Positron Emission Tomography Modeling Research Articles

Quantitative autoradiography of ligands for dopamine receptors and transporters in brain of Göttingen minipig: Comparison with results in vivo

The pig has been used as animal model for positron emission tomography (PET) studies of dopamine (DA) receptors and pharmacological perturbations of DA neurotransmission. However, the binding properties of DA receptors and transporters in pig brain have not been characterized in vitro. Therefore, the saturation binding parameters of [3H]SCH 23390 for DA D1 receptors and [3H]raclopride for DA D2/3 receptors were measured by quantitative autoradiography in cryostat sections from brain of groups of 8 week old and adult female Göttingen minipigs. The magnitudes of Bmax and Kd for these ligands were similar in young and old pigs, and were close to those reported for rat and human brain. Furthermore, gradients in the concentrations of D1 and D2/3 sites in striatum measured in vitro agreed with earlier findings in PET studies. However, the dopamine transporter (DAT) ligand [3H]GBR12935 did not bind in pig brain cryostat sections. Whereas the tropane derivative [125I]RTI-55 labeled serotonin transporters (serotonin transporter (SERT)) in pig brain, use of the same ligand under conditions specific for DAT, revealed a pattern of binding similar to that observed for SERT conditions. Parallel studies revealed the presence of DAT in rat and ferret brain. The distribution volume (Vd) of the selective DAT ligand [11C]NS2214 ([11C]Brasofensine) was mapped in groups of normal and MPTP-lesioned Göttingen miniature pigs. The in vivo pattern of Vd matched the distribution of SERT in vitro, and did not differ between the normal pigs and the lesioned animals with documented 60% DA depletions. However, the pattern of specific binding of the selective noradrenaline transporter ligand (S,S)-[11C]MeNER in a single Landrace pig showed that, of the three monoamine transporters, only DAT could not be detected in pig brain. We conclude that the pig is a suitable model for PET studies of DA D1 and D2/3 binding sites, which are fully developed on the eighth postnatal week. However, well-characterized piperazine and tropane radioligands failed to recognize DAT in pig brain; the two tropane radioligands lacked pharmacological specificity for DAT and SERT in pig brain in vitro and in vivo.

Positron emission tomography, Borel measures and weak convergence

In this paper, we develop a refined version of the usual Poisson model for positron emission tomography (PET), in which the data space is finite dimensional, but the unknown emission intensity is represented by a Borel measure on the region of interest. We demonstrate that maximum likelihood (ML) estimators exist in the space of Borel measures and analyse an extension of the finite dimensional EM algorithm for reconstructing the emission intensity. We present evidence that convergence of this functional iteration should be considered in the weak topology and obtain partial convergence results, which contain all the known convergence results to date as special cases. General conditions are obtained under which an ML estimator can be represented by a bounded function. In particular, we show that the regularity of ML estimators depends heavily on properties of the probabilities governing the PET mathematical model. We also show that, in some cases, no ML estimator can be represented by a bounded function. Although this paper is motivated by PET, the results apply to general inverse problems in which the unknown measure, and the kernel representing the blurring operator are all positive.

Discussion of the Papers by Atkinson, and Bates et al.

Discussion of the Papers by Atkinson, and Bates <i>et al.</i>

Discussion of the Paper by Vardi and Lee

Discussion of the Paper by Vardi and Lee