Automatic polyp detection and measurement with computed tomographic colonography: A phantom study
1 Philips Healthcare, Ohio, United States
2 Department of Radiology, Hospital of the University of Pennsylvania, Philadelphia, United States
Purpose: The purpose of this study is to assess the
performance of computer-aided detection (CAD) software in detecting and
measuring polyps for CT Colonography, based on an in vitro phantom
Material and methods: A colon phantom was
constructed with a PVC pipe of 3.8 cm diameter. Nine simulated polyps of
various sizes (3.2mm-25.4mm) were affixed inside the phantom that was placed in
a water bath. The phantom was scanned on a 64-slice CT scanner with tube
voltage of 120 kV and current of 205 mAs. Two separate scans were performed,
with different slice thickness and reconstruction interval. The first scan
(thin) had a slice thickness of 1mm and reconstruction interval 0.5mm. The
second scan (thick) had a slice thickness of 2mm and reconstruction interval of
1mm. Images from both scans were processed using CT Colonography software that
automatically segments the colon phantom and applies CAD that automatically
highlights and provides the size (maximum and minimum diameters, volume) of
each polyp. Two readers independently measured each polyp (two orthogonal
diameters) using both 2D and 3D views. Readers� manual measurements (diameters)
and automatic measurements from CAD (diameters and volume) were compared to
actual polyp sizes as measured by mechanical calipers.
Results: All polyps except the smallest (3.2mm)
were detected by CAD. CAD achieved 100% sensitivity in detecting polyps
≥6mm. Mean errors in CAD automated volume measurements for thin and thick
slice scans were 8.7% and 6.8%, respectively. Almost all CAD and manual
readers� 3D measurements overestimated the size of polyps to variable extent.
Both over- and underestimation of polyp sizes were observed in the readers�
manual 2D measurements. Overall, Reader 1 (expert) had smaller mean error than
Reader 2 (non-expert).
Conclusion: CAD provided accurate size measurements
for all polyps, and results were comparable to the two readers' manual
measurements. � 2009 Biomedical Imaging and Intervention Journal. All
Keywords: Polyp measurement; CT colonography; CAD; automatic
Colorectal cancer (CRC) is the second leading cause of
cancer death in the United States. The American Cancer Society estimates that
about 148,810 new cases of colorectal cancer will be diagnosed in 2008. In the
vast majority of cases, colorectal cancer develops slowly from precancerous
polyps and can be prevented if precancerous polyps are removed. This indicates
the importance of screening for colorectal cancer. However, patient compliance
with screening recommendations remains low, due, at least in part, to the
limitations of current screening techniques (e.g. optical colonoscopy, flexible
sigmoidoscopy, barium enema and fecal occult blood test etc.).
More than a decade ago, CT Colonography was introduced as
a non-invasive technique for the detection of colonic polyps and colorectal
cancer. Since then tremendous advancements have occurred, including
improvements in the examination technique itself and also in the interpretation
methods.� CT Colonography studies can be read primarily using 3-dimensional
visualisation techniques with 2-dimensional images used for lesion
characterisation or by means of primary 2‑dimensional reading for
detection and characterisation. Polyp size is the most important criteria for
assessing the risk of malignancy and the need for follow up in CT Colonography
(CTC). Size was used as the most important criterion for risk stratification of
polyps during the development of CT Colonography: Reporting & Data System
(CRADS) . Diameter (maximum linear dimension) has been the standard
parameter for reporting polyp size. Based on size (maximum diameter), polyps
are typically classified into three categories: Diminutive (≤ 5 mm),
Intermediate (6-9 mm) and Large (≥ 10 mm). Recommendations for patient
follow-up change significantly, based on the number and more importantly, the
size category of polyps that are detected.� Therefore, accurate and
reproducible diameter measurement both during 2D and 3D interpretation of CTC
studies is critical. Polyp volume has also been proposed as a better measure
than linear dimension or diameter . Further investigational studies are
needed to study the value of polyp volume compared to maximum diameter. During
the past decade, several studies [3-5] have demonstrated high sensitivity and
specificity of CTC for polyp detection but few other studies [6, 7] have
questioned that and created disparity in the results. For widespread acceptance
of CTC, methods to improve its accuracy and reproducibility are required.
Computer Aided Detection (CAD) has been proposed as a possible solution .
