Yttrium-90 is widely used to label radiopharmaceuticals with curative, or at least palliative intent. At present, both the quantitative assay, /spl plusmn/10% variation, and the in vivo imaging observe bremstrahllung from the 2.3 MeV /spl beta//sup -/. Detection of the positron, which is created in the E/sub 0/-decay of the 1.78 MeV excited state with a 34/spl plusmn/4 ppm branching ratio, are resolved with two detector systems. A high purity Ge spectrometer with calibrated efficiency clearly resolves the 511 keV annihilation with a 2/1 peak over the continuous bremstrahllung, providing /spl plusmn/5% precision in 10 min of multichannel analysis. Secondly, a simple NaI detector pair in fast coincidence yields similar precision, with trues/randoms /spl ap/1000/1 evident in the time spectrum and the on/off-axis counting rate. The Ge detector, more tolerant to distributed sources, was used to assay the filling of a micro-Derenzo phantom, charged with 50 MBq/ml of Y-90 chloride, imperfectly stripped from unused /spl mu/-spheres. Phantoms were scanned with a Concorde /spl mu/PET P4 and a CTI 933/04. The phantom's 1.6-2.4-3.2-4.0-4.8 mm holes are clearly resolved, and participate residue is evident as a settled deposit. The /spl mu/PET sensitivity to a centered 37 MBq point source of Y-90 is /spl ap/24 cps trues, 37 cps randoms and 270 kcps singles, with the 4-decade drop being the result of the miniscule positron branching. Imaging is slow, but the promise of quantitative dosimetry is essential for a rational application of Y-90 radiotherapeutic agents.