Fig. 1

a Example of photomultiplier tubes (PMT)-bismuth germanate (BGO) block detector from a clinical PET system. Readout is performed using the PMTs that are connected to the pixellated scintillator block. Light sharing is used to distribute light originating from a single pixel between the readout PMTs (P1-P4). The position of the incident annihilation photon event can be calculated using an Anger-weighting of the measured signals (b). b Schematics of the detection process from annihilation to stopping the annihilation of photons in the crystal and signal transformation inside the PMT. c Conventional PET detectors (see a) work only outside magnetic fields (B = 0). If a PMT is operated inside a magnetic field (B > 0), then the multiplier step is distorted and the readout map severely distorted. d Avalanche photodiode (APD)-based detectors are semiconductors that can be operated in magnetic fields, even at higher field strengths. Images courtesy Prof. B. Pichler, Tübingen