I. Optimize the pocket phantom source design and the bias estimation algorithm: determine the tolerances and variations in the source sphere geometry and radioactive filing process; evaluate the need for higher-activity sources in the presence of increased attenuation (i.e. thicker patients); optimize CT contrast levels for source boundary detection and localization from CT images; evaluate the robustness for use with different scanner parameters (e.g. CT slice thicknesses); modify bias estimation algorithm as needed to take into account changes.