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.

NBT-II carcinoma behaviour is not dependent on cell-cell communication through gap junctions.

To study the mechanism(s) underlying the proliferation of heterogeneous cell populations within a solid tumour, the NBT-II rat bladder carcinoma system was used. It has been first investigated whether the different cell populations are coupled through gap junctions (GJIC). Cells overexpressing the Cx43 were generated to test for any tumour suppressive activity in vivo. To determine whether GJIC is essential for tumour proliferation and the establishment of a cooperative community effect, NBT-II cells that are incompetent for cell coupling were generated. The data report that (i) carcinoma cells expressing or not FGF-1 are coupled through GJIC in vitro and in coculture and express the gap junction protein Cx43, (ii) overexpression of Cx43 in these cells does not affect their in vitro coupling capacities and in vivo tumourigenic growth properties, (iii) inhibition of GJIC through antisense strategy has no in vivo obvious consequence on the tumour growth properties of the carcinoma, and (iv) the community effect between two carcinoma cell populations does not critically involve cell coupling through gap junctions.

Micro-emboli detection: an ultrasound Doppler signal processing viewpoint.

Several studies have been carried out in the last twenty years on the characterization and detection of cerebral artery emboli. From the detection point of view, the existing methods are largely based on the classical Fourier analysis of which the well known limitations provide poor accuracy. This paper first recalls existing methods based on Fourier, Wigner-Ville and wavelet approaches. It then presents new emboli detection methods based on parametric signal processing approaches. The basic idea of these parametric methods is to compare the Doppler embolic signal to its autoregressive model. The detection principle consists in constructing a decision information which contains the signature of the micro-embolus being sought. The detection is finally evaluated using receiver operating characteristic (ROC) curves. Comparison between the new methods and classical approaches is performed using a realistic embolic signal simulation. Furthermore, to validate our theoretical study, we tested our new algorithms using in vivo signals. This comparison shows the significant inaccuracy of existing methods to detect micro-emboli.

Estimation of the blood Doppler frequency shift by a time-varying parametric approach.

Doppler ultrasound is widely used in medical applications to extract the blood Doppler flow velocity in the arteries via spectral analysis. The spectral analysis of non-stationary signals and particularly Doppler signals requires adequate tools that should present both good time and frequency resolutions. It is well-known that the most commonly used time-windowed Fourier transform, which provides a time-frequency representation, is limited by the intrinsic trade-off between time and frequency resolutions. Parametric methods have then been introduced as an alternative to overcome this resolution problem. However, the performance of those methods deteriorates when high non-stationarities are present in the Doppler signal. For the purpose of accurately estimating the Doppler frequency shift, even when the temporal flow velocity is rapid (high non-stationarity), we propose to combine the use of the time-varying autoregressive (AR) method and the (dominant) pole frequency. This proposed method performs well in the context where non-stationarities are very high. A comparative evaluation has been made between classical (FFT based) and AR (both block and recursive) algorithms. Among recursive algorithms we test an adaptive recursive method as well as a time-varying recursive method. Finally, the superiority of the time-varying parametric approach in terms of frequency tracking and delay in the frequency estimate is illustrated for both simulated and in vivo Doppler signals.

Tumour suppressive properties of fibroblast growth factor receptor 2-IIIb in human bladder cancer.

FGFRs (fibroblast growth factor receptors) are encoded by four genes (FGFR1-4). Alternative splicing results in various receptor isoforms. The FGFR2-IIIb variant is present in a wide variety of epithelia, including the bladder epithelium. Recently, we have shown that FGFR2-IIIb is downregulated in a subset of transitional cell carcinomas of the bladder, and that this downregulation is associated with a poor prognosis. We investigated possible tumour suppressive properties of FGFR2-IIIb by transfecting two human bladder tumour cell lines, J82 and T24, which have no endogenous FGFR2-IIIb expression, with FGFR2-IIIb cDNA. No stable clones expressing FGFR2-IIIb were isolated with the J82 cell line. For the T24 cell line, stable transfectants expressing FGFR2-IIIb had reduced growth in vitro and formed fewer tumours in nude mice which, in addition, grew more slowly. The potential mechanisms leading to decreased FGFR2-IIIb mRNA levels were also investigated. The 5' region of the human FGFR2 gene was isolated and found to contain a CpG island which was partially methylated in more than half the cell lines and tumours which do not express FGFR2-IIIb. No homozygous deletion was identified in any of the tumours or cell lines with reduced levels of FGFR2-IIIb. Mutational analysis of the entire coding region of FGFR2-IIIb at the transcript level was performed in 33 bladder tumours. In addition to normal FGFR2-IIIb mRNA, abnormal transcripts were detected in two tumour samples. These abnormal mRNAs resulted from exon skipping which affected the region encoding the kinase domain. Altogether, these results show that FGFR2-IIIb has tumour growth suppressive properties in bladder carcinomas and suggest possible mechanisms of FGFR2 gene inactivation.

