Silicon photomultipliers (SiPMs) are of great interest to positron emission tomography (PET), as they enable new detector geometries, for e.g., depthof-interaction (DOI) determination, are MR compatible, and offer faster response and higher gain than other solid-state photosensors such as avalanche photodiodes. Here we present a novel detector design with DOI correction, in which a position-sensitive SiPM array is used to read out a monolithic scintillator. Initial characterization of a prototype detector consisting of a 4 × 4 SiPM array coupled to either the front or back surface of a 13.2 mm × 13.2 mm × 10 mm LYSO:Ce 3+ crystal shows that front-side readout results in significantly better performance than conventional back-side readout. Spatial resolutions <1.6 mm full-width-at-half-maximum (FWHM) were measured at the detector centre in response to an ∼0.54 mm FWHM diameter test beam. Hardly any resolution losses were observed at angles of incidence up to 45• , demonstrating excellent DOI correction. About 14% FWHM energy resolution was obtained. The timing resolution, measured in coincidence with a BaF 2 detector, equals 960 ps FWHM.

The timing performance of scintillation detectors is ultimately limited by photon counting statistics. In fact, photon counting statistics form a dominant contribution to the overall timing resolution of many state-of-the-art detectors. A common approach to investigate this contribution is to calculate the variance in the registration times of individual scintillation photons within the photosensor. However, in general the single-photon variance is not equal to the intrinsic limit on the timing resolution, since in principle one can make use of the timing information carried by all photons detected. In this work, the Cramér-Rao lower bound on the timing resolution of a scintillation detector, based on the information contained in the full set of registered photons, is calculated. The results appear to be in good agreement with trends observed in the literature. Furthermore, it is shown that the timestamp obtained from any single scintillation photon never yields the optimum timing resolution for realistic scintillation detectors. Yet, it appears that the intrinsic timing resolution limit can be approached closely by making use of the timestamps from a relatively small number of photons emitted during the initial part of the scintillation pulse.

The use of time-of-flight (TOF) information in positron emission tomography (PET) enables significant improvement in image noise properties and, therefore, lesion detection. Silicon photomultipliers (SiPMs) are solid-state photosensors that have several advantages over photomultiplier tubes (PMTs). SiPMs are small, essentially transparent to 511 keV gamma rays and insensitive to magnetic fields. This enables novel detector designs aimed at e.g. compactness, high resolution, depth-of-interaction (DOI) correction and MRI compatibility. The goal of the present work is to study the timing performance of SiPMs in combination with LaBr 3 :Ce(5%), a relatively new scintillator with promising characteristics for TOF-PET. Measurements were performed with two, bare, 3 mm × 3 mm × 5 mm LaBr 3 :Ce(5%) crystals, each coupled to a 3 mm × 3 mm SiPM. Using a 22 Na point source placed at various positions in between the two detectors, a coincidence resolving time (CRT) of ∼100 ps FWHM for 511 keV annihilation photon pairs was achieved, corresponding to a TOF positioning resolution of ∼15 mm FWHM. At the same time, pulse height spectra with well-resolved full-energy peaks were obtained. To our knowledge this is the best CRT reported for SiPM-based scintillation detectors to date. It is concluded that SiPM-based scintillation detectors can provide timing resolutions at least as good as detectors based on PMTs.

Digital silicon photomultiplier (dSiPM) arrays have favorable characteristics for application in monolithic scintillator detectors for time-of-flight positron emission tomography (PET). To fully exploit these benefits, a maximum likelihood interaction time estimation (MLITE) method was developed to derive the time of interaction from the multiple time stamps obtained per scintillation event. MLITE was compared to several deterministic methods. Timing measurements were performed with monolithic scintillator detectors based on novel dSiPM arrays and LSO:Ce,0.2%Ca crystals of 16 × 16 × 10 mm(3), 16 × 16 × 20 mm(3), 24 × 24 × 10 mm(3), and 24 × 24 × 20 mm(3). The best coincidence resolving times (CRTs) for pairs of identical detectors were obtained with MLITE and measured 157 ps, 185 ps, 161 ps, and 184 ps full-width-at-half-maximum (FWHM), respectively. For comparison, a small reference detector, consisting of a 3 × 3 × 5 mm(3) LSO:Ce,0.2%Ca crystal coupled to a single pixel of a dSiPM array, was measured to have a CRT as low as 120 ps FWHM. The results of this work indicate that the influence of the optical transport of the scintillation photons on the timing performance of monolithic scintillator detectors can at least partially be corrected for by utilizing the information contained in the spatio-temporal distribution of the collection of time stamps registered per scintillation event.

