In PET imaging a radiotracer is administered to a subject such as a patient or an animal prior to its positioning in a PET imaging region. The radiotracer is preferentially absorbed by regions in the subject and its distribution is imaged following an uptake period. Subsequently a clinician interprets the relative uptake in particular regions in the images and may perform a diagnosis of the subject. The radiotracer undergoes radioactive decay which results in the production of positrons. Each decay event produces one positron which travels up to a few millimeters in human tissue where it subsequently interacts with an electron in an annihilation event that produces two oppositely-directed gamma photons. The two gamma photons each have an energy of 511 keV and are subsequently detected by gamma photon detectors disposed radially around the PET imaging region which each produce an electrical signal when struck by an incident gamma photon. In a gamma photon detector, defined herein to comprise a scintillator element in optical communication with an optical detector, the scintillator element converts the high energy gamma photon into a scintillation light pulse comprising a number of optical photons, and the electrical signal is generated by the optical detector. A timestamp is issued to each electrical signal by a timestamping unit and compared to other timestamps in a coincidence determination unit. Two gamma photons are identified as coincident if their timestamps occur within a narrow time interval of each other; typically if they are within +/−3 ns. The positions of the two detectors receiving the coincident gamma photons define a line in space along which the annihilation event occurred, the line being termed a line of response (LOR). Such LORs are subsequently reconstructed to produce an image illustrative of the distribution of the radiotracer within the imaging region.
In such systems the identification of pairs of gamma photons as coincident events is often further supported by an integration unit. The integration unit computes the energy of each incident gamma photon by integrating the total number of optical photons present in each scintillation light pulse. If the energies of each of the time-wise coincident gamma photons are within a predetermined range that is characteristic of a gamma photon the time-wise coincident scintillation light pulses are processed as coincident events. However, if the energy of one or both of the gamma photons lies outside the predetermined range the pair of time-wise coincident events are rejected. Such rejected events may be the consequence of gamma photon scattering; a phenomenon which changes the gamma photon trajectory as well as its energy and therefore leads to an erroneous LOR.
The integration and timing of light pulses from directly-detected radiation quanta in applications such as microscopy and Cherenkov radiation detection is carried out in much the same way. In fluorescence microscopy for example the optical photons are detected directly, thus in the absence of a scintillator element. Cherenkov radiation is likewise detected directly, thus in the absence of a scintillator element, the optical photons being generated by a dielectric medium.
The integration and timing of light pulses is therefore a common feature of such imaging systems. Both factors affect the image resolution of images generated by such systems. Conventionally an array of optical detectors is used to detect such a light pulse. The scintillation light pulse is distributed across the detectors in the array, each of which is typically capable of distinguishing the detection of individual optical photons, and the signals from the detectors are analyzed by the separate integration and timing units. Silicon photomultiplier (SiPM) arrays, and single photon avalanche diode (SPAD) arrays have both been used in this respect. Both analogue and digital SiPMs and SPADs have been used, wherein the analogue devices generate an avalanche current pulse in response to the detection of an optical photon, and the digital devices include additional electronic circuitry which causes an output signal to transition between two voltage levels.
Patent application WO2006/111883A2 discloses a circuit for use with a SiPM detector array in integrating and timing such light pulses. In WO2006/111883A2, digital triggering circuitry is configured to output a trigger signal indicative of a start of an integration time period responsive to a selected number of detector cells transitioning from a first digital value to a second digital value. Readout digital circuitry accumulates a count of a number of transitions of detector cells of the array of detector cells from the first digital state to the second digital state over the integration time period.
Another, more conventional, circuit used with analogue silicon photomultiplier (SiPM) optical detectors in connection with integrating and timing optical pulses is disclosed in document Photo-Detectors for Time of Flight Positron Emission Tomography (ToF-PET), Spanoudaki and Levin, Sensors 2010, 10, 10484-10505. In this document, a group of analogue SiPM detectors are connected in parallel to generate a composite signal from such an optical pulse. The composite signal may then be integrated, and may cause a timing unit to generate a timestamp when a threshold level is exceeded.
However there remains room for improvement in terms of the integration of the number of optical photons in such an optical pulse, and further in the timing of the optical pulse.