DOI:10.2214/AJR.05.0255
AJR 2006; 186:1458-1467
© American Roentgen Ray Society
Truncation Artifact on PET/CT: Impact on Measurements of Activity Concentration and Assessment of a Correction Algorithm
Osama Mawlawi1,
Jeremy J. Erasmus2,
Tinsu Pan1,
Dianna D. Cody1,
Rachelle Campbell2,
Albert H. Lonn3,
Steve Kohlmyer3,
Homer A. Macapinlac2 and
Donald A. Podoloff1
1 Department of Imaging Physics, M. D. Anderson Cancer Center, 1515 Holcombe
Blvd., Box 56, Houston, TX 77030.
2 Department of Diagnostic Imaging, M. D. Anderson Cancer Center, Houston,
TX.
3 GE Healthcare, Waukesha, WI.
Received February 14, 2005;
accepted after revision April 6, 2005.
Presented in part at the 2005 meeting of the American Roentgen Ray Society,
New Orleans, LA.
Address correspondence to O. Mawlawi
(omawlawi{at}mdanderson.org).
Abstract
OBJECTIVE. Discrepancy between fields of view (FOVs) in a PET/CT
scanner causes a truncation artifact when imaging extends beyond the CT FOV.
The purposes of this study were to evaluate the impact of this artifact on
measurements of 18F-FDG activity concentrations and to assess a
truncation correction algorithm.
MATERIALS AND METHODS. Two phantoms and five patients were used in
this study. In the first phantom, three inserts (water, air, bone equivalent)
were placed in a water-filled cylinder containing 18F-FDG. In the
second phantom study, a chest phantom and a 2-L bottle fitted with a bone
insert were used to simulate a patient's torso and arm. Both phantoms were
imaged while positioned centrally (baseline) and at the edge of the CT FOV to
induce truncation. PET images were reconstructed using attenuation maps from
truncated and truncation-corrected CT images. Regions of interest (ROIs) drawn
on the inserts, simulated arm, and background water of the baseline truncated
and truncation-corrected PET images were compared. In addition, extremity
malignancies of five patients truncated on CT images were reconstructed with
and without correction and the maximum standard uptake values (SUVs) of the
malignancies were compared.
RESULTS. Truncation artifact manifests as a rim of high activity
concentration at the edge of the truncated CT image with an adjacent
low-concentration region peripherally. The correction algorithm minimizes
these effects. Phantom studies showed a maximum variation of -5.4% in the
truncation-corrected background water image compared with the baseline image.
Activity concentration in the water insert was 6.3% higher while that of air
and bone inserts was similar to baseline. Extremity malignancies showed a
consistent increase in the maximum SUV after truncation correction.
CONCLUSION. Truncation affects measurements of 18F-FDG
activity concentrations in PET/CT. A truncation-correction algorithm corrects
truncation artifacts with small residual error.
Keywords: attenuation correction • nuclear medicine • oncologic imaging • PET/CT • truncation artifact
Introduction
PET is increasingly being used in oncology patients to stage the primary
malignancy, assess therapeutic response and prognosis, and detect recurrence
[1-7].
More recently, integrated PET/CT has been used to evaluate oncology patients
[2,
8-10].
A major advantage of integrated PET/CT over dedicated PET is that it allows
the use of CT for attenuation correction
[11] and for anatomic
localization [12]. Directly
combining functional and morphologic information has been shown to improve the
accuracy and efficiency of PET
[13]. However, the use of CT
for attenuation correction of PET images has introduced artifacts that can
affect the interpretation of the PET scan. In this regard, a truncation
artifact can occur when the acquisition field of view (FOV) of the CT data is
smaller than the PET FOV.
Current commercially available PET/CT scanners have a CT FOV of 45-50 cm
whereas that of PET scanners is 60-70 cm. When large patients undergo imaging,
the small CT FOV causes some CT projection views to be truncated, and this
manifests on the CT image as a rim of high-attenuation values combined with
characteristic streaking (Fig.
1A). Furthermore, the discrepancy between the two FOVs causes some
sections of the PET emission data not to have any corresponding
attenuation-correction factors. The net result of this artifact is an
overestimation of the activity concentration corresponding to the image rim
and an underestimation corresponding to the region without
attenuation-correction factors (Fig.
