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At least 1,243 records · Page 69Linked to original sources
PubMed1989

Modelling paired survival data with covariates.

The objective of this paper is to consider the parametric analysis of paired censored survival data when additional covariate information is available, as in the Diabetic Retinopathy Study, which assessed the effectiveness of laser photocoagulation in delaying loss of visual acuity. Our first approach is to extend the fully parametric model of Clayton (1978, Biometrika 65, 141-151) to incorporate covariate information. Our second approach is to obtain parameter estimates from an independence working model together with robust variance estimates. The approaches are compared in terms of efficiency and computational considerations. A fundamental consideration in choosing a strategy for the analysis of paired survival data is whether the correlation within a pair is a nuisance parameter or a parameter of intrinsic scientific interest. The approaches are illustrated with the Diabetic Retinopathy Study.

Actuarial Analysis↗
PubMedDate not supplied

[Analysis of the principal factors which influence cerebral circulation and intracranial cerebrospinal fluid dynamics].

Cerebral blood flow and intracranial pressure dynamics are affected by two main factors. A first factor is represented by the action of cerebrovascular regulatory mechanisms (controls of cerebral blood volume and cerebral blood flow). Furthermore, intracranial circulation occurs within a rigid space, whose volume must, therefore, remain constant. This second factor explains the presence of a brain volume control, in order to maintain a normal value of intracranial pressure. The mean intracranial pressure value is mainly affected by the pressure-volume characteristic of the intracranial compartment, by the mean cerebral blood volume and by cerebrospinal fluid production and reabsorption processes. On the contrary, the pulsating intracranial pressure component is mainly affected by the intracranial pressure volume relationship and by cerebrovascular compliance; this last, in its turn, is modified by the action of regulatory mechanisms. A better understanding of the relationship occurring between cerebral hemodynamics, cerebrospinal fluid kinetics and intracranial pressure can be achieved with the aid of mathematical models. In particular, mathematical modelling allows the way intracranial pressure and cerebral blood flow are related to the main physical parameters of the intracranial compartment to be pointed out. Moreover, with a mathematical model it is possible to discriminate between the great number of different factors which act simultaneously "in vivo" on the cerebrovascular bed.

Cerebrovascular Circulation↗
PubMedDate not supplied

A computerized physiologic pulse duplicator for in-vitro hydrodynamic and ultrasonic studies of prosthetic heart valves.

A physiologic pulse duplicator for the simultaneous in-vitro hydrodynamic and ultrasonic evaluation of aortic prosthetic heart valves is described. The system is interfaced to a personal computer, which provides greater efficiency over manual techniques in system calibration, data acquisition, and analysis. The data analysis program aids selection of start and end systole and calculates pressure difference across the valve, the closing and regurgitant volumes, flow rates, and the Gabbay and Swanson performance indices. The pulse duplicator is designed to accommodate the ultrasonic measurement of fluid velocities, including pressure difference via the Bernoulli equation, and color-flow imaging. In tests of 19-mm, 23-mm, and 27-mm Bjork-Shiley spherical occluder valves, continuous-wave Doppler ultrasound was found to overestimate by about 50% the pressure difference measured directly by pressure transducers, a finding that is clinically important.

Aortic Valve↗
PubMed1989

Vascular resistance vs. perfusate osmolarity: the short term microvascular effect of hypotonic and hypertonic perfusion in the isolated kidney.

Vascular resistance changes were measured in response to alteration in perfusate osmolarity in isolated rabbit kidneys perfused at 10 degrees C. The data obtained were found to fit a simple mathematical model of the vascular resistance of the microcirculation in which it was assumed that variation in this parameter depended solely upon osmotic alterations in the size of the cells within and around the blood vessel walls. The model predicts that the volumetric changes due to the different osmolarities are produced in a tissue layer whose thickness is 30% relative to a fixed outer radius. This result is compatible with the hypothesis that the effects are predominantly due to changes of endothelial cell volume and other perivascular capillary cells. The analysis illustrates the significance of perfusate osmolarity as a determinant of vascular resistance, can be used to investigate the hemodynamic effects that occur during the introduction and removal of cryoprotective agents, and is relevant to the interpretation of results obtained with hypertonic solutions in blood volume restoration after hypovolemic shock.

Animals↗
PubMed1989

Correction of nonuniform attenuation in cardiac SPECT imaging.

Correction for photon attenuation in cardiac SPECT imaging using a measured attenuation distribution with an iterative expectation maximization (EM) algorithm and an iterative Chang algorithm were compared with the conventional filtered backprojection and an iterative EM algorithm without attenuation correction. The attenuation distribution was determined from a transmission computed tomography study that was obtained using an external collimated sheet source. The attenuation of the emitting photons was modeled in the EM algorithm by an attenuated projector-backprojector that used the estimated attenuation distribution to calculate attenuation factors for each pixel along each projection and backprojection ray. Results from a heart-lung phantom study and a 201Tl patient study demonstrated that the iterative EM algorithm with attenuation correction provided improved image quality in terms of reduced streak artifacts and noise, and more accurate quantitative information in terms of improved radioactivity distribution uniformity where uniformity existed, and better anatomic object definition.

Algorithms↗
PubMed1988

[Coronary artery blood flow velocity non-invasively measured using a vessel-tracking pulsed Doppler system].

