Segmentation of dynamic PET images with kinetic spectral clustering.

Segmentation is often required for the analysis of dynamic positron emission tomography (PET) images. However, noise and low spatial resolution make it a difficult task and several supervised and unsupervised methods have been proposed in the literature to perform the segmentation based on semi-automatic clustering of the time activity curves of voxels. In this paper we propose a new method based on spectral clustering that does not require any prior information on the shape of clusters in the space in which they are identified. In our approach, the p-dimensional data, where p is the number of time frames, is first mapped into a high dimensional space and then clustering is performed in a low-dimensional space of the Laplacian matrix. An estimation of the bounds for the scale parameter involved in the spectral clustering is derived. The method is assessed using dynamic brain PET images simulated with GATE and results on real images are presented. We demonstrate the usefulness of the method and its superior performance over three other clustering methods from the literature. The proposed approach appears as a promising pre-processing tool before parametric map calculation or ROI-based quantification tasks.

Comparative study of Ru(bpz)3(2+) Ru(bipy)3(2+) and Ru(bpz)2Cl2 as photosensitizers of DNA cleavage and adduct formation.

The efficiency of ruthenium complexes for photosensitizing DNA damage depends on the oxidizing character of their ligands. Here we report on the difference in behavior of tris(2.2'-bipyrazyl)ruthenium(II) (Ru[bpz]3(2+)), tris(2,2'-bipyridyl)ruthenium(II) (Ru[bipy]3(2+)) and cis-dichlorobis (2,2'-bipyrazyl)ruthenium(II) (Ru[bpz]2Cl2). Upon irradiation at 436 nm, Ru(bpz)3 (2+) was far less stable than Ru(bipy)3(2+). Ru(bpz)3(2+) in phosphate buffer containing NaCl undergoes a photoanation reaction leading to the formation of Ru(bpz)2Cl2, as previously reported also in organic media. In the presence of phage phi X174 DNA, Ru(bpz)3(2+) photosensitized the formation of single strand breaks with an efficiency that was, at the beginning of irradiation, similar to that of Ru(bipy)3(2+). After 8 min of irradiation, the cleavage efficiency of Ru(bpz)3(2+) reached a plateau that may correspond to its photode composition. For the same conditions, Ru(bpz)2Cl2 did not induce DNA breakage. Scavenging experiments showed that, in the presence of oxygen, DNA cleavage induced by Ru(bpz)3(2+) partly resulted from the formation of singlet oxygen and hydroxyl radical while in the absence of oxygen an additional mechanism involving electron transfer between the excited state of the ruthenium complex and DNA is proposed. The ICP measurement showed that Ru(bpz)3(2+) and Ru(bpz)2Cl2 gave rise to covalent binding onto DNA in contrast with Ru(bipy)3(2+), which did not bind to DNA under the experimental conditions. The results are discussed with regard to the potential use of these photosensitizers in phototherapy.