CT colonoscopy electronic cleansing software reduces polyp size

Electronic cleansing software algorithms applied to computed tomography colonoscopy (CTC) images display polyps submerged in tagged fecal residue and helps improve colonic evaluation. But in the process, polyp size is significantly reduced, caution radiologists from the Medical University of Vienna in Austria. They recommend that polyp measurements should be performed in unsubtracted colon windows, and describe research findings in the November 2018 issue of European Radiology.

Lead author Christian Bräuer, MD, of the Department of Biomedical Imaging and Image-guided Therapy, and colleagues analyzed a database of 894 colonoscopy-validated CTC datasets of average-risk participants in three clinical colonoscopy screening trials. This data set had 48 polyps completely submerged within tagged fecal residue.

Ten radiologists with moderate experience in CTC independently measured the size of the largest two-dimensional (2D) linear diameter of a polyp in all 894 datasets. Six measurements were made, within three standardized CT-window/level settings both before and after the electronic cleansing algorithm was applied.

The authors calculated differences of polyp size for the three standardized CT-window/level settings.They then averaged the polyp sizes, the reduction of the polyp sizes for different CT windows, and the reduction of the polyp sizes from the electronic cleansing algorithm provided by the 10 radiologists.

The polyps displayed significantly different measurements for each CT window setting, with a mean 9.8 mm in the colon window, 9.9 in the bone window, and 8.2 mm in a soft-tissue window. With the application of the electronic cleansing algorithm, mean polyp size decreased by 0.4 mm in the colon window, 0.8 mm in the bone window, and 1.1 mm in a soft-tissue window.

The authors noted that because polyp size represents the key criterion by which to estimate clinical relevance using CTC criteria, the size of a polyp as measured by a radiologist has a direct influence on the selection of therapeutic procedures. They explained that patients with polyps 10 mm and larger require colonoscopy with resection.

Both electronic cleansing and narrow window-level settings reduced significantly the size measurements of polyps. Polyp size was largest when measured in wide window settings, which the authors attributed to be related to a partial volume effect. The authors believe size reductions caused by the electronic cleansing algorithm occur from a slight transformation of the shape of the line attenuation profile of the polyps during the digital subtraction process.

The size changes in CT windows led to a shift of three large polyps (20%) to the small category and 21 small polyps to the diminutive category. Size reductions from electronic cleansing caused downward shifts in all CT window categories. The combination of changing the window setting and electronic cleansing algorithm utilization increased the shift of polyps to smaller sized categories.

The authors determined that “compared to measurements taken in an unsubtracted colon window, which is generally agreed to display the largest diameter of the polyp most accurately, electronic cleansing, performed in a soft-tissue window, led to a size reduction of 27.6% for polyps equal to or larger than 6 mm and of 13.3 for polyps equal to or larger than 10 mm.

However, the authors reported that there was no relationship between the reduction of polyp size and the CT density of tagged residue, the tube current, and morphological and histological polyp subgroups.

They recommend that radiologists be alert to these phenomena, and to measure submerged polyps on unsubtracted image data in a colon-window setting to avoid underestimation of polyp size.

REFERENCE

1. Bräuer C, Lefere P, Gryspeerdt S, et al. CT colonography: size reduction of submerged colorectal polyps due to electronic cleansing and CT-window settings. Eur Radiol. 2018;28(11):4766-4774.