1B).
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Fig. 1A —48-year-old man with lymphoma. CT image with truncation of arms
(A) and CT attenuation-corrected PET scan (B) show truncation
artifact. Truncation artifact causes rim of high attenuation values at edge of
CT field of view (FOV), leaving objects beyond edge with no attenuation. These
artifacts result in overestimation of 18F-FDG activity
concentration in rim region (arrows, B) and underestimation in
regions beyond edge (asterisks, B) of CT FOV. Note marked
uptake of 18F-FDG in chest wall mass (M).
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Fig. 1B —48-year-old man with lymphoma. CT image with truncation of arms
(A) and CT attenuation-corrected PET scan (B) show truncation
artifact. Truncation artifact causes rim of high attenuation values at edge of
CT field of view (FOV), leaving objects beyond edge with no attenuation. These
artifacts result in overestimation of 18F-FDG activity
concentration in rim region (arrows, B) and underestimation in
regions beyond edge (asterisks, B) of CT FOV. Note marked
uptake of 18F-FDG in chest wall mass (M).
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Several techniques have been proposed to correct for this truncation
artifact
[14-17].
One of the proposed techniques is that the missing data can be estimated by
extrapolation of the missing detector data using the fact that the total
attenuation of the slice should be the same in all parallel beam projections
[14,
17]. In this article, we
evaluate how truncation affects measurements of 18F-FDG activity
concentrations in PET/CT and the impact of a new truncation-correction
algorithm, as implemented on the Discovery ST PET/CT scanner (GE Healthcare),
on the measurement of standard uptake values (SUVs) using two phantoms and
five patient studies.
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Fig. 3B —Phantom 2. and C, CT (left) and PET (right)
images of same phantom positioned at edge of FOV to cause truncation in
simulated arm (asterisk) before (B) and after (C)
truncation correction.
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Materials and Methods
Integrated PET/CT Scanner
Data acquisition for this study was performed on a Discovery ST Integrated
PET/CT scanner. The CT component of this scanner has a 50-cm transaxial FOV
and can acquire 8 slices per X-ray tube rotation. The CT slice thickness can
range from 1.25 to 10 mm. The X-ray tube current can be varied between 10 and
440 mA, and the peak tube voltage setting can be 80, 100, 120, or 140 kVp. The
table feed rate of the CT scanner ranges from 1.25 to 30 mm per 360°
rotation of the X-ray tube. The minimum and maximum scanning times per gantry
rotation are 0.5 and 4 sec, respectively, with highest in-plane spatial
resolution of 0.32 mm.
The PET component of the Discovery ST PET/CT scanner is composed of 24
rings of bismuth-germanate (BGO) detectors. The dimensions of each detector
element are 6.3 x 6.3 x 30 mm in the tangential, axial, and radial
directions, respectively. The scanner has a transaxial FOV of 70 cm and an
axial FOV of 15.7 cm. The scanner is also capable of acquiring data in 2D and
in 3D modes by retracting tungsten septa (54 mm long and 0.8 mm thick) from
the FOV. The performance characterization of this scanner has been described
elsewhere [18].
Phantom Studies
Two phantom studies were conducted. In the first study, a cylindric phantom
(depth = 20 cm, length = 20 cm) fitted with three inserts (water, air, and
bone equivalent) was filled with water having an activity concentration of 3.2
kBq/mL of 18F-FDG. The water and air inserts were 4.3 cm in
diameter, and the bone equivalent insert was 2.5 cm in diameter. The water
insert was filled with water at an 18F-FDG activity concentration
of 7.8 kBq/mL giving an insert-to-background ratio of 2.4:1. The three inserts
were positioned in the cylindric phantom around the circumference of a
12.5-cm-diameter circle. The phantom was then placed centrally in the FOV of
the scanner and a CT scan (Fig.
2A), followed by a PET scan, was acquired. This acquisition was
then repeated with the phantom placed at the edge of the CT FOV in such a way
as to cause truncation along the center of the three inserts
(Fig. 2B). The CT data of the
second acquisition were then processed with and without the new
truncation-correction algorithm. The two resulting CT image sets were then
used to generate two attenuation maps to correct the corresponding PET
emission data.
