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Biomedical subjects

R L Viana

Publications and source records attributed to R L Viana.

9 recordsLinked to original sources
PubMed2006

Spatial recurrence plots.

We propose an extension of the recurrence plot concept to perform quantitative analyzes of roughness and disorder of spatial patterns at a fixed time. We introduce spatial recurrence plots (SRPs) as a graphical representation of the pointwise correlation matrix, in terms of a two-dimensional spatial return plot. This technique is applied to the study of complex patterns generated by coupled map lattices, which are characterized by measures of complexity based on SRPs. We show that the complexity measures we propose for SRPs provide a systematic way of investigating the distribution of spatially coherent structures, such as synchronization domains, in lattice profiles. This approach has potential for many more applications, e.g., in surface roughness analyzes.

Journal Article↗
PubMed2004

Multistability and phase-space structure of dissipative nonlinear parametric four-wave interactions.

We investigate the phase-space structure displayed by a system of four waves interacting by means of nonlinear coupling between two wave triplets, which results in a dissipative high-dimensional vector field presenting an invariant manifold, wherein the dynamics is essentially conservative. The focus is on the coexistence of a large number of periodic attractors in the phase space, with an interwoven structure of the basins of attraction, where low-period attractors have predominance. The time behavior of nearly conserved quantities and the properties of the Lyapunov spectra are used to imply the existence of a lower-dimensional invariant manifold where the dynamics is nearly conservative. A three-dimensional map is used to illustrate these findings.

Journal Article↗
PubMed2003

Validity of numerical trajectories in the synchronization transition of complex systems.

We investigate the relationship between the loss of synchronization and the onset of shadowing breakdown via unstable dimension variability in complex systems. In the neighborhood of the critical transition to strongly nonhyperbolic behavior, the system undergoes on-off intermittency with respect to the synchronization state. There are potentially severe consequences of these facts on the validity of the computer-generated trajectories obtained from dynamical systems whose synchronization manifolds share the same nonhyperbolic properties.

Journal Article↗
PubMed2000

Synchronization plateaus in a lattice of coupled sine-circle maps.

Frequency synchronization is a common phenomenon in spatially extended dynamical systems, like oscillator chains and coupled map lattices. We study the distribution of synchronization plateaus in a sine-circle map lattice with a variable range coupling and randomly distributed natural frequencies. A transition between synchronized and nonsynchronized phases is observed as the coupling range is varied. The lengths of the synchronization plateaus are found to obey an exponential distribution for local coupling.

Kinetics↗
PubMed1999

Argon plasma coagulation therapy for hemorrhagic radiation proctosigmoiditis.

BACKGROUND: Radiation-induced proctosigmoiditis is a serious complication of pelvic radiation therapy. Rectal bleeding occurs among 6% to 8% of these patients and is extremely difficult to manage. Pharmacotherapy is generally ineffective, whereas surgical treatment is associated with high morbidity and mortality. Argon plasma coagulation is a new method of noncontact electrocoagulation well suited for hemostasis of large bleeding areas. METHODS: From December 1996 through March 1998, we used argon plasma coagulation to treat 28 patients with hemorrhagic radiation-induced proctosigmoiditis. Indications for treatment were anemia (n = 18) and persistent bleeding despite pharmacotherapy (n = 10). Argon flow and electrical power were set at 1.5 L/min and 50 W. The severity of rectal bleeding was graded from 0 to 4 (highest), and hemoglobin levels were recorded before and after treatment. RESULTS: Eighty-two therapeutic sessions were performed (median 2.9 sessions per patient). The severity score for rectal bleeding dropped at least 1 point for all but 2 patients, and the mean value decreased from 2.96 to 0.68. Average hemoglobin level increased 1.2 gm/dL (1.9 gm/dL among anemic patients). No serious complications were observed. CONCLUSIONS: Argon plasma coagulation appears to be a simple, safe, and effective technique in the management of hemorrhagic radiation-induced proctosigmoiditis.

Adult↗
PubMed1975

Selective arteriography in the diagnosis and evaluation of amebic abscess of the liver.

Hepatic selective arteriography was performed in 20 patients with an amebic hepatic abscess. In most cases, arterial hepatic vascularization showed a characteristic change in the benign space-occupying lesion. The arteriogram revealed displacement of intrahepatic arteries, with a bare area outlined by curved, narrowed, and stretched vessels. The hepatogram phase showed a different radiolucency of the liver parenchyma in the area of the amebic abscess. It is possible to differentiate between an abscess and an hepatic malignant tumor by hepatic arteriography. This technique is valuable in that it can be used to determine whether the abscess has disappeared after medical treatment.

Celiac Artery↗