Article

Investigation of gated cone-beam CT to reduce respiratory motion blurring

Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York 10065.
Medical Physics (Impact Factor: 2.64). 04/2013; 40(4):041717. DOI: 10.1118/1.4795336
Source: PubMed

ABSTRACT

Purpose:
Methods of reducing respiratory motion blurring in cone-beam CT (CBCT) have been limited to lung where soft tissue contrast is large. Respiration-correlated cone-beam CT uses slow continuous gantry rotation but image quality is limited by uneven projection spacing. This study investigates the efficacy of a novel gated CBCT technique.

Methods:
In gated CBCT, the linac is programmed such that gantry rotation and kV image acquisition occur within a gate around end expiration and are triggered by an external respiratory monitor. Standard CBCT and gated CBCT scans are performed in 22 patients (11 thoracic, 11 abdominal) and a respiration-correlated CT (RCCT) scan, acquired on a standard CT scanner, from the same day serves as a criterion standard. Image quality is compared by calculating contrast-to-noise ratios (CNR) for tumors in lung, gastroesophageal junction (GEJ) tissue, and pancreas tissue, relative to surrounding background tissue. Congruence between the object in the CBCT images and that in the RCCT is measured by calculating the optimized normalized cross-correlation (NCC) following CBCT-to-RCCT rigid registrations.

Results:
Gated CBCT results in reduced motion artifacts relative to standard CBCT, with better visualization of tumors in lung, and of abdominal organs including GEJ, pancreas, and organs at risk. CNR of lung tumors is larger in gated CBCT in 6 of 11 cases relative to standard CBCT. A paired two-tailed t-test of lung patient mean CNR shows no statistical significance (p = 0.133). In 4 of 5 cases where CNR is not increased, lung tumor motion observed in RCCT is small (range 1.3-5.2 mm). CNR is increased and becomes statistically significant for 6 out of 7 lung patients with > 5 mm tumor motion (p = 0.044). CNR is larger in gated CBCT in 5 of 7 GEJ cases and 3 of 4 pancreas cases (p = 0.082 and 0.192). Gated CBCT yields improvement with lower NCC relative to standard CBCT in 10 of 11, 7 of 7, and 3 of 4 patients for lung, GEJ, and pancreas images, respectively (p = 0.0014, 0.0030, 0.165).

Conclusions:
Gated CBCT reduces image blurring caused by respiratory motion. The gated gantry rotation yields uniformly and closely spaced projections resulting in improved reconstructed image quality. The technique is shown to be applicable to abdominal sites, where image contrast of soft tissues is low.

Full-text

Available from: Gig Mageras, May 28, 2014
Investigation of gated cone-beam CT to reduce respiratory motion blurring
Russell E. Kincaid, Jr. and Ellen D. Yorke
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York 10065
Karyn A. Goodman, Andreas Rimner, and Abraham J. Wu
Department of Radiation Oncology, Memorial Sloan-Kettering Cancer Center, New York, New York 10065
Gig S. Mageras
a)
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York 10065
(Received 31 August 2012; revised 28 December 2012; accepted for publication 27 February 2013;
published 20 March 2013)
Purpose: Methods of reducing respiratory motion blurring in cone-beam CT (CBCT) have been
limited to lung where soft tissue contrast is large. Respiration-correlated cone-beam CT uses slow
continuous gantry rotation but image quality is limited by uneven projection spacing. This study
investigates the efficacy of a novel gated CBCT technique.
Methods: In gated CBCT, the linac is programmed such that gantry rotation and kV image acquisi-
tion occur within a gate around end expiration and are triggered by an external respiratory monitor.
Standard CBCT and gated CBCT scans are performed in 22 patients (11 thoracic, 11 abdominal)
and a respiration-correlated CT (RCCT) scan, acquired on a standard CT scanner, from the same
day serves as a criterion standard. Image quality is compared by calculating contrast-to-noise ra-
tios (CNR) for tumors in lung, gastroesophageal junction (GEJ) tissue, and pancreas tissue, relative
to surrounding background tissue. Congruence between the object in the CBCT images and that in
the RCCT is measured by calculating the optimized normalized cross-correlation (NCC) following
CBCT-to-RCCT rigid registrations.
Results: Gated CBCT results in reduced motion artifacts relative to standard CBCT, with better
visualization of tumors in lung, and of abdominal organs including GEJ, pancreas, and organs at
risk. CNR of lung tumors is larger in gated CBCT in 6 of 11 cases relative to standard CBCT. A
paired two-tailed t-test of lung patient mean CNR shows no statistical significance (p = 0.133). In
4 of 5 cases where CNR is not increased, lung tumor motion observed in RCCT is small (range
1.3–5.2 mm). CNR is increased and becomes statistically significant for 6 out of 7 lung patients with
> 5 mm tumor motion (p = 0.044). CNR is larger in gated CBCT in 5 of 7 GEJ cases and 3 of 4
pancreas cases (p = 0.082 and 0.192). Gated CBCT yields improvement with lower NCC relative to
standard CBCT in 10 of 11, 7 of 7, and 3 of 4 patients for lung, GEJ, and pancreas images, respectively
(p = 0.0014, 0.0030, 0.165).
Conclusions: Gated CBCT reduces image blurring caused by respiratory motion. The gated
gantry rotation yields uniformly and closely spaced projections resulting in improved recon-
structed image quality. The technique is shown to be applicable to abdominal sites, where im-
age contrast of soft tissues is low. © 2013 American Association of Physicists in Medicine.
[http://dx.doi.org/10.1118/1.4795336]
Key words: image guided radiation therapy, cone-beam CT, respiratory motion, motion management
I. INTRODUCTION
Modern linear accelerators use onboard cone-beam CT
(CBCT) for visualizing tumors and organs at risk (OAR), and
to correct patient position, just prior to radiation treatment.
1
Current clinical CBCT scan acquisition time is approximately
1 min for a 360
scan. Since the typical patient breathing pe-
riod is 10–25 times shorter, respiratory motion during a free-
breathing scan is unavoidable. Image quality in CBCT is ad-
versely affected by respiratory motion. Respiratory motion
blurs the tumor and nearby organs in the images which makes
visualization of organ boundaries difficult. Motion of high
contrast objects and interfaces causes streak artifacts in the
reconstructed images which also reduce image quality. There-
fore, motion mitigation strategies are needed during imaging.