Metastatic rat carcinoma cells express a new retrotransposon.

Genes differentially expressed by a rat bladder carcinoma NBT-II cells and their in-vivo-selected metastatic M-NBT-II variant were analysed. Amplification and cloning of a 277-bp B sequence, exclusively expressed by the M-NBT-II cells, were performed, and this sequence was detected as a 6.7-kb RNA. This fragment shares 46-50% identities with the gag-related protein of mouse and hamster Intracisternal A Particles (IAPs). Screening of a M-NBT-II cDNA library with the B probe selected a 1671-bp sequence corresponding to the 5' end of a novel retrotransposon member of the rat IAP family. This sequence has a strong identity with the Ecker Rat IAP (ERA-IAP) except for the B portion and has an open reading frame potentially encoding a 114-amino-acid gag retrovirus-related protein. Rearrangement of this new retrotransposon could be relevant with the tumor progression in our model system since it is only expressed in the M-NBT-II in-vivo-selected carcinoma metastasis.

Xenopus cadherin-11 is expressed in different populations of migrating neural crest cells.

We cloned the Xenopus homologue of cadherin-11 and studied its spatiotemporal expression pattern during early development. The messenger RNA is present from the mid-gastrulation through embryo development. It is expressed in different neural crest cell populations, during their migration and differentiation. This pattern, unexpected for an adhesion molecule, reinforces the idea of novel functions for type II cadherins.

Time-varying autoregressive spectral estimation for ultrasound attenuation in tissue characterization.

In the field of biological tissue characterization, fundamental acoustic attenuation properties have been demonstrated to have diagnostic importance. Attenuation caused by scattering and absorption shifts the instantaneous spectrum to the lower frequencies. Due to the time-dependence of the spectrum, the attenuation phenomenon is a time-variant process. This downward shift may be evaluated either by the maximum energy frequency of the spectrum or by the center frequency. In order to improve, in strongly attenuating media, the results given by the short-time Fourier analysis and the short-time parametric analysis, we propose two approaches adapted to this time-variant process: an adaptive method and a time-varying method. Signals backscattered by an homogeneous medium of scatterers are modeled by a computer algorithm with attenuation values ranging from 1 to 5 dB/cm MHz and a 45 MHz transducer center frequency. Under these conditions, the preliminary results obtained with the proposed time-variant methods, compared with the classical short-time Fourier analysis and the short-time auto-regressive (AR) analysis, are superior in terms of standard deviation (SD) of the attenuation coefficient estimate. This study, based on nonstationary AR spectral estimation, promises encouraging perspectives for in vitro and in vivo applications both in weakly and highly attenuating media.

[Hypoplasia of the inferior vena cava with intrahepatic continuation. Color Doppler ultrasonography and MRI]. / Hypoplasie de la veine cave inférieure avec continuation intrahépatique. Echo-Doppler couleur et IRM.

We report a rare case of inferior vena cava hypoplasia discovered fortuitously, with intra-hepatic venous shunts, explored only by color-doppler ultrasound and MRI. These noninvasive imaging tests demonstrated that the intra hepatic collateral pathway arose from an inferior (accessory) right hepatic vein and flowed into the right and middle hepatic veins. Due to these findings a cavography was avoided in this asymptomatic patient.

[Bilateral adrenal hemorrhage induced by stress. Radiological diagnosis. Apropos of a case]. / Hémorragie surrénalienne bilatérale par stress. Diagnostic radiologique. A propos d'une observation.

Bilateral hemorrhage of the adrenal gland is a rare disease in adults. Although stress is often said to be the cause, it is rarely observed. The non specific clinical manifestations, and the absence of adrenal insufficiency demonstrate the importance of radiology. The diagnosis is made by ultrasonography and CT scan, which reveals an oval adrenal mass of high density that subsequently decreases in both size and density.

[Agyria-pachygyria and pachygyria in children. Contribution of imaging]. / Agyries-pachygyries et pachygyries de l'enfant. Apport de l'imagerie.