In the second study, a thorax phantom (Alderson, Radiology Support Devices)
and a 2-L cylindric plastic bottle fitted with a bone insert were used to
simulate a patient's torso and arm (Fig.
3A). The phantom and plastic bottle were filled with
18F-FDG and water having activity concentrations of 11.5 and 7.4
kBq/mL, respectively. The phantom was then placed centrally inside the FOV of
the scanner, and a CT scan and then a PET scan were acquired. The phantom was
then repositioned in a way to cause truncation in the simulated arm along the
bone insert (Fig. 3B), and a
second CT and PET acquisition was performed. The CT data of the second
acquisition were processed in a similar manner to that in the first phantom
study, thus generating two attenuation maps to correct the corresponding PET
emission data.
The phantom designs that were used in both studies were selected to
evaluate whether the algorithm can correct truncated tissues of different
densities (phantom 1: air, water, bone) and geometries that mimic a real
clinical setting (phantom 2: chest with simulated arm). In both phantom
studies, the central positioning of the phantom in the FOV on the scanner will
be referred to from here onward as the "baseline scan" because it
represents the acquired data without truncation artifacts. The CT acquisition
parameters of all phantom studies were as follows: 120 kVp, 180 mAs, helical
mode, and a pitch of 1.35 with a detector configuration of 8 x 1.25 mm.
Images were then reconstructed using a soft filter with a slice thickness of
3.75 mm. The PET emission data were acquired for 3 min in the 2D mode and were
corrected for attenuation using the CT images. PET image reconstruction was
performed using the weighted least-squares ordered subset expectation
maximization (OSEM) iterative techniques
[19] with two iterations and
30 subsets using a post filter and a loop filter of 6 and 5.47 mm,
respectively.
To evaluate the impact of the truncation-correction algorithm on
quantitative measurements, several regions of interest (ROIs) of 1.5 cm in
diameter were drawn on the two different attenuation-corrected PET image sets
of both phantom studies and their corresponding baseline scans. The ROIs were
drawn in the three inserts and background water of the first phantom study and
the background water and bone insert of the simulated arm in the second
phantom study. The mean and SD of the activity concentration in all these ROI
measurements were then recorded, and the percentage error between the baseline
and truncation-corrected images was calculated.
Patient Studies
In addition to the phantom experiments, integrated PET/CT scans of five
patients that exhibited a truncation artifact were also evaluated. Permission
to perform this study was obtained through the institutional retrospective
chart review process, according to the ethics requirement of our institution.
Two of the patients had melanoma and lymphoma and, according to standard
institutional protocol, were imaged with their arms adjacent to their bodies.
Both of these patients had a forearm lesion that was truncated. The third
patient had a mid thigh osteosarcoma. Because the patient could not straighten
his leg, the tumor was truncated in the CT image. The remaining two patients
were large and extended beyond the 50-cm CT FOV of the Discovery ST Integrated
PET/CT scanner.
The CT scans of all five patients were processed with and without the
truncation-correction algorithm, and the resulting CT attenuation maps were
used to correct the emission PET data. ROIs were then drawn in the lesion or
truncated area on the truncation-corrected and uncorrected PET
attenuation-corrected images, and the maximum SUVs of the lesions were
evaluated.
All patients had fasted for 4 hr before injection of approximately 555 MBq
of 18F-FDG. Patients were scanned 75 ± 10 min (mean ±
SD) after injection. All PET data were acquired in the 2D mode with 3 min for
each bed position and were reconstructed using the same parameters as those
used for the phantom studies.
Truncation-Correction Algorithm
The method for extending the CT reconstruction FOV has been described by
Hsieh et al. [14]. Briefly,
the method extends the truncated projections by relying on the fact that the
total attenuation of each ideal projection remains constant over all views. By
comparing the total measured attenuation in each view, the truncated
projections can be identified and the amount of truncation can be quantified.
The truncated projection can then be corrected by assuming that the missing
attenuation is made of a partial cylindric water object that is fitted to
match the slope and intercept of the measured attenuation values at the edge
of the detector.
