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Sezione di Napoli Univ. “Federico II” Experimental study of beam hardening artefacts in photon counting breast computed tomography M.G. Bisogni a, A. Del Guerra a,N. Lanconelli b, A. Lauria c, G. Mettivier c, M.C. Montesi c, D. Panetta a, R. Pani d, M.G. Quattrocchi a, P. Randaccio e, V. Rosso a, P. Russo c a Università di Pisa and INFN, Pisa, Italy b Università di Bologna and INFN, Bologna, Italy c Università di Napoli Federico II and INFN, Napoli, Italy d Università La Sapienza and INFN, Roma, Italy e Università di Cagliari and INFN, Cagliari, Italy

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Sezione di Napoli Univ. “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

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Sezione di Napoli Univ. “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.

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Sezione di Napoli Univ. “Federico II” Bimodal energy model For a polychromatic beam the X-ray attenuation in a material is described by two effective energies (E 1, E 2 ; E 2 >E 1 ) and, correspondingly, by two effective attenuation coefficients 1 and 2 (< 1): the lower value 2 at the beam effective energy E 2 accounts for the effective attenuation in large material thicknesses E. Van de Casteele et al., Phys. Med. Biol. 47, (2002) 4181 –ln(I x /I 0 )= 2 x + ln{[1+ ]/[1+ exp( 2 x- 1 x)]} = f(E 1 ) (E 1 )/ f(E 2 ) (E 2 ) Source-Detector efficiency

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Sezione di Napoli Univ. “Federico II” Bimodal energy model: measurements –ln(I x /I 0 )= 2 x + ln(1+ ) for large thickness - a stack of 1 up to 14 PMMA sheets (20×20 cm 2, 1 cm thick) - CdTe diode detector (mod. XR-100T-CdTe) Amptek Inc.

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Sezione di Napoli Univ. “Federico II” CdTe detector Spectra X-ray attenuation in PMMA as a function of material thickness: effective attenuation coefficient eff = 0.244 cm -1 (E eff =51.0 keV) I0I0 I 14 cm 1 (cm -1 ) E 1 (Kev) 0.60221.3 2 (cm -1 ) E 2 (Kev) 0.24451.0

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Sezione di Napoli Univ. “Federico II” Experimental set-up W-anode X-ray tube 80 kVp 4°×56° fan beam A B C PMMA Phantoms 14 cm thick 0.3 mm Si Hybrid pixel detector 256 x 256 pixels, 55 x 55 m 2 Detector intrinsic resolution: 110 m Sensitive area 14.08×14.08 mm 2 Readout: Single photon counting Medipix2 chip* * Developed by the Medipix2 collaboration, www.cern.ch\medipix

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Sezione di Napoli Univ. “Federico II” 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

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Sezione di Napoli Univ. “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 ) - low detection efficiency - the charge sharing effect of the silicon pixel detector

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Sezione di Napoli Univ. “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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Sezione di Napoli Univ. “Federico II” Beam hardening in PMMA ellipsoid phantom ( edge - center )/ edge = 18% ( edge - center )/ edge = 4% ( edge - cente r)/ edge = 12%

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Sezione di Napoli Univ. “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

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Sezione di Napoli Univ. “Federico II” Bimodal Energy Model Calculated attenuation coefficient as a function of PMMA thickness

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Sezione di Napoli Univ. “Federico II” 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 Anode 80 kVp, 0.25 mA 4.2 mm Al 51.5 cm 36 cm15.5cm CdTe detector (mod. XR-100T-CdTe) 14 PMMA sheets 1cm thick

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