Sezione di NapoliUniv. “Federico II”

Experimental study of beam hardening artefacts in photon counting breast

computed tomography

M.G. Bisognia, A. Del Guerraa,N. Lanconellib, A. Lauriac, G. Mettivierc, M.C. Montesic,

D. Panettaa, R. Panid, M.G. Quattrocchia, P. Randaccioe, V. Rossoa, P. Russoc

aUniversità di Pisa and INFN, Pisa, ItalybUniversità di Bologna and INFN, Bologna, Italy

c Università di Napoli Federico II and INFN, Napoli, ItalydUniversità La Sapienza and INFN, Roma, ItalyeUniversità di Cagliari and INFN, Cagliari, Italy

Sezione di NapoliUniv. “Federico II”

Summary

• Beam hardening effect • Bimodal energy model• Beam hardening in PMMA slabs• Experimental CT set-up• Beam hardening in PMMA breast phantoms• Conclusions and future work

Sezione di NapoliUniv. “Federico II”

Motivation and beam hardening effect

X-ray Computed Tomography (CT) system on the gantry of a dedicated, scintillator based single photon emission tomography (SPECT) system for breast 99m-Tc imaging (see presentation S. Vecchio at this Conference);

the breast would be scanned in a pendant geometry, i.e. with the patient in a prone position and the breast uncompressed;

the beam energy distribution becomes more abundant in high energy photons and this effect causes an under-estimation or “cupping” artefact in the reconstructed attenuation coefficient at the center of the volume sample .

Sezione di NapoliUniv. “Federico II”

Bimodal energy model

For a polychromatic beam the X-ray attenuation in a material is described by two effective energies (E1, E2; E2>E1) and, correspondingly, by two effective attenuation coefficients 1 and 2 (<1): the lower value 2 at the beam effective energy E2 accounts for the effective attenuation in large material thicknesses

E. Van de Casteele et al., Phys. Med. Biol. 47, (2002) 4181

–ln(Ix/I0)=2x + ln{[1+]/[1+exp(2x-1x)]}

= f(E1)(E1)/ f(E2)(E2)

Source-Detector efficiency

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Bimodal energy model:measurements

–ln(Ix/I0)=2x + ln(1+) for large thickness

0 2 4 6 8 10 12 140.00.51.01.52.02.53.03.54.04.55.0

Y = A + B*XA=0.221+/-0.03

B=0.244+/-0.003 cm-1

R= 0.999 (p<0.0001)

PMMA thickness, x (cm)

-ln

(I x/I 0)

- a stack of 1 up to 14 PMMA sheets (20×20 cm2, 1 cm thick)- CdTe diode detector (mod. XR-100T-CdTe) Amptek Inc.

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CdTe detector Spectra

20 30 40 50 60 70 800

10000

20000

30000

40000

50000

60000

x 10

Emean

= 51.2 keV

Emean

= 47.3 keV

Direct Beam (51.5 cm air) After 29 cm air + 14 cm PMMA + 8.5 cm air

Inte

nsity

(cou

nts

s-1 m

m-2 k

eV-1 m

A-1)

Photon energy (keV)

X-ray attenuation in PMMA as a function of material thickness:effective attenuation coefficient eff = 0.244 cm-1 (Eeff=51.0 keV)

I0

I14 cm

1 (cm-1)E1 (Kev)

0.60221.3

2 (cm-1)E2 (Kev)

0.24451.0

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Experimental set-up

W-anode X-ray tube 80 kVp 4°×56° fan beam

A

B C

PMMA Phantoms14 cm thick

0.3 mm Si Hybrid pixel detector256 x 256 pixels, 55 x 55 m2

Detector intrinsic resolution: 110 mSensitive area 14.08×14.08 mm2

Readout: Single photon counting Medipix2 chip* * Developed by the Medipix2 collaboration, www.cern.ch\medipix

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Beam hardening in PMMA cylinder phantom

-3D view of the reconstructed* transaxial slice of the 14 cm diameter PMMA cylinder;

- isotropic voxel side= 0.232 mm;- total thickness = 7.4 mm; - 180 views on 360°

- 2D reconstruction of a single slice (thickness = 0.232 mm);

*Custom algorithm implementing the filtered

backprojection fan beam reconstruction algorithm

Sezione di NapoliUniv. “Federico II”

Beam hardening in 14 cm thick PMMA cylinder phantom

the drop of the attenuation coefficient (edge-center)/edge=18%

( 0.33 cm-1 0.27 cm-1)

0 2 4 6 8 10 12 140,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

18%

Atte

nuat

ion

coef

ficie

nt (

cm-1)

Distance along a diameter (cm)

- low detection efficiency - the charge sharing effect of the silicon pixel detector

Sezione di NapoliUniv. “Federico II”

Beam hardening in PMMA ellipsoid phantom

3D view of the CT reconstruction of three different sections of the PMMA ellipsoid phantom related to three different distances from the phantom top (“nipple”)

A) distance = 10.5 cm, = 14 cm B) distance = 4.5 cm, = 11.5 cm C) distance = 0.5 cm, = 4 cm

7.6 mm

5 mm

7.6 mm

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Beam hardening in PMMA ellipsoid phantom

0 2 4 6 8 10 12 140.25

0.30

0.35

Profile at 10.5 cm from the top, = 14.0 cm Profile at 4.5 cm from the top, = 11.5 cm Profile at 0.5 cm from the top, = 4.0 cm

A

tte

nu

atio

n C

oe

ffic

ien

t (c

m-1)

Distance along the diameter (cm)

(edge-center)/edge = 18% (edge-center)/edge = 4%

(edge-center)/edge = 12%

Sezione di NapoliUniv. “Federico II”

Conclusions and future work

• Preliminary tests for beam hardening “cupping” artefact in photon counting X-ray breast CT system using PMMA phantoms and a very fine pitch silicon pixel detector have been shown

• Drop of the attenuation coefficient of 4% when the PMMA thickness is 4-cm and of 18% for 14-cm PMMA thick material

• A bimodal energy model for beam hardening artefact in CT has been shown applicable to our data and produce an estimate of 19% for the attenuation coefficient drop for the 14-cm-diameter phantom

• Correction of the CT data in the pre-reconstruction phase will be applied and tests will be reported of this photon counting system, in comparison with an integrating flat panel detector

Sezione di NapoliUniv. “Federico II”

Bimodal Energy Model

0 2 4 6 8 10 12 140.26

0.27

0.28

0.29

0.30

0.31

CB

A: (4 cm, 0.284 cm-1) drop=7%

B: (11.5 cm, 0.264 cm-1) drop=13%

C: (14 cm, 0.261 cm-1) drop=19%

A

0.244 0.276 0.602

Att

en

ua

tion

co

eff

icie

nt

(cm-1)

PMMA thickness (cm)

Calculated attenuation coefficient as a function of PMMA thickness

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Experimental set-up for PMMA attenuation coefficient evaluation

• X-ray tube: W anode with a 40 m focal spot size (Source-Ray, Inc., mod. SB-80-250, NY, USA). • 35 kVp to 80 kVp with an anode current in the range 10−250 A• fan beam irradiation geometry (4 deg horizontal × 56 deg vertical)• CdTe diode detector (mod. XR-100T-CdTe) associated at power supply

amplifier (mod. PX2T-CR) from Amptek Inc., Bedford, MA, USA

W Anode80 kVp, 0.25 mA4.2 mm Al

51.5 cm

36 cm 15.5cm

CdTe detector (mod. XR-100T-CdTe)

14 PMMA sheets1cm thick