CAD techniques not only provide the capability of detecting polyps but also
provide automatic measurement of the volume and diameter of polyps.
Materials and Methods
The primary objective of our study was to evaluate the
accuracy of automatic diameter and volume measurements provided by Computer
Assisted Detection (CAD) software (Extended Brilliance Workspace, Philips
Healthcare, Andover, Massachusetts) for polyp-simulated structures in a colon
phantom. The secondary objective includes comparison of 2D and 3D manual
measurements performed by two human readers with the automatic measurements of
CAD. Furthermore, we sought to assess the effect of scanning parameters (slice
thickness and reconstruction interval) on both automatic and manual
A colon phantom was constructed using a 1.5" diameter
PVC pipe. Ten phantom polyps comprised of glass beads of various diameters were
glued to the inner surface of the pipe to mimic polyps on the colon wall (Fig
2). A very thin layer of glue was applied to the phantom polyps to avoid any
over-estimation of the polyp size by the software. The glass beads were chosen
as the phantom polyps because their CT density was in a similar range to real
polyps, which vary between 20‑100 HU .� The phantom was submerged
in water as shown in Fig 1 to simulate the attenuation of the x-ray beam by the
soft tissues surrounding the colon in vivo. The phantom was sealed at
both ends to avoid any water flowing into the tube. Wooden planks were used to
hold the submerged hollow phantom in place in the water bath.
The phantom polyps were spherical in shape and were of
different sizes. The reference diameter of the phantom polyps were measured
using mechanical calipers and their volume was mathematically calculated. The
distribution of the phantom polyps by diameter and their classification into
clinically relevant polyp size classes are provided in Tables 1 and 2
respectively. The placement of the polyps within the phantom is shown in Figure
3. The polyp sizes were chosen such that there is at least one representative
phantom polyp from each of the standard polyp size categories: diminutive
(≤ 5 mm), intermediate (6-9 mm) and large (≥ 10 mm).
The phantom was scanned on a Brilliance 64-slice CT
scanner (Philips Healthcare, Andover, Massachusetts, USA). The scan was
performed in a helical mode at a collimation of 64x0.625, 120 kV, pitch of 0.95
and 205 mAs. The data was reconstructed into thick and thin images using slice
thickness of 2 mm/ reconstruction interval of 1 mm and slice thickness 1
mm/reconstruction interval 0.5 mm, respectively. This was done to analyse the
effect of slice thickness on the accuracy of manual and automatic measurements.
Human readers for manual measurements
For manual measurements, two readers, both familiar with
the CT colonography software but with different levels of experience, were
employed. Reader 1 was an abdominal radiologist with 22 years of experience in
radiology and fellowship training in gastrointestinal imaging, and was also an
expert in CT Colonoscopy (4 years of experience in CT Colonoscopy and read
over 200 cases at the time of the study). Reader 1 was also an expert with the
workstation and software used in the study. Reader 2 was a radiologic
technologist with no clinical experience interpreting CT Colonoscopy but
with more than 5 years of experience in advanced CT post-processing and 3D
imaging. The two readers interpreted the images separately and were blinded to
the results from each other and also to the ground truth.
Image Processing and Interpretation
The reconstructed images of the phantom were transferred
to a clinical post-processing workstation (Extended Brilliance Workspace,
Philips Healthcare, Andover, Massachusetts, USA). The workstation includes an
advanced Virtual Colonoscopy application with varied displays including 2D
multi-planar, endoluminal and a Perspective-Filet View (dissection view) to
optimize interpretation and Colon CAD software for automatic detection,
segmentation and measurement of polyps.
The Colon CAD software (Philips Healthcare, Andover, Massachusetts, USA) used in the study is a feature-based technique that identifies
potential polyps based on morphology and density. The CAD algorithm performs this
operation in three steps. First, the algorithm identifies convex elevated
regions with positive curvature throughout the colonic surface. Second, it
calculates the likelihood value based on morphology (including size, convexity
and compactness) and Hounsfield Unit (intensity) average and standard deviation
of each of those candidates. Finally, a subset of those candidates are
classified and highlighted as possible polyps based on a pre-defined threshold
for the likelihood value and on an optimal setting on the Free-receiver
operating curve (FROC).