Consider a projection view p(i, k), where i is
the distance from the isocenter and k is the projection angle. The
projection sum function 
(k) is then obtained by
A projection is considered to be truncated if (k) is less than a
preset percentage of the maximum attenuation, m. The
value of m is determined by averaging projections that
do not exhibit truncation that are identified by determining if the boundary
samples of these projections are constrained to a predetermined threshold
(rather than zero to account for nonideal calibration). The missing
attenuation is then equal to:
(k) is then modeled as two partial cylindric water objects
fitted to match the slope and intercept at both ends of the truncated
projection (k). The total area of the added cylinders for the
k-th projection is given by 
(k):
where pl and pr are the magnitude of
the left and right intercepts, respectively, and xl,
Rl, xr, and Rr are
the location and radius of the left and right fitted cylinders.
Ideally the ratio 
(k) = (k)µw /
(k), where µw is the attenuation coefficient of
water, should be equal to unity. If (k) 
1, then additional
adjustments to the estimated missing projections are performed
[14].
Although the extended region of the projection does not contain any fine
structure, this information is supplied by other views that pass through the
region, and the reconstructed image can reveal anatomic detail in the extended
FOV. These extended reconstructions are then used to create attenuation maps
for correction of the PET emission activity.
Results
Truncation artifacts appear in PET images as a rim of high activity
concentration due to the elevated CT pixel intensity at the edge of the
truncated CT image, and an adjacent region of low activity concentration due
to attenuation losses beyond the edge of the CT FOV. ROI measurements of
18F-FDG activity concentration on the truncated PET images
confirmed visual assessment, showing increased activity concentration in the
rim region and decreased activity concentration peripherally. The
truncation-correction algorithm minimizes these effects and restores the shape
of the imaged object. Both truncation-corrected phantom studies showed a
maximum variation of 5.4% in the truncation-corrected background water when
compared with baseline. Activity concentration in the water insert was higher
by 6.3%, whereas that of the air and bone inserts was similar to baseline. The
ROI measurements before and after correction are shown in
Table 1. The percentage error
between the corrected and the baseline measurements is also shown in
Table 1.
Analysis of the patient data sets showed a consistent increase in maximum
SUV after truncation correction for malignancies and normal tissue located in
the truncated regions (Figs.
4A,
4B,
5A,
5B,
6A,
6B,
7A, and
7B and
Table 2). In the three patients
with peripherally located malignancies, six measurements of maximum SUVs after
truncation correction showed an increase of 43-520% (average, 279%). In the
two patients with truncation artifact involving the extremities and normal
tissues in these regions, measurements of the maximum SUVs of the soft tissues
after truncation correction showed an increase of 147% and 98%,
respectively.
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Fig. 4A —55-year-old man with history of metastatic melanoma. Reprinted from
[23]. Coronal CT
(left), PET (middle), and coregistered PET/CT
(right) images obtained before (A) and after (B)
truncation correction. Metastasis (arrow) in upper right extremity is
located in truncated region of CT image. Before truncation correction, maximum
standard uptake value (SUV) measurement in 1.5-cm region of interest drawn on
metastasis was 3.25, and after truncation correction maximum, SUV was
6.05—a difference of 86%.
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Fig. 4B —55-year-old man with history of metastatic melanoma. Reprinted from
[23]. Coronal CT
(left), PET (middle), and coregistered PET/CT
(right) images obtained before (A) and after (B)
truncation correction. Metastasis (arrow) in upper right extremity is
located in truncated region of CT image. Before truncation correction, maximum
standard uptake value (SUV) measurement in 1.5-cm region of interest drawn on
metastasis was 3.25, and after truncation correction maximum, SUV was
6.05—a difference of 86%.
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Fig. 5A —64-year-old man with history of diffuse large B-cell lymphoma.
Coronal CT (left), PET (middle), and coregistered PET/CT
(right) images obtained before (A) and after (B)
truncation correction. Before truncation correction, four measurements of
18F-FDG-avid tumor (within ellipse, A) had maximum standard
uptake value (SUV) range of 1.3-2.8 (2.51, 1.28, 1.31, 2.74). After truncation
correction, maximum SUV range (within ellipse, B) was 3.6-11.1 (3.59,
7.94, 7.13, 11.12)—an average difference of 328%.
