performances of different digital mammography imaging systems: evaluation and comparison maria...
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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