Townsend began his work on PET instrumentation development in the early eighties, designing and building the first rotating partial ring PET scanner using bismuth germanate–or BGO–block detectors. This necessitated new reconstruction algorithms, new approaches to detector normalization, close attention to scatter correction, and much work to optimize and demonstrate the value of imaging in 3D. Townsend’s contributions to 3D PET imaging were influential in improving the signal-to-noise of reconstructed PET images without increasing the amount of detector material, which to a large extent determines cost. As a result, the impact on the overall cost-effectiveness of PET as a clinical tool has been substantial.