Feasibility of using beta-gamma coincidence for 3D PET attenuation correction

Better attenuation correction is needed in 3D PET in order to reduce image noise. It is well known that positrons must lose an appreciable amount of energy before annihilation with an electron can occur. If this energy is lost in a plastic scintillator or PIN photo-diode it can provide a "start pulse" to a coincidence circuit, while the conventional PET detectors provide a stop pulse and identify the line of response. Cheaper, non-positron emitting isotopes, which decay by beta-gamma cascade (e.g. Co-60) could also be used. The authors have shown that the energy loss of positrons emitted from F-18 can be detected with a PIN diode. The recorded energy is above the noise for most events, and the timing spectrum from a PIN diode detecting the positron and an LSO crystal detecting the gamma ray show excellent timing resolution. A narrow coincidence window allows high count rates. Multiple sources and post-injection scans can be used. Based on preliminary data presented here it should be possible to adapt this technique to modern PET scanners which now use 2D acquisitions, and extended septa only for performing transmission scans.