Search

Monolithic scintillator blocks provide an efficient way to significantly increase the sensitivity of high-resolution positron emission tomography (PET) systems. Using statistical or neural network-based positioning schemes, the measured intrinsic detector resolution for perpendicular incident photons is 1.7 mm full-width at half-maximum (FWHM) for 10 mm thick LSO blocks and 2.0 mm FWHM for 20 mm thick LSO blocks. Because the positioning algorithms determine the incidence position as opposed to the combination of interaction position and interaction depth, a very good parallax correction can be achieved. Energy and detector time resolution were 11.5% FWHM and 1.6 ns FWHM, respectively.

The absence of very small crystal pixels in monolithic scintillation detectors has a number of potential advantages such as higher sensitivity, better energy resolution and continuous coordinates. In such detectors, the photon incidence position on the detector surface is derived from the measured scintillation light distribution using artificial neural networks (NNs). To this end, each detector module has to be position-calibrated by training the NNs. An automated procedure to simultaneously obtain the calibration data to train NNs for all detector modules in a fully assembled PET system has been developed and evaluated on a simulator set-up.

We have compared the performance of PET detector modules using continuous scintillator blocks with modules based on a large number of small crystal pixels. We show that a good spatial resolution can be obtained in undivided scintillators. One of the main advantages of the undivided scintillators is the excellent depth of interaction identification capability. There is almost no degradation of the spatial resolution upto incidence angles of 40° away from normal incidence.

The Crystal Clear Collaboration has designed and is building a high-resolution small animal PET scanner. The design is based on the use of the Hamamatsu R7600-M64 multi-anode photomultiplier tube and a LSO/LuYAP phoswich matrix with one to one coupling between the crystals and the photo-detector. The complete system will have 80 PM tubes in four rings with an inner diameter of 137 mm and an axial field of view of 110 mm. The PM pulses are digitized by free-running ADCs and digital data processing determines the gamma energy, the phoswich layer and even the pulse arrival time. Single gamma interactions are recorded and coincidences are found by software. The gantry allows rotation of the detector modules around the field of view. Simulations, and measurements a 2×4 module test set-up predict a spatial resolution of 1.5 mm in the centre of the field of view and a sensitivity of 5.9% for a point source in the centre of the field of view.

A 3D small animal PET scanner, designed and built at the University of Brussel, has been operational since the beginning of 1996. The scanner has a transaxial field of view (FOV) of 110 mm and an axial FOV of 52 mm. The absolute sensitivity is around 35,000 coincidences s−1 MBq−1 for a point source at the center of the scanner and the time resolution is 29 ns FWHM. To measure the achievable spatial resolution, a thin 22Na source was scanned at various distances from the scanner axis. The resolution in a reconstructed image for a source close to the center is 3.0 mm FWHM. This figure can be improved at the expense of the sensitivity by lowering the voltage on the anodes in the wire chamber. Finally, to assess the overall image quality, scans were made of a cylindrical phantom with holes of varying diameter.

This work is part of the design study for a medical positron emission tomography (PET) system based on the use of BaF 2 scintillator and photosensitive wire chambers with TMAE. The number of photoelectrons per MEV (phe/MeV) is measured in the wire chamber and on a photomultiplier for crystals from five different suppliers. For the best crystals 26.7±2.0 phe/MeV in TMAE are observed, corresponding to an average quantum efficiency of TMAE for the fast component of BaF 2 of (7.6±0.7)%. The performance of a PET scanner based on this principle was evaluated with a detailed Monte Carlo simulation. It is shown that the reduced energy resolution with wire chambers has only a marginal effect on the scatter rejection in the scanner. Finally LaF 3 :Nd 3+ as an alternative to BaF 2 was tested and found to give a disappointing photoelectron yield.

This article has been removed, consistent with Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). The Publisher apologizes for any inconvenience this may cause.