Biomedical subjects

C Falcón

Publications and source records attributed to C Falcón.

12 recordsLinked to original sources

PubMed2006

Front propagation sustained by additive noise.

The effect of noise in a motionless front between a periodic spatial state and an homogeneous one is studied. Numerical simulations show that noise induces front propagation. From the subcritical Swift-Hohenberg equation with noise, we deduce an adequate equation for the envelope and the core of the front. The equation of the core of the front is characterized by an asymmetrical periodic potential plus additive noise. The conversion of random fluctuations into direct motion of the core of the front is responsible of the propagation. We obtain an analytical expression for the velocity of the front, which is in good agreement with numerical simulations.

Journal Article↗

PubMed2003

Dynamic model of the left ventricle for use in simulation of myocardial perfusion SPECT and gated SPECT.

Simulation is a useful tool in cardiac SPECT to assess quantification algorithms. However, simple equation-based models are limited in their ability to simulate realistic heart motion and perfusion. We present a numerical dynamic model of the left ventricle, which allows us to simulate normal and anomalous cardiac cycles, as well as perfusion defects. Bicubic splines were fitted to a number of control points to represent endocardial and epicardial surfaces of the left ventricle. A transformation from each point on the surface to a template of activity was made to represent the myocardial perfusion. Geometry-based and patient-based simulations were performed to illustrate this model. Geometry-based simulations modeled (1) a normal patient, (2) a well-perfused patient with abnormal regional function, (3) an ischaemic patient with abnormal regional function, and (4) a patient study including tracer kinetics. Patient-based simulation consisted of a left ventricle including a realistic shape and motion obtained from a magnetic resonance study. We conclude that this model has the potential to study the influence of several physical parameters and the left ventricle contraction in myocardial perfusion SPECT and gated-SPECT studies.

Algorithms↗

PubMed2001

Characterisation of fan-beam collimators.

Fan-beam collimators offer a good balance between resolution and noise. The collimator response may be included in iterative reconstruction algorithms in order to improve single-photon emission tomography (SPET) resolution. To this end, accurate determination of the focal region and characterisation of the collimator response as a function of the source co-ordinates must be performed. In this paper, a method to characterise fanbeam collimators is evaluated. First, we calculated the real focal region and the accuracy of the collimator convergence. Then, we confirmed the hypothesis that Gaussian distributions adequately fit the collimator responses, although no individualised treatment was performed for the tails of detector response which are associated with scattering and septal penetration. Finally, analytical functions were used to model the resolution and sensitivity. The parameter values in these functions were obtained from experimental measures by non-linear regression fitting. Our findings show differences of 1.43% between nominal and real focal length and standard deviations of 2.5 mm in the x-direction and 7.1 mm in the y-direction for the focal convergence. The correlation coefficients between experimental and predicted values were 0.994 for resolution and 0.991 for sensitivity. As a consequence, the proposed method can be used to characterise the collimator response.

Algorithms↗

PubMed1999

Evaluation of algorithms for the registration of 99Tcm-HMPAO brain SPET studies.

Five functions of merit were used for the registration of 99Tcm-HMPAO brain SPET studies. The correlation coefficient (COR), the stochastic sign change (SSC), the standard deviation of ratios (SDR), the sum of the absolute differences (SAD) and a new function based on a local correlation coefficient (LOC) were tested in the registration of photic neuroactivation (ACT), epilepsy (EPL) and Wada (WAD) SPET studies. The comparison included simulated and real studies. The translation error in registration was 0.1 +/- 0.1 pixels (mean +/- S.D.) for all functions of merit for the complete set of simulated studies (10 runs for each ACT, EPL and WAD). For rotation, LOC yielded the best results with a mean error of 0.3 +/- 0.2 degree and a maximum error of 0.6 degree. Slightly higher errors were found with SAD (0.4 +/- 0.2 degree, maximum 1.0 degree) and COR (0.5 +/- 1.0 degree, maximum 1.0 degree). The highest errors were found with SDR (0.8 +/- 1.0 degree, maximum 4.8 degrees) and SSC (0.8 +/- 1.1 degrees, maximum 4.7 degrees). The results obtained from five real studies of ACT, of EPL and of WAD were in agreement with the findings from the simulated studies, thus confirming the robustness of LOC, SAD and COR for the registration of 99Tcm-HMPAO brain SPET studies.

Algorithms↗

PubMed1998

Evaluation of a cross-validation stopping rule in MLE SPECT reconstruction.

