm

Mammographic facility of the Istituto di Radiologia, Ospedale S. Chiara, Pisa, Italy

RMI 156 phantom is recommended for quality checks in mammography(as suggested by American College of Radiology (ACR) Phantom)

dimension : 8 x 8 cm2 a Lucite block 3.3 cm thick a wax block 0.6 cm thick a 0.3 cm thick cover

Mammographic Phantom: RMI 156Mammographic Phantom: RMI 156

16 test objects embedded in wax:5 groups of simulated micro-calcifications of different diameter (0.54 mm, 0.40 mm, 0.32 mm, 0.24 mm and 0.16 mm),

5 different thickness tumor-like masses (2.00 mm, 1.00 mm, 0.75 mm, 0.50 mm and 0.25 mm)

6 different size nylon fibers that simulate fibrous structures (1.56 mm, 1.12 mm, 0.89 mm, 0.75 mm, 0.54 mm and 0.40 mm)

It simulates a compressed breast (4.2 cm)

Acquisition Geometry

detector

Pb collimator

focus

105 mm

3 mm

440 mm

42 mm

50 mm

phantom

scan direction

Move and tile technique 72 different acquisitions scanning performed with stepper

motors controlled by the PC

Move and tile technique 72 different acquisitions scanning performed with stepper

motors controlled by the PC

0 200 400 600 800 10000

5000

10000

15000

20000

25000

0 coll 1 coll

coun

ts

26 kV 250 mAs

Exposure conditions optimizationExposure conditions optimizationThe effect of the collimator has been evaluated by measuring the contrast of a W edge with and without a collimator

•The use of a collimator close to the beam focus allows a significant improvement in the detected contrast and therefore in the image quality. •The system equipped with 2 collimators shows a slight contrast improvement (~ 0.6%).

With one collimator

With one collimatorN1 N2

contrast

(65.0 + 0.7) % (95.11 + 0.03) %

MEDIPIX I –Film Images Comparison

kodak trimatic film• kodak min-r 2190 screen• Digitized 12 bits, 85 m pitch

Medipix 1 • Detector Si • 525 m thick

Medipix 1 • Detector Si • 525 m thick

Tube settings25 kV 80 mAs

MedipixI-Film SNR comparison

12 13 14 15

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0 film MedipixI

SN

R

Particular #

The Signal to Noise Ratio (SNR) of the details imaged with the MEDIPIXI system is systematically higher than the SNR of the same details imaged by the film

The SNR of the detail 15 (0.24 mm thick) is not measurable on film

The commercial digital systems The commercial digital systems

•GE Senographe 2000D based on CsI(Tl)-aSi•Diagnostic Centre Prevenia in Torino (Italy)•Ge Senographe 2000D is a full field digital mammography system:•19 x 23 cm2 amorphous silicon detector coupled to CsI (Tl) (flat panel) with a pixel size of 100 m

•Fuji FCR 5000MA Computed Radiography (CR) based on imaging plates with a photostimulable phosphor screen

•C.S.P.O. in Firenze (Italy)•Image Plate with a photostimulable phosphor screen •Senographe 800T X-ray unit: •18 x 24 cm2 with a pixel size of 50 m

•Giotto Image MD Internazionale Medico Scientifica Srl (I.M.S. Bologna, Italy) based on aSe

•Istituto di Radiologia APGD in Udine (Italy)•Amorphous selenium technology (flat panel)•active area 17.4 x 23.9 cm2 with a pixel size of 85 m •Mo and Rh filter

SNR comparisonSNR comparison•SNR as a function of the mAs for the detail 12 (2 mm thick tumor mass) of the RMI 156 phantom, obtained with the different acquisition systems

0 50 100 150 200 2500

1

2

3

4

5

6

7

8

9

10

25 kVolt Si 525 m GE Senographe 2000D FCR 5000 MA Giotto Image MD

SN

R

mAs

25 kVolt

0 20 40 60 80 100 120 1400,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

GaAs Si 525m

SN

R

mAs

GaAs- Si detectors SNR comparison GaAs- Si detectors SNR comparison

•SNR as a function of the mAs for the detail 12 (2 mm thick tumor mass) of the RMI 156 phantom with GaAs and Si 525 m thick detectors.