Furthermore, when treatment localization is affected by res-
piratory motion, and strategies are applied to manage the mo-
tion for treatment, the same strategies should be applied for
imaging.
2, 3
Respiration-correlated CBCT (RC-CBCT), also termed
4D-CBCT, on a linac, using slow continuous gantry rotation
to produce a series of images over the breathing cycle, has
been studied by several investigators.
46
These references dis-
cuss degradation of imaging quality in the images by view
aliasing streak artifacts caused by uneven projection spacing.
For this reason, RC-CBCT has been limited to lung where soft
tissue contrast is large.
7, 8
Alternative acquisition methods have been used to reduce
motion in CBCT of the abdomen. One method is breath-hold
(BH) at end expiration (EE), accomplished by breaking the
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Page 1
041717-2 Kincaid
et al.
: Investigation of gated cone-beam CT to reduce respiratory 041717-2
acquisition into several breath-holds. This approach requires
patient cooperation and compliance. In one study, 38% of
the patients were not eligible for the active breathing con-
trol (ABC) treatment and imaging protocol.
9
Also, anatomical
positions can be different between BH and free breathing,
10
so BH CBCT is normally used in conjunction with BH treat-
ment. The other method used in CBCT of the abdomen is ab-
dominal compression, but recent data i n the literature show
the amount of motion reduction that is achieved at the tumor
site is modest on average.
11, 12
Prior studies at our institution investigated different meth-
ods of gating a megavoltage (MV) CBCT system to reduce
motion artifacts. Sillanpaa et al.
13
demonstrated the feasi-
bility of synchronizing gated operation of a linac and MV-
CBCT acquisition to verify tumor position for gated lung
treatment. Chang et al.
14
investigated two methods of respi-
ratory gated MV CBCT, using gated image acquisition and
slow continuous gantry rotation, the other with both gated im-
age acquisition and gated rotation. The use of gated rotation
avoided nonuniform angular spacing of projection images and
thus eliminated the view aliasing streak artifacts when recon-
structed with a filtered backprojection algorithm. However,
the linac was not designed for gated gantry rotation, thus
gantry movement resulted in excessive vibration and mechan-
ical stress. A subsequent patient study by Chang et al.
15
used
a prototype computer-controlled system to operate the linac,
which reduced the vibration during gated rotation but did not
eliminate it.
This study investigates a gated CBCT technique in which
both gantry rotation and acquisition of kV image projections
occur within a respiratory gate. This technique is now pos-
sible using a new type of computer-controlled linac (Varian
TrueBeam
TM
) which allows programmed gantry motion. The
precise control of gantry motion yields uniformly angular-
spaced CBCT projection images without gaps. We investigate
the ability of gated CBCT to reduce motion blurring in phan-
tom studies and patient studies in both lung and abdominal
sites.
II. METHODS
II.A. Gated CBCT overview
Gated CBCT scans are performed on a computer-
controlled linac (TrueBeam v1.5, Varian Medical Systems,
Palo Alto, CA), which has capabilities for synchronizing mo-
tions of all mechanical axes with MV dose delivery and kV
image acquisition, either with or without respiratory gating.
In a gated CBCT scan, gantry rotation and kV image acquisi-
tion occur only within a gate generated from an external res-
piratory monitoring system (Real-time Position Management
System, RPM). The monitoring system consists of a block
with four infrared reflective markers, placed on the patient’s
abdomen and monitored by a stereoscopic camera, and is sen-
sitive to six degrees-of-freedom motion (three translations,
three rotations). For gated CBCT, the gate interval is speci-
fied in terms of the phase of the respiratory signal.
In the current TrueBeam software, gated CBCT is possi-
ble only in a research mode of operation and programmed
by means of a script file. Gated rotation of the gantry is en-
abled only when the MV beam is on, which is programmed
as an arc over 360
. In order to minimize patient exposure
to MV irradiation, the total beam-on time over the arc is set
to 10 monitor units, the jaws are closed to the minimum al-
lowed field size of 1 × 1cm
2
, and the multileaf collimator
is closed such that the position of leaf abutment does not co-
incide with the jaw-defined opening. The resultant dose from
the 6 MV beam, confirmed with ion chamber measurement, is
less than 0.025 cGy at the isocenter. Imaging parameters for
gated CBCT are similar to those for the nongated CBCT, i.e.,
125 kVp, 80 mA, 20 ms, 11 images/s half-fan acquisition (i.e.,
detector laterally offset 16 cm to obtain a 46 cm reconstructed
diameter), 360
rotation. During the gated CBCT scan and at
the start of each gate, the gantry accelerates under computer
control to reach its target speed of approximately 1 rpm. At
the end of each gate the gantry decelerates, reverses direc-
tion, and repositions at its location at the time of gate’s end.
The result yields narrowly spaced projection images without
gaps between consecutive gates, although there is sometimes
overlap of images at gantry angles at the abutment between
gates. Because the gantry starts from a stationary position at
the start of each gate, the average gantry speed with gated
CBCT is lower and the number of projections is greater than
with standard nongated CBCT.
In gated CBCT, the gate is adjusted to minimize the scan
time while limiting motion within the gate to less than 30%
of the peak to peak actual tumor or surrogate (ungated)
motion extent as measured in the respiration-correlated CT
(RCCT) scan obtained at treatment simulation. Increasing the
gate width shortens the s can time, partly because of the in-
creased duty cycle, and partly because fewer breath cycles
are needed, thereby reducing the number of gantry decelera-
tion/acceleration cycles. This increases average rotation speed
and results in fewer redundant images at the abutment be-
tween gates, and hence less patient imaging dose (discussed
further below).
Gated CBCT projections are preprocessed using custom
programs written in
MATLAB (MathWorks, Inc., Natick, MA)
to reorder images by gantry angle, average any redundant im-
ages occurring at the same angle, and correct any errors in
the x-ray source intensity measurement. The intensity mea-
surement, from a sensor at the x-ray source, is used to nor-
malize the intensity of each projection for all CBCT scans.
This is to compensate for variations in x-ray intensity for
each projection. We note that at the start of any s can, and
particularly at the start of each gate in gated CBCT, the x-
ray intensity can be lower by 10% or more. A set of stan-
dard CBCT calibration files obtained for the same imaging
parameters as the gated CBCT is used to calibrate and cor-
rect for attenuation by the bow-tie filter (using an in-air nor-
malization image), Hounsfield units (HU), x-ray spectrum,
and scatter. All projection sets are reconstructed using a re-
search version of the Feldkamp-Davis-Kress (FDK) filtered
backprojection algorithm
16
(Varian iTools version 1.0.32.0),
which includes the same preprocessing, reconstruction,
Medical Physics, Vol. 40, No. 4, April 2013
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041717-3 Kincaid
et al.
