Activity per unit volume: a pitfall in predicting size.

TO THE EDITOR: A number of texts and articles indicate that dilatation of the hepatobiliary ducts may be suggested from findings on the HIDA study. This should be tempered by the well-known fact that an increase in radioactivity per unit volume in an object makes the image of an object appear larger (1). When there is stasis in the hepatobiliary tree, not necessarily accompanied by dilatation this condition is fulfilled as shown in Fig. 1. Figure 2 shows the image of four capillary tubes filled with the same volume of fluid but differing in activity per unit volume by factors of two. While the images suggest differing sizes, the objects are, in fact, identical.