Yttrium-90 is a promising radionuclide for radioimmunotherapy of cancer because of its energetic beta emissions. Therapeutic management requires quantitative imaging to assess the pharmacokinetics and radiation dosimetry of the 90Y-labeled antibody. Conventional gamma photon imaging methods cannot be easily applied to imaging of 90Y-bremsstrahlung because of its continuous energy spectrum. Methods: The sensitivity, resolution and source- to-background signal ratio (S/B) of the detector system for 90Y- bremsstrahlung were investigated for various collimators and energy windows in order to determine optimum conditions for quantitative imaging. After these conditions were determined, the accuracy of quantitation of 90Y activity in an Alderson abdominal phantom was examined. Results: When the energy-window width was increased, the benefit of increased sensitivity outweighed degradation in resolution and S/B ratio until the manufacturer's energy specifications for the collimator were exceeded. Using the same energy window, we improved resolution and S/B for the medium-energy (ME) collimator when compared to the low-energy, all-purpose (LEAP) collimator, and there was little additional improvement using the high-energy (HE) collimator. Camera sensitivity under tissue equivalent conditions was 4.2 times greater for the LEAP and 1.7 times greater for the ME collimators when compared to the HE collimator. Thus, the best, most practical selections were found to be the ME collimator and an energy window of 55-285 keV. When we used these optimal conditions for image acquisition, the estimation of 90Y activity in organs and tumors was within 15% of the true activities. Conclusions: The results of this study suggest that reasonable accuracy can be achieved in clinical radioimmunotherapy using 90Y-bremsstrahlung quantitation.