: Investigation of gated cone-beam CT to reduce respiratory 041717-3
and postprocessing procedures as the standard clinical
software.
II.B. Motion phantom study
We evaluate efficacy and performance of gated CBCT with
a respiratory motion phantom
17
(Quasar, Modus Medical De-
vices, Inc., London, ON, Canada). This phantom consists of
a torso shaped section of acrylic with interchangeable cylin-
drical motion inserts. It moves a platform (representing the
chest wall) in the anterior-posterior (AP) direction and syn-
chronously moves the insert in the superior-inferior (SI) di-
rection to simulate patient breathing. The phantom can be
programmed to follow a patient’s respiratory trace or for ad-
justable sinusoidal oscillation. The motor driven acrylic in-
sert contains embedded higher density objects. In this study,
we consider 10 and 20 mm diameter spheres, and a 30 mm
cube. The RPM block is placed on the platform to provide the
optical signal for gated operation.
Increasing the gate width shortens the scan time and re-
duces patient dose but increases the residual motion within
the gate. To test and quantify the effects of increasing the gate
width, we perform test scans of the respiratory motion phan-
tom with 25%, 35%, and 45% gate widths.
In the first set of measurements, the Quasar phantom is
programmed for simple sinusoidal motion, with 3.2 cm peak-
to-trough excursion of the cylindrical insert. Three gated
CBCT scans are performed, using 25%, 35%, and 45% gates,
centered at EE. The gate is widened symmetrically about the
50% (EE) phase, corresponding to the insert being at the most
superior position, to isolate the effects of gate width on total
number of projection images acquired and scan time.
In the second set of measurements, the Quasar phantom is
programmed to follow a patient respiratory trace resulting in
1.4 cm mean excursion of the insert. Four CBCT scans are
performed, all 360
scans with 125 kVp, as follows:
1. Clinical “free-breathing” CBCT (standard CBCT),
where the gantry rotates continuously at approxi-
mately 6
/s while kV projection images are acquired
at the maximum rate (11 fps), resulting in 656 projec-
tion images, resulting in 1478 mAs.
2. RC-CBCT, where t he gantry rotates continuously and
slowly at 1
/s while kV projection images are ac-
quired only within a respiratory gate for 35% duty cy-
cle around EE. This is meant to provide the approxi-
mate equivalent of one bin of sorted RC-CBCT pro-
jections resulting from application of the RC-CBCT
technique.
46
This experimental scan was done only
with the phantom, in research mode, using a custom
script file. Using the patient trace chosen for this ex-
periment, this scan resulted in 901 projection images
(1442 mAs), but the exact number of projections can
vary with breathing pattern, which therefore affects the
total mAs for the scan.
3. Gated CBCT, with 35% gate centered at EE. Using the
patient trace, this scan resulted in 924 projection im-
ages (1478 mAs), but the exact number of projections
can vary, which therefore affects the total mAs for the
scan.
4. Clinical CBCT of static phantom, with cylinder insert
fixed at EE to serve as a criterion standard. The scan
parameters are the same as scan (#1).
Phantom image quality and the effect of motion blurring is
evaluated by comparing contrast-to-noise ratios
4, 5
(CNR) of
the 20 mm spherical object relative to the surrounding back-
ground, μ
o
and μ
b
are mean pixel i ntensities inside the spher-
ical and background volumes, respectively, and σ
o
is the stan-
dard deviation of the intensities inside the spherical volume
CNR =
(
μ
o
μ
b
)
o
. (1)
Analysis is performed using a treatment planning system de-
veloped at this institution. We wish to compute CNR based
on the ground truth spherical volume. The 20 mm spherical
object is delineated in the stationary phantom scan and a 5
mm 3D annulus of the surrounding background is constructed
around the object. The stationary scan is rigidly registered
to each of the three motion phantom scans by matching in
a rectangular volume of interest (VOI) containing the spheri-
cal object using a normalized cross-correlation cost function
18
(NCC) where g1 and g2 are pixel intensities, and μ
1
and μ
2
are mean pixel intensities, within the VOI of n voxels on the
two images, respectively,
NCC =−
n
i=1
(
g1
i
μ
1
)
(
g2
i
μ
2
)
n
i=1
(
g1
i
μ
1
)
2
n
i=1
(
g2
i
μ
2
)
2
. (2)
The automated rigid registration optimization uses downhill
simplex to minimize the NCC. The delineated sphere and an-
nulus contours are transferred to the other scans using the
rigid registrations. CNR is measured for each of the three mo-
tion phantom scans. A comparatively larger CNR indicates
better agreement of the study image with the spherical volume
defined on the stationary phantom (criterion standard) image.
Phantom image quality and the effect of motion blurring are
evaluated by comparing contrast-to-noise ratios. Congruence
between the object in the three motion phantom images and
that in the static CBCT is measured by calculating the NCC
at the completion of each rigid registration. Axial localization
accuracy is evaluated in sagittal images by comparing mean
voxel intensity profiles through the 20 mm diameter sphere.
II.C. Patient study
We evaluated the performance of gated CBCT in IRB ap-
proved patient studies. Eligible patients were those receiving
radiation treatment of a malignancy in lung, gastroesophageal
junction (GEJ), or pancreas that exhibited at least 5 mm mo-
tion in a RCCT scan obtained at treatment simulation (de-
scribed below).
Patients received a RCCT scan (eight-slice Lightspeed, GE
Medical Systems, Waukesha, WI), typically 2–3 weeks before
Medical Physics, Vol. 40, No. 4, April 2013
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041717-4 Kincaid
et al.
: Investigation of gated cone-beam CT to reduce respiratory 041717-4
the treatment day research scans, as part of their treatment
simulation. This RCCT was also used to predict the optimal
patient-specific gate position and width for the gated CBCT
scan. The RCCT is obtained from a cine scan which is ac-
quired while recording the patient’s respiration (Varian RPM).
Acquisition time per couch position is set to the patient’s res-
piration period plus 1 s, with gantry rotation period of 0.5 s.
