Studies of Organ Motions Resulting From Respiration by
4D CT Imaging
Andrew Wu, Edward Brandner, Hungcheng Chen, Dwight Heron, George Henning, Kristina Gerszten, Steve
Burton and Shalom Kalnicki
Department of Radiation Oncology, UPMC Cancer Centers, Pittsburgh, Pennsylvania, 15232, USA
Abstract
A series of measurements of motions of internal organs such as kidneys, liver, spleen, and pancreas resulting from respiration
were performed using 4D CT imaging technique for treatment planning purposes.
Keywords
Organ motion, radiation therapy, treatment planning, 4D CT scanning, respiration
Introduction
Organ motion must be considered during radiation therapy
planning in order to avoid overdosing critical organs near
targets. In particular, organs in the upper abdomen including
the liver, kidneys, pancreas, and spleen move significantly
as a result of respiration. It is these organs that often limit
the maximum dose that can be delivered to a target. By
accounting for organ motion during treatment, acute
complications resulting from dose delivered to critical
organs can be reduced and higher doses can be delivered to
targets to improve tumor control (1).
Knowledge of this motion has previously been difficult to
acquire (2), but now, with 4D CT scanning, we have been
able to observe and quantify this motion throughout the
respiratory cycle. This paper provides an analysis of organ
motion including the liver, kidneys, pancreas, and spleen
resulting from respiration for 15 patients.
Methods
The 4D CT scans are acquired using a high-speed and
multiple-slice CT scanning technique that is synchronized
with the patient’s respiratory cycle that is monitored by the
respiratory gating technology. First, the respiratory cycles
of each patient are monitored by an external marker taped to
the patient (Figure 1). Each cycle is divided into numerous
breathing phases. One phase is full inspiration; another
phase is full expiration; and several phases are defined
between them. The phases are evenly distributed in time
from full inspiration, through expiration, and back to full
inspiration. From the CT scans, multiple 3D images of a
patient at a different phase of the breathing cycle are
obtained.
The patient’s respiration is monitored by measuring the
position of an external marker block taped to the patient’s
abdomen approximately 5 cm below the xiphoid (Figure 1).
A camera mounted on the foot of the CT table relays the
image of the marker to the “Respiratory Position Monitor”
(RPM) computer (Figure 2). This computer provides output
traces of the respiratory pattern, a measure of the amplitude
of the marker, and times for inspiration, expiration, and the
breathing cycle of the patient (Figure 3).
Figure 1. Reflective Marker Taped to Patient’s Abdomen.
Figure 2. Camera Mounted to CT Table.
Figure 3. RPM Computer Screen Output.
These 3D images are sorted retrospectively according to the
breathing phase during which they were acquired. It
identifies the best image for each breathing phase at each
location. All of the images for each breathing phase are
then assembled together to create a complete 3D image for
each breathing phase. The transverse, saggital, and coronal
views for each of these 3D images can be reviewed and
studied as a function of the breathing phase.
This 4D CT technology provides a very powerful and
precise method for studying motion of tumors and organs
resulting from respiration. In order to quantify the motion
of organs from full inspiration to full expiration, superior to
inferior (S-I), anterior to posterior (A-P), and left to right
(L-R) motion were measured by noting the position of each
organ at full inspiration and full expiration. These positions
were measured in one of two ways:
The organ was contoured on the planning system at both
full inspiration and full expiration. With fixed center, the
motions in three directions can be determined from the
contours.
The axial images were stepped through until the image that
displays the most superior slice of the organ was identified.
The S-I position was recorded as the superior edge of the
organ. Then the axial images were stepped through again
until the inferior slice of the organ was displayed, and this
S-I position was identified as the inferior edge of the organ.
The same process was repeated for the coronal and saggital
views so that the anterior to posterior (A-P) positions were
identified and the left to right (L-R) positions were
identified respectively. The process was repeated for the
full inspiration and full expiration phases. Because the
borders of the pancreas are not as well defined in the CT
images as those of the kidneys, liver, and spleen, each
pancreas was contoured in order to identify its edges on the
full inspiration and full expiration phases.
Results
The motion of abdominal organs has been studied in 15
patients. These include 5 males and 10 females. The
patients were scanned in the supine position with their feet
tied and with their hands either above their heads in a wing
board or folded across their chests. Table I lists the average
S-I motion, L-R motion, and A-P motion for the female
patients along with the patient population used in each
average. Table II reports the same data for male patients.