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Fig. 5B —64-year-old man with history of diffuse large B-cell lymphoma.
Coronal CT (left), PET (middle), and coregistered PET/CT
(right) images obtained before (A) and after (B)
truncation correction. Before truncation correction, four measurements of
18F-FDG-avid tumor (within ellipse, A) had maximum standard
uptake value (SUV) range of 1.3-2.8 (2.51, 1.28, 1.31, 2.74). After truncation
correction, maximum SUV range (within ellipse, B) was 3.6-11.1 (3.59,
7.94, 7.13, 11.12)—an average difference of 328%.
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Fig. 6A —35-year-old man with osteosarcoma of left thigh. Coronal oblique CT,
PET, coregistered PET/CT, and CT attenuation maximum-intensity-projection
images from left to right, respectively. Patient was unable to extend leg and
this resulted in truncation artifact. Before truncation correction (A),
maximum standard uptake value (SUV) of tumor in truncated region
(arrow, A) was 3.37, and after truncation correction
(B), maximum SUV was 12.74 (arrow, B)—an increase
of 278%.
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Fig. 6B —35-year-old man with osteosarcoma of left thigh. Coronal oblique CT,
PET, coregistered PET/CT, and CT attenuation maximum-intensity-projection
images from left to right, respectively. Patient was unable to extend leg and
this resulted in truncation artifact. Before truncation correction (A),
maximum standard uptake value (SUV) of tumor in truncated region
(arrow, A) was 3.37, and after truncation correction
(B), maximum SUV was 12.74 (arrow, B)—an increase
of 278%.
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Fig. 7A —Large 73-year-old woman with lymphoma. Coronal CT (left),
PET (middle), and coregistered PET/CT (right) images
obtained before (A) and after (B) truncation correction.
Truncation artifact affects soft tissue only, and truncated area on CT shows
rim of increased attenuation value with corresponding decrease in
18F-FDG activity concentration. Before truncation correction,
maximum standard uptake (SUV) value of soft tissue was 0.32 (arrow,
A), and after truncation correction, maximum SUV was 0.79
(arrow, B)—an increase of 147%.
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Fig. 7B —Large 73-year-old woman with lymphoma. Coronal CT (left),
PET (middle), and coregistered PET/CT (right) images
obtained before (A) and after (B) truncation correction.
Truncation artifact affects soft tissue only, and truncated area on CT shows
rim of increased attenuation value with corresponding decrease in
18F-FDG activity concentration. Before truncation correction,
maximum standard uptake (SUV) value of soft tissue was 0.32 (arrow,
A), and after truncation correction, maximum SUV was 0.79
(arrow, B)—an increase of 147%.
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Discussion
Integrated PET/CT improves lesion localization and the accuracy of tumor
staging when compared with PET and CT performed independently
[8-10].
However, dual-technique PET/CT has also introduced artifacts that can affect
the interpretation of the PET scan
[12,
20-22].
These artifacts are mainly due to the use of CT rather than a transmission
source for the attenuation correction of PET images. One of these artifacts is
truncation; this artifact is seen more frequently in large patients or when
patients are imaged with their arms at their sides and is due to the
difference in the size of the data acquisition FOV of the CT and PET
scanners.
Current commercially available integrated PET/CT scanners have a CT FOV of
45-50 cm, whereas that of PET is 60-70 cm. When imaging large patients, the
small CT FOV causes some CT projection views to be truncated and this can
manifest on the CT image as a rim of high attenuation values combined with
characteristic streaking. Furthermore, the discrepancy between the two FOVs
results in the absence of attenuation-correction factors in some sections of
the PET emission data. The net result of this artifact is an overestimation of
the activity concentration corresponding to the image rim and an
underestimation corresponding to the region without attenuation-correction
factors.
Several techniques have been proposed to correct for the truncation
artifact
[14-17].
In this study, we evaluated the CT truncation-correction algorithm that was
developed and characterized by Hsieh et al.