The time interval between consecutive images is the greater
of either 1/20 of the couch position acquisition time or 0.25 s.
Slice width is 0.25 cm. The slices are sorted into ten phase
bins (GE Advantage 4D) that comprise the RCCT image set.
Patient simulation day RCCT images and breathing traces
were screened for image artifacts due to irregular breath-
ing, namely, discontinuity artifact in images at end inspira-
tion (EI), and irregular period or amplitude in their breath-
ing trace. During the accrual of the patient data presented in
this work, no patients were excluded because of this require-
ment. Three GEJ patient data sets were not included in this
analysis, for the following reasons. One patient had fiducial
streaking throughout all CBCT slices in the GEJ making it
impossible to visually align the region for analysis. A second
patient had an esophageal stent that extended through the GEJ
into the stomach making the images inappropriate for similar
analysis and comparison to the others. The third patient did
not understand the instructions and breathed extremely errat-
ically which made the CBCT and RCCT images unusable.
GEJ patients allergic to oral barium contrast or pancreas pa-
tients allergic to IV iodine contrast were excluded from the
study. Patient data analyzed in the study included 11 in lung,
seven GEJ, and four pancreas. The 22 patients included 10 fe-
males and 12 males. The mean age was 65.9 years (standard
deviation 12.8, range 43–95).
In order to determine an appropriate gate width and loca-
tion, a tumor motion trajectory vs RPM phase is determined
from the simulation RCCT as follows. The 50% phase bin
image is chosen as the nominal EE image and the other nine
images are each registered rigidly to it, without rotation, us-
ing an automatic rigid registration algorithm that minimizes
a normalized cross-correlation function of voxel intensities
[Eq. (2)] within a VOI that includes either the tumor or im-
planted markers near the tumor. The 3D registration displace-
ments are plotted, as a function of phase bin, in the AP/SI
plane, and in the left-right (LR)/SI plane, to represent the 3D
tumor respiratory motion trajectory. In addition, abdominal
displacement (RPM block height) recorded during the x-ray
on intervals of the scan is plotted as a function of phase. Both
plots are considered for t he choice of gate: the former to min-
imize internal target motion within the gate, and the latter
to confirm that abdominal motion correlates with the phase
assignments. A phase gate is chosen of approximately 35%
(25%–50%, mean 35.6%) duty cycle around EE.
Patients enrolled in the protocol underwent research scans
on one day in the first week of treatment, consisting of a gated
CBCT in nonclinical mode, a standard CBCT in clinical mode
prior to treatment, and a standard CBCT in clinical mode af-
ter treatment. In addition, they received a RCCT scan on a
multislice CT scanner (GE Lightspeed), either before or af-
ter the treatment session. Patients were advised to relax and
breathe regularly. The research scans per patient resulted in
about 20 cGy to the tumor and surrounding tissues. Because
the gated CBCT was carried out in a nonclinical mode of op-
eration, a medical physicist experienced in gated CBCT op-
eration was always present to guide the radiation therapist
through the procedure. Following completion of the gated
CBCT, the linac was returned to the clinical mode of oper-
ation prior to acquiring the standard CBCT and administering
treatment. The IRB approval of the protocol included the ac-
quisition of gated CBCT in the nonclinical mode under the
conditions described here.
For the purposes of the analysis in this paper, comparison
of gated CBCT and standard CBCT was different for each
disease site: (1) In lung, the gated CBCT was compared to
the standard CBCT prior to treatment. (2) In GEJ, patients re-
ceived 200 cc oral contrast containing 2% barium sulfate sus-
pension prior to the gated CBCT scan. The gated CBCT was
compared to the standard CBCT prior to treatment, such that
the oral contrast was visible in both CBCT images. (3) In pan-
creas, the gated CBCT was compared to the standard CBCT
after treatment. We note parenthetically that pancreatic pa-
tients received intravenous (IV) contrast prior to the pretreat-
ment standard CBCT scan. Therefore, in order to minimize
the influence of residual IV contrast, the pretreatment stan-
dard CBCT was not used in the analysis of pancreatic cases.
In all three disease sites, the RCCT image acquired on the
same day served as a criterion standard in the comparison.
We visually assess the differences in image quality, be-
tween standard CBCT and gated CBCT, by comparing tumor
and organ visibility and boundary sharpness, the presence of
streak artifacts in transaxial images, and motion blurring in
coronal and sagittal images. In addition, we quantitatively as-
sess the improvement in image quality and accuracy by com-
paring standard CBCT and gated CBCT images to the RCCT
image acquired on the same day. In order to minimize irreg-
ular breathing artifacts, the cine scan is amplitude binned to
create the RCCT images. For some patients, there are gaps
(missing data) at some couch positions caused by variable
breathing amplitudes, and in these cases we use motion pre-
dicted RCCT images as described in Hertanto et al.
19
Image
quality is evaluated using the CNR [Eq. (1)] of the gross tu-
mor volume (GTV), or a portion of the diseased organ, rel-
ative to the surrounding background tissue. The GTV and
background volume for determining CNR are delineated on
the same-day RCCT and transferred to the CBCT images, in
the following fashion.
In the lung tumor cases, the GTV delineated by the physi-
cian on the planning CT is used as a guide to delineate the
GTV on the RCCT image at EE, and a 5 mm 3D annulus con-
taining parenchymal lung is constructed around the object.
Rigid registration of the RCCT image to each of the CBCT
images is performed within a VOI containing the GTV and
by minimizing NCC [Eq. (2)] as previously described. The
automatic registration i s visually inspected and if needed, a
manual adjustment is made to visually align the tissue bor-
ders in t he region of interest. The GTV and annulus contours
are transferred from the RCCT to the CBCT images. The pro-
cess of registration and contour transfer is repeated 3–4 times,
Medical Physics, Vol. 40, No. 4, April 2013
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041717-5 Kincaid
et al.
: Investigation of gated cone-beam CT to reduce respiratory 041717-5
on different days, to determine the variability of the CNR
measurement.
For computation of CNR in the GEJ cases, the GTV,
esophagus, and stomach delineated by the physician on the
planning CT are used as a guide to delineate, on the RCCT
image at EE, a portion of the GEJ excluding implanted fidu-
cial markers, and a 5 mm background region of lower density
neighboring tissue. The portion chosen is along the anterior
border of the GEJ with the two contoured regions inside and
outside the GEJ tissue plane. Care is taken to avoid the lumen
of the esophagus and the gas within it, which can affect the
CNR measurement. The RCCT image is rigidly registered to
each of the CBCT images in a VOI enclosing the GEJ and
contoured regions, but excluding fiducial markers, and the
contours are transferred as described for the lung cases.
