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Small-animal imaging is rapidly becoming an essential tool for preclinical development of new compounds for imaging and therapy. Although imaging performance evaluations and regular quality control are both important, surprisingly, there are relatively few appropriate phantoms available for such procedures. Phantoms can also be used with liquid or gel dosimeters for measuring radiation doses in different compartments, as well as cross-doses to other compartments. Both imaging and therapy applications, however, could benefit from phantoms with walls that are thinner than those currently available (~1 mm). Using high-resolution stereolithography (SL), we produced phantoms with 2-cm-long cold rods or hot channels ranging from 0.5 to 1.0 mm in diameter. Hollow spheres with 3- mm inner diameter were also produced, with wall thickness from 100 to 350 mum. All phantoms were imaged by muCT, with 27 mum resolution. SL performance was estimated by measuring the dimensions of many structures in the micro-CT images. We also evaluated the degree of water absorption by two different SL resins, Somosreg 11120 and Accurareg 40, after curing. The average bias (and precision) of the cold-rod and hot-channel structures over the size range 0.5 to 1.0 mm, were 0.75% (1.93%) and 0.9% (1.92%), respectively. Water absorption by the resins after 4 hours was minimal (0.1 to 0.15% weight change) for both materials; however, after 66 hours in water, the change was greater for Accurareg 40 (1.5%) than for Somosreg 11120 (0.25%); thus, the latter is a more suitable material for nuclear medicine applications. The muCT images demonstrated that the minimum acceptable spherical wall thickness was ~150 mum; this will allow beta particles from low-energy beta emitters, e.g., Lu-177, Cu-67, or 1-131, to deliver a greater radiation dose to neighboring compartments. SL is a robust and accurate method for fabrication of phantoms with thin walls for small-animal - imaging systems and dosimetry measurements.