An algorithm for metal streaking artifact reduction in cone beam CT
M. Bazalova1,2, G. Landry1, L. Beaulieu3, F. Verhaegen1,4
1-McGill University, Montreal, Canada, 2-Stanford University, Stanford, CA, Centre Hospitalier Univ de Quebec, Quebec, QC, Canada, 4-Maastro, Maastricht, The Netherlands
Introduction
A number of patients undergoing HDR brachytherapy in our institution are
imaged with cone beam computed tomography (CBCT). CBCT helps
identify patient anatomy and the position of the catheter or vaginal
cylinder in which the radiation source is inserted and the tumor treated.
Image quality in CBCT can suffer from severe artifacts if metals, such as
vaginal cylinders or hip prostheses, are present in the patient body. Metal
artifacts in CT have been studied extensively, however, research in CBCT
imaging is sparse. This abstract presents a correction algorithm for CBCT
In the next step, the interpolated projections were reconstructed using an
The patient study was the most challenging test for our correction method.
x-ray imaging simulation program (ImaSim) developed at our institute. The
The identification of metal traces based on identification of metal traces in
distances source-to-isocenter, detector-to-isocenter, and the data overlap
the artifact corrupted images worked very well, as demonstrated in fig 4a
for the half fan geometry were taken into account. CBCT images were
and 4b. Fig 4c represents the interpolated projection and the vaginal
filtered and backprojected with the Feldkamp algorithm. To reduce
cylinder rod is made invisible.
calculation speed and noise, projection data were downsampled by a
factor of two before the backprojection. The reconstruction of a single
886×886 pixel slice takes approximately 35 minutes on a 3 GHz
processor. The deleted metals were superimposed on the reconstructed
images based on the interpolated projections which produced the final
artifact corrected image with the metals.
b)
a)
metal artifacts based on sinogram interpolation methods used in CT.
Results
c)
Figure 4: Original patient projection (a), masked projection (b) and interpolated
projection (c).
Materials and Methods
CBCT images of a 15 cm diameter water phantom with three 2 cm
diameter steel cylinders and two teflon inserts and a 30 cm diameter solid
water phantom with tissue equivalent inserts and with or without two large
steel cylinders mimicking hip prostheses were scanned on a CBCT
scanner (Simulix, Nucletron). CBCT images of a GYN patient with metallic
The small water phantom CBCT images suffered from severe artifacts, as
Due to the vaginal cylinder geometry, the interpolation is hardly
demonstrated with 3D volume rendering in fig 2a and with an axial slice in
noticeable. Fig. 5a and 5b show an original and artifact corrected slice,
fig 2b. Our reconstruction algorithm produced images with less, however
respectively. The reduction of streaking artifacts in the vicinity of the
still significant, streaking artifacts that closely resemble CT metal artifacts
cylinder is evident. The coronal slices at the position of the vaginal
(fig 2c). The correction algorithm introduced in this paper significantly
cylinder (fig 5c and 5d ) also demonstrate the effectiveness of the artifact
diminished the metal streaking artifacts (fig 2d). The teflon cylinders can
correction algorithm. Note that the streaks caused by the metal rings are
be identified more easily than in the original CBCT images.
not corrected for since this was not the aim of our method. Only the
vaginal cylinder were also used in this study. The correction technique
f)
was designed to reduce metal artifacts caused by the steel cylinders and
a)
g)
b)
by the central rod of the vaginal cylinder. It can be easily modified for
projections corresponding to the central rod were interpolated. To correct
for the rings, a different threshold for identification of metal voxels in the
a)
b)
image domain has to be chosen.
broader applications.
a)
b)
c)
a)
b)
d)
c)
c)
d)
Figure 1: Small water phantom: original CBCT projection (a), projection with deleted
metal traces (b), interpolated projection (c).
c)
d)
The Nucletron CBCT scanner operates with a 120 kV x-ray tube rotating
at a distance of 100 cm from the isocenter and the x-rays are detected by
an amorphous silicon flat panel with 1024×1024 pixels of 0.4×0.4 mm2.
The detector-to-isocenter distance is 52 cm. CBCT images are
reconstructed from approximately 500 views. The scanner can operate in
half fan or full fan modes requiring two different reconstruction techniques.
The artifact correction algorithm is described here. First, the metal traces
in the projection data were identified. In the case of the small water
phantom, the metal traces could be segmented directly in the projection
space due to the simple geometry (fig 1a-b). However, a more
sophisticated approach had to be taken in the large phantom and the
patient studies. Metals were first segmented in the original reconstructed
images using a fixed threshold, which worked well in the studied cases.
Figure 2: 3D volume rendering of the small phantom based on CBCT scanner
reconstructed images (a). An axial slice reconstructed by the CBCT scanner (b), using
our reconstruction algorithm (b) and the original projections and CBCT image
reconstructed using the modified interpolated projections (d).
The CBCT images of the large phantom are presented in fig 3. Fig 3a
shows streaking artifacts around the steel cylinders, however, the artifacts
phantom. Nevertheless, the corrected image in fig. 3b reduces the
artifacts in the vicinity of the steel cylinders and the image quality is similar
The artifact correction algorithm introduced in this study significantly
to the CBCT image taken without the steel inserts.
improves image quality and enables to define phantom geometry and
patient anatomy in the regions obscured by the artifacts. The ultimate test
projection of the voxels from the source onto the flat panel and the
of the method will be correction of artifacts for a patient with bilateral hip
corresponding detector signal was deleted. This was done for each x-ray
prostheses. This study has the potential to be translated into the clinic.
tube position. Consequently, the deleted data were filled in using
The interpolated projections can be uploaded to the CBCT reconstruction
interpolation of the neighboring data in the direction perpendicular to the
direction for correction of artifacts caused by long objects parallel to the SI
direction, such as vaginal cylinders, tungsten shielding or hip prostheses.
Conclusions
between the steel cylinders are less pronounced than in the small
Metal traces of these voxels in the projection space were found by
scanner rotation axis (fig 1c). This direction of interpolation is the optimal
Figure 5: Axial original (a) and corrected (b) slice. Coronal original (c) and corrected
(c) slice. The arrows indicate artifact suppression.
PC and corrected CBCT images can be reconstructed directly on the
a)
b)
c)
Figure 3: Original CBCT image (a), artifact corrected CBCT image (b) of the large
phantom with steel cylinders and CBCT image of the large phantom without steel. All
images reconstructed by our algorithm with no scatter correction.
scanner computer. Further investigation into the resulting image quality is
warranted.