For computation of CNR in the pancreatic cases, a portion
of the tail of the pancreas is delineated on the RCCT image
atEE,alongwitha5mmbackground region of lower den-
sity neighboring adipose tissue. The RCCT image is rigidly
registered to each of the CBCT images in a VOI enclosing
the pancreas tail and contoured regions and the contours are
transferred as described for the lung cases. For all patients,
all contours were delineated by a physicist and reviewed by a
physician.
In all cases, similarity in object size and shape between
the CBCT and RCCT is measured by calculating the opti-
mized NCC after alignment of the object using CBCT-to-
RCCT rigid registration. The rigid registration serves to re-
move any object displacement caused by setup error and pa-
tient movement that may have occurred between the RCCT,
gated CBCT, and standard CBCT scans.
III. RESULTS
III.A. Motion phantom study
Table I summarizes the measurements with the motion
phantom programmed for sinusoidal motion. As the gate is
increased from 25% to 45% of the motion cycle, there is
a reduction in scan time, total projections, and x-ray expo-
sure. For a gate of 35% (45%), relative to a 25% gate, actual
scan time is reduced by 38% (53%), compared to the 29%
(44%) reduction predicted by the increase in gate width. Sim-
ilarly, the total number of projections for the 35% (45%) gate
width is reduced 13% (17%) relative to that for the 25% gate
width. These further reductions, caused by fewer breath cy-
cles per scan, are due to fewer repeat images at the abutment
between consecutive gates and to the larger average gantry
TABLE I. Scan parameters and statistics for scans of Quasar motion phantom
using sinusoidal motion with 3.8 s period.
Width
of gate [%
of cycle]
Projections/
scan
Scan
duration
[min:s]
Cycles/
scan
Projections/
cycle
Scan
exposure
[mAs]
25 990 5:58 94.2 10.5 1584
35 857 3:42 58.4 14.7 1371
45 823 2:47 43.9 18.7 1317
speed within the longer gates. We note that because a sinu-
soidal motion trace was used, a smaller fraction of the time
was spent near EE than is more typically the case for patient
breathing. A motion trace more typical of patients would have
resulted in a larger number of images per gate, and reduced
the relative effects described above.
Figure 1 compares transaxial and sagittal CBCT images
acquired using various CBCT acquisition modes when the
phantom motion is programmed to follow a patient breathing
trace (Fig. 1). A scan of the stationary phantom serves as a
criterion standard (top left and bottom l eft panels). The gated
CBCT image (middle left and second bottom panels) shows
less streak artifacts in the transaxial image than RC-CBCT
due to more uniformly spaced projections, and less motion
blurring than standard CBCT in the sagittal image due to the
smaller amount of motion within the gate. The RC-CBCT
image (top right and third bottom panels) shows pronounced
streak artifacts in the transaxial image and more noise in the
sagittal image than any other mode due to unevenly spaced
projections. Motion blurring in the sagittal RC-CBCT image
is similar to gated CBCT and less than standard CBCT. The
standard CBCT (middle right and bottom right panels) shows
slightly more motion induced streak artifacts in the transax-
ial image than gated CBCT, and more motion blurring in the
sagittal image than any other mode.
Figure 2 compares profiles of mean voxel intensity, aver-
aged over six pixels in the AP direction, along the direction
of motion (SI) through the 20 mm diameter sphere in the
sagittal images in Fig. 1. The three profile intensities were
shifted to minimize their root mean squared (RMS) deviation
from the static phantom profile. The root mean squared devi-
ation, in CT numbers, is 31.7, 90.7, and 56.7 for gated CBCT,
standard CBCT, and RC-CBCT, respectively. The shallower
falloff near the sphere boundary in the gated CBCT and RC-
CBCT profiles, relative to the stationary phantom image and
reflected in the nonzero RMS deviation, is owing to the resid-
ual motion within the gate. The profile for the RC-CBCT im-
ages shows the effect of noise caused by reconstruction arti-
facts from unevenly spaced projections resulting in a larger
RMS deviation than for the gated CBCT. The standard CBCT
profile shows more shallow falloff and larger RMS deviation
than the gated CBCT.
CNR in the motion phantom images, for a VOI containing
the 20 mm diameter sphere and surrounding background an-
nulus, is calculated using the criterion-standard contours from
the static phantom images. CNR for gated CBCT, RC-CBCT,
and standard CBCT is 3.44, 2.11, and 2.68, respectively. CNR
for gated CBCT is larger than for RC-CBCT (ratio 1.63) and
for standard CBCT (1.28). NCC for gated CBCT, RC-CBCT,
and standard CBCT is 0.944, 0.858, and 0.870, respec-
tively (where 1 corresponds to perfect correlation). NCC for
gated CBCT is improved over RC-CBCT (ratio 1.10) and over
standard CBCT (1.09).
III.B. Patient studies in lung
Figure 3 compares standard and gated CBCT images of
a free-breathing patient with a tumor in the right lung that
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FIG. 1. CBCT images of the motion phantom using various acquisition modes. Transaxial images (top and middle rows) include the entire phantom with higher
density 10 and 20 mm diameter spheres. (Top left) Standard CBCT scan of stationary phantom; (middle left) gated CBCT with gate centered at one extreme of
the motion (“EE”); (top right) RC-CBCT at EE; (middle right) standard CBCT. Sagittal images (bottom row) are 2× magnification of a region with the 20 mm
diameter sphere and a 30 mm cube. Motion in three rightmost panels is along the axial (S-I) direction (vertical in image). (Left to right) Standard CBCT scan of
stationary phantom, gated CBCT with gate centered at one extreme of the motion (“EE”), RC-CBCT at EE, standard CBCT.
undergoes respiratory motion. The RPM phase gate used for
this patient (patient 4) is 25%–65%. The number of projec-
tions acquired is 1244 and the scan duration is 4:53 [min:s].
The image planes intersect at the tumor. The same win-
dow/level settings are used for both images. A visual com-
parison of gated CBCT images to standard CBCT images of
this patient shows features including the tumor and the di-
aphragm appear sharper and clearer in the gated CBCT. Stan-
dard CBCT shows more motion induced streak artifact in the
transaxial image and more motion blurring in the coronal im-
age than gated CBCT (Fig. 3).
