[en] A properly designed circular ring of NaI(Tl) crystals and suitable reconstruction algorithms can provide rapid, high-resolution transverse section images of positron-labeled compounds in the human body. The spatial resolution is limited by the crystal center-to-center spacing and by the positron range in tissue. Using 280 crystals with 10-mm center-to-center spacing and isotopes such as ⁶⁸Ga, ¹¹C, ¹⁸F, or ⁵²Fe, a resolution of 7.5 mm full width at half maximum (FWHM) is possible. Improvements in this technology will involve larger numbers of smaller crystals, more efficient crystals (possibly bismuth germanate), or faster detectors (such as loaded organic scintillators). 41 references