The core of the twilite system is a very compact measuring head made from two LYSO crystals enclosed by medical grade tungsten, which is fully MR compatible. The head has an opening into which the catheter templates are inserted to ensure reproducible geometry. There are templates available for different catheter dimensions (currently PE10 0.61 mm OD, PE50 0.96 mm OD, and 2 mm OD to suit common low-volume extension set tubing used in human studies). The illustration below shows the measuring head and a template with a fitted human-size catheter. Note that the (standard) catheter enters the detector head as an unbroken loop, allowing detection of coincidences without any interaction between the twilite and the subject blood.
The scintillations in the LYSO crystals are conveyed to the photon detection unit via two flexible high efficiency light guides. (BGO crystals are also available for specific applications). This design is without any electronics in the sensor head and thus avoids any potential problems of electromagnetic interference with other devices, in particular MR scanners. Furthermore, as the detector itself does not interact with the blood or influence flow this design minimizes any potential risks for use in human research experiments.