Timing pickoff circuit

#According to conventional positron-emission-tomography (PET) imaging, a radionuclide tracer is introduced into a patient body. Radioactive decay of the tracer generates positrons which eventually encounter electrons and are annihilated thereby. An annihilation produces two photons which travel in approximately opposite directions.

#A ring of detectors surrounding the body detects the photons, identifies “coincidences” based thereon, and reconstructs PET images based on the identified coincidences. A coincidence is identified when two photons arrive at two detector elements within a particular coincidence time window. Because the two “coincident” photons travel in approximately opposite directions, the locations of the two detector elements determine a Line-of-Response (LOR) along which an annihilation may have occurred. Time-of-flight (TOF) PET additionally measures the difference between the detection times of the two photons arising from the annihilation. This difference may be used to estimate a particular position along the LOR at which the annihilation event occurred. Both coincidence detection and TOF measurements require extremely accurate and consistent determination of photon arrival times.

#Arrival of a photon at a detector element causes a transducer to generate an electrical signal, or pulse. Generally, a photon arrival time (i.e., “event time”) is identified as a time at which the generated pulse crosses a threshold voltage. A coincidence is identified when the event times determined from two generated pulses are within the coincidence time window.

#FIGS. 1A and 1B illustrate detection of coincidences according to some embodiments.

#FIG. 2 is a block diagram of a coincidence determination system according to some embodiments.

#FIG. 3 illustrates a detector block according to some embodiments.

#The following description is provided to enable any person in the art to make and use the described embodiments. Various modifications, however, will remain apparent to those in the art.

#The present inventors have discovered that the performance of existing timing pickoff circuits for PET detectors is limited due to power mismatching (i.e., both impedance mismatching and noise mismatching) between the very low (e.g., a few ohms) “source impedance” of a transducer array (e.g., several transducers connected in parallel) disposed in a “common cathode” arrangement and higher (e.g., 50 ohm) input impedance of the following gain stage. Impedance mismatching causes most of the timing signal to be “self-absorbed” in the transducer array. Consequently, only a small fraction of the signal is output, and this small fraction exhibits low signal amplitude and poor signal-to-noise ratio. In the high frequency signal region, the mismatch is particularly troublesome since the high frequency components of the timing signals are crucial for accurate PET timing pickoff.

#According to some embodiments a “buffer stage” is added to the cathode output of each transducer of an array of transducers (e.g., 4×4 array). outputs of the buffer stages are summed together to form a single timing signal. This timing signal is sent to a leading edge discriminator (LED) circuit and a time-to-digital converter (TDC) to resolve event times based on the timing signal.