Determining the true size of a polyp found during a CT colonoscopy procedure is of considerable clinical importance, because the size of the polyp serves as a rough indicator of the risk of carcinoma. Polyp size measured on 3D endoluminal views with CT colonography is substantially affected by the threshold value for surface rendering, according to a recent article published in the journal Radiology.
“Although the current consensus proposal recommends that the single greatest dimension of the polyp should be obtained regardless of the use of 2D or 3D images, 3D endoluminal measurement is likely to have the advantage of being more suited in finding the long axis of the polyp, as one may instinctively visualize the long axis of the polyp by using the endoluminal view,” the authors of the multinational study wrote.
Researchers from the University of Ulsan College of Medicine and Hallym University College of Medicine in Seoul, Korea, and Weill Medical College in New York City, conducted automated and manual polyp measurements with endoluminal views in an effort to determine the optimal surface rendering threshold.
“Of the various factors that affect the accuracy of polyp measurement on endoluminal images, the threshold value for endoluminal rendering is most likely a critical factor,” they wrote.
The group conducted its study using pig colonic specimens as well as in vivo human polyps. CT exams were performed with a 16-slice system (Somatom Sensation, Siemens Medical Solutions). Manual 3D endoluminal polyp measurement was performed using the AW4.2_06 application (GE Healthcare). Automated measurement was performed using the syngo Colonography PEV application (Siemens).
A radiologist obtained measurements of the porcine-simulated and human polyps by using the automated software and by manually measuring them at four selected surface-rendering threshold values of -800, -700, -600, and -500 Hounsfield Units (HU). The interpreting clinician was blinded to the threshold value and the size range and specific size of polyps but was informed of the location of each polyp, according to the authors.
The team reported that there was a general decline in polyp size with manual measurement as the threshold value changed from -800 to -500 HU on both the porcine and human polyp models. The scientists noted that this pattern was in accordance with what was expected on the basis of the principles of surface rendering.
Their research found that the optimal surface-rendering threshold value for accurate polyp measurement by using endoluminal views is approximately -500 HU.
“In CT colonographic screening protocols where polyp size dictates clinical treatment, this overlooked source of measurement error caused by surface-rendering threshold values for 3D visualization can cause problems,” the authors wrote. “The clinical consequence of this error is overt, especially in patients who have polyps with sizes that are on the borderline between size categories (i.e., ≤5 mm, 6–9 mm, and ≥10 mm); in these patients, a slight size disparity may lead to changes in polyp size categorization, as indicated with our data.
The researchers observed that various CT colonographic systems may use different techniques for endoluminal visualization and that their results may be vendor-specific and may not be directly applicable to other systems.
Their study indicated that automated CT colonographic systems produced similar polyp measurement results, with a few caveats.
“Automated measurement with the CT colonographic system evaluated is completely reproducible and agrees closely with the manual measurement at the optimal surface-rendering threshold value for well-circumscribed smooth rounded polyps; however, such measurement is prone to macroscopic error especially in polyps that are large, lobulated, irregularly shaped, or located adjacent to a bulbous edge of the haustral fold,” they wrote.