BACKGROUND: Lissencephaly (agyria-pachygyria) is a defect in migration of cerebral neurons resulting in failure of cortical gyri to develop. Progress in imaging techniques improves its diagnosis. POPULATION AND METHODS: The files of 17 patients (ten boys and seven girls), aged 7 months to 16 years, were retrospectively studied. The clinical picture consisted of mental retardation (17 patients), seizures (eight patients), facial dysmorphia (seven patients), axial hypotonia (four patients). CT scan was performed in 16 cases and MRI with T1 and T2 weighted images in all 17. RESULTS: The CT scan identified pachygyria in 12 cases. Cerebral calcifications were seen in four cases. MRI detected typical changes in all 17 cases: thickened cortex and gyri, loss of cortical white matter interdigitations, lack of operculisation of the sylvian fissure. Pachygyria was generalized (six patients) or localized (11 patients). Associated abnormalities were dysgenesis of corpus callosum in three patients, cerebellar hypoplasia in one, deep grey matter heterotopia in one; hypersignal of the white matter was identified on T2 weighted images in five patients. CONCLUSION: MR imaging permits precise analysis of abnormalities secondary to a defect in neuronal migration.

Sequence and distribution of Xenopus laevis E-cadherin transcripts.

Cadherins are calcium-dependent adhesive glycoproteins implicated in histogenetic processes. In Xenopus laevis, the distribution of classical cadherins N, E, EP, XB and U has been determined by immunofluorescence labeling or by in situ hybridization. In this study, we report the full-length sequence of the E-cadherin cDNA. Comparison with the other cadherin sequences available indicates that Xenopus E-cadherin is as homologous to Xenopus EP-cadherin as to the chicken L-CAM and to the mammalian E-cadherin. Although Xenopus E-cadherin protein sequence exhibits many short conserved motifs present in other E-cadherins, it differs remarkably from the chicken L-CAM and the mammalian E-cadherin in its appearance after gastrulation. In situ hybridization data showed that E-cadherin transcripts are homogenously distributed in all differentiating epithelia from early tailbud to post-metamorphic stage. In contrast to mouse E-cadherin, Xenopus E-cadherin was not detected transiently in the nervous system during embryogenesis and in the post-metamorphic stages.

Plasticity of astroglial glutamate and gamma-aminobutyric acid uptake in cell cultures derived from postnatal mouse cerebellum.

The plasticity of astroglial glutamate and gamma-aminobutyric acid (GABA) uptakes was investigated using mouse cerebellar cell cultures. The influence of external factors, such as different sera and/or the presence of neurons, was examined. Control autoradiography experiments showed that after short-term exposure to radioactive amino acids, granule cells took up neither glutamate nor GABA, and beta-alanine predominantly inhibited astroglial GABA uptake. Astroglial uptake was quantified by measuring the radioactivity taken up by the cells in the culture and relating this measurement to the number of glial fibrillary acidic protein-positive cells present. Glutamate uptake was investigated in astroglial cultures and subcultures and in neuronal-astroglial cultures derived from postnatal day 4 mouse cerebella. In the absence of neurons, glutamate uptake increased during the first 9 days after plating and then leveled off. At 14 days in vitro in horse serum, which favors the differentiation of fibrous-like astrocytes, glutamate uptake related to astrocyte number was twice as high as in fetal calf serum. In the presence of cerebellar neurons, this rate was even higher. The specificity of the responsiveness of astrocytes to neurons with respect to glutamate uptake was investigated by comparing GABA uptake in the different culture conditions. Neurons also increased the rate of GABA uptake by astrocytes. Another component of the astroglial plasma membrane, the density of beta-adrenergic receptors, was, however, not markedly affected by the presence of neurons. Hence, these results showed that in astrocytes plated from postnatal day 4 mouse cerebella, the level of neurotransmitter uptake can be regulated in vitro by factors present in sera and by cerebellar neurons in the culture. However, this plasticity declined during development because astrocytes plated from postnatal day 8 cerebella and cultured under identical conditions were less active in glutamate uptake and were insensitive to the presence of horse serum. The latter observation suggested that the metabolic plasticity of astrocytes is restricted to a period defined early in cerebellar development and is no longer evident by postnatal day 8.

[Adhesion and mobility of embryonic and tumoral cells]. / Adhérence et mobilité des cellules embryonnaires et tumorales.

Regulation of a number of adhesion molecules during neural crest cell migration was studied. The neural crest, a transient embryonic neural epithelium structure, undergoes mesenchymal transformation (epithelial-mesenchymal transition). The cells then migrate, giving rise to a variety of elements including the peripheral nervous system and melanocytes. During migration, neural crest cells do not express functional cell Adhesion Molecules but interact specifically with cell-binding domains in fibronectin molecules. A rat bladder carcinoma cell line was used as an in vitro model to study conversion of epithelial cells to a migratory fibroblast-like state. Conversion can be induced by culture on collagen or exposure to acidic Fibroblast Growth Factor (aFGF). Furthermore, constitutive fibroblast-like transformation can be induced by transfection with cDNA encoding aFGF. Growth factor-producing clones exhibit increased invasive and metastatic properties as compared with non-FGF-producing control cells. This model may provide increased understanding of the role of the different adhesion molecules in processes involving cell remodeling, such as tumor spread and development of metastases.