A newly-developed noninvasive method was used to measure left coronary blood flow during phantom experiments. Two techniques were used in which: (1) the sample position can always be set in a fluctuating vessel using a wall echo-tracking method with a phase-locked-loop, and (2) the Doppler reference signal was generated separately synchronous with the wall echo signal. These techniques were combined, using a commercially available pulsed Doppler apparatus (SSH-40B: Toshiba). Basic experiments were performed using a blood vessel phantom to verify the validity of these systems. Blood flow velocity in the fluctuating tube could be measured clearly using a vessel-tracking method. The blood flow velocity of the left anterior descending artery was measured in three normal subjects and in seven patients from the third intercostal space along the left sternal border. The velocity pattern was characterized by a crescendo-decrescendo shape in diastole. The peak velocity which appeared in diastole ranged from 19 to 69 cm/sec, with no difference by disease entity. However, in all cases, the blood flow velocity signals were marred by extraneous signals, making it impossible to measure blood flow velocity during systole. Further improvement of the system is mandatory in order to use this flowmeter clinically.

Adult↗
PubMed1988

[Analysis of myocardial texture in two-dimensional echocardiographic images].

To evaluate tissue changes, we studied myocardial texture using two-dimensional echocardiographic images. We investigated 19 normal subjects, 28 patients with left ventricular hypertrophy, and 12 patients with old anteroseptal myocardial infarction of longer than one year duration. Using 2.5, 3.5, and 5.0 MHz transducers, two-dimensional echocardiograms in the parasternal long-axis view were obtained, and the textures of the interventricular septal images were classified in three types; type I, with a nearly uniformly speckled or echolucent appearances; type II, with multiple, discrete, small (2 to 4 mm) highly refractile echoes; type III, with larger highly refractile echoes (greater than 4 mm) appearing as a cluster of broad patches or band-like echoes. Normal subjects belonged to the type I texture, while many with left ventricular hypertrophy belonged to the type II category. Type III was often observed in patients with old anteroseptal infarction. Using a transducer of higher frequency, there tended to be a shift from type II to I or type III to II. In phantom experiments using carbolandam granules instead of the myocardium, the echocardiographic texture became rough when the phantom was farther from the transducer or the transducer was of low frequency. We suggest that the texture in two-dimensional images may reflect myocardial tissue changes, when other factors including the apparatus and technique remain stable.

Adult↗
PubMed1986

Carotid bifurcation imaging model for more accurately comparing imaging techniques.

For comparing current and presubably forthcoming imaging modalities, a carotid bifurcation model was made from cadaveric specimens. Perfusing and pulsing the immersed common carotid artery and its proximal branches via a Harvard pump simulated clinical imaging conditions. Film-screen (F-S) and digital subtraction (DS) angiography, computed tomography (CT), ultrasound and magnetic resonance imaging (MRI) were compared. For CT and MRI, scanning parameters such as slice thickness, degree of overlapping, amount of contrast medium needed, scanning mode and multiplanar and three-dimensional techniques enabled enhancing the capacity for CT in the clinical setting. Direct axial CT proved to be most accurate for assessing the contours and magnitude of carotid narrowing. Nonetheless, these serial segments were not readily compared with F-S and DS angiographic full length images. The use of multiplanar reconstruction (MPR) and three-dimensional (3-D) CT achieved this and furthermore showed the external contour of the diseased segment. Concerning carotid ulceration, our carotid model study showed CT to be equally accurate with DS and superior to F-S angiography. However, in our clinical study of 34 carotid arteries in 17 patients ulcerations were equally well identified by CT and angiography but DS angiography proved superior in identifying ulcers not seen with CT. Perhaps this discrepancy is explained by the clinical routine of attaining multiple fluoroscopically positioned views of the common carotid bifurcations in DS catheter angiography unlike the complexity of attaining optimal views of tortuous vessels on CT.

Angiography↗
PubMed1988

[Comparison of methods for computer and visual assessment of stenosing lesions of the coronary arteries. 1].

A system of computer analysis of coronary arteries stenotic lesions developed in the USSR Cardiology Research Centre, Academy of Medical Sciences of the USSR, is described. Reliability of the computer analysis data was assessed, and computer and visual methods were compared on the basis of phantom study. The study was performed in 5 models of stenotic coronary arteries, the main parameters of which were measured mechanically as well as by computer and visual analysis of their images. Both methods proved to give highly reproducible intraobserver and interobserver results. Comparison to mechanically obtained data showed that the results of computer analysis are highly significant while visual evaluation allows no accurate evaluation of the stenosis degree (it is overestimated).

Cardiac Care Facilities↗
PubMedDate not supplied

The effects of hydropenia, volume expansion, interstitial pressures, excluded volumes, and venous pressures on the robustness of capillary filtration coefficient determination.

The measurement of peripheral capillary filtration coefficient (Kf) by venous occlusion methods presupposes that filtration equilibrium does not occur along the length of the filtering surface, since this would result in an unpredictably low estimate for Kf. Filtration equilibrium occurs if the ratio of plasma flow (Q) to Kf is less than a critical value previously estimated to be between 200 and 250 mmHg. The minimum Q/Kf was reevaluated, taking into account the nonlinear relationship between protein concentration and oncotic pressure, changes in interstitial fluid hydrostatic and oncotic pressure, interstitial excluded volume, whether hydropenia or volume expansion is present, and whether the induced venous pressure change is low (10 mmHg) or high (40 mmHg). In all conditions the minimum Q/Kf ranged from 10 to 20 mmHg, while experimental conditions commonly have Q/Kf ratios from 1200 to 1500 mmHg. Thus, filtration equilibrium is unlikely to occur during Kf determinations under any circumstances in a living organism. Kf methodology is considerably more robust than previously reported.

Animals↗