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
Summary Univ. Federico II 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
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.
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 μ 1 and μ 2 (<μ1): the lower value μ 2 at the beam effective energy E 2 accounts for the effective attenuation in large material thicknesses ln(i x /I 0 )=μ 2 x + ln{[1+α]/[1+αexp(μ 2 x-μ 1 x)]} α = f(e 1 )γ(e 1 )/ f(e 2 )γ(e 2 ) Source-Detector efficiency E. Van de Casteele et al., Phys. Med. Biol. 47, (2002) 4181
-ln (I x /I 0 ) Bimodal energy model: 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 measurements Y = A + B*X A=0.221+/-0.03 B=0.244+/-0.003 cm -1 R= 0.999 (p<0.0001) 0 2 4 6 8 10 12 14 PMMA thickness, x (cm) ln(i x /I 0 )=μ 2 x + ln(1+α) 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.
CdTe detector Spectra Intensity (counts s -1 mm -2 kev -1 ma -1 ) 60000 50000 40000 30000 20000 Direct Beam (51.5 cm air) After 29 cm air + 14 cm PMMA + 8.5 cm air E mean = 47.3 kev E mean = 51.2 kev 10000 x 10 0 20 30 40 50 60 70 80 Photon energy (kev) I 0 I 14 cm E 2 (Kev) 51.0 E 1 (Kev) 21.3 μ 2 (cm -1 ) 0.244 μ 1 (cm -1 ) 0.602 X-ray attenuation in PMMA as a function of material thickness: effective attenuation coefficient μ eff = 0.244 cm -1 (E eff =51.0 kev)
Experimental set-up W-anode X-ray tube 80 kvp 4 56 fan beam A B C 0.3 mm Si Hybrid pixel detector 256 x 256 pixels, 55 x 55 μm 2 Detector intrinsic resolution: 110 μm Sensitive area 14.08 14.08 mm 2 Readout: Single photon counting Medipix2 chip* * Developed by the Medipix2 collaboration, www.cern.ch\medipix PMMA Phantoms 14 cm thick
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
Beam hardening in 14 cm thick PMMA cylinder phantom Attenuation coefficient (cm -1 ) 0,35 0,30 0,25 0,20 0,15 0,10 0,05 0,00 18% 0 2 4 6 8 10 12 14 Distance along a diameter (cm) the drop of the attenuation coefficient (μ edge -μ center )/μ edge =18% ( 0.33 cm -1 0.27 cm -1 ) - low detection efficiency - the charge sharing effect of the silicon pixel detector
Beam hardening in PMMA ellipsoid phantom 5 mm 7.6 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 ) A) distance = 10.5 cm, φ = 14 cm B) distance = 4.5 cm, φ = 11.5 cm C) distance = 0.5 cm, φ = 4 cm 7.6 mm
Attenuation Coefficient (cm -1 ) 0.35 0.30 0.25 Beam hardening in PMMA ellipsoid phantom Profile at 10.5 cm from the top, φ = 14.0 cm Profile at 4.5 cm from the top, φ = 11.5 cm Profile at 0.5 cm from the top, φ = 4.0 cm 0 2 4 6 8 10 12 14 Distance along the diameter (cm) (μ edge -μ center )/μ edge = 18% (μedge -μ center )/μ edge = 4% (μ edge -μ cente r)/μ edge = 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
Bimodal Energy Model Attenuation coefficient (cm -1 ) 0.31 0.30 0.29 0.28 0.27 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 μ2 0.244 α 0.276 μ1 0.602 0.26 0 2 4 6 8 10 12 14 PMMA thickness (cm) B C Calculated attenuation coefficient as a function of PMMA thickness
Experimental set-up for PMMA attenuation coefficient evaluation Univ. Federico II W Anode 80 kvp, 0.25 ma 4.2 mm Al 14 PMMA sheets 1cm thick CdTe detector (mod. XR-100T-CdTe) 36 cm 15.5cm 51.5 cm X-ray tube: W anode with a 40 μm 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 μa 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