Figure 4 compares tumor CNR between standard and gated
CBCT in 11 patients with lung tumors. Gated CBCT shows
increase of CNR in 6 out of 11 patients. A paired two-tailed
t-test of lung patient mean CNR shows no statistical signif-
icance (p = 0.133). For 4 of t he 5 patients where there is
no CNR improvement tumor motion extent is small (mean
3.2 mm, range 1.3–5.2 mm), as measured in the RCCT, and
so gating brings little or no benefit. Gated CBCT shows in-
crease of CNR in 6 out of 7 patients with >5 mm tumor mo-
tion extent (patients 1–6 and 10). A paired two-tailed t-test
of lung patient mean CNR for these 7 patients shows there is
statistical significance (p = 0.044).
Figure 5 compares, for standard and gated CBCT, the
normalized cross-correlation following rigid registration of
the CBCT to the RCCT image. Gated CBCT yields lower
NCC in 10 of 11 patients. A paired two-tailed t-test of
lung patient mean NCC shows there is statistical significance
(p = 0.0014). This indicates higher congruence with the
criterion-standard RCCT at EE for gated CBCT than standard
CBCT. Gated CBCT yields lower NCC in 7 of 7 patients with
> 5 mm tumor motion extent (patients 1–6 and 10). A paired
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FIG. 2. Mean voxel intensity profiles in longitudinal direction across 20
mm diameter sphere in motion phantom. “RC” denotes respiration correlated
CBCT. Mean intensity is calculated from six voxels in direction perpendic-
ular to motion (i.e., along the horizontal in the bottom row of Fig. 1). Each
profile is normalized by minimizing the RMS deviation from the static phan-
tom profile.
two-tailed t-test of lung patient mean NCC for these 7 patients
shows there is statistical significance (p = 0.00037).
III.C. Patient studies in gastroesophageal junction
Figure 6 compares standard and gated CBCT images of a
free-breathing patient with a GEJ tumor that undergoes respi-
ratory motion. The RPM phase gate used for this patient (pa-
tient 1) is 35%–65%. The number of projections acquired is
1072 and the scan duration is 5:56 [min:s]. The image planes
intersect at the GEJ. The patient was given oral barium con-
FIG. 4. CNR for 11 patients with tumor in lung, in order of decreasing dif-
ference in CNR between gated and standard CBCT. Inset shows example
tumor and background regions for calculation of CNR.
trast before the start of the gated CBCT scan. The standard
CBCT scan started 3:52 [min:s] after the end of the gated
scan. The same window/level settings are used for both im-
ages. A visual comparison of gated CBCT images to standard
CBCT images of this patient show finer features and sharper
organ boundaries in the gated CBCT. Motion artifacts in stan-
dard CBCT severely limit visibility of the GEJ, whereas the
GEJ is clearly visible in gated CBCT. Standard CBCT shows
more motion induced streak artifact in the transaxial image
and more motion blurring in the coronal image than gated
CBCT (Fig. 6).
Figure 7 (left panel) compares GEJ-to-background CNR
between standard and gated CBCT in seven patients with
malignancy in GEJ. Gated CBCT shows larger CNR rela-
tive to standard CBCT in five out of seven patients. Cases
of negative CNR are those in which the mean intensity of the
FIG. 3. (Top and bottom) Transaxial and coronal CBCT images of free-breathing patient with tumor in right lung. Image planes intersect in the tumor. Left
column is standard CBCT, right is gated CBCT. Arrows indicate lung tumor in the gated CBCT.
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FIG. 5. Zero suppressed plot of NCC for 11 patients with tumor in lung.
background is greater than that of the object. A paired two-
tailed t-test of CNR in these patients shows no statistical sig-
nificance (p = 0.082).
Figure 7 (right panel) compares, for standard and gated
CBCT, the normalized cross-correlation following rigid regis-
tration of the CBCT to the RCCT image at EE. Gated CBCT
yields lower NCC than standard CBCT in seven of seven
patients. A paired two-tailed t-test of NCC in these patients
shows there is statistical significance (p = 0.0030). This indi-
cates higher congruence with the criterion-standard RCCT at
EE for gated CBCT than standard CBCT.
III.D. Patient studies in pancreas
Figure 8 compares standard and gated CBCT images of a
free-breathing patient treated for pancreatic cancer. The RPM
FIG. 7. (Left panel) Gastroesophageal junction CNR for seven patients, in
order of decreasing difference in CNR between gated and standard CBCT.
Inset shows example regions used for calculating CNR (G - GE junction, B -
background). (Right panel) NCC for same patients.
phase gate used for this patient (patient 1) is 35%–70%. The
number of projections acquired is 889 and the scan duration is
3:54 [min:s]. The image planes intersect in the pancreas, infe-
rior to the fiducial. The patient was given intravenous iodine
contrast after the gated CBCT scan, at the start of the stan-
dard CBCT scan before treatment. For the purposes of this
analysis, the gated CBCT is compared to the post-treatment
standard CBCT (approximately 15 min after the intravenous
contrast was administered), in order to minimize differences
in the two scans caused by the contrast. The residual iodine
causes the standard CBCT to appear brighter than the gated
CBCT. The same window/level settings are used for both im-
ages. A visual comparison of gated CBCT images to stan-
dard CBCT images of this patient shows finer features and
sharper organ boundaries in the gated CBCT. Motion artifacts
in standard CBCT severely limit visibility of the pancreas
and nearby organs, whereas the pancreas is clearly visible in
FIG. 6. (Top and bottom) Transaxial and coronal CBCT images of free-breathing patient with tumor in GE Junction. Image planes intersect in the GEJ. Left
column is standard CBCT, right is gated CBCT. Arrows indicate GEJ in the gated CBCT.
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FIG. 8. Transaxial and sagittal CBCT images of a free-breathing patient treated for pancreatic cancer. Image planes intersect in the pancreas, inferior to the
fiducial. Left column is standard CBCT, right is gated CBCT. Arrows indicate pancreas in the gated CBCT.
the transaxial and sagittal gated CBCT images. In addition,
there is a clear reduction in the blurring of the fiducial. Stan-
dard CBCT shows more motion induced streak artifact in the
transaxial image and more motion blurring in the sagittal im-
age than gated CBCT (Fig. 8).
