• Introduction– Why digital?– Why dual energy?

• Experimental setup• Image processing and results

– Alvarez-Macovski algorithm– Subtraction imaging with contrast medium

• Conclusion and outlook

A silicon microstrip system with A silicon microstrip system with the RX64DTH ASIC for dual the RX64DTH ASIC for dual

energy radiologyenergy radiology

1) University of Eastern Piedmont and INFN, Alessandria, Italy L. Ramello;

2) University and INFN, Torino, Italy P. Giubellino, A. Marzari-Chiesa, F. Prino;

3) University and INFN, Ferrara, Italy; M. Gambaccini, A. Taibi, A. Tuffanelli, A. Sarnelli;

4) University and INFN, Bologna, Italy G. Baldazzi, D. Bollini;

5) AGH Univ. of Science and Technology, Cracow, Poland W. Dabrowski, P. Grybos, K. Swientek, P. Wiacek;

6) University of Antwerp, Antwerp, Belgium P. Van Espen;

7) Univ. de los Andes, Colombia C. Avila, J. Lopez Gaitan, J.C. Sanabria;

8) CEADEN, Havana, Cuba A.E. Cabal, C. Ceballos, A. Diaz Garcia;

9) CINVESTAV, Mexico City, Mexico L.M. Montano;

The CollaborationThe Collaboration

Introduction: why digital ?Introduction: why digital ?• Digital radiography has well known advantages over

conventional screen-film systems

– Enhance detecting efficiency w.r.t. screen-film

– Image analysis– Easy data transfer

• Dual energy techniques

• GOAL: improve image contrast

Based on different energy dependence

of different materials

Enhance detail visibility (SNR)

Decrease dose to the patient

Decrease contrast media concentration

Introduction: why dual energy ?Introduction: why dual energy ?

Example 1: dual energy Example 1: dual energy mammographymammography

E 15-20 keV:Signal from cancer tissue deteriorated by the adipose tissue signal

E 30-40 keVCancer tissue not visible, image allows to map glandular and adipose tissues

Example 2: angiography at the Example 2: angiography at the iodine K-edgeiodine K-edge

Iodine injected in patient vessels acts as radio-opaque contrast medium

Dramatic change of iodine absorption coeff. at K-edge energy (33 keV)

Image subtraction (2 images taken below and above the K-edge energy)

Experimental setupExperimental setup• To implement dual energy imaging we need:

• a dichromatic beam

• a position- and energy-sensitive detector

Quasi-monochromatic beams • ordinary X-ray tube + mosaic

crystals • instead of truly monochromatic

synchrotron radiationAdvantages: cost, dimensions, availability in hospitals

Linear array of silicon microstrips + electonics for single photon counting•Binary readout

•1 or 2 discriminators (and counters) per channel

•Integrated counts for each pixel are readout

• Scanning required to build 2D image

Experimental setup: beam (1)Experimental setup: beam (1)

Bd

chnBE

sin2

..

Bragg Diffraction on Highly Oriented Pyrolitic Grafite Crystal

W anode tube

1st and 2nd Bragg harmonics E and 2E are obtained in the same beam

Collimator

Experimental setup: beam (2)Experimental setup: beam (2)

Bd

chnBE

sin2

..

Bragg Diffraction on Highly Oriented Pyrolitic Grafite Crystal

W anode tube

Double slit collimator

Two spatially separated beams with different energies E-E and E+E obtained in 2 separate beams

More on the dichromatic beamMore on the dichromatic beam

incidentspectraat 3 energysettings …

… spectra after 3 cm plexiglass

(measured with HPGe detector)

• Fully parallel signal processing for all channels• Binary architecture for readout electronics

1 bit information (yes/no) is extracted from each stripThreshold scans needed to extract analog information

• Counts integrated over the measurement period transmitted to DAQ

• Fully parallel signal processing for all channels• Binary architecture for readout electronics

1 bit information (yes/no) is extracted from each stripThreshold scans needed to extract analog information

