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Position-sensitive scintillation-detector arrays (PSSDAs) are used in nuclear-imaging methods such as PET. The kind of technique selected in producing the PSSDA determines the imaging resolution, sensitivity, labor/part cost, and reliability of the system. Production of PSSDA is especially challenging and costly for ultra-high-resolution systems that have large numbers of very small crystal needles, so we developed a new slab-sandwich-slice (SSS) production method. Instead of using individual crystal needles, the construction started with crystal slabs that are 15-crystal-needles wide and 1-needle thick. White-paint was deposited onto slab surfaces to form shaped optical windows. The painted slabs were grouped into two crystal-sandwich types. Each sandwich type consisted of a stack of seven slabs painted with a distinctive set of optical windows, held together with optical glue. For a 40 000-crystal system, only 192 type A and 144 type B sandwiches are needed. Sandwiches were crosscut into another slab formation ("slices"). Each slice was again 1-needle thick; each slice was basically a stack of needles glued together, optically coupled by the glue and the painted windows. After a second set of white-paint optical-windows was applied on the slices' surface, three slices of type B were grouped between four slices of type A to form a 7 × 7 PSSDA. We used SSS production method to build 7 × 7, 7 × 8 and 8 × 8 crystal blocks needed for a high-resolution 12-module prototype PET camera. The method reduced the more than 400 000 precision painting and gluing steps into 55 500 steps for a 40 000-BGO-crystal system, thus lowering the labor cost. The detectors fabricated with the method were of high quality: 2.66 mm × 2.66 mm crystals were separated by only a 0.06-mm gap for a 98% linear detector packing fraction or 96% area packing fraction. Compared to 90% linear-packing (81% area) from conventional methods, the 20% increase in packing density translates into as much as a 1.2 to 1.4 coincidence sensitivity in PET. Crystal cost was halved, and production yield increased to 94%. It generated very small crystal-positioning errors (σ=0.09mm), required for ultrahigh resolution detectors.