Color coding aids 3D interpretation in virtual colonoscopy

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The application of moderate cathartic preparation with color-coded tagged stool can facilitate primary 3D interpretation of CT colonography and ease patient discomfort experienced during exam preparations, according to research published this month in Radiology.

A team from the department of radiology at the University of Ulsan College of Medicine, Asian Medical Center in Seoul, Korea, evaluated a newly developed technique for discriminative color coding of tagged stool during 3D endoluminal fly-through CT colonography and determined its effect on reading efficiency.

“In our study, we developed volume rendering with an attenuation-dependent color-enhancement technique as a tool to assist the primary 3D interpretation of CT colonographic images obtained with reduced cathartic preparation,” the authors wrote.

Two tagged stool pieces are white (arrowheads) after application of color-coding technique. Polyp (arrow) remains with the same color before and after application of color-coding technique. Image and caption courtesy of the Radiological Society of North America.  

The team evaluated the diagnostic accuracy of their technique on a prospective and randomly selected group of 14 men and 16 women referred for CT colonography at their institution. Three patients dropped out of the test, leaving a cohort of 27.

The patients were given a moderate cathartic preparation consisting of 20 mg bisacodyl and three doses of 200 mL of 5 percent (weight/volume) barium sulfate, which was administered orally for stool tagging on the day before CT colonography.
Colonic distention during the procedure was achieved with an automated CO2 insufflator (PROTOCO2L, E-Z-Em) and the scans were performed on a Siemens Medical Solutions 16-slice Somatom Sensation CT system.
The technique for 3D color coding developed by the scientists is a modification of the commonly used ray-casting method in which multiple rays of light are cast from an imaginary light source to the target volumetric CT data set. Their algorithm checks the attenuation values of data points along each ray path beyond the point of 95 percent cumulative opacity from the light source.

“Then, if there are data points greater than 200 HU within the next 2 mm from the point of 95 percent cumulative opacity along the path of a particular ray, the screen display of the color of the volume-rendered area that corresponds to that particular ray is changed from red-to-brown color to white (to tag stool) by applying weights to the green and blue primary color components,” the authors wrote.

The researchers noted that because only the color is changed, every other 3D feature of the original volume-rendered image is maintained.

In clinical evaluation, the CT data sets were independently reviewed by two radiologists experienced in CT colonography evaluation using a modified software program (Lucion, Infinitt). One reading session was performed using the 3D color-coded technique. A second 3D review was performed without the color coding technique.

“Both reviewers preferred the combination of primary 3D interpretation with the discriminative color coding of tagged stool and subsequent limited 2D review to the conventional primary 3D interpretation, because with the former method, many of the repetitive 2D correlations were avoided,” the authors wrote.

The researchers observed that the two review methods were not significantly different in terms of per-lesion sensitivity and per-patient specificity. In addition, they found that the use of the color-coded method reduced interpretation time significantly, by 2.5 to 3 minutes per case.

“Volume rendering with the attenuation-dependent color-enhancement technique facilitates the primary 3D interpretation of CT colonographic images obtained with moderate cathartic preparation, which yields a large number of residual tagged stool, by allowing easier and more time-efficient review of images while maintaining diagnostic accuracy,” the team reported.