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We describe a radiotracer imaging system for measuring the biochemical production rates of organic compounds from animals or plants. It uses a high performance liquid chromatography (HPLC) column to separate the compounds and a parallel-plane radiation detector to measure the disintegrations from each compound over a period of time. Because the measurement time is much longer than conventional techniques, the sensitivity is greatly improved. This high-sensitivity radiation detector can be used to image radioactivity in the HPLC flow cell or column and can be used for a variety of analytical HPLC applications. The detector is comprised of 8 Siemens ECAT EXACT HR+ PET detector modules arranged into a parallel plane and read out with modified Siemens ECAT HRRT electronics. This high-sensitivity radiation detector was placed in line after a conventional HPLC radiation detector (a small CsI:Tl scintillator crystal coupled to a PIN photodiode) to allow a direct comparison. If we inject 9.3 μCi of [18F]FDG into the system, we see consistently shaped peaks with an excellent signal-to-noise ratio from both radiation detectors. If we inject only 5.4 nCi of [18F]FDG, we measure a signal-to-noise ratio of 28:1 with the high-sensitivity radiation detector and 5:1 with the conventional radiation detector. We have therefore achieved a sensitivity gain of 32 at low radioactivity concentrations using our high-sensitivity radiation detector compared to a conventional radiation detector. We believe that a high-sensitivity radiation detector, using parallel-plane PET detector modules, could become an important tool for analytical HPLC research.