One of the problems in the routine use of the maximum-likelihood estimator method-expectation maximization (MLE-EM) algorithm is to decide when the iterative process should be stopped. We studied a cross-validation stopping rule to assess its usefulness in SPECT. We tested this stopping rule criterion in the MLE-EM algorithm without acceleration as well as in two accelerating algorithms, the successive substitutions algorithm (SSA) and the additive algorithm (AA). Different values of an acceleration factor were tested in SSA and AA. Our results from numerical and physical phantoms show that the stopping rule based on the cross-validation ratio (CVR) takes into account the similarity of the reconstructed image to the ideal image, noise and the contrast of the image. CVR yields reconstructed images with balanced values of the figures of merit (FOM) employed to assess the image quality. The CVR criterion can be used in the original MLE-EM algorithm as well as in SSA and AA. The reconstructed images obtained with SSA and AA showed FOM values that were very similar. These results were justified by considering AA to be an approximate form of SSA. The range of validity for the acceleration factor in SSA and AA was found to be [1, 2]. In this range, an inverse function connects the acceleration factor to the number of iterations needed to attain prefixed values of FOMs.

Algorithms↗

PubMed1996

The influence of a relaxation parameter on SPECT iterative reconstruction algorithms.

Algebraic reconstruction techniques (ARTs) have been intensively studied in image reconstruction. However, reconstructed images often exhibit a characteristic noise when applied to emission tomography. In this work, we study the influence of the ART relaxation parameter and identify values of that parameter which yield smooth images and good convergence. The study includes both simulated and real single photon emission computed tomographic (SPECT) data. In the simulated studies, scattering, attenuation, noise and detector response were included. Relaxation factors ranging between 0.01 and 0.35 were considered. Total counts in projections were between 2.5 x 10(5) and 10(6) in one slice. The goodness of the reconstructions was assessed by using the correlation coefficient between the pattern and the actual reconstruction (CC), the background coefficient of variation (CV), the contrast (CON) and the signal-to-noise ratio (SNR). The results show that values for the relaxation factor and number of interations close to 0.1 and 8, respectively, yield reconstructed images in which CC, CV, CON and SNR have well balanced values and CC, CV and SNR are very close to the extremal. An increase in either the number of iterations or in the relaxation factor results in a rise of CON but the other three FOMs deteriorate. The results from the real phantom, which are in agreement with those of the simulated studies, validate the simulation method and demonstrate the influence of the relaxation parameter.

Algorithms↗

PubMed1993

Immunological evaluation of patients with invasive carcinoma of the gallbladder.

Forty-three patients with invasive adenocarcinoma of the gallbladder were postoperatively studied in order to determine their general immunological status as well as the local immunohistological reaction to the tumor. At the end of the follow-up, they formed two groups: 19 living patients (group GL) and 24 dead patients (group GD). As a control group (GC), 21 patients with cholecystectomy or cholelithiasis and without carcinoma were simultaneously evaluated. In GL, most of the tumors were limited to the gallbladder wall, and in GD, most of the tumors were already disseminated at the time of diagnosis. GD presented a lower percentage of peripheral blood B lymphocytes, as compared to GL and GC cases. Skin tests of delayed hypersensitivity were significantly more reactive in GL cases than in GD cases, and less reactive in GD than in GC cases. The immunohistological evaluation of the gallbladder yielded a lower B lymphocyte infiltration in GD tumors than in the control cases. GL cases showed a higher intratumoral lymphocytic and mononuclear cell infiltration than GD cases. Although the clinical stage was higher in GD than in GL cases, there were also significant differences in the local immune response and the general immunological status. Patients with invasive gallbladder adenocarcinoma showing longer postoperative survival revealed normal or increased local and general immunological reactions, whereas patients with disseminated tumors showed an important humoral and cellular secondary immunodeficiency.

Adenocarcinoma↗

PubMed1993

[Antiphospholipid antibodies and cerebral ischemic infarction in a 6-year-old boy].

Lupus anticoagulant activity and anti-phospholipid antibodies (aPL) were found in a six-year-old child with cerebral ischemic infarction in the absence of any underlying disease. The association of these antibodies with thrombosis has been well documented in adult patients. In view of our observation, we believe that aPL may also be involved in the pathogenesis of arterial thrombotic events in childhood, and aPL should be systematically searched in these cases.

Antibodies, Antiphospholipid↗

PubMed1992

Placental alterations, intrauterine growth retardation and teratogenicity associated with enalapril use in pregnant rats.