The organometallic precursor (NEt(4))(2)[ReBr(3)(CO)(3)] was reacted with bidendate dithioethers (L) of the general formula H(3)C-S-CH(2)CH(2)-S-R (R = -CH(2)CH(2)COOH, CH(2)-C&tbd1;CH) and R'-S-CH(2)CH(2)-S-R' (R' = CH(3)CH(2)-, CH(3)CH(2)-OH, and CH(2)COOH) in methanol to form stable rhenium(I) tricarbonyl complexes of the general composition [ReBr(CO)(3)L]. Under these conditions, the functional groups do not participate in the coordination. As a prototypic representative of this type of Re compounds, the propargylic group bearing complex [ReBr(CO(3))(H(3)C-S-CH(2)CH(2)-S-CH(2)C&tbd1;CH)] Re2 was studied by X-ray diffraction analysis. Its molecular structure exhibits a slightly distorted octahedron with facial coordination of the carbonyl ligands. The potentially tetradentate ligand HO-CH(2)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(2)-OH was reacted with the trinitrato precursor [Re(NO(3))(3)(CO)(3)](2-) to yield a cationic complex [Re(CO)(3)(HO-CH(2)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(2)-OH)]NO(3) Re8 which shows the coordination of one hydroxy group. Re8 has been characterized by correct elemental analysis, infrared spectroscopy, capillary electrophoresis, and X-ray diffraction analysis. Ligand exchange reaction of the carboxylic group bearing ligands H(3)C-S-CH(2)CH(2)-S-CH(2)CH(2)-COOH and HOOC-CH(2)-S-CH(2)CH(2)-S-CH(2)-COOH with (NEt(4))(2)[ReBr(3)(CO)(3)] in water and with equimolar amounts of NaOH led to complexes in which the bromide is replaced by the carboxylic group. The X-ray structure analysis of the complex [Re(CO)(3)(OOC-CH(2)-S-CH(2)CH(2)-S-CH(2)-COOH)] Re6 shows the second carboxylic group noncoordinated offering an ideal site for functionalization or coupling a biomolecule. The no-carrier-added preparation of the analogous (99m)Tc(I) carbonyl thioether complexes could be performed using the precursor fac-[(99m)Tc(H(2)O)(3)(CO)(3)](+), with yields up to 90%. The behavior of the chlorine containing (99m)Tc complex [(99m)TcCl(CO)(3)(CH(3)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(3))] Tc1 in aqueous solution at physiological pH value was investigated. In saline, the chromatographically separated compound was stable for at least 120 min. However, in chloride-free aqueous solution, a water-coordinated cationic species Tc1a of the proposed composition [(99m)Tc(H(2)O)(CO)(3)(CH(3)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(3))](+) occurred. The cationic charge of the conversion product was confirmed by capillary electrophoresis. By the introduction of a carboxylic group into the thioether ligand as a third donor group, the conversion could be suppressed and thus the neutrality of the complex preserved. Biodistribution studies in the rat demonstrated for the neutral complexes [(99m)TcCl(CO)(3)(CH(3)CH(2)-S-CH(2)CH(2)-S-CH(2)CH(3))] Tc1 and [(99m)TcCl(CO)(3)(CH(2)-S-CH(2)CH(2)-S-CH(2)-C&tbd1;CH)] Tc2 a significant initial brain uptake (1.03 +/- 0.25% and 0.78 +/- 0.08% ID/organ at 5 min. p.i.). Challenge experiments with glutathione clearly indicated that no transchelation reaction occurs in vivo.