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FIGURE 1. Diagrammatic representation of geometric center approach for estimating colonic transit time. According to Temple University protocol, colon is subdivided into 6 regions with weighting factors increasing from 1 to 6 proximally to distally (cecum and ascending colon to rectosigmoid). Highest weighting factor (7) is assigned to radioactivity unaccounted for in images, which is therefore assumed to have been evacuated in stools. Each black dot represents 1% (or 0.01 fraction) of radioactivity that has reached colon. Examples of calculation are given in inserts for 2 different geometric centers, 2.50 on left and 5.62 on right. In actual colonic transit study performed on healthy individual, most radioactivity would be seen in transverse colon at 24 h (geometric center, approximately 3), progressing to descending colon at 48 h (geometric center, about 5), and being mostly evacuated at 72 h (geometric center, about 6–7). In patient with colonic inertia, radioactivity would not significantly progress past hepatic flexure at 48 h or at 72 h (geometric centers, about 1.5–2 and 2–3, respectively). In patient with functional rectosigmoid obstruction, progression of radioactivity would be nearly normal until 48 h after ingestion, combined however with little further progression at 72 h (geometric center, about 5–6), thus indicating obstructed defecation.