• Counts integrated over the measurement period transmitted to DAQ

data, control

Silicon strip detector Integrated circuit

100 m

current pulses

X-rays

PC

N. I. I/O cards PCI-DIO-N. I. I/O cards PCI-DIO-96 96

and DAQCard-DIO-24and DAQCard-DIO-24

Experimental seup: Single Photon Experimental seup: Single Photon Counting SystemCounting System

Experimental setup: silicon detectorExperimental setup: silicon detector

Parameter Value

Depth 300 μm

Strip length 10 mm

Number of strips 400

Strip pitch 100 μm

Depletion voltage 20-23 V

Leakeage curr. (22º C)

50-60 pA

Inactive region thickn.

765 μm

Designed and fabricated by ITC-IRST, Trento, Italy

Detector efficiencyDetector efficiency

• Front geometry– Strip orthogonal to the beam– 70 m of Al light shield

• Edge-on geometry– Strip parallel to the beam– 765 m of inactive Si– Better efficiency for E > 18 keV

• Efficiency calculation– X-ray absorbed if interacts in passive regions– X-ray detected if makes photoelectric effect in active regions

Experimental setup: RX64 chipExperimental setup: RX64 chipCracow U.M.M. design - (28006500 m2) - CMOS 0.8 µm process

(1) (1) 64 front-end channels a) preamplifierb) shaperc) 1 or 2 discriminators

(2)(2) (1 or 2)x64 pseudo-random counters (20-bit)

(3)(3) internal DACs: 8-bit threshold setting and 5-bit for bias settings

(4)(4) internal calibration circuit (square wave 1mV-30 mV)

(5)(5) control logic and I/O circuit (interface to external bus)

Cracow U.M.M. design - (28006500 m2) - CMOS 0.8 µm process

(1) (1) 64 front-end channels a) preamplifierb) shaperc) 1 or 2 discriminators

(2)(2) (1 or 2)x64 pseudo-random counters (20-bit)

(3)(3) internal DACs: 8-bit threshold setting and 5-bit for bias settings

(4)(4) internal calibration circuit (square wave 1mV-30 mV)

(5)(5) control logic and I/O circuit (interface to external bus)

11 22

3344

55

Det

ecto

rD

etec

tor

Experimental setup: PCBExperimental setup: PCB

detectorpitch adapter

ASICs

PCB:- One 400 strip detector- Pitch adapter- 6 RX64 chips

384 equipped channels- connector to DAQ card

2 protoype detectors:a) 6 x Single threshold RX64b) 6 x Dual threshold RX64

PCB:- One 400 strip detector- Pitch adapter- 6 RX64 chips

384 equipped channels- connector to DAQ card

2 protoype detectors:a) 6 x Single threshold RX64b) 6 x Dual threshold RX64

System calibration setup in AlessandriaSystem calibration setup in Alessandria

Detector in Front config.Fluorescence target

(Cu, Ge, Mo, Nb, Zr, Ag, Sn)

Cu anode X-ray tube

→ X-ray energies = characteristic lines of target material

150

100

50

0

Co

un

ts

500400300200100

Threshold (mV)

Source Am+Rb target Source Am+Mo target Source Am+Ag target Tube+Cu target Tube+Ge target Tube+Mo target Tube+Ag target Tube+Sn target

Cu K

Mo K

Ge K

Rb K

Ag K

Sn K

Ag K

Mo K

Sn K

System Tp

GAINV/el.

