Introduction
Introduction
Cross sectional imaging techniques including magnetic resonance imaging (MRI) and
computed tomography (CT) are increasingly being considered as alternative imaging
modalities for colorectal screening [1 2 3
Currently MRI colonography is restricted by limited availability of scanners and high
procedural costs. MRI as well as single-slice CT suffer from restrictions in spatial
resolution and motion artefacts, which explain insufficient detection rates for masses
smaller than 10 mm [2 1
Recently introduced multi-slice CT (MSCT) scanners represent a significant improvement
in CT technology, combining high resolution thin slice imaging with high speed volume
coverage [4 5 6
Materials and Methods
Materials and Methods
We prospectively studied 48 patients (20 women, 28 men, mean age 61.5 years) for colorectal
cancer screening (low risk, no symptoms) or for evaluation of symptoms (including
hematochezia, abdominal pain, weight loss, metastases of unknown primary) using a
four slice CT scanner (Somatom Volume Zoom, Siemens AG, Forchheim, Germany). The study
was approved by the ethics committee of the Medical Board Münster. From all patients
a signed informed consent was obtained. The examination was performed after standard
colonoscopy preparation (clear liquid diet and drinking of 4 L of polyethylene glycol
solution the day prior to the scheduled examination), intravenous injection of butylscopolamin
(Buscopan®, Boehringer, Ingelheim, Germany) and colonic distension with room air to
maximum patient tolerance (average of 30 bulb compressions). Distension was assessed
by scout CT images, when necessary further air insufflation was performed. We obtained
images in prone and supine position [7 8
Results
Results
Concerning polyps conventional colonography was normal in 33 patients. A total of
30 polyps and one carcinoma were identified in 15 patients. MSCT colonoscopy detected
23 polyps (77 %) and the carcinoma. All polyps greater than 10 mm were detected. 6
of 7 polyps from 9.9 to 5.1 mm and 14 of 20 polyps smaller than or equal to 5 mm were
identified. 13 false positive findings affected 8 patients (Table [1
Discussion
Discussion
MSCT has multiple technical advantages over single-slice CT as already shown for CT
angiography or lesion detection in the liver [9 10 6
In the largest prospective study to date by Yee et al. [12 1 1 2 12 13 14 15 11
Although sensitivity for polyp detection is improved, a limitation of better size
resolution with MSCT colonography is the number of false positive findings, with the
majority of these lesion smaller than or equal to 5 mm (10 lesions < 6 mm in 5 patients).
In our group this would result in an approximate rate of 10 % of unnecessary conventional
colonoscopies, consequently raising cost, and radiation exposure. However, results
of follow-up colonoscopy in one patient suggest that the specificity of MSCT colonography
may be higher than reported. Indeed conventional colonoscopy misses 24 % of polyps
smaller than 5 mm, 27 % of polyps 6 - 9 mm and 13 % of polyps 10 mm or more [11
In cases of missed polyps, reevaluation of the CT-data set revealed colonic collapse
or inadequate bowel preparation as the most common cause for false negative results,
suggesting, that observer independent factors predominantly limit accuracy of CT colonography.
However, the question whether MSCT improves colonic distention has not been subject
of this study.
Multiple image display techniques are available to view CT colonography data. We found
virtual flights as primary diagnostic method a time-consuming approach that requires
more time than the 2D approach. According to our experience polyp detection is just
as effective by using 2D as by using 3D images. However, endoluminal 3D images are
useful in problem solving situations (for example differentiation of small polyps
on folds from nodular folds), and for demonstration as well as documentation purposes.
MSCT offers a wide range of possible scan parameter combinations. The 4 × 1 mm detector
configuration used in our study generates 3 mm slices with a slice sensitivity profile
that is superior to 3 mm slices from a 4 × 2.5 mm detector configuration or single
slice CT [4
Furthermore, these results from a study of symptomatic and asymptomatic patients cannot
be extrapolated to a screening population. Hence, the validity of MSCT colonography
has to be established in multicenter studies in persons with average risk.
Table 1 Multi-slice CT colonoscopy compared to conventional colonoscopy: Results (no. of polyps,
in brackets no of patients) according to polyp size and occurrence of cancer. Sensitivity
was calculated on a lesion-per-lesion basis, specificity was a result of an analysis
on a patient-per-patient basis.
Total
True
False
False
Sensi-
Specificity
All sizes
30
23 (10)
13 (8)
7 (5)
77 %
78 %
≥ 10 mm
3
3 (3)
1 (1)
0
100 %
97 %
5.1 - 9.9 mm
7
6 (2)
2 (2)
1 (1)
86 %
94 %
< 5 mm
20
14 (5)
10 (5)
6 (4)
70 %
85 %
cancer
1
1
0
0
100 %
100 %
Fig. 1 Multi-slice CT colonoscopy: Axial (a ) and endoluminal (b ) view of a small 5 mm lesion (arrow) in the coecum displaying the polyp located on
a fold.
