AQUA
ADVANCED QUALITY ASSURANCE:
Positron Emission Tomography with Crystals CRYSTAL - PET

Positron-emitting isotopes are produced all along the primary hadron beam by interaction with the tissues; the positrons annihilate with an electron producing two 511 keV photons, anti-parallel in the positron frame and almost co-linear in the laboratory frame. The photons have a mean attenuation length of ~ 10 cm in water, so most are absorbed by surrounding tissues and typically only 15-20% emerge from the body over the solid angle. Due to the relatively small production rate and fast decay time of the isotopes, on-beam PET imaging has low event rates and is very demanding on efficiency and solid angle covered by the detectors.
Inorganic scintillator crystals are the most commonly used detectors for PET. Annihilation photons interact within the scintillator through Compton and photoelectric effects, releasing energy to the detector. The absorbed energy causes the crystal lattice to make a transition to higher energy state, from which it may decay emitting lower energy photons. The scintillation is converted into measurable signals usually using photomultiplier tubes (PMTs). The high gain, stability and low noise of PMTs have rendered these devices the standard ones used in PET scanners. The most common detector configuration used in commercial scanners couples each detector block to four PMTs with light sharing, providing the position of the conversion with an accuracy of a few mm or better. The development of multi-anode position-sensitive photomultiplier tubes (PSPMTs) of square shape allows accurate positional and energy information to be derived from the PMT itself.
The basic approach is to use thin and long scintillator crystals in contact with a multi-anode photomultipier; the light emitted by a gamma conversion in the crystal produces signals on several adjacent anode pads; analysis of the signal profile allows to reconstruct both the position of the conversion in the plane of the sensor, and the depth of interaction in the direction perpendicular to the anodes. Results of simulation and measurements with a detector, oparated in coincidence with a small crystal to detect the 511 keV photins from a 22Na source are given un the CNAO report.

Single-head PET setup	Correlation betweenreal and measured gamma conversion coordinate (X-direction, along pad rows)	Correlation between real and measured Depth of Interaction (Z-direction, perpendicular to anode pad plane)

A dual-head system making use of a Micro-Channel Multi-Anode Photomultiplier and LYSO cristals has been built and tested, and provides reasonable results in terms of position accuracy and time resolution: