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At least 1,225 records · Page 68Linked to original sources
PubMed1991

[A study on crosstalk correction in dual energy acquisition of 123I-MIBG and 201TlCl in myocardial SPECT].

In the simultaneous dual energy acquisition, energy spectrums of two radionuclides crosstalk each other and this phenomenon is a cause of the poor quality of images. In order to obtain the image of high quality in dual energy acquisition of 123I-MIBG and 201TlCl, a crosstalk correction method was originated. The crosstalk from 201Tl to 123I window (R1) and the crosstalk from 123I to 201Tl window (R2) were determined by the cardiac phantom studies. R1 and R2 showed almost constant value throughout the myocardial wall. The crosstalk correction was performed using R1 and R2. After the crosstalk correction, the defect region placed in the cardiac phantom was detected more clearly both in visual interpretation and in quantitative analysis. The crosstalk correction method with R1 and R2 was applied to some clinical cases. By the crosstalk correction, the quality of image was improved and a false defect caused by crosstalk disappeared in a clinical case. The crosstalk correction was considered to be useful for improving the quality of image on dual energy acquisition.

3-Iodobenzylguanidine↗
PubMed1991

Estimation of entire cardiac left ventricular pressure cycle from brachial arterial pressure during systemic hypoxia.

In functional study of the cardiac pump during systemic hypoxia, noninvasive measurement of the cardiac left ventricular pressure (LVP) during hypoxia is a tempting problem. The authors found from point of view of systems physiology that there existed correlation between brachial arterial blood pressure (BP) and LVP. Using method of normalization and stepwise regression, models were established for estimating LVP upon BP. For the LVP estimated with single hypoxia condition models the average relative area errors e less than 4.3%, with the general hypoxia condition model, e = 5.6%. This approach may be expected to be developed into a method of noninvasive measuring of the LVP.

Animals↗
PubMedDate not supplied

Quantitative measurement of high flow velocities by a spin echo MR technique.

A new method of flow measurement using a spin echo (SE) technique has been developed on the basis of the flow effect that at high velocities signal intensity decreases linearly with increasing flow velocity. Flow velocity is calculated from the signal intensity ratio of the flowing material in two images with the same imaging parameters but different echo times. The linear relationship between the signal intensity and flow velocity was examined with a steady flow phantom. When assessed with steady flows in the phantom, flow velocities calculated by this method were in good agreement with velocities measured by a flow meter. This method was used with ECG gating to measure the blood flow of the right common carotid artery of a healthy volunteer. The measured peak flow velocity and the pattern of flow velocities during systole correlated well with the results obtained by Doppler ultrasound.

Blood Flow Velocity↗
PubMedDate not supplied

Mechanisms of MR signal alteration by acute intracerebral blood: old concepts and new theories.

We reviewed the current literature and theories pertaining to the appearance of hemorrhage on MR images. New theories were formulated to explain the etiology of spin-echo (SE) and gradient-echo contrast on clinical MR images of hematomas at low and intermediate field strengths. It is our hope that these theories will prompt further experiments to prove or disprove their validity. The discussion of SE contrast focuses on the powerful MR effects mediated by changes in protein concentration. These changes are postulated to occur as a result of (1) clot matrix formation; (2) settling of RBCs, which increases the number of RBCs per unit volume and the total protein concentration; and (3) alterations in intracellular protein concentration caused by changes in RBC hydration. The contribution of acid methemoglobin to image contrast via the proton electron dipole-dipole mechanism is also included. The discussion of gradient-echo contrast focuses on the inhomogeneity that may be present within voxels that contain a blood clot. This inhomogeneity results when the voxel contains regions with different magnetic susceptibilities (i.e., clumped RBCs and serum). Two temporally arranged schemata for T2-weighted and T1-weighted SE scans are presented that illustrate how these factors might interact to explain the diverse appearance of hematomas on clinical SE images. The signal intensity on T2-weighted SE scans appears to be dependent primarily on the state of hydration of the RBC. This factor and the presence or absence of methemoglobin appear to be the major factors that govern contrast on T1-weighted SE images.

Acute Disease↗
PubMedDate not supplied

A surrogate arm for evaluating the accuracy of instruments for indirect measurement of blood pressure.

A surrogate arm, containing an artificial artery, has been developed for evaluating the accuracy of automatic, non-invasive, blood-pressure measuring devices. The surrogate arm produces all of the events associated with the auscultatory and oscillometric methods of measuring blood pressure. The surrogate arm includes a flat artificial artery in a plastic cylindrical chamber, 30 cm in circumference and 25 cm long, that contains an air/water mixture that allows matching the compliance of the human arm. Linear regression lines were obtained to correlate the indirectly (y) and directly (x) measured pressures. For auscultatory systolic pressure y = 1.01x - 1.60, with a correlation coefficient of 0.99. For mean oscillometric pressure y = 1.00x + 0, with a correlation coefficient of 1.00. For phase IV auscultatory (diastolic) pressure, y = 1.02x - 1.40, with a correlation coefficient of 0.96. For phase V auscultatory (diastolic) pressure y = 0.79x + 10.9, with a correlation coefficient of 0.98.

Arm↗
PubMedDate not supplied

[Mathematical model of intracranial blood-cerebrospinal fluid dynamics system applied to the study of extreme conditions].

A mathematical model of cerebral blood and lymph circulation that can be used to study various effects (simulated space flight factors, functional loads) on the human body was developed. Biophysical patterns that determine relationships between cerebral blood and lymph circulation as well as between fluid volumes and pressures (P and V) are discussed. Involvement of the mechanism of brain circulation autoregulation in cerebral blood and lymph circulation is considered. The model was applied to study responses of cerebral blood and lymph circulation to various exposures such as orthostatic and antiorthostatic tests and provocative tests which led to intracranial pressure shifts and modified the ratio between volumes and pressures in different hydrodynamic systems. The modelling data were consistent with clinical, physiological and experimental observations which demonstrate an adequacy of the above model.

Aerospace Medicine↗
PubMed1990

Sustained peripheral vasoconstriction while working in continuous intense noise.

The study examined the effects of exposure to continuous noise on pulse volume (vasoconstriction) and rate, as well as four-choice serial reaction time. There were 2 separate groups of 12 subjects that performed the 20-min task during simultaneous monitoring of their physiological response to continuous 93 dBA white noise or 70 dBA quiet control conditions. Pulse volume showed marked reduction, implying increased arousal, in the first 3-min exposure to noise. Although some habituation of the vasoconstriction response then occurred, pulse volume continued to remain significantly higher than in quiet throughout the work period. Serial choice performance was unaffected by noise. The results confirm that there is a physiological cost incurred when working in noise and one may be concerned for long-term effects on noise-exposed people.

Adult↗
PubMed1986

Membrane currents underlying bursting pacemaker activity and spike frequency adaptation in invertebrates.

Invertebrate systems have proved to be quite useful for the development of an understanding of some processes in the central nervous system (CNS). An understanding of the basic mechanisms of epilