Biomedical subjects

D Ros

Publications and source records attributed to D Ros.

At least 19 recordsLinked to original sources

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↗

PubMed2002

Demonstration of a Ni-like Kr optical-field-ionization collisional soft x-ray laser at 32.8 nm.

We report the first experimental demonstration of a Ni-like optical-field ionization collisional soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 760 mJ, 30 fs, 10-Hz Ti:sapphire laser beam in a few mm cell filled with krypton. We have measured a gain coefficient of 78 cm(-1) on the 3d(9)4d 1S0-3d(9)4p(1)P1 transition at 32.8 nm, which is here amplified for the first time. This radiation source represents the shortest wavelength optical-field ionization collisional soft x-ray laser ever produced. The influence of the gas pressure and the pumping energy on the lasing output are also presented.

Journal Article↗

PubMed2001

Saturated amplification of a collisionally pumped optical-field-ionization soft X-ray laser at 41.8 nm.

We report the first saturated amplification of an optical-field-ionization soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 330-mJ, 35-fs, 10-Hz Ti:sapphire laser system in a few-mm cell filled with xenon. A gain of 67 cm(-1) on the 4d(9)5p-4d(9)5d transition at 41.8 nm in Pd-like xenon and a gain-length product of 15 have been inferred at saturation. This source delivers about 5 x 10(9) photons per pulse. The influence of the pumping energy and the laser polarization on the lasing output are also presented.

Journal Article↗

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↗

PubMed1997

Estimation of 99Tcm-MAG3 clearance using a single blood sample obtained over a continuous time interval.

Simplified methods based on a single blood sample have been proposed to estimate the clearance of 131I-hippuran (HIP) and 99Tcm-mercaptoacetyltriglycine (MAG). The blood sample is usually drawn at that time which yields a minimum error between the estimated clearance and that obtained by the standard nine-sample method. In this paper, we establish the regression equations to obtain the HIP and MAG clearances using one sample withdrawn during a fixed time interval. As HIP has long been the agent of choice, we have also established regression equations to estimate the clearance of HIP using one sample after MAG administration. Our results suggest that it is possible to obtain plasma samples 30-50 min post-injection which result in an error of the estimate only slightly higher than the minimum.

Adult↗

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↗

PubMed1996

Baseline, visual deprivation and visual stimulation 99TCm-HMPAO-related changes in visual cortex can be detected with a single-head SPET system.

To determine the sensitivity of 99TCm-hexamethylpropylene amine oxime (99TCm-HMPAO) and a single-head SPET (single photon emission tomography) system in the detection of perfusion changes in the visual cortex due to different visual conditions, six normal healthy volunteers were studied under conditions of visual deprivation (blindfolded), visual stimulation (stroboscopic light) and baseline (dim light and eyes open). Visual cortex/whole-brain activity ratios, and the percentage of activity change between the different visual conditions were calculated after three-dimensional realignment of the images. The activity in the visual cortex was higher during visual stimulation than during the visual deprivation (P = 0.002, 17.6 +/- 8.6% increase) and baseline conditions (P = 0.009, 8.8 +/- 5.6% increase). Furthermore, the activity in the visual cortex was lower during the visual deprivation than in the baseline condition (P = 0.001, 8.1 +/- 2.9% decrease). 99TCm-HMPAO SPET, even with a single-head system, is capable of detecting changes in rCBF in the striate cortex, not only between conditions of visual stimulation and deprivation, but also between these two conditions and the baseline state.

Adult↗

PubMed1995

Tc-99m galactosyl-neoglycoalbumin hepatic scintigraphy in fulminant hepatic failure.

Tc-99m galactosyl-neoglycoalbumin (NGA) is a new liver imaging agent that specifically binds to a hepatocyte specific membrane receptor, the hepatic binding protein. Scintigraphy with Tc-99m NGA is a noninvasive method that provides functional images of the liver. This report deals with one case of fulminant hepatic failure in which hepatic scintigraphy with Tc-99m NGA predicted hepatic recovery before clinical and biochemical parameters, being a prognostic index in this patient.

