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It has become widely recognized, that besides qualitative imaging, the absolute quantification of tissue parameters is essential for the understanding of biological systems. One of the gold standard methods in quantification is PET, which has sensitivity down to the picomolar range, and a unique specifity due to the targeting by molecular probes. Quantitative PET requires the measurement of the arterial input function (AIF), the unchanged radiotracer in arterial plasma, and modeling of the tissue response measured in the PET image.

Swisstrace developed the "twilite" coincidence detector for the online monitoring of the tracer concentration in whole blood. The design of the system was optimized for animal and human research using PET. The twilite features excellent sensitivity, linearity and signal-to-noise, even in the presence of significant external radiation, resulting in whole blood radioactivity curves with high temporal resolution. An experimental setup using an arteriovenous shunt allows arterial input curves to be acquired without blood loss in rodents [1]. The twilite and arteriovenous shunt have been used as gold-standard to image-derived input function in mice [2] and in quantitative imaging of high-grade glioma and radiation necrosis in rats [3]. In humans, the twilite has been used as part of a quantitative perfusion PET protocol for comparison to MRI arterial spin labeling [4]. Version three of the twilite was released in late 2018, and features upgraded base unit ergonomics, improved touch-screen operation and a choice between direct data acquisition or internal storage and FTP-based transfer.

PMOD's PSAMPLE software is the acquisition software of the twilite, and supports the correction and calibration of the raw data (coincidence counts per second) to derive the whole blood tracer activity concentration. It can be easily forwarded into PMOD's Kinetic Modeling tool for fitting, WB-to-plasma and metabolite correction. The resulting AIF is subsequently used for modeling purposes in the PKIN and PXMOD quantification tools.