Both phantom scans were processed by the Colon CAD
algorithm. Automatic diameter (two orthogonal) and volume measurements provided
by CAD were recorded. An example of CAD detection and automatic measurement is
shown in Figure 4. Both readers read the scans using a combination of
perspective filet view and endoluminal 3D view, making measurements on 2D
images and 3D endoluminal images on separate occasions. 3D measurements were
made on the endoluminal view and 2D measurements were made on either the axial,
coronal or sagittal 2-dimensional reconstructed images (whichever provides the
optimized �maximal� dimension), as in clinical practice. A substantial time gap
(approximately 1 month) was built in between 2D and 3D measurements to avoid
The accuracy of measurements from CAD and readers (2D and
3D) was measured using absolute mean error (%) calculations compared to the
actual measurements of the simulated-polyps. A Student T Test was used to
compare the volume and diameter measurements from CAD and readers, between
thick and thin slices. Bland-Altman Analysis  was used to evaluate the
inter-observer agreement between the readers for the repeated manual
measurements (2D and 3D).
Diameter 1 is the maximum diameter of a polyp measured by
readers or CAD, and Diameter 2 is the measurement orthogonal to the maximum
diameter. For 2D measurements, readers used axial 2D slices and the maximum
diameter measurements were made on the slice with the largest visible diameter.
The orthogonal measurement to the maximum diameter was recorded as Diameter 2.
For 3D measurements, readers used the endoluminal view. Sample measurements are
shown in Figure 5.
All polyps except one (3.2mm) were detected by Colon CAD
in both thin and thick scans. There was no difference in the CAD standalone
sensitivity between thick and thin slices, with CAD achieving 100% sensitivity
for polyps ≥ 6 mm. The absolute mean error in automatic Colon CAD volume
measurements for thin and thick slice scans were 8.7% and 6.8%, respectively.
The difference in the CAD volume measurements between thick and thin slice
datasets is not statistically significant (P>0.05). The absolute mean error
for Colon CAD diametric measurements was lesser in thin slice datasets than
thick slice datasets but the difference was not statistically significant. The
absolute mean error for readers� manual 2D and 3D measurements was in most
cases lesser for thin-slice datasets compared to thick slice datasets. The
absolute mean errors (%) for all measurements are shown in Table 3. The
differences between measurement were found to be statistically insignificant
(P>0.05). Almost all CAD and readers� 3D measurements overestimated the size
of polyps but it was noticed that the overestimation was relatively higher for
small polyps. Both over- and underestimation of polyp sizes were observed in
the readers� 2D measurements.
The Bland-Altman analysis computed the interobserver
agreement between the readers for repeat manual 2D and 3D measurements of
simulated polyps. The mean difference between the observer measurements and the
95% Bland-Altman limits of agreement are shown in Table 4.
The Colon CAD software for CT Colonography may potentially
improve readers� detection performance and reduce variability among readers
. The CAD software can be used in a concurrent reading (CAD findings
highlighted during the radiologist�s primary read) or a sequential/second
reading paradigm (CAD findings highlighted only after the radiologist�s primary
read is complete). Some studies have suggested that CAD may benefit novice
readers more than the experienced ones . Colon CAD provided accurate size
measurements for a wide-ranging size� of phantom polyps, and results were
comparable to the manual measurements made by two independent readers. The
absolute % (mean) error for CAD and for the readers was relatively higher for
smaller polyps (6-9 mm diameter) than large polyps (≥10 mm diameter).
Overall, Reader 1, the expert in interpretation of CT colonography, had a
smaller mean error than Reader 2. As mentioned above, polyps can be categorised
based on their size (diameter) and three clinically relevant categories have
been defined . In our study, based on readers' and CAD measurements, no
phantom polyp was misclassified in a larger or smaller size category due to
over- or under-estimation of size respectively. But this may be due to the low
number of sample polyps with sizes that are close to the polyp size categorical
boundaries. This is an important issue since the recommended patient follow-up
may change substantially as the size of the polyp detected increases . For
example, in some clinical settings, a patient with a single 6-9 mm polyp may be
triaged to a follow-up CT colonography while a patient with a polyp above 10mm
in diameter is generally considered to have an advanced adenoma for which
optical colonoscopy would be recommended.� Therefore, this finding needs
further analysis with a larger sample size.
