vertor (ADC) modules and a PXI (the Compact PCI standard from. National Instruments) enclosure for the data readout from the ADCs. Two interface modules ...
Chapter 10 RECONSTRUCTION ALGORITHM WITH RESOLUTION DECONVOLUTION FOR 3-D IMAGE IN A SMALL ANIMAL PET IMAGER ————–
E. N. Tsyganov, A. I. Zinchenko, N. V. Slavine, P. P. Antich, S. Y. Seliounine, O. K. Oz, P. V. Kulkarni, M. A. Lewis, R. P. Mason, R. W. Parkey The University of Texas Southwestern Medical Center at Dallas
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1.
Experimental Set-Up
A small Animal PET Imaging device has been developed at the University of Texas Southwestern Medical Center at Dallas using scintillating 1 mm round BCF-10 fibers and small admixture of CsF microcrystals between the fibers[1]. The fiber core is polystyrene (C8 H8 )n doped with butyl-PBD and dPOPOP. The fiber are clad in a nonscintillating lucite cladding. The scintillation mechanism can be either from the excitation of π-electrons in the butyl-PBD benzene ring in the fiber, or from excitation within the micro-crystals. In both cases, the emitted light is compatible with the optimal spectral response of standard photomultiplier cathodes. For a 511 keV photon in plastic, the photo-absorption is small and Compton scatter interactions are dominant. The scattered electrons give up their energy well within a fiber diameter, but wave-shifting produces light in proximal fibers. The current imager uses the 2-fold coincident detection of a single event in 2 orthogonal fibers of 1 mm diameter to detect the location and the energy transferred at a point within the detector. Two sets of fibers each 60 cm in length and 1 mm in diameter, were used to construct two alternating, mutually orthogonal sets of 14 planar arrays of 135 fibers each. In this detector, the planar fiber arrays are arranged along two alternat-
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ii ing mutually orthogonal (X and Y) axes, and are stacked along a third (Z). Scintillating light from the fibers is detected by two (X and Y directions) Hamamatsu R-2486 Position Sensitive Photomultiplier Tubes (PSPMT). A single ended readout scheme is used, where the X,Z and Y,Z interaction positions in a detector are determined from coincident detection in the two PSPMT. The precision of the detection of the interaction point depends upon PSPMT performance and software filters. The current Data Acquisition (DAQ) system is based on a multistandard platform: a custom backplane for the Analog-to-Digital Convertor (ADC) modules and a PXI (the Compact PCI standard from National Instruments) enclosure for the data readout from the ADCs. Two interface modules (PXI-6508 for slow control, and PXI-6533 for the fast data transfer) are included in this enclosure. The current data transfer rate is about 6 MB/s (∼40 K events per second), but we expect a final data transfer rate of 40 MB/s. For PET imaging two planar detectors are required. Each planar detector is positioned to measure one of the two 511 keV annihilation photons. By requiring a coincidence between the two detectors (i.e. 4 PSPMT), the position of an electron-positron interaction can be reconstructed. The two detectors can be rotated around the central axis to approximate a truncated cylindrical detector. The performance of the system is shown in Table 1. The object spatial resolution is unchanged across the entire field of view, while the sensitivity varies between 40% and 100% of the central maximum over a 10 x 10 cm2 field of view. The data in Table 1 shows that the system has both high resolution and sensitivity, and achieves a level of performance comparable to that of other current animal imaging systems [1]. These results are significant considering the construction from non-standard materials, the novel design, and the fact that only two of the four detectors necessary for closed 2π geometry have been completed at this time. Noise reduction avails itself of the very fast scintillation time of plastic (1-2 ns), which permits a narrow coincidence window. Software algorithms have also been implemented to further reduce noise on the data itself.
2.
List-mode EM Algorithm With Convolution Model
In order to fully exploit the advantages offered by the experimental apparatus, an appropriate image reconstruction method should be used. The method should also offer reasonable processing time and manageable data volumes for practical applications. One of the most attractive approaches appears to be the recently
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iii Table 10.1.
PERFORMANCE CHARACTERISTICS OF THE SYSTEM Detector type
Scintillating optical fibers and CsF micro-crystals in Teflon matrix 135 x 135 x 28 (3780 fibers) 4 Position-sensitive PMTs 128 channels Variable, 15 to 45 cm
