Measuring sphere-like structures using transrectal ultrasonography

Sphere-like structures are commonly measured for clinical or research purposes. We measured two diameters, the circumference (perimeter) and cross sectional area of 113 equine ovarian follicles in a water bath then determined the relationships between those measurements and the actual volume of fluid in the follicles. Cross-sectional area, diameter3 and area2/3 were all highly correlated with the volume of the structures and superior (p < 0.05) to diameter or perimeter as predictors of volume. As predictors of volume, there was no significant difference (p > 0.05) between cross-sectional area, diameter3, and area2/3. Because of the ease of measuring diameter, we suggest that diameter3 be used to estimate the actual size of sphere-like structures by ultrasonographic measurement. A table was generated for clinical use to show how much the largest diameter of a fluid-filled structure (follicle, cyst, hematoma, etc.) would have to increase or decrease to indicate a significant change in the volume of the structure.