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Experimental study of beam hardening artefacts in photon counting breast computed tomography

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

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Experimental study of beam hardening artefacts in photon counting breast computed tomography

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  1. 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, Italy bUniversità di Bologna and INFN, Bologna, Italy c Università di Napoli Federico II and INFN, Napoli, Italy dUniversità La Sapienza and INFN, Roma, Italy eUniversità di Cagliari and INFN, Cagliari, Italy

  2. 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

  3. 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 .

  4. Source-Detector efficiency 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 m1 and m2 (<m1): the lower value m2 at the beam effective energy E2 accounts for the effective attenuation in large material thicknesses –ln(Ix/I0)=m2x + ln{[1+a]/[1+aexp(m2x-m1x)]} a = f(E1)g(E1)/ f(E2)g(E2) E. Van de Casteele et al., Phys. Med. Biol. 47, (2002) 4181

  5. Bimodal energy model:measurements –ln(Ix/I0)=m2x + ln(1+a)for large thickness - a stack of 1 up to 14 PMMA sheets (20×20 cm2, 1 cm thick) - CdTe diode detector (mod. XR-100T-CdTe) Amptek Inc.

  6. E1 (Kev) E2 (Kev) m1 (cm-1) m2 (cm-1) 21.3 51.0 0.602 0.244 CdTe detector Spectra I0 I14 cm X-ray attenuation in PMMA as a function of material thickness: effective attenuation coefficient meff = 0.244 cm-1(Eeff=51.0 keV)

  7. Experimental set-up • W-anode X-ray tube 80 kVp • 4°×56° fan beam C B A 0.3 mm Si Hybrid pixel detector 256 x 256 pixels, 55 x 55 mm2 Detector intrinsic resolution: 110 mm Sensitive area 14.08×14.08 mm2 Readout: Single photon counting Medipix2 chip* PMMA Phantoms 14 cm thick * Developed by the Medipix2 collaboration, www.cern.ch\medipix

  8. 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

  9. the drop of the attenuation coefficient (medge-mcenter)/medge=18% ( 0.33 cm-1 0.27 cm-1) Beam hardening in 14 cm thick PMMA cylinder phantom • low detection efficiency • the charge sharing effect of the silicon pixel detector

  10. Beam hardening in PMMA ellipsoid phantom 5 mm • 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”) • distance = 10.5 cm,  = 14 cm • distance = 4.5 cm,  = 11.5 cm • distance = 0.5 cm,  = 4 cm 7.6 mm 7.6 mm

  11. Beam hardening in PMMA ellipsoid phantom (medge-mcenter)/medge = 18% (medge-mcenter)/medge = 4% (medge-mcenter)/medge = 12%

  12. 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

  13. Bimodal Energy Model Calculated attenuation coefficient as a function of PMMA thickness

  14. X-ray tube: W anode with a 40 mm 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 mA 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 14 PMMA sheets 1cm thick W Anode 80 kVp, 0.25 mA 4.2 mm Al CdTe detector (mod. XR-100T-CdTe) 36 cm 15.5cm 51.5 cm Experimental set-up for PMMA attenuation coefficient evaluation

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