ENC Energy resolution

6 x RX64 0.7 s 64 ≈170 el. ≈0.61 keV

6 x RX64DTH 0.8 s 47 ≈ 200 el. ≈0.72 keV

241241Am source with rotary target holder (targets: Cu, Rb, Mo, Ag, Ba)Am source with rotary target holder (targets: Cu, Rb, Mo, Ag, Ba)Cu-anode X-ray tube with fluorescence targets (Cu, Ge, Mo, Ag, Sn)Cu-anode X-ray tube with fluorescence targets (Cu, Ge, Mo, Ag, Sn)

System calibrationSystem calibration

• K-edge subtraction imaging with contrast medium Cancel background structures by subtracting 2 images taken at energies just

below and above the K-edge of the contrast medium Suited for angiography at iodine (gadolinium) K-edge

- Cancel background structures to enhance vessel visibility Possible application in mammography (study vascularization extent)

- Hypervascularity characterizes most malignant formations

• Dual energy projection algorythm Make the contrast between 2 chosen materials vanish by measuring the

logarithmic transmission of the incident beam at two energies and using a projection algorithm [Lehmann et al., Med. Phys. 8 (1981) 659]

Suited for dual energy mammography

– remove contrast between the two normal tissues (glandular and adipose), enhancing the contrast of the pathology

– Single exposure dual-energy mammography reduces radiation dose and motion artifacts

Dual energy imagingDual energy imaging

X-ray tube with dual energy output

Phantom

Detector box with 2 collimators

1.1. X-ray tube + mosaic crystal and 2 collimators to provide dual-energy output X-ray tube + mosaic crystal and 2 collimators to provide dual-energy output

- E1= 31.5 keV, E2 =35.5 keV (above and below iodine k-edge)- E1= 31.5 keV, E2 =35.5 keV (above and below iodine k-edge)

2.2. Detector box with two detectors aligned with two collimatorsDetector box with two detectors aligned with two collimators

3.3. Step wedge phantom made of PMMA + Al Step wedge phantom made of PMMA + Al with 4 iodine solution filled with 4 iodine solution filled cavities of 1 or 2 mm diametercavities of 1 or 2 mm diameter

Angiography setupAngiography setup

15

10

5

0

pixe

ls

3002001000pixels

-0.8

-0.6

-0.4

-0.2

0.0

log

con

tegg

i

0 50 100 150 200 250 300 350

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2 Conc(I) = 370 mg/ml Measurement Simulation

ln[c

ou

nt(

E=

35.5

Kev)]

- ln

[co

un

t(E

=31.5

Kev)]

Strip Number

15

10

5

0

pix

els

3002001000pixels

161412108642

Co

nte

gg

i (

x1

03 )

0 50 100 150 200 250 300 350

0,0

0,2

0,4

0,6

0,8

1,0 Conc(I) = 370 mg/ml E = 31.5 KeV

Measurement Simulation

Co

un

ts / M

ax.C

ou

nts

Strip Number

E = 31.5 keVE = 31.5 keV

15

10

5

0

pix

els

3002001000pixels

6

5

4

3

2

1

Co

nte

gg

i (x

103 )

0 50 100 150 200 250 300 350

0,0

0,2

0,4

0,6

0,8

1,0

Strip Number

Measurement Simulation

Conc(I) = 370 mg/ml E = 35.5 KeV

Co

un

ts / M

ax.C

ou

nts

E = 35.5 keV

5.3125.351 lnln NCNC logarithmic subtraction

Phantom structure not

visible in final image

Angiographic test results (I)Angiographic test results (I)

15

10

5

0

pix

els

3002001000pixels

-0.8

-0.6

-0.4

-0.2

0.0

log

co

nte

gg

i

Conc = 370 mg / mlConc = 370 mg / ml

15

10

5

0

pix

els

3002001000pixels

-0.3

-0.2

-0.1

0.0

0.1

0.2

log

co

nte

gg

iConc = 92.5 mg / mlConc = 92.5 mg / ml

15

10

5

0

pix

els

3002001000pixels

-0.15

-0.10

-0.05

0.00

0.05

0.100.15

log

co

nte

gg

i

Conc = 23.1 mg / mlConc = 23.1 mg / ml

100

80

60

40

20

0

SN

R

4003002001000Concentrazione (mg/ml)

cavità 4 teor. cavità 4 cavità 3 teor. cavità 3 cavità 2 teor. cavità 2 cavità 1 teor. cavità 1

Possible decrease of iodine concentration keeping the same rad. dose

Angiographic test results (II)Angiographic test results (II)