The error listed in the table is the resolution of the
measurement. The S-I motion for males is greater than that
for females.
The resolution of the measurements is limited by the scan
parameters and the software applications. For the S-I
direction, the resolution is the slice spacing (2.5 mm). For
the L-R and A-P directions, the resolution is the step size
used by the software when panning through coronal and
sagittal images (1 mm). Error can be introduced if the full
expiration and full inspiration phases are not properly
identified. In order to estimate this error, the displacement
between breathing phases must be estimated at full
inspiration and full expiration. This displacement follows
the breathing cycle which will be approximated by a cosine
wave for calculation purposes:
Acos(2t/T): inspiration
where time will be estimated as one seventh of the period
(T) because of the 7 phases into which the breathing cycle is
divided.
Acos(2/7) = A0.62: inspiration
For the largest average displacement (1.64 cm), its
amplitude (A) is equal to half the displacement (0.82 cm).
This would yield an error of 0.5 cm if the full inspiration
and full expiration phases were erroneously identified as
being 1 phase removed from the absolute full inspiration
and full expiration phases. Because neighboring phases are
directly compared, 0.5 cm is a greater error than can be
expected for our measurements.
Conclusion
The S-I motions of the kidneys, liver, spleen, and pancreas
can be 1.5 cm or more for both males and females. As
expected, all organs moved inferiorly on inspiration (from
2.5 mm to over 30 mm). All kidneys moved anteriorly on
inspiration from 1 to over 12 mm. The liver also moved
anteriorly on inspiration (from 1.5 mm to over 14 mm).
Likewise, the spleen moved anteriorly on inspiration (from
5 mm to over 11 mm). For all but two patients, the
pancreases moved anteriorly on inspiration (from 1 mm
posteriorly to over 10 mm anteriorly). The right kidneys
moved from 3 mm right to 7 mm left during inspiration.
The left kidneys moved from over 3 mm left to less than 1
mm right on inspiration. The livers moved from less than 3
mm left to nearly 6 mm right on inspiration. The spleens
moved from less than 2 mm right to over 6 mm left on
inspiration. The pancreases moved left over 7 mm to right
over 10 mm left. Furthermore, the maximum extent of the
marker block displacement with the organ motions did not
correlate with the maximum extent of organ displacement
for any organ studied.
References
1) Page: 3
Balter, JM; Ten Haken, RK; Lawrence, TS; Lam, KL;
Robertson, JM. Uncertainties in CT-Based Radiation
Therapy Treatment Planning Associated with Patient
Breathing. Int. J. Radiat. Oncol. Biol. Phys. 36:167-174;
1996.
2) Page: 3
Mageras, GS; Kutcher, GJ; Leibel, SA; Zelefsky, MJ;
Melian, E; Mohan, R; Fuks, Z. A Method of Incorporating
Organ Motion Uncertainties into Three-Dimensional
Conformal Treatment Plans. Int. J. Radiat. Oncol. Biol.
Phys. 35:333-342; 1996
Table I
Average Organ Motion – Female Patients – Supine
Position at Expiration Minus Position at Inspiration (Superior, Left, and Anterior are positive)
Displacement
S-I
# Patients
Displacement
L-R
# Patients
Displacement
A-P
# Patients
(cm)
(cm)
(cm)
Left
Kidney
Right
Kidney
Liver
Spleen
Pancreas
1.18 0.25
1.48 0.25
1.57 0.25
1.28 0.25
1.14 0.25
10
10
9
9
7
-0.14 0.1
0.07 0.1
0.15 0.1
-0.10 0.1
0.02 0.1
10
10
9
9
7
-0.48 0.1
-0.68 0.1
-0.66 0.1
-0.74 0.1
-0.38 0.1
10
10
9
9
7
Table II
Average Organ Motion – Male Patients – Supine
Position at Expiration Minus Position at Inspiration (Superior, Left, and Anterior are positive)
Left Kidney Right Kidney
Liver
Spleen
Pancreas
Displacement
(cm)
0.98 0.25
1.20 0.25
1.64 0.25 1.44 0.25 1.03 0.25
S-I
# Patients
5
5
5
5
5
Displacement
(cm)
-0.10 0.1
-0.10 0.1
0.01 0.1
-0.34 0.1
-0.24 0.1
L-R
# Patients
5
5
5
5
5
Displacement
(cm)
-0.39 0.1
-0.57 0.1
-0.56 0.1 -0.51 0.1
-0.42 0.1
A-P
# Patients
5
5
5
5
5