Performance Upgrade of the Small Animal PET/SPECT Scanner

YAP-(S)PET+ Scanner Design Improvement

The new detector design is aimed at the improvement of both sensitivity and spatial resolution of the YAP-(S)PET without increasing the scanner complexity.
The idea is to reduce the head-to-head distance, i.e. scanner "diameter", from 15 cm down to 10 cm or less. This will strongly increase the solid angle coverage of each head, thus improving the sensitivity. In order to limit the higher image blurring, due to the parallax effect, we are reducing the crystal thickness.
In order to further improve the spatial resolution we will make use of YAP:Ce crystal pixels with a reduced pitch (1.5 mm x 1.5 mm) without changing the photodetection system. Preliminary laboratory tests have demonstrated that the Hamamatsu R2486 PS-PMT, read out with a resistive chain and our readout circuitry, is able to identify each 1.5 mm pixel with a reasonable resolution, even at typical SPECT energies.
Figure 1 shows the flood-field image of a 8 x 8 YAP:Ce crystal matrix (1.5 mm pitch) obtained with the R2486 PS-PMT as used in the YAP-(S)PET scanner. The matrix was uniformly illuminated with a 140 keV ^99mTc source.
The larger solid angle coverage of each head exposes the system to a higher pile-up effect. For this reason we have design a faster readout electronics that includes a 100% pile-up rejecting circuitry. The new electronics also offers better count rate performance as required due to the increased sensitivity.

Fig. 1a. Flood field image at 140 keV of the 8 x 8 YAP:Ce crystal matrix (1.5 mm pitch) obtained with the R2486 PS-PMT.

Fig. 1b. Flood field image at 140 keV of the 8 x 8 YAP:Ce crystal matrix (1.5 mm pitch) obtained with the R2486 PS-PMT.

The spatial resolution in SPECT modality will also improve by reducing the distance between the detector heads, as the spatial resolution obtained with parallel hole collimators is improved by reducing the distance between object and the collimator itself.

Work in progress: YAP-(S)PET+ Performance Evaluation

The new YAP-(S)PET+ scanner with the above described performance improvement will be assembled and fully tested to verify the expected performance upgrade. The tentative NEMA standard for small animal PET scanner performance evaluation will be used to this aim. The performance evaluation will be presented.
The evaluation of the effectiveness of the new pile-up rejection technique is in progress on the comparison of the uniformity of the reconstructed images of an uniform phantom, at various count rates, with and without the pile-up rejection.

Literature

S. Moehrs, M. Defrise, N. Belcari, A. Del Guerra, A. Bartoli, S. Fabbri and G. Zanetti, "Multi-Ray based system matrix generation for 3D PET reconstruction", Physics in Medicine and Biology, 53(23): 6925 - 6945.

N.Belcari, A. Del Guerra, A.Bartoli,, D.Bianchi, M.LAzzarotti, L.Sensi, L.Menichetti, M.Lecchi, P.Erba, G.Mariani, G.U.Corsini. P.Sgado, "Evaluation of the performance of the YAP-(S)PET scanner and its application in neuroscience", Nuclear Instr Meth A 2007, 571, 18-21.

A.Del Guerra, A.Bartoli, N.Belcari, D.Herbert, A.Motta, A.Vaiano, G.Di Domenico, N.Sabba, E.Moretti, G.Zavattini, M.Lazzarotti, L.Sensi, M.Larobina, "Performance Evaluation of the fully engineered YAP-(S)PET Scanner for Small Animal Imaging", IEEE Trans. Nucl. Sci. (2006) Vol. 53(3), 1078-1083.

A. Del Guerra, C. Damiani, G. Di Domenico, A. Motta, L. Sartori, G. Zavattini, "An integrated PET-SPECT small animal imager: preliminary results", IEEE Trans Nucl Sci 47 1537-1540 (2000).

A. Del Guerra, Di Domenico, M. Scandola, G. Zavattini, "High spatial resolution small animal YAPPET", Nucl. Instr. and Meth. Phys. Res. A409 537-541 (1998).

I.S.E. Ingegneria dei Sistemi Elettronici s.r.l., Vecchiano, Pisa, Italy, Web Page.