Figure 9 (left panel) compares pancreas CNR between
standard and gated CBCT in four patients with malignancy
in pancreas. Gated CBCT shows larger CNR relative to stan-
dard CBCT in three out of four patients. A paired two-tailed
t-test of CNR in these patients shows no statistical signifi-
cance (p = 0.192). We note that for the one patient where
gated CBCT does not bring CNR improvement, disease has
caused noticeable atrophy to the tail region of the pancreas,
which was used for this study. For the other three patients,
where there is CNR improvement, a paired two-tailed t-test
of mean CNR improves statistically (p = 0.096).
FIG. 9. (Left panel) CNR for four patients treated for pancreatic cancer, in
order of decreasing difference in CNR between gated and standard CBCT.
Inset shows example regions used for calculating CNR (P - pancreas tail, B -
background). (Right panel) NCC for same patients.
Figure 9 (right panel) compares, for standard and gated
CBCT, the normalized cross-correlation following rigid reg-
istration of the CBCT to the RCCT image at EE. Gated
CBCT yields lower NCC than standard CBCT in three of
four patients, indicating higher congruence with the criterion-
standard RCCT at EE than for standard CBCT. A paired two-
tailed t-test of NCC in these patients shows no statistical sig-
nificance (p = 0.165). When we exclude the one patient with
atrophy to the tail region of the pancreas, NCC for the other
three patients becomes statistically significant (p = 0.020).
IV. DISCUSSION AND CONCLUSIONS
Our studies in phantom and patient data indicate that gated
CBCT reduces image blurring and streaking artifacts caused
by respiratory motion. By using recent computer controlled
linac technology to gate gantry rotation, projection images
within a RPM-defined gate interval that are uniformly and
closely s paced can be acquired. Gated CBCT images of a mo-
tion phantom show a reduction of streak artifacts and noise,
relative to RC-CBCT images, that are acquired with uneven
projection spacing. In patient and motion phantom images,
gated CBCT shows less motion induced streak artifact in
transaxial images, and less motion blurring in coronal and
sagittal images, than standard CBCT.
The technique is applicable to respiratory sites such as
lung and abdomen. Gated CBCT images in lung show sharper
definition of tumor and diaphragm than standard CBCT. In
abdomen, where soft tissue visibility in standard CBCT is
more difficult due to motion-induced streaking artifacts, gated
CBCT shows finer features and sharper organ boundaries than
standard CBCT. Gated CBCT in abdomen provides improved
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ability to visualize and localize tumor-bearing organs and
nearby organs at risk. Our findings indicate improved CNR,
and congruence of lung tumors and abdominal organs with
those in RCCT as measured by NCC, in most patients in
which motion of the target organ exceeds 5 mm.
In gated CBCT, the goal is to limit residual target mo-
tion within the gate to approximately 30% or less of the to-
tal motion extent. To achieve this, our study uses simula-
tion day RCCT images to predict the optimal patient-specific
gate position and width. A possible limitation of this tech-
nique is the use of an external marker on the abdomen which
is dependent on correlation with the internal anatomy. Feng
et al.
20
reported that pancreatic tumor border position did not
correlate with abdominal wall or diaphragm position. How-
ever, Wang et al.
21
found that variations in tidal volume and
diaphragmatic excursion correlated strongly with superior-
inferior GEJ displacement. Although our studies showed im-
proved CNR and NCC in three out of four pancreas patients,
they require confirmation with a larger number of patients.
Changes in time lag between internal and external mo-
tion signals can affect the performance of gating based on an
external signal. Published studies have reported on internal-
external time lag and changes in time lag between treatment
sessions.
2224
We account for time lag by selecting a RPM
phase-based gate that is centered on tumor motion about the
end expiration position in the simulation RCCT. The proce-
dure followed in this study therefore assumed constancy in
time lag between simulation and the first week of treatment,
in which the gated CBCT scan occurs. Change in time lag,
between simulation and treatment, or between treatment ses-
sions, is a potential problem because the gate might no longer
be centered on the extremum of internal motion. Within-gate
verification images during treatment provide an important
check of gating accuracy.
25
Along those lines one could ac-
quire and analyze a short fluoroscopic sequence at the treat-
ment machine just prior to the gated scan, determine the time
lag between internal motion, by visualizing the tumor, a fidu-
cial, or the diaphragm, and motion of the external marker and
compare it with the time lag measured in the prior RCCT.
In the event of change, the gate position for the CBCT can
be adjusted accordingly. Furthermore, this updated gate posi-
tion could then be used in the case of gated treatment. Such
an approach would require software that quickly analyses the
fluoro images prior to the scan.
Another limitation of the method is longer scan duration,
mean 5 min (range 3–8 min), depending on gate width and
breathing period. The actual scan duration depends on the pa-
tient breathing behavior during the scan.
Because gated CBCT acquires projections over only a part
of the respiration cycle, it potentially results in less patient
dose than RC-CBCT. It yields a 3D image at one motion
state, thus providing no motion information, in contrast to
RC-CBCT which provides motion information but whose ap-
plicability has been limited to tumors in lung. Studies in lung,
by Sonke et al.
7
and independently by Bissonnette et al.,
8
found that interfraction baseline variations, i.e., variations in
the respiration-averaged position of lung tumors, were larger
than amplitude variations. These studies suggest that in most
cases it may be sufficient to acquire a CBCT of improved im-
age quality, gated at a single phase interval in the breathing
cycle, and correct for tumor position at this phase interval, in-
stead of measuring the full tumor trajectory using RC-CBCT.
For small changes in amplitude, for example, tumor position
at end expiration will correlate closely with the respiration-
averaged position. Changes in breathing amplitude could in-
stead be monitored using a short fluoroscopic sequence and
comparing tumor or diaphragm excursion to that observed in
a prior (planning day) RCCT. If the amplitude has changed
such that the respiration-averaged tumor position may be dif-
ferent, then a correction to account for this could be calculated
using the amplitude difference in the fluoroscopic sequence
and comparing it to the amplitude difference in the RCCT.
The resultant scale factor could be applied to calculate the
respiration-averaged tumor position using the tumor position
observed in the gated CBCT at end expiration. Furthermore, if
lung anatomical changes that may affect tumor motion extent
are observed in the gated CBCT, such as tumor shrinkage or
resolution of atelectasis, a RC-CBCT could be acquired t o re-
establish the tumor-diaphragm motion relationship. As well,
better images allow better assessment of tumor shrinkage (or
growth) during the course of treatment, providing information
for adaptive treatment changes.