[14] and implemented by GE
Healthcare on the Discovery ST Integrated PET/CT scanner to study its impact
on SUV measurements using phantom and patient data. The correction technique
extends the CT FOV based on information obtained from nontruncated projections
of the object and the knowledge that the total attenuation of an object should
be the same independent of the projection angle. It is also important to note
that the correction algorithm is not dependent on the extent of truncation, as
was shown by Hsieh et al.
[14].
Both of our phantom studies showed that after correction the activity
concentration in the water insert was 97% higher than those measurements
without any correction. Furthermore, the patient data obtained using the
correction algorithm showed that the SUV measurements corresponding to the
truncated regions increased as would be expected. In some cases, this increase
was as high as 278% and would significantly have affected interpretation
regarding treatment response and could potentially have had a major impact on
the patient's clinical management.
The effect of the truncation artifact on SUV measurements and the changes
that occur when a correction algorithm is used can potentially have
significant clinical ramifications. In this regard, besides being used for the
diagnosis, staging, and restaging of a wide range of tumors,
18F-FDG PET is being increasingly used clinically to determine the
response to therapy and to predict prognosis. This assessment requires an
objective measurement of 18F-FDG uptake in the tumor, and SUV
measurements are the most commonly used parameter to quantify this uptake.
Because these measurements can have a major impact on the clinical management
of oncology patients, their accuracy and reproducibility are important.
However, in malignancies with a propensity to be located peripherally, such as
soft-tissue sarcomas, melanomas, and osteosarcomas, detection of locoregional
metastases and accurate determination of SUV measurements can be compromised
by the presence of the truncation artifact associated with integrated PET/CT.
Fortunately, correction of SUV measurements for tumors located in a truncated
region improves the accuracy of these measurements. Consequently, knowledge
and recognition of this artifact are important so that the correction
algorithm can be applied to prevent misinterpretation and inappropriate
assessment of tumor grade, staging, and response to therapy.
Although our results show an improvement in the semiquantitative SUV
measurements in the truncated region, a close evaluation of the CT images
shows that the shape of the truncated object is not fully recovered. This is
mainly due to an error in the estimation of the boundary of the truncated
projection. The correction algorithm estimates the missing attenuation in a
projection based on the difference between the maximum attenuation and the
projection under consideration. This difference is then modeled as a partial
water cylinder with an area equivalent to the missing region. The only
information available for the fitting process is the intercept and the slope
at the truncated edge of the projection. No information about the natural
boundary of the projection is available. Such information is important for
more accurate fitting of the missing attenuation. An extension of the current
truncation-correction technique, which is currently under investigation, is to
provide the missing boundary information from the nonattenuated corrected PET
data. It is well known that reconstruction of the emission PET data without
attenuation correction results in an enhancement of the boundaries of the
patient's body. This information could then be incorporated during the fitting
process of the partial water cylinder to more accurately define the distal
boundary of the missing attenuation in a truncated projection.
Other limitations of the algorithm that will cause the correction technique
to fail occur whenever all projection views are truncated, such as in the case
of a large patient who has been malpositioned in the FOV of the scanner. In
this case, the algorithm cannot estimate the maximum attenuation of the object
and hence fails to extend the truncated projection accurately. Another
limitation occurs whenever the truncated portion of the object cannot be
modeled as a smooth rolloff, which is a characteristic of a water phantom.
This situation occurs when there is a discontinuity between the patient and
the truncated object, such as scanning patients who do not put their arms
close to their body. If the truncation occurs within the patient's body
boundary, then the smooth water phantom cannot appropriately model the air gap
between the patient and the arms. In this case, a modification of the
reconstruction algorithm that is based on iterative reconstruction techniques
combined with information about the shape of the truncated object obtained
from the nonattenuated PET images would be of great interest to further
improve the CT reconstruction
In conclusion, the discrepancy between PET and CT FOVs in an integrated
PET/CT scanner when imaging extends beyond the CT FOV can cause a truncation
artifact that affects SUV measurements. Because these measurements can have a
major impact on the clinical management of oncology patients, recognition of
this artifact is important so that the correction algorithm can be applied to
prevent an inappropriate assessment of tumor grade, staging, and response to
therapy. The new truncation-correction algorithm used in this study is capable
of correcting PET/CT truncation artifacts in tissues with different densities
and activity concentrations with small residual error.
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Abstract
Introduction
