Oxyorthosilicates

New materials with higher sensitivity or resolution have been designed A 3-dimensional PET scanner using gadolinium oxyorthosilicate (GSO) crystals (Surti). 

The detection efficiency of a detector is another important property in PET technology. Since it is desirable to have shorter scan times and low tracer activity for administration, the detector must detect as many of the emitted photons as possible. The 511-keV photons interact with detector material by either photoelectric absorption or Compton scattering, as discussed in Chap. 1. Thus, the photons are attenuated (absorbed and scattered) by these two processes in the detector, and the fraction of incident 7 rays that are attenuated is determined by the linear attenuation coefficient (/x) given in Chap. 1 and gives the detection efficiency. At 511 keV, /x = 0.92 cm-1 for bismuth germanate (BGO), 0.87 cur1 for lutetium oxyorthosilicate (LSO), and 0.34 cm-1 for Nal(Tl) (Melcher, 2000). Consequently, to have similar detection efficiency, Nal(Tl) detectors must be more than twice as thick as BGO and LSO detectors. 

A recently introduced detector, yttrium-activated lutetium oxyorthosilicate (LYSO), has the physical properties similar to LSO and has been used in PET scanners by a commercial vendor. 

Cerium-doped yttrium oxyorthosilicate (YSO) is a new type of detector, but no commercial manufacturer has yet used it in PET technology. 

Bushberg JT, Seibert JA, Leidholdt Sr EM, Boone JM (2002). The essential physics of medical imaging, 2nd ed. Lippincott, Williams and Wilkins, Philadelphia Chatziioannou AF, Cherry SR, Shao Y et al (1999). Performance evaluation of microPET a high-resolution lutetium oxyorthosilicate PET scanner for animal imaging. J Nucl Med 40 1164... 

As observed in the oxyorthosilicate structure of the trivalent rare earths, there are also two types of anions present in this structure. One... 

Bartram RH, Hamilton DS, Kappers LA, Lempicki A, Glodo J, Schweitzer JS et al (1999) Electron traps and transfer efficiency in cerium-doped lutetium oxyorthosilicate sdntillatms. 

Differences between PET-CT systems apply to the number of detector rows with which the CT component is equipped. Currently, up to 64 detector rows have been integrated in PET-CT. For the PET component, different detector materials have been available. The three most common detector materials are bismuth ger-manate (BGO), lutetium oxyorthosilicate (LSO), and gadolinium oxyorthosilicate (GSO). All available PET-GT systems are equipped with a full-ring PET detector (360° coverage). The size of each detector crystal defines the spatial resolution of the tomograph, which may range from 2 to 5 mm. 

Melcher CL, Schweitzer IS. Cerium-doped lutetium oxyorthosilicate a fast, efficient new scintillator. IEEE Trans Nucl Sci 1992 39 502-505. 

The 1 1 compound between RjOj and SiOj is called rare earth oxyorthosilicate with a formula R2(8104)0 because it contains isolated Si04 tetrahedra and extra oxygen atoms that are not bonded to silicon. The oxyorthosilicates can be prepared directly by a solid state reaction between rare earth oxides and silicon dioxide. The reaction is slow and high temperatures are needed (1700°C) to get 100% conversion in a reasonable time (Keler and Kuznetsov, 1962). The kinetics of the reaction between Y2O3 and Si02 have been studied and the reaction appears to proceed via diffusion (Keler and Kuznetsov, 1962 Leskela and Niskavaara, 1982). The polymorphic transitions of Si02 complicate the interpretation of the reaction mechanism, however. 

The crystal structure of the rare earth oxyorthosUicates has been determined from single crystals prepared using Bi203 as a flux material (Buisson and Michel, 1968). The oxyorthosilicates from praseodymium to terbium are isomorphic and crystallize in the monocUnic space group P2 /c with Z = 4. According to Ananeva et al. (1981) dysprosium can also have this structure type (A) too, but more often has another structure type (B). It is not clear whether or not lanthanum has a structure of its own. The unit cell dimensions of lanthanide oxyorthosilicates, except those of cerium and promethium compounds, are presented in table 9. 

The B-type oxyorthosilicate is also monoclinic with a space group of B2/b and there are pight formula units in a unit cell approximately double the size of the A-type ceU. This structure type is characteristic of the heavier lanthanides (Dy - Lu) and yttrium. Crystal structures have been determined for the yttrium and ytterbiiun... 

In addition to the well-established A and B forms, Wanklyn et al. (1975) have found a new form of oxyorthosilicate for gadolinium, terbium and dysprosium. Single crystals have been obtained by the flux growth method from a mixture containing PbFj, RjOj and SiOj. Only the X-ray powder patterns of these crystals have been published. It should also be noted that the new structure was obtained from only 4 of 40 batches prepared. Wanklyn et al. concluded that the structure must be somewhat unstable, yet upon heating to 1700°C the diffraction pattern remained unchanged. The crystals contained lead only as a trace impurity. 

According to Warshaw and Roy (1964), Lazarev et al. (1968) and Ananeva et al. (1981), lanthanum oxyorthosilicate has a structure of its own that is not similar to the monochnic A-type of the lighter lanthanides. Felsche, however (table 9), did not place lanthanum oxyorthosilicate into a separate group but included it among the A-type structures. As no structural study is available for La 2(8104)0 the question of its separate structure cannot yet be resolved. 

Reports of scandium oxyorthosilicates are limited to an IR spectroscopic study from the early 1960s (Lazarev et al., 1963). 

The main focus of research on the rare earth silicates has been their preparation and structure. A summary of the structural data available is presented in table 14. In many cases the physical properties are unknown. An important application of the rare earth silicates is the use of yttrium oxyorthosilicate activated with terbium as a luminescent material in fluorescent lamps. Several patents have been published in this field. 

Among the general characteristics of rare earth silicates high melting points (table 15) and high refraction indices may be mentioned. The hardness of oxyorthosilicates is 6-6.5 (Ananeva et aL, 1981) and that of oxyapatite siUcates with divalent metals 6.6-7 in Mohs scale (Hopkins et al, 1971a, b). 

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