Adult↗

PubMed1994

Three-dimensional realignment of activation brain single-photon emission tomographic studies.

Two methods for 3D realignment of activation brain single-photon emission tomographic (SPET) studies are analyzed. The first is based on principal axes transformation (PAT). The second uses the results of the first method as initial values to start a least-squares iterative process (LS) to search for the maximum value of the correlation function. Both methods were tested with simulated and real studies. The results of the PAT method showed a maximum translation error of 0.3 +/- 0.1 pixels and a rotational error of 1.2 +/- 0.7 degrees in a total of 100 runs. For the LS method these errors were 0.2 +/- 0.1 and 0.6 +/- 0.3. The realignment for 34 real studies was assessed by three expert observers. The alignment was found to be satisfactory in all cases for the LS method, and in 18 cases (53%) for the PAT method. From the results we conclude that a combination of both methods allows the accurate realignment of SPET neuroactivation studies.

Algorithms↗

PubMed1994

Pulmonary vascular abnormalities and ventilation-perfusion relationships in mild chronic obstructive pulmonary disease.

Morphologic changes in pulmonary muscular arteries may modify the mechanisms that regulate the pulmonary vascular tone and contribute to maintaining an adequate ventilation-perfusion (VA/Q) matching in patients with chronic obstructive pulmonary disease (COPD). To analyze the relationships between the abnormalities of pulmonary muscular arteries and the degree of VA/Q inequality, and to assess the effect of these abnormalities on the changes in VA/Q relationships induced by oxygen breathing, we studied a group of patients with mild COPD undergoing resective lung surgery. According to the degree of airflow obstruction and the increase in VA/Q mismatch produced by 100% O2 breathing (delta logSD Q), patients were divided into three groups: (A) patients with normal lung function, (B) patients with airflow obstruction and a high response to oxygen (delta logSD Q > 0.4), and (C) patients with airflow obstruction and a low response to oxygen (delta logSD Q < 0.4). Pulmonary arteries in Groups B and C showed narrower lumens and thicker walls than in Group A. These morphologic changes were produced mainly by an enlargement of the intimal layer and were more pronounced in Group C than in Group B. The assessment of intimal area as a function of artery diameter showed that the increase in intima in Group C took place predominantly in arteries with small diameters (< 500 microns). The mean intimal area on each subject correlated with both the PaO2 value (r = -0.46, p < 0.05) and the overall index of VA/Q mismatching (r = 0.51, p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Female↗

PubMed1994

Estimate of relative function and transit time in renographic studies.

Renal transit time and relative renal function are the most commonly used parameters in the study of renal function. In clinical practice the determination of these two parameters is carried out either using the renal retention function or directly from the renogram. This study seeks to compare the values for the transit time and the relative function, as calculated from the renogram and from the renal retention function, in renographic studies using 131I-OIH and 99mTc-MAG3. For both tracers it was found that the estimated renal transit time from the renogram (OIH: 289 +/- 118 s, MAG3: 297 +/- 110 s) generated values that were higher than those obtained from the renal retention function (OIH: 245 +/- 85 s, MAG3: 274 +/- 97 s), with significant differences between the two estimates (p < 0.001). As regards the relative function, there were no significant differences between the estimates obtained from the renogram (58.5 +/- 8.3%) and the renal retention function (59.5 +/- 9.3%) in the case of OIH. For MAG3 the estimate obtained from the renogram (58.3 +/- 6.3%) and from the renal retention function (59.9 +/- 7.1%) were significantly different (p < 0.025).

Humans↗

PubMed1994

Prefrontal and temporal blood flow in schizophrenia: resting and activation technetium-99m-HMPAO SPECT patterns in young neuroleptic-naive patients with acute disease.

