Original ArticleA feasibility study of unidirectional 240°-angle 3D CT colonography
Introduction
Computed tomography colonography (CTC) has been accepted as an option for colorectal cancer screening [1]. CTC is an evolving diagnostic tool and is rapidly developing technically. There are two strategies for interpreting CT colonographic data sets: a primary two-dimensional (2D) approach and a primary three-dimensional (3D) approach. The primary 3D approach is advantageous compared with the primary 2D approach for reader intensiveness, polyp conspicuity, and ease of learning [2], [3], [4], [5], [6]. In addition, a higher sensitivity for polyp detection of the primary 3D approach compared to the primary 2D approach has been suggested [7]. However, the primary 3D approach has a drawback of creating blind spots behind the colonic folds, for which the readers have to perform four sets of fly-through navigations per case including bidirectional prone and supine series [8]. A relative long reading time of the primary 3D approach may prevent the widespread introduction of CTC in routine clinical practice [9]. A variety of 3D panoramic displays such as the virtual dissection and the filet view methods have been developed to review larger mucosal coverage at a glance and to promote time efficiency of CTC [10], [11], [12], [13], [14], [15]. These panoramic displays provided us with a new way of inspecting virtually dissected colon like a pathologic specimen. The common goal of these new alternative 3D methods would be the increase of time efficiency by performing only one-way evaluation of the colon while minimizing blind spots.
Another method to increase mucosal coverage is by widening the viewing angle, which is familiar to radiologists because we can see similar images to conventional endoluminal images. In such cases, a change in reconstruction algorithms is not necessary. East et al. [16] reported that a unidirectional 170°-angle review visualized 92% of colonic surface. If there are no significant blind spots during forward review with a widened angle exceeding 180°, we can eliminate the need for backward review as the whole colon has already been evaluated on the forward review. To the best of our knowledge, there has been no study on the polyp detection performance of unidirectional 3D CTC with an angle exceeding 180°. Our investigation was focused on the feasibility of the unidirectional 3D CTC with a 240° angle. Therefore we compared the performance of polyp detection and time efficiency between unidirectional 3D CTC with 240° and bidirectional CTC with 120°.
Section snippets
Materials and methods
Our institutional review board approved this retrospective study and waived informed patient consent. This study included 40 patients (17 men and 23 women, mean age 58 years) who underwent CTC followed by colonoscopy within 3 months. No patients underwent prior colonic resection.
We used 4 L of polyethylene glycol solution for colon cleansing and automated CO2 insufflators for bowel distension. Contrast agents for fecal or fluid tagging were not used. CT was performed using a 64-row detector
Results
Twenty-six of 40 patients had polyps. A total of 53 polyps were found: 43 benign (10 flat, 28 sessile, 5 pedunculated) and 10 malignant. Per-polyp sensitivity and PPV did not show a significant difference between the two review methods in both reviewers. The difference between the two readers was not significant in either method (P>.05; Table 1).
As for the acceptability of the wide-angle CTC, all 40 fly-through navigations with a wide angle were regarded as acceptable for polyp detection by the
Discussion
Because the drawback of the primary 3D approach is the fact that there can be missed areas, a radiologist should use backward fly-through reviews as well as forward reviews. Approximately 80% of colonic surface coverage can be obtained by unidirectional 3D endoluminal fly-through with a 120° CTC [16]. In order to see the remaining 20% of the colonic surface, we should perform a backward review, which needs a large amount of time to perform. A variety of alternative 3D display methods have been
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