In our study, readers� 2D measurements were more accurate
than 3D measurements.� All polyps except one (3.2mm) were detected by Colon CAD
in both thin and thick scans. This polyp was missed in both thin and thick
slice datasets. This may be due to the fact that the manufacturer default
threshold for the Colon CAD algorithm is set at detecting polyps ≥6 mm.
The polyp was visible to the naked eye in both the datasets since the
reconstruction of both datasets were� thin enough to reveal such small polyps.
The size of the missed polyp being very close to this threshold could have been
the reason for non-detection. In fact, after the study was completed, the 3.2mm
polyp was shown to be detectable when the diameter threshold for CAD was
reduced below the default used for the study. The clinical significance of
detecting polyps in the 3-4 mm size range remains a subject of controversy.�
The published consensus proposal for reporting CT colonography considers
colonic polyps less than or equal to 5 mm in diameter or so-called diminutive
polyps as not clinically significant . CAD achieved 100% sensitivity in
detecting polyps ≥6mm in this phantom.
A typical CT Colonoscopy patient prep includes ingestion
of a low-density barium suspension for tagging solid fecal residue and
iodine-based contrast material for tagging the fluid that remains inside the
patient�s colon. This helps in differentiating between polyps and other
non-polyp material. The presence of tagged or untagged fluid/fecal material on
or close to the polyps may affect the manual and automatic measurements. In
this study design, this effect was not measured due to the limitations in the
design of the phantom. Future studies to evaluate the effect of fluid and fecal
material (tagged or untagged) in the colon on these measurements are needed.
Due to the small sample size (number of phantom polyps) and use of only two
dissimilar readers, we did not analyse inter- and intra-observer variability
for the manual 2D and 3D measurements. Future studies with larger sample size
are needed to perform this analysis. Lastly, the phantom used in this study is
a simplistic approach to a real patient colon. To verify that these findings
are reproducible in a clinical setting, a similar study is needed using a more
complex phantom with a morphology that mimics colonic folds and flexures as
well as non-spherical phantom polyps that mimic the irregular shapes of real
CAD provided accurate size measurements (diameter and
volume) for all simulated polyps. The CAD automated measurements were
comparable to the two readers' manual measurements. This proves the ability of
CAD to provide automated measurements of diameter which may help decrease
interpretation time for CT Colonography. CAD was also very sensitive to the
polyps that are considered clinically relevant. This study shows that CAD may
prove to be beneficial for CT Colonography.
Figure 1 Phantom polyps used in the colon phantom with their actual diameters measured using mechanical calipers
Figure 1 (a) Experimental Set-up: Colon phantom submerged in water placed on the scanner table (Brilliance 64, Philips Healthcare, Andover, Massachusetts, USA); (b) Close-up view of the colon phantom inside the hollow container.
Figure 2 Glass beads (to mimic polyps) glued to the inner surface of the colon phantom.
Figure 3 Placement of all polyps shown in a 3-dimensional overview image reconstructed from CT data.
Figure 4 Colon Computer Aided Detection (CAD) software automatically detecting and measuring the polyp diameters and volume.
Figure 5 Sample measurements: (a) Image shows an axial 2D slice with the largest visible diameter (Diameter 1 = 22.1 mm) measured along with the orthogonal diameter (Diameter 2 = 21.6 mm); (b) Image shows a polyp on the 3D endoluminal view with two diameter measurements. The image also shows CAD volume measurements that are computed automatically.
Table 2 Defining the clinically relevant ranges of polyp size. Recommended patient follow-up changes substantially for polyps detected in the higher size ranges.
Table 3 Results show the absolute mean errors for automatic measurements and manual measurements (2D and 3D) by readers when compared with actual polyp sizes.
Table 4 Interobserver Agreement Analysis: Comparing manual 2D and 3D measurements from two readers on thick and thin slice datasets
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|Received 16 December 2008; received in revised form 27 April
2009, accepted 15 May 2009
Correspondence: CT Clinical Science, Philips Healthcare, 595 Miner Road, Highland Heights, OH 44143, United States of America. Tel.: (440) 483-2517; Fax: (440) 483-7024; E-mail: email@example.com (Sunny Virmani).
Please cite as: Virmani S, Lev-Toaff AS, Ciancibello LM,
Automatic polyp detection and measurement with computed tomographic colonography: A phantom study, Biomed Imaging Interv J 2009; 5(3):e15