UNLABELLED: This study assesses prefrontal and temporal regional cerebral blood flow (rCBF) changes in young, neuroleptic-naive schizophrenic patients with acute disease. METHODS: A selected population of 10 young, never-treated schizophrenic women with acute disease was studied by two hexamethylpropyleneamine oxime (HMPAO) brain SPECT sessions, performed 48 hr apart, both at rest and during a prefrontal activation task using the Wisconsin Card Sort Test (WCST). All patients met Diagnostic and Statistical Manual of Mental Disorders, 3rd edition-revised criteria for schizophrenia or schizophreniform disorder, were neuroleptic-naive and had acute symptoms. RESULTS: Under resting conditions, the schizophrenic group had significantly higher rCBF in the prefrontal regions, mainly in the left side and including the anterior cingulate, than did the controls. In addition, schizophrenic patients showed significant interhemispheric differences in prefrontal and posterior temporal index values at rest (left hyperfrontality and left hypotemporality). During WCST activation, the control group showed significant increases in prefrontal blood flow, whereas the schizophrenic group did not. CONCLUSION: These results support a physiologic dysfunction of the prefrontal cortex in schizophrenia that is present at the onset of the illness prior to neuroleptic treatment. Furthermore, both left hyperfrontality and left hypotemporality may indicate a brain lateralization defect in schizophrenia.

Acute Disease↗

PubMed1991

Comparative study of the kinetics of the renal tracers 99Tcm-MAG3 and 131I-hippuran.

Serial measurements of plasma activity, plasma protein binding and urine excretion were obtained in order to study 99Tcm-MAG3 (MAG) and 131I-Hippuran (OIH) kinetics after simultaneous injection of both tracers in 21 patients with various renal diseases. Results were compared on the basis of a compartmental model, calculating the rate constants as well as the clearance and volume of distribution. Protein binding was calculated in 10 patients (mean: MAG = 54.7%, OIH = 33.8%). The dependence of time, tracer and patient factors was shown by ANOVA. Time was independent, with tracer and patient factors and their interaction being significant. The mean value of the renal excretion constant was equal for the two tracers (k12 = 0.052 min-1). The clearance values were found to be highly correlated (r = 0.982) with a ratio of 0.57 between them. The volumes of distribution in litres were 4.1 (MAG) and 7.0 (OIH). One-hour urine excretion was nearly the same for both tracers (MAG: 64%, OIH: 63% of the injected dose).

Blood Proteins↗

PubMed1991

Evaluation of thallium myocardial-perfusion imaging by means of washout parametric images: a simulation study.

A method for the evaluation of myocardial perfusion with 201Tl scintigraphic images is presented. The method is based on making a pixel-by-pixel comparison of the stress and the redistribution images aligned by using bidimensional correlation techniques between the nonpathologic regions of the myocardium. The comparison is made by means of parametric images of the washout, which are evaluated by calculating separately the mean values in the normal and the abnormal regions. The study is carried out on controlled models that contain myocardium, an artifact, and noise. The results show the suitability of the method and confirm the validity of its application to individualized studies of clinical interest.

Computer Simulation↗

PubMed1989

Thallium-201 myocardial planar scintigrams. Alignment of stress and rest images by correlation.

The utility of the correlation for alignment of myocardial perfusion imaging with 201TI was studied. The tests were made by numerical simulation of a heart model with an artefact and a theoretical background superimposed in normal and pathological situations. The methods used were bidimensional and unidimensional correlation techniques and were applied either with or without previous background subtraction. Our results show the suitability of this alignment technique, useful in finding the displacement between images with an average error inferior to one pixel in different situations of myocardial damage.

Computer Simulation↗

PubMed1989

Methods of background subtraction in myocardial perfusion imaging with thallium-201: a comparative study in the presence of an artefact.

For thallium imaging quantitation, a simulation study has been made using different methods of interpolation for background subtraction and studying their behaviour in the presence of an artefact. Two artefacts were used, one simulating hepatic or splenic uptake and the other diaphragmatic activity. The interpolative methods used were three classic techniques using standard masks. Their algorithms were modified so that they could also be used with arbitrarily shaped masks enclosing myocardium, and a comparison between the original and modified methods was made. Our results show an improvement when using the modified algorithms adapted to the arbitrarily outlined mask.

Computer Simulation↗