Introduction –Why digital? –Why dual energy? Experimental setup Image acquisition Image processing and results A silicon microstrip system with the RX64DTH.

• Introduction– Why digital?– Why dual energy?

• Experimental setup• Image acquisition• Image processing and results

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

dual energy radiologydual energy 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, L. Bolaños;

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 Introduction: why dual energy ?energy ?

Example 1: dual energy Example 1: dual energy mammographymammography

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: angiographyExample 2: angiography•Angiography = X-ray examination of blood vessels

determine if the vessels are diseased, narrowed or blocked Injection of a contrast medium (Iodine) which absorbs X-ray

differently from surrounding tissues

•Coronary angiographyIodine must be injected into the heart or very close to itA catheter is inserted into the femoral artery and managed up

to the heart→Long fluoroscopy exposure time to guide the catheter→Invasive examination

•Why not to inject iodine in a peripheral vein?Because lower iodine concentration would be obtained,

requiring longer exposures and larger doses to obtain a good image

But, if the image contrast could be enhanced in some way…

Example 2: angiography at Example 2: angiography at the iodine K-edge (II)the iodine K-edge (II)

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

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

Subtraction of 2 images taken with photons of 2 energies (below and above the K-edge)→ in the resulting image only the iodine signal remains and all other materials are canceled

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 + electronics 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)

Bdchn

BE 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)

Bdchn

BE 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

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 setup: Single Photon Experimental setup: Single Photon Counting SystemCounting System

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

Detecting systemDetecting system

Chip RX64 → counts incident photons on each strip of the detector

4 cm

6.4 mm10 strip = 1 mm

micro-bondings

Silicon microstrip detectoreach strip is an independent detector which gives an electric signal when an X-ray photon crosses it and interacts with a silicon atom

Knowing from which strip the electric signal comes from,the position of the incoming X-ray phonton is reconstructed.

Why silicon detectors?Why silicon detectors?Main characteristics of silicon detectors:• speed of the order of 10 ns• spatial resolution of the order of 10 m•small amount of material

0.003 X0 for a typical 300 m thickness

• excellent mechanical properties• good resolution in the deposited energy

3.6 eV of deposited energy needed to create a pair of charges, vs. 30 eV in a gas detector

Silicon sensor diodeSilicon sensor diode•The impinging ionizing particles generate electron-hole pairs •The impinging photons which interact in the detector volume create an electron (via Photoelectric, Compton or Pair Production)

•The electron ionizes the surrounding atoms generating electron-hole pairs

• Electron and holes drift to the electrodes under the effect of the electric field present in the detector volume. •The electron-hole current in the detector induces a signal at the electrodes on the detector faces.Metal contact

n+-type implant

n-type bulk

Charged particle -V

+V

electron

hole

P+-type implant

photon

photoelectron

ReversebiasE

Why reverse biased diode?Why reverse biased diode?•The amount of charge deposited in the silicon detector is very small

≈5500 electrons are produced by a 20 keV photons making photoelectric effect in the silicon

Forward-biased junction: the signal would be masked by the fluctuations of the current which the applied field makes flow even in high resistivity, hyper-pure silicon.Reverse-biased junction: allows to obtain the necessary electric field and only a very small “dark” current also at room temperature.

-V

+V

depleted region

Increasing the polarization voltage, it is possible to extend the depletion layer down to the backplane.

To have full efficiency, the polarization voltage must be high enough to deplete the full detector thickness (typically 300 m)

junction

NOT GOOD

Silicon Microstrips detectorsSilicon Microstrips detectors• A micro-strip detector is a silicon detector segmented in long, narrow elements.

•Each strip is an independent p-n reverse-biased junction• Provides the measurement of one coordinate of the particle’s crossing point with high precision (down to 1 m).

N-type substrate

P+n+

Al

P+

SiO2

AC coupling to electronics

SiO2AlDC coupling to electronics

Experimental setup: silicon Experimental setup: silicon detectordetector

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

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)

11 22

3344

55Det

ecto

r

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

Imaging testImaging test1-dimensional array of strips → 2D image obtained by scanning

Cd-109 source (22.24 keV)

Detector

Collimator (0.5 mm)

Tes

t O

bjec

t

5 mm

Imaging testImaging test1-dimensional array of strips → 2D image obtained by scanning

0 1 0 2 0 3 0 4 0 5 0 6 0

5 0

6 0

7 0

8 0

9 0

1 0 0

1 1 0

1 2 0

1 3 0

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2 1 0

C a n a le s

Pasos

0

3 , 0 0 0

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Scan

nin

g

System calibration setup in System calibration setup in AlessandriaAlessandria

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

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nts

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 TpGAINV/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

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ln[c

ount

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ev)]

- ln[

coun

t(E=3

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ev)]

Strip Number

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Measurement Simulation

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E = 31.5 keVE = 31.5 keV

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Measurement Simulation

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5.3125.351 lnln NCNC logarithmic subtraction

Phantom structure not

visible in final image

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

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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 Results with a second phantomphantom

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PhantomDigital Subtraction

Angiography

Dual Energy Angiography

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

Iodine conc. = 95 mg/ml

Dual energy projection Dual energy projection algorithmalgorithm

The 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

aEaE

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

EEEEEMEMA

EEEEEMEM

A

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

C90°

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 Dual energy projection algorithmalgorithmThe 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

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Image processing (1)Image processing (1)

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Low thr. High thr.

Measured (raw)

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16 keV

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32 keVHE 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

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1= PMMA 2=water3=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

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

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Results (1): SNR vs. proj. Results (1): SNR vs. proj. angleangle100

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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

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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* RX64DTH

PMMA-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 Results (3): Projected imagesimagesRX64DTH 16 – 32 keV simulation 16 – 32 keV

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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