SPECT is a powerful clinical tool. However, the low spatial resolution and ill-defined boundaries associated with SPECT require special consideration in visualization. Quantitative geometric and magnitude information are areas of particular usefulness in evaluating disease states. In this paper, we describe a set of practical 3D visualization tools to display and analyze SPECT data, and present interactive methods to measure (1) the relative position, size and shape of regions of interest and (2) the magnitude and distribution of radioactive count information. Interactive pick tools allow users to extract values at selected points, distance between points, or value profiles along selected line segments. In the three-dimensional reconstruction, transparent and opaque isosurfaces are formed simultaneously at specified activity levels, and the volume enclosed by the opaque surface is displayed. The utility of these tools is demonstrated with two types of patient studies: those using tumor-avid agents to identify active tumor in the chest and abdomen, and those used for evaluating the volume of perfused myocardium.