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CT Colonography: Influence of display methods on reviewing time

White Paper
Philips CT Clinical Science Philips Healthcare

Summary

As the value of CT colonoscopy remains to be defined in the fight against colon cancer, it is continuously discussed as the only realistic and practical alternative screening tool compared to conventional colonoscopy. Beside the fact that there is still no established standard and accepted examination and reviewing technique, the widespread use of this promising technique - with the potential of more than 70 million patients (age > 50 years) in the USA alone - is mainly limited through one simple fact: with traditional tools it takes an average of about 30 to 40 minutes to review the two datasets of the supine and prone exam. As long as there are no reliable computer aided diagnostic systems for the detection of polyps available, it has to be the task of all new software applications to improve the reading time without losing diagnostic accuracy.

 

There are multiple papers published about the influence of the different 2D and 3D display methods and their influence on the overall sensitivity and specificity of the test. Little attention has been paid until now on comparing the influence of the different display tools on reviewing time. An analysis of the published data revealed that there has been only little progress regarding the reviewing time needed to analyze a whole colon, although the reconstruction rate and especially the 3D visualization - usually mimicking the view of a conventional endoscope with a limited view-angle - directly improved from increasing processor speed of the standard PC hardware. Therefore, it is less likely that the current visualization technique will allow reducing the reviewing time significantly enough to finally get the reviewing time reduced to those needed for the analysis of a conventional CT exam.

 

To improve the day-to-day routine usability of CT colonoscopy - especially with respect to the upcoming reimbursement for the whole exam - it must be possible to finalize the reading in less than 10 minutes. In other words: a method is needed to compress the reading time to one-third of the current standard.

 Conventional vs. New Method (Virtual Colonoscopy)
Conventional vs. New Method (Virtual Colonoscopy)

A new viewing method has been developed with the goal of reducing read time. The main part of the improvement is related to a new 3D visualization of the colon CT dataset. In our opinion, it does not make much sense to copy the viewing method of a conventional endoscope and recreate a virtual representation that has all of the same limitations, especially with the limited view-angle. With this method it is impossible to display views of both sides of the folds and in an antegrade and retrograde fashion at the same time. The main advantage of the stack of CT images of the colon is not used in these cases: it is a real, volumetric data-set and the view-angles on or into this data are just limited by the display method used. A possible solution to overcome the limited view-angle of a conventional endoscope solution is to create multiple such views ensuring a 360o display at any given point in the data set. Several combined methods have been developed to provide that kind of information in combined windows displaying the different view-angles of the multiple virtual cameras in real-time. The main shortcoming of these solutions is that they are widely ignoring a simple fact: the human eye is not able to focus onto different window areas containing permanently changing content representing different views and motion-directions of the same volume-data. Those solutions will therefore never be really successful.

 

Another discussable feature of most current software solutions is the attempt to preserve the 3D anatomy in the main endoluminal viewing mode in order to visualize curves and flexures of the colon - although this kind of information is perfectly represented in the commonly used external overview displays with semi-transparent colon as radiologists are used to from conventional double contrast colonography. The focus for the endoluminal view should be to detect polyps and cancer-like lesions.

 

A new developed visualization method - the so called interactive "dissection" mode - promises to overcome some of the above mentioned shortcomings. The main display is representing an approximately 400o circular view of the colon at any given point along an automatically calculated centerline through all colon segments. The main difference between the new display and other solutions is that it is displaying the internal surface of the colon in a linear fashion unfolded like a pathology dissection and sequential like a film-stripe or tape. The reviewer can concentrate on one specific window to analyze the surface of the colon as it scrolls by. As the whole circumference of the colon is displayed, it is not necessary to review the colon several times like it is mandatory to do with a simulated virtual endoscopic view. Secondary windows are used to display multiplanar cuts through the data at the current position of the "virtual" camera. If a suspicious irregularity is detected in the colon wall, these windows are used for a quick first reference to differentiate between real findings and artifacts. If a more detailed analysis is required, the reviewer can simply click on the structure to obtain a focused 3D view with interactively changeable view angles.

 Philips Filet View is viewed as if the viewing area is pushed
across a tube, rounding the center of the viewing area. This
allows you to see both the retrograde (on the right side of
the screen) and antegrade (on the left side of the view port)
without having to manipulate the image at all. Philips Filet View vs. Previously Described Filet - Note that competitive methods of the Filet View are flat and do not allow the user to see around and between folds of the colon. A lesion or polyp on a fold can easily be missed.
Philips Filet View is viewed as if the viewing area is pushed across a tube, rounding the center of the viewing area. This allows you to see both the retrograde (on the right side of the screen) and antegrade (on the left side of the view port) without having to manipulate the image at all.
Philips Filet View vs. Previously Described Filet - Note that competitive methods of the Filet View are flat and do not allow the user to see around and between folds of the colon. A lesion or polyp on a fold can easily be missed.

An initial trial of this new viewing method in comparison to a traditional display method in patients with endoscopic proven polyps led to a reduction of necessary review time from an average of 38 minutes to less than 10 minutes. At the same time, it was possible to increase the sensitivity for small lesion (< 5 mm) from 35.29% to 47.06%. The needed review time was significantly shorter using the new dissection mode compared to the endoluminal display. The initial test included 25 patients and data sets acquired using both a 4-slice and 16-slice scanner. The value of the results is therefore limited.

 

Nevertheless, there was a trend that the new methods perform better on thin-sliced CT data (1 mm) compared to the traditional method. The principle behind the dissection mode promises to improve even further through the availability of higher resolution CT data that are derived from the newest scanner generations (Brilliance CT 40-channel). As the test comparison was made between the 3D dissection mode alone versus all combined 3D and 2D display methods of the traditional colon visualization package, it is to be expected that the combination of the new 3D view with parallel displayed multiplanar reformations will result in additional improvements.

The dissection mode alone leads to a significant acceleration of the CT colonoscopy review and will enable a wider routine use of this promising screening tool for colorectal cancer. Further studies are needed to establish this method as a new reliable standard.
 This view of the descending section of the colon with
extensive diverticulosis is the most challenging to view the
colon surface and Filet View demonstrates 100 percent of
the surface. Retrograde sides of colonic folds are viewed on
the right side of the screen and antegrade sides viewed on
the left while the top and center portions are mostly
viewable in the center of the viewport.
This view of the descending section of the colon with extensive diverticulosis is the most challenging to view the colon surface and Filet View demonstrates 100 percent of the surface. Retrograde sides of colonic folds are viewed on the right side of the screen and antegrade sides viewed on the left while the top and center portions are mostly viewable in the center of the viewport.


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Aug 1, 2005

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White Paper
3D, abdomen, Body, colon, colorectal cancer, CT colonography, MPR, polyp, Virtual Colonoscopy
 

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