Enalapril (15 mg/kg/day p.o.) was given to 11 pregnant rats from day 1 to 9 (E1-9) and to 11 rats from day 10 to 20 (E10-20) of pregnancy; 12 rats were the control group. Fifteen animals were sacrificed on day 20 of pregnancy and 19 were allowed to progress into partum. Placentas were smaller in E10-20 rats (-15%, p less than 0.05) and had a simple hypocellular cordonal structure; in E1-9 animals the predominant pattern was a combination of complex and simple structure. At day 20 the fetuses in the treated groups were smaller than the controls (-5% in E1-9 and -16% in E10-20, p less than 0.05); differences disappeared on the 13th day postpartum. Two fetuses from treated mothers presented incomplete skull ossification. We believe this report adds arguments to preclude converting enzyme inhibitors in pregnancy.

Animals↗

PubMed1990

Inhibition of human platelet aggregation and thromboxane-B2 production by melatonin: evidence for a diurnal variation.

The effects of melatonin on platelet aggregation and thromboxane-B2 (TxB2) production induced by 1-4 x 10(-6) M adenosine diphosphate (ADP) or 0.6 x 10(-3) M arachidonic acid (AA) were assessed in platelet-rich plasma (PRP). Micromolar concentrations of melatonin inhibited in a dose-dependent way ADP-induced platelet aggregation with individual inhibitions 40% or more at 10(-6)-10(-5) M. A significant depression of AA-induced platelet aggregation was observed only at 10(-5)-10(-4) M melatonin. Morning (0830 h)-evening (1800 h) studies of ADP-induced platelet aggregation in seven normal men showed a higher sensitivity at 1800 h when analyzed as a global inhibitory effect of melatonin (P less than 0.01). Moreover, only during the evening hours did melatonin induce reversible aggregation, an index of inhibition of the platelet secretory process elicited by ADP exposure. No diurnal variability in melatonin inhibition of AA-induced aggregation was detected. TxB2 production elicited by AA in the evening was inhibited significantly in a concentration-related manner by a 2-min preincubation with 10(-9)-10(-5) M melatonin, while during the morning hours the inhibition was significant only at 10(-6) M or higher melatonin concentrations. In the case of ADP, the inhibition of TxB2 release attained significance at 10(-5)-M (0830 h) or 10(-6)-M concentrations (1800 h). In the presence of either stimulatory agent, melatonin depression of TxB2 generation was about 2-fold greater at 1800 h than at 0830 h. The diurnal changes in melatonin effect on TxB2 production were also observed in thrombin-stimulated washed platelets. The present data indicate the existence of circadian variations in platelet responsiveness to melatonin in humans.

Adult↗

PubMedDate not supplied

[Advances in the study of primary auditory cortex. Demostration of its activation by functional magnetic resonance imaging].

PURPOSE: The purpose of this preliminary study has been to demonstrate and investigate the activation patterns of the primary auditory cortex (Heschl's gyrus = HG) using functional magnetic resonance imaging (fMRI). MATERIAL AND METHODS: A 2500 Hz tone stimulus was delivered monoaurally to the right and left ear of 15 normal-hearing right-handed volunteers in 20-second on-off cycles. FMRI data were obtained using a 1.5-Tesla scanner and processed with SPM2. RESULTS: Activated pixels were identified in the transverse temporal gyrus (Heschl's gyrus) of both hemispheres in response to pure tone stimuli using cross-correlation analysis (P < 0.001). Bilateral hemispheric activation was observed in all subjects and there was a trend towards contralateral HG activation to the stimulated ear. CONCLUSION: These results demonstrate directly that fMRI is a new and useful imaging technique to study the complex auditory cortex and it will have potential clinical applications in the next future.

Adult↗

PubMedDate not supplied

[Cerebral activation during Stroop's test in a case of early focal brain injury].

INTRODUCTION: Functional magnetic resonance imaging (fMRI) offers an important advantage over other functional neuroimaging techniques used with children because of its harmlessness. Previous studies conducted with adults with alexia suggested two ways the brain can reorganise reading after a brain injury affecting the left hemisphere, one contralateral and the other ipsilateral. CASE REPORT: We describe a study carried out using fMRI of a 10 year old girl with an injury to the left hemisphere caused by a fishing harpoon when she was 6 years old. As a result of the accident the girl presented a right hemiparesia. The girl s parents and teachers also reported difficulties in the acquisition of reading writing and arithmetic, as well as a certain degree of attentional deficit. An fMRI exploration was performed while the girl was doing the Stroop test. The structural MR images showed left hemisphere cortical lesions in the orbital and angular gyrus regions, in addition to the caudate and putamen nuclei, and in the inferior longitudinal bundle. The fRMI revealed a strong overactivation of the right dorsolateral frontal cortex, in the evaluation of interference, and activations of the right angular and bilateral supramarginal gyri, in the evaluation of word reading. CONCLUSION: The functional study suggests the existence of a reorganisation of reading that is both intra and inter hemispheric.

Brain↗