Ventrolateral regionalization of Xenopus laevis mesoderm is characterized by the expression of alpha-smooth muscle actin.

Mesodermal patterning in the amphibian embryo has been extensively studied in its dorsal aspects, whereas little is known regarding its ventrolateral regionalization due to a lack of specific molecular markers for derivatives of this type of mesoderm. Since smooth muscles (SM) are thought to arise from lateral plate mesoderm, we have analyzed the expression of an alpha-actin isoform specific for SM with regard to mesoderm patterning. Using an antibody directed against alpha-SM actin that recognized specifically this actin isoform in Xenopus, we have found that the expression of alpha-SM actin is restricted to visceral and vascular SM with a transient expression in the heart. The overall expression of the alpha-SM actin appears restricted to the ventral aspects of the differentiating embryo. alpha-SM actin expression appears to be activated following mesoderm induction in animal cap derivatives. Moreover, at the gastrula stage, SM precursor cells are regionalized since they will only differentiate from ventrolateral marginal zone explants. Using the animal cap assay, we have found that alpha-SM actin expression is specifically induced in treated animal cap with bFGF or a low concentration of XTC-MIF, which induce ventral structures, but not with a high concentration of XTC-MIF, which induces dorsal structures. Altogether, these results establish that alpha-SM actin is a reliable marker for ventrolateral mesoderm. We discuss the importance of this novel marker in studying mesoderm regionalization.

[Primary mediastinal choriocarcinoma. Apropos of 3 cases]. / Choriocarcinomes médiastinaux primitifs. A propos de 3 cas.

We report three cases about the diagnostic imaging features of mediastinal choriocarcinoma and during follow up. Computed tomography proved to be more accurate than plain X ray to show the topography and volume of the mediastinal mass hence providing a better approach for biopsy and surgical ablation of the residual tumoral tissues. However, CT cannot differentiate fibrous tumoral remnants from active malignancies and beta HCG is an important tumoral marker in the follow up.

EP-cadherin in muscles and epithelia of Xenopus laevis embryos.

EP-cadherin is a novel Xenopus Ca+2-dependent adhesion molecule, which shares comparable homology with mouse E- and P-cadherins (Ginsberg, De Simone and Geiger; 1991, Development 111, 315-325). We report here the patterns of expression of this molecule in Xenopus laevis embryos at different developmental stages ranging from cleavage to postmetamorphic. EP-cadherin is already expressed in the oocyte and egg and can then be detected in close association with the membrane of all blastomeres up to late blastula stages. Starting at late gastrula stages, the level of EP-cadherin expression increases sharply in non-neural ectodermal cells, in the somites and in the notochord; it persists in endodermal cells and decreases rapidly in all migratory cells. During neurulation the level of EP-cadherin expression declines gradually in the nervous system and is undetectable here throughout later development except in the optic nerve and in the neural part of the olfactory organ. This pattern continues during later development so that in the tailbud stage and up to metamorphosis the most prominent staining is detected in the epidermis and skeletal muscle. After metamorphosis, the molecule gradually disappears from the muscle tissue and the major site of expression remains the skin. EP-cadherin is invariably present in close association with the cell membrane. In the muscle it is associated with the sarcolemma at regions of myoblast-myoblast or myotube-myotube contact. In epidermal cells, EP-cadherin is usually coexpressed with E-cadherin. Yet, while E-cadherin staining is always restricted to the basolateral aspects of the cells, EP-cadherin is often distributed throughout the plasmalemma including the apical surface.

Thymotaxin, a chemotactic protein, is identical to beta 2-microglobulin.

Thymotaxin, an 11-kilodalton protein chemotactic for rat bone marrow hematopoietic precursors, was purified from media conditioned by a rat thymic epithelial cell line. The NH2-terminal sequence of thymotaxin was identical to that of rat beta 2-microglobulin (beta 2m). Antibodies to beta 2m removed thymotaxin activity from the fraction containing the 11-kilodalton protein. Chemotactic activity was observed with rat plasma beta 2m, human beta 2m, and mouse recombinant beta 2m, further supporting the identity of thymotaxin with beta 2m. The directional migration, as opposed to random movement, of the cells was also confirmed. The only rat bone marrow cells that migrated toward beta 2m were Thy1+ immature lymphoid cells devoid of T cell, B cell, and myeloid cell differentiation markers.