A potentially important application of gated CBCT is the
improved soft tissue visualization for assessing target cover-
age and OAR sparing, particularly in abdomen. This would
be possible because one could more accurately register the
gated CBCT with the EE scan from simulation and verify that
anatomical changes during the course of treatment have not
shifted an OAR into the high dose region. Gated CBCT thus
provides information not available through implanted fiducial
markers, which are generally limited to a few locations in or
near the target.
Gated CBCT is also potentially applicable to gated treat-
ment, although in this study we did not compare tumor or sur-
rogate positions with images acquired during treatment deliv-
ery. For gated treatment, a consistent motion state is desirable
for all imaging and treatment, thus the same gating system
and gating conditions are preferable.
2, 3
In this study, gated
CBCT was investigated under free-breathing conditions with-
out coaching. In the case of gated treatment with coaching,
the simulation RCCT and gated CBCT would also be carried
out with coaching.
ACKNOWLEDGMENTS
This work was supported in part by Award No. R01-
CA126993 from the National Cancer Institute. The content
is solely the r esponsibility of the authors and does not nec-
essarily represent the official views of the National Can-
cer Institute or the National Institutes of Health. Memorial
Sloan-Kettering has a research agreement with Varian Med-
ical Systems. The authors thank Michelle Svatos and Stefan
Scheib for assistance with the research mode of the Varian
TrueBeam, Timo Berkus for assistance with the Varian re-
search cone-beam tomographic reconstruction software, As-
sen Kirov for assistance with gated CBCT scan dosimetry,
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and Joseph McNamara for assistance with and helpful discus-
sions concerning the Quasar motion phantom.
a)
Author to whom correspondence should be addressed. Electronic mail:
magerasg@mskcc.org; Telephone: 646-888-5615.
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Medical Physics, Vol. 40, No. 4, April 2013
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  • [Show abstract] [Hide abstract] ABSTRACT: Purpose: 4d cone-beam computed tomography (CBCT) scans are usually reconstructed by extracting the motion information from the 2d projections or an external surrogate signal, and binning the individual projections into multiple respiratory phases. In this "after-the-fact" binning approach, however, projections are unevenly distributed over respiratory phases resulting in inefficient utilization of imaging dose. To avoid excess dose in certain respiratory phases, and poor image quality due to a lack of projections in others, the authors have developed a novel 4d CBCT acquisition framework which actively triggers 2d projections based on the forward-predicted position of the tumor. Methods: The forward-prediction of the tumor position was independently established using either (i) an electromagnetic (EM) tracking system based on implanted EM-transponders which act as a surrogate for the tumor position, or (ii) an external motion sensor measuring the chest-wall displacement and correlating this external motion to the phase-shifted diaphragm motion derived from the acquired images. In order to avoid EM-induced artifacts in the imaging detector, the authors devised a simple but effective "Faraday" shielding cage. The authors demonstrated the feasibility of their acquisition strategy by scanning an anthropomorphic lung phantom moving on 1d or 2d sinusoidal trajectories. Results: With both tumor position devices, the authors were able to acquire 4d CBCTs free of motion blurring. For scans based on the EM tracking system, reconstruction artifacts stemming from the presence of the EM-array and the EM-transponders were greatly reduced using newly developed correction algorithms. By tuning the imaging frequency independently for each respiratory phase prior to acquisition, it was possible to harmonize the number of projections over respiratory phases. Depending on the breathing period (3.5 or 5 s) and the gantry rotation time (4 or 5 min), between ∼90 and 145 projections were acquired per respiratory phase resulting in a dose of ∼1.7-2.6 mGy per respiratory phase. Further dose savings and decreases in the scanning time are possible by acquiring only a subset of all respiratory phases, for example, peak-exhale and peak-inhale only scans. Conclusions: This study is the first experimental demonstration of a new 4d CBCT acquisition paradigm in which imaging dose is efficiently utilized by actively triggering only those projections that are desired for the reconstruction process.
    No preview · Article · Sep 2013 · Medical Physics
  • [Show abstract] [Hide abstract] ABSTRACT: Cone-beam computed tomography (CBCT) images are currently used for patient positioning and adaptive dose calculation; however, the degree of CBCT uncertainty in cases of respiratory motion remains an interesting issue. This study evaluated the uncertainty of CBCT-based dose calculations for a moving target. Using a phantom, we estimated differences in the geometries and the Hounsfield units (HU) between CT and CBCT. The calculated dose distributions based on CT and CBCT images were also compared using a radiation treatment planning system, and the comparison included cases with respiratory motion. The geometrical uncertainties of the CT and the CBCT images were less than 0.15 cm. The HU differences between CT and CBCT images for standard-dose-head, high-quality-head, normal-pelvis, and low-dose-thorax modes were 31, 36, 23, and 33 HU, respectively. The gamma (3%, 0.3 cm)-dose distribution between CT and CBCT was greater than 1 in 99% of the area. The gamma-dose distribution between CT and CBCT during respiratory motion was also greater than 1 in 99% of the area. The uncertainty of the CBCT-based dose calculation was evaluated for cases with respiratory motion. In conclusion, image distortion due to motion did not significantly influence dosimetric parameters.
    No preview · Article · Mar 2014 · Journal- Korean Physical Society
  • [Show abstract] [Hide abstract] ABSTRACT: In image-guided radiotherapy (IGRT) of disease sites subject to respiratory motion, soft tissue deformations can affect localization accuracy. We describe the application of a method of 2D/3D deformable registration to soft tissue localization in abdomen. The method, called Registration Efficiency and Accuracy through Learning a Metric on Shape (REALMS), is designed to support real-time IGRT. In a previously developed version of REALMS, the method interpolated 3D deformation parameters for any credible deformation in a deformation space using a single globally-trained Riemannian metric for each parameter. We propose a refinement of the method in which the metric is trained over a particular region of the deformation space, such that interpolation accuracy within that region is improved.We report on the application of the proposed algorithm to IGRT in abdominal disease sites, which is more challenging than in lung because of low intensity contrast and non-respiratory deformation. We introduce a rigid translation vector to compensate for non-respiratory deformation, and design a special regionof- interest around fiducial markers implanted near the tumor to produce a more reliable registration. Both synthetic data and actual data tests on abdominal datasets show that the localized approach achieves more accurate 2D/3D deformable registration than the global approach.
    No preview · Article · Apr 2014
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