Results with a second phantomResults with a second phantom

140 140

120 120

100 100

80 80

60 60

40 40

20 20

0 0

300

um p

ixel

300

300

200

200

100

100

0

0

100 um pixel

140 140

120 120

100 100

80 80

60 60

40 40

20 20

0 0

300

um p

ixel

300

300

200

200

100

100

0

0

100 um pixel

140 140

120 120

100 100

80 80

60 60

40 40

20 20

300

um p

ixel

300

300

200

200

100

100

0

0

100 um pixel

Phantom

Digital SubtractionAngiography

Dual Energy Angiography

smaller cavity (=0.4 mm) visible in DEA and not in DSA

Iodine conc. = 95 mg/ml

Dual energy projection algorithmDual energy projection algorithmThe mass attenuation coefficient μ of any material at a given energy E is expressed as a combination of the coefficients of any two suitable materials and :

E

aE

aE

21

The logarithmic attenuation M = μξtξ of the material of thickness tξ is measured at two different energies: low (El) and high (Eh):

lhlh

lhhl

lhlh

hllh

hhh

lll

EEEEEMEM

A

EEEE

EMEMA

EAEAM

EAEAM

2

1

21

21

A1 and A2 represent the thicknesses of the two base materials which would provide the same X-ray attenuation as material ξ.

C

C

90°

M1

R

1

M2

2

If a monochromatic beam of intensity I0 goes through material ξ which is partly replaced by another material ψ …

I0

I1 I2

ξψ

… then the vertexes of log. attenuation vectors M2 (material ξ) and M1 (mat. ξ + ψ) lie on a line R which is defined only by the properties of materials α, β, ξ and ψ. Projecting along direction C, orthogonal

to R, with the contrast cancellation angle :

… it is possible to cancel the contrast between materials ξ and ψ: both M1 and M2 will project to the same vector

A2

A1

Dual energy projection algorithmDual energy projection algorithmThe logarithmic attenuation M in a given pixel can be represented as a vector having components A1 and A2 in the basis plane, the modulus will then be proportional to the gray level of that pixel

sincos 21 AAC

Mammographic phantomMammographic phantom• Three components: polyethylene (PE), PMMA

and water to simulate the attenuation coeff. (cm-1) of the adipose, glandular and cancerous tissues in the breast

S. Fabbri et al., Phys. Med. Biol. 47 (2002) 1-13

E _fat _gland _canc

20 .456 .802 .844

40 .215 .273 .281

E μ_PE μ_PMMA μ_water

20 .410 .680 .810

40 .225 .280 .270

300pixel

350

300

250

200

150

100

50

0

pixe

l

Image processing (1)Image processing (1)

300pixel

350

300

250

200

150

100

50

0

pixe

l

Low thr. High thr.

Measured (raw)

300pixel

350

300

250

200

150

100

50

0

pixe

l

16 keV

300pixel

350

300

250

200

150

100

50

0

pixe

l

32 keV

HE and LE imagesCorrect for:

1. pixels with huge n. of counts (bad counter conversion)

2. dead pixels3. X-ray beam fluctuations4. subtract high threshold

image from low threshold one

5. correct for spatial inhomogeneities of beam and detector extracted from flat-field profiles

300pixel

350

300

250

200

150

100

50

0

pixe

l

300pixel

350

300

250

200

150

100

50

0

pixe

l

16 – 32 keV 18 – 36 keV

300pixel

350

300

250

200

150

100

50

0

pixe

l

300pixel

350

300

250

200

150

100

50

0

pixe

l

1= PMMA 2=water

3=PE 4=(water+PE)

Image processing (2)Image processing (2)

Low statistics due to:1) 2nd order harmonic2) dectecting efficiency

Simulation with MCNPSimulation with MCNP

1=detector2=PMMA3=water4=PE

MCNP-4C simulation with ENDF/B-VI library• Photons and electrons

tracked through the phantom and the detector (including the inactive region in front of the strips)

