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in terms of low contrast visibility. This limits the
application of CBCT mainly to patient setup based on high
contrast structures. We address these limitations by
applying advanced preprocessing and reconstruction
algorithms to improve patient setup and facilitate
advanced applications like adaptive radiotherapy.
Material and Methods
The
commercially
available
TrueBeam
CBCT
reconstruction pipeline removes scatter usi ng a kernel-
based correction followed by filtered bac k-projection-
based reconstruction (FDK). These reconstruction n
pipeline steps are replaced by a physics-based scatter
correction (pelvis only) and an iterative reconstruction.
We use statistical reconstruction that takes the Poisson
distribution of quantum noise into account, an d applies
an edge preserving image regularization. The advanced
scatter correction is based on a finite-ele ment solver
(AcurosCTS) to model the behavior of photons as they pass
(and scatter) through the object. Both algorit hms have
been implemented on a GPU cluster pla tform, and
algorithmic acceleration techniques are utilized to
achieve clinically acceptable reconstruction times. The
image quality improvements have been an alyzed on
TrueBeam kV imaging system phantom scans, as well as
on daily CBCT scans of head/neck and prostate cancer
patients acquired for image-guided localization.
Results
Artifacts in head/neck FDK reconstructions (Fig . 1) e.g.
resulting from photon starvation in the shoulder region or
cone-beam are highly reduced in the iterative
reconstructions. The iterative reconstruction s show
enhanced soft tissue definition providing better cl arity for
boundary definition (see the level 2 lymph node located in
the contoured region of the axial view, Fig. 1). The
advanced scatter correction applied for pelvis scans
removes residual scatter artifacts, increasing the mean
homogeneity from 78.2 HU ± 18.0 HU to 20.9 HU ± 10.9 HU
within the bladder region of 9 daily CBCT scans of typical
prostate patients. Iterative reconstruction provides
further benefit by reducing image noise as well as
eliminating streak and cone-beam artifacts, thereby
significantly improving soft-tissue visualization, as noted
in the clinical pelvis CBCT scan (Fig. 2). The noise level
was reduced to 45% of the original value.
Conclusion
Statistical reconstruction in combination with advanced
scatter correction substantially improves CBCT image
quality by enabling removal of artifacts caused by
remaining scatter, projection noise, photon starvation,
and cone-beam angle. These artifact reductions improve
soft tissue definition that is necessary for accurate
visualization, contouring, dose calculation, and
deformable image registration in clinical practice. The
presented improvements are expected to facilitate soft
tissue-based patient setup. Promise has been
demonstrated for new applications, such as adaptive
radiotherapy.
PO-0894 Comparing the spatial integrity of 7T and 3T
MR images for image-guided radiotherapy of brain
tumors
J. Peerlings
1,2
, I. Compter
1
, F.M. Janssen
1
, C.J. Wiggins
3
,
F.M. Mottaghy
2,4
, P. Lambin
1
, A.L. Hoffmann
1,5,6,7
1
Maastricht University Medical Center+- GROW - School
for Oncology and Developmental Biology, Department of
Radiation Oncology - MAASTRO, Maastricht, The
Netherlands
2
Maastricht University Medical Center+, Department of
Radiology and Nuclear Medicine, Maastricht, The
Netherlands
3
Maastricht Brain Imaging Center - Scannexus, Maastricht
University, Maastricht, The Netherlands
4
University Hospital RWTH Aachen University,
Department of Nuclear Medicine, Aachen, Germany
5
University Hospital Carl Gustav Carus at the Technische
Universität Dresden, Department of Radiotherapy,
Dresden, Germany
6
OncoRay, National Center for Radiation Research in