•Tube for general radiography (W anode and an Al filter 2.5 mm) •Tube settings: 40 kV and 16 - 125 mAs

• The performance of the GaAs detector in terms of the SNR is superior to the 525 m thick Si one

• The statistics on the images is in agreement with the different detection efficiencies of Si and GaAs at the energies of the X ray beam (average = 28 keV)

MTF evaluation: edge methodMTF evaluation: edge method

•The MTF of the system has been evaluated using the image of the W edge with the mammographic tube at 25 kV and 12 mAs.

0 2 4 6 8 10 120,0

0,2

0,4

0,6

0,8

1,0with collimator, 25 kV 12 mAs

MTF Si 525 m

MT

F

lp/mm

At the Nyquist frequency (2.94 lp/mm) of the MedipixI the MTF is 60 %

ESF: from the image of a W edge LSF: the ESF’ derivativeMTF: normalized Fourier Trasform of the LSF

MTF comparisonMTF comparison•Comparison among the MTF of the different digital systems is presented (the vertical lines represent the value of the Nyquist frequency for each system).

0 2 4 6 8 10 12 140.0

0.2

0.4

0.6

0.8

1.0 GE Senographe 2000D Si 525 m Fuji FCR 5000MA Giotto Image MD

MT

F

lp/mm

Nyquist frequency MTF

Giotto Image MD 5.88 lp/mm 46 %

Single photon counting 2.94 lp/mm 60 %

GE Senographe 2000 D 5 lp/mm 20 %

FCR 5000MA 10 lp/mm 1 %

These results are in agreement with data published in literature.Bloomquist et al. “Acceptance Testing of Digital Mammography Units for the ACRIN/DMIST Study”, Proceedings of the IWDM 2002, Bremen, Germany, pp. 85-89

MTF evaluation: slit methodMTF evaluation: slit method Tantalum phantom,

1.5 mm of thickness, 10 m width slit.

Presampling MTF • line slightly tilted with respect to the

perpendicular to the pixel lines.• Composition of digital LSFs.• Computation of the FFT for the MTF

Edge –slit methods comparison The two curves have the same

behavior up to the Nyquist frequency

For higher frequencies the slit method is more accurate• Good agreement with the

theoretical curve sinc(fA) of an ideal imaging system of sampling aperture A

The first minimum position provides the information on the sampling aperture A • For the PPC A = 0.17 mm which

corresponds to the pixel dimension

The MTF does not depend on the detector material and thickness

0 1 2 3 4 5 6 7 8 9 100.0

0.2

0.4

0.6

0.8

1.0 MTF edge PPC MTF slit PPC

MT

F

spatial frequency (lp/mm)

Sampling aperture

o Development of Si and GaAs pixel semiconductor detectors, which have been bump bonded to single photon counting chips (PCC-MedipixI system).

o These assemblies have shown very good imaging capabilities in terms of:

SNR and MTF in comparison with commercial digital systems

o The MTF measurements with the slit method for the commercial systems have still to be done

o Assemblies based on Si pixel detectors and the new chip Medipix2 (256 x 256 pixel, 55 m in side), are currently under test and an improvement in terms of SNR and MTF is expected

ConclusionsConclusions

Medipix I -Medipix II MTF comparison

MTF measured with the slit method W anode tube, 40 kV, 125 mAs

0 5 10 15 20 250.0

0.2

0.4

0.6

0.8

1.0

MedipixI MedipixII

MT

F

Spatial Frequency (lp/mm)Sampling aperturea = 0.055 mm

fNy=9.1 lp/mm

fNy=2.9 lp/mm

Sampling aperturea = 0.17 mm