Sezione di Napoli
Univ. “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, 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
Summary
Univ. “Federico II”
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•
•
•
•
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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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Univ. “Federico II”
Motivation and
beam hardening effect
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 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
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Univ. “Federico II”
 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)
Source-Detector efficiency
E. Van de Casteele et al., Phys. Med. Biol. 47, (2002) 4181
-ln (Ix/I0)
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Bimodal energy model:
measurements
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Y = A + B*X
A=0.221+/-0.03
-1
B=0.244+/-0.003 cm
R= 0.999 (p<0.0001)
0
2
4
6
8
10
12
PMMA thickness, x (cm)
14
–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.
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CdTe detector Spectra
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-1
Intensity
-2
-1
-1
(counts s mm keV mA )
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60000
Direct Beam (51.5 cm air)
After 29 cm air + 14 cm PMMA + 8.5 cm air
Emean= 47.3 keV
50000
I0
E2 (Kev)
m2 (cm-1)
51.0
0.244
E1 (Kev)
m1 (cm-1)
21.3
0.602
40000
30000
I14 cm
20000
Emean= 51.2 keV
10000
0
20
x 10
30
40
50
60
70
Photon energy (keV)
80
X-ray attenuation in PMMA as a function of material thickness:
effective attenuation coefficient meff = 0.244 cm-1 (Eeff=51.0 keV)
Experimental set-up
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Univ. “Federico II”
 W-anode X-ray tube 80 kVp
 4°×56° fan beam
B
C
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
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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°
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- 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 Sezione di Napoli
Univ. “Federico II”
-1
Attenuation coefficient (cm )
0,35
18%
0,30
0,25
0,20
the drop of the
attenuation coefficient
(medge-mcenter)/medge=18%
( 0.33 cm-1  0.27 cm-1)
0,15
0,10
0,05
0,00
0
2
4
6
8 10 12 14
Distance along a diameter (cm)
- low detection efficiency
- the charge sharing effect of the silicon pixel detector
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Beam hardening in
PMMA ellipsoid phantom
5 mm
7.6 mm
7.6 mm
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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
Beam hardening in
PMMA ellipsoid phantom
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
-1
Attenuation Coefficient (cm )
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0.35
0.30
0.25
0
2
4
6
8
10
12
Distance along the diameter (cm)
(medge-mcenter)/medge = 18%
(medge-mcenter)/medge = 12%
(medge-mcenter)/medge = 4%
14
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Conclusions and future work
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• 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 14cm-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
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Univ. “Federico II”
-1
Attenuation coefficient (cm )
-1
A: (4 cm, 0.284 cm ) drop=7%
-1
B: (11.5 cm, 0.264 cm ) drop=13%
-1
C: (14 cm, 0.261 cm ) drop=19%
0.31
m
a
m
A
0.30
0.244
0.276
0.602
0.29
B
0.28
C
0.27
0.26
0
2
4
6
8
10
12
PMMA thickness (cm)
14
Calculated attenuation coefficient as a function of PMMA thickness
Experimental set-up for PMMA
attenuation coefficient evaluation
Sezione di Napoli
Univ. “Federico II”
14 PMMA sheets
1cm thick
CdTe detector
(mod. XR-100T-CdTe)
W Anode
80 kVp, 0.25 mA
4.2 mm Al
36 cm
15.5cm
51.5 cm
• 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