Development of Methods for Scatter Artifact Correction in Industrial X

145 Pages·2012·12.96 MB·English

by  Karsten Schörner

P (cid:7)D
HYSIK EPARTMENT

Development of Methods for Scatter
Artifact Correction in Industrial X-ray
Cone-beam Computed Tomography

Dissertation
von

Karsten Schörner

T U
ECHNISCHE NIVERSITÄT
M
ÜNCHEN

TECHNISCHE UNIVERSITÄT MÜNCHEN
Physik Department E21 (Lehrstuhl für Experimentalphysik III)

Development of Methods for Scatter Artifact Correction
in Industrial X-ray Cone-beam Computed Tomography

Dipl.-Phys. Univ. Karsten Schörner

Vollständiger Abdruck der von der Fakultät für Physik der Technischen Universität München
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
genehmigten Dissertation.

Vorsitzender: Univ.-Prof. Dr. Martin Zacharias
Prüfer der Dissertation: 1. Univ.-Prof. Dr. Peter Böni
2. Univ.-Prof. Dr. Franz Pfeiffer

Die Dissertation wurde am 29.02.2012 bei der Technischen Universität München
eingereicht und durch die Fakultät für Physik am 04.05.2012 angenommen.

Meinen Eltern gewidmet.

Abstract
Scattered radiation represents a major source of image degradation in industrial X-ray
cone-beam computed tomography (CBCT) leading to the formation of scatter artifacts
in the reconstructed volume. These artifacts include cupping artifacts, streaks between
high-contrast details, and a general loss of contrast. They hamper qualitative and quanti-
tative analysis in industrial nondestructive inspection tasks: Reduced contrast decreases
detectability of flaws such as cracks and voids while the cupping artifact as well as streaks
affect dimensional measurements. This motivates the correction of scatter artifacts in in-
dustrial CBCT.
In this work, we investigate two novel experimental methods for a-posteriori correction
of scatter artifacts in CBCT: first the beam-hole array technique, and second a different
approach that is based on temporal primary modulation.
The beam-hole array technique is considered as complementary in experimental practice
to the better-known beam-stop array technique: The beam-hole array is manufactured
as a sheet of a highly absorbing material such as lead with small apertures for measuring
primary signals at a number of sampling points in a first scan. In a second scan without
beam-hole array, for each pixel, the total signal is measured. Pixel-wise subtraction of
the primary signal from the total signal yields scatter estimates at the sampling points.
Subsequently, spline interpolation is used in order to compute entire scatter images which
are subtracted from original CT projections.
Comparison measurements between the new beam-hole array and the more established
beam-stop array technique generally show good agreement of both techniques. However,
they also indicate that with the beam-stop array technique, scatter-to-total ratios are
constantly larger by 1-2%-points at object-covered sampling points. This is due to scat-
tered radiation from a support plate which is only to be used in the beam-stop array
technique. From this point of view, the beam-hole array is particularly suited for series
CT scans whereby scattered radiation, and hence noise can be reduced.
We successfully demonstrate scatter correction by the beam-hole array technique applied
to the CT of an industrial, ceramic specimen. In this application, scatter artifacts can be
eliminated in certain regions within the reconstructed volume. Furthermore, in contrast
to the common assumption that the scatter distribution is of low spatial frequency con-
tent, we observe high spatial frequencies contained within the calculated scatter images.
We attribute this observation to a strong scatter contribution from detector-internal X-
ray scatter and light spread effects. From basic experimental investigations, we deduce
thattheseeffectsamounttoonefifthofthetotalmeasuredsignalinatypicalCTsituation
(220kVp polychromatic X-ray spectrum).
Scatter correction by temporal primary modulation is the second method which we have
developedandexperimentallystudied. Thismethodofferstwoimportantadvantagesover
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other techniques: First, it can be performed without additional scan time, and second, it
offers high spatial resolution for sampling scatter data. From a theoretical point of view,
the resolution is only limited by the detector.
For scatter estimation, the primary fluence is temporally amplitude-modulated in small
pixel clusters which are phase-shifted to each other whereas the total scatter distribu-
tion virtually remains constant. This enables a separation of modulated primary signals
and unmodulated scatter signals by coherent demodulation afterwards. The assump-
tion of a virtually constant scatter distribution is supported by Monte-Carlo simulations
and requirements concerning the spatial modulation pattern are derived in a theoretical
analysis.
For modulation of the primary fluence, we use a so-called primary modulator, a spatially
repetitive attenuation pattern in front of the object which imprints its pattern on the
primary fluence. Temporal modulation is realized by moving the attenuation pattern and
recording a number of modulated projections. In our experimental investigations, we em-
ploy a checkerboard pattern with 99 99 squares as primary modulator which is shifted
×
or slided, respectively, by one square length within two modulated projections. The pro-
posed method of temporal primary modulation is verified in a comparison measurement
with the standard beam-stop array technique. Scatter estimates from both methods are
generally in good agreement, i.e. deviations are less than 6% at direct sampling points of
the beam-stop array.
Additionally, we demonstrate the application of the proposed method for the correction
of scatter artifacts within a single CT scan of an aluminum test phantom. For this
scan including scatter correction, no additional scan time is necessary and the scatter
images are sampled at 95 95 points within the region of interest. Scatter artifacts
×
are greatly suppressed and almost eliminated compared to a normal CT scan without
scatter correction. For example, the contrast of specific slits cut in the test phantom is
enhanced whereby deviations of contrast values to ideal contrast values of a simulated
CT decrease from about 35% to 10% and less. Furthermore, the cupping artifact is
completely removed, i.e. line profiles in the corrected CT slices show an almost perfect
rectangular shape.
In summary, we present two novel experimental methods for scatter correction in CBCT
whereby scatter artifacts can be greatly suppressed. Particularly, temporal primary mod-
ulation has been shown to be a favorable method for scatter correction since it offers high
spatial resolution and does not extend scan times.
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List of Abbreviations
BHA Beam-Hole Array
BHC Beam-Hardening Correction
BHD Beam-Hardening
BSA Beam-Stop Array
CBCT Cone-Beam Computed Tomography
CFRP Carbon-Fiber-Reinforced Polymer
CT Computed Tomography
FBP Filtered Back-Projection (reconstruction algorithm)
FDK FBP algorithm by Feldkamp, Davis and Kress for 3D cone beam CT
FPD Flat-Panel Detector
MC Monte-Carlo (simulation)
NDT Nondestructive Testing
PSF Point-Spread Function
ROI Region Of Interest
SDD Source-to-Detector Distance
SNR Signal-to-Noise Ratio
SOD Source-to-Object Distance
SPM Spatial Primary Modulation (scatter correction method)
SPR Scatter-to-Primary Ratio
SSS Single Scatter Sources
STR Scatter-to-Total Ratio
TFT Thin-Film Transistor
TPM Temporal Primary Modulation (scatter correction method)
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