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. 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

2 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

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.

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

4 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

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

5 -ln (I x /I 0 ) Bimodal energy model: measurements Y = A + B*X A=0.221+/-0.03 B=0.244+/ cm -1 R= (p<0.0001) 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.

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

6 CdTe detector Spectra Intensity (counts s -1 mm -2 kev -1 ma -1 ) Direct Beam (51.5 cm air) After 29 cm air + 14 cm PMMA cm air E mean = 47.3 kev E mean = 51.2 kev x Photon energy (kev) I 0 I 14 cm E 2 (Kev) 51.0 E 1 (Kev) 21.3 μ 2 (cm -1 ) μ 1 (cm -1 ) X-ray attenuation in PMMA as a function of material thickness: effective attenuation coefficient μ eff = cm -1 (E eff =51.0 kev)

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.

7 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 mm 2 Readout: Single photon counting Medipix2 chip* * Developed by the Medipix2 collaboration, PMMA Phantoms 14 cm thick

resolution: 110 μm Sensitive area 14.08 14.

8 Beam hardening in PMMA cylinder phantom -3D view of the reconstructed* transaxial slice of the 14 cm diameter PMMA cylinder; - isotropic voxel side= mm; - total thickness = 7.4 mm; views on 360-2D reconstruction of a single slice (thickness = mm); *Custom algorithm implementing the filtered backprojection fan beam reconstruction algorithm

9 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% Distance along a diameter (cm) the drop of the attenuation coefficient (μ edge -μ center )/μ edge =18% ( 0.33 cm cm -1 ) - low detection efficiency - the charge sharing effect of the silicon pixel detector

the drop of the attenuation coefficient (μ edge -μ center )/μ edge =18% ( 0.33 cm -1 0.

10 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

ellipsoid phantom related to three different distances from the phantom top (

11 Attenuation Coefficient (cm -1 ) 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 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%

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

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

13 Bimodal Energy Model Attenuation coefficient (cm -1 ) A: (4 cm, cm -1 ) drop=7% B: (11.5 cm, cm -1 ) drop=13% C: (14 cm, cm -1 ) drop=19% A μ α μ PMMA thickness (cm) B C Calculated attenuation coefficient as a function of PMMA thickness

264 cm -1 ) drop=13% C: (14 cm, 0.261 cm -1 ) drop=19% A μ2 0.244 α 0.276 μ1 0.

14 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 , NY, USA). 35 kvp to 80 kvp with an anode current in the range 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

5 cm X-ray tube: W anode with a 40 μm focal spot size (Source-Ray, Inc., mod. SB-80-250, NY, USA).

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