• Energy deposition in each strip recorded

• histogram of counts vs. strip number filled

Top View

Side View

Experiment vs. Simulation (1)Experiment vs. Simulation (1)RX64DTH 16 – 32 keV

300pixel

350

300

250

200

150

100

50

0

pixe

l

300pixel

350

300

250

200

150

100

50

0

pixe

l

simulation 16 – 32 keV

300pixel

350

300

250

200

150

100

50

0

pixe

l

300pixel

350

300

250

200

150

100

50

0

pixe

l

Experiment vs. Simulation (1)Experiment vs. Simulation (1)

3400

3200

3000

2800

2600

2400

2200

3002001000

Simul.16 keV Left Part Meas.16 keV Left Part

3400

3200

3000

2800

2600

2400

2200

3002001000

Simul.16 keV Right Part Meas.16 keV Right Part

3100

3000

2900

2800

2700

2600

3002001000

Simul.32 keV Left Part Meas.32 keV Left Part

3100

3000

2900

2800

2700

2600

3002001000

Simul.32 keV Right Part Meas.32 keV Right Part

Results (1): SNR vs. proj. angleResults (1): SNR vs. proj. angle100

80

60

40

20

0

SN

R

706050403020Angle (deg)

SNR PE-Water SNR PMMA-Water SNR PMMA-PE

5x5 pixels area

SNR = 9.6287 theta = 36.5deg

SNR = 4.7246 theta = 52.5degSNR = 3.1887

theta = 43deg

RX64DTH 16 – 32 keV

MCNP simulation160

140

120

100

80

60

40

20

0

SN

R

706050403020Angle (deg)

SNR_PE_225_23_WAT225_3_5x5 SNR_PMMA_2_20_WAT225_3_5x5 SNR_PMMA_20_2_PE225_23_5x5

SNR = 23.176 theta = 35deg

SNR = 14.521 theta = 44.5deg

SNR = 9.2112 theta = 39deg

Cancellation angle for a pair given by SNR=0

Theoretical cancellation angles: PMMA-water 36.5° PE-water 40.5° PMMA-PE 45°

Results (2): SNR summaryResults (2): SNR summaryEnergy Canceled Contrast SNR SNR

(keV) materials material RX64* RX64DTHPMMA-water

PE 8.11 9.63

16-32 PE-water PMMA 2.53 3.19PE-PMMA water 3.96 4.72PMMA-water

PE 7.43 5.14

18-36 PE-water PMMA 2.70 2.10PE-PMMA water 3.85 3.13PMMA-water

PE 2.55 3.27

20-40 PE-water PMMA 0.67 1.07PE-PMMA water 0.89 1.58

* Previous version of ASIC, exposure with about 2x more incident photons

Results (3): Projected imagesResults (3): Projected imagesRX64DTH 16 – 32 keV simulation 16 – 32 keV

300pixel

350

300

250

200

150

100

50

0

pix

el

35º

300pixel

350

300

250

200

150

100

50

0

pix

el

44.5º

PMMA-water cancellation

PMMA-PE cancellation

300pixel

350

300

250

200

150

100

50

0

pixe

l

36.5º

300pixel

350

300

250

200

150

100

50

0

pixe

l

52.5º

Conclusion and OutlookConclusion and Outlook• We have developed a single photon counting silicon

detector equipped with the RX64DTH ASIC, with two selectable energy windows

• The energy resolution of 0.8 keV (rms) is well adapted for dual energy mammography and angiography

• We have performed mammography imaging tests with a three-material phantom– We have demonstrated the feasibility of contrast cancellation between two

materials, enhancing the visibility of small features in the third one• We have performed angiography imaging tests with 2

different phantoms and iodine contrast medium– We have demonstrated the feasibility of logarithmic subtraction between two

images, enhancing contrast of small vessels also with lower iodate solution concentrations

• OUTLOOK: – Increase photon statistics at high energy, optimize exposure conditions– New detector materials, CZT?– Tests with a more realistic mammographic phantom