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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 - PowerPoint PPT Presentation
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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
Sezione di NapoliUniv. “Federico II”
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.
Sezione di NapoliUniv. “Federico II”
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
Sezione di NapoliUniv. “Federico II”
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
Sezione di NapoliUniv. “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
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
Sezione di NapoliUniv. “Federico II”
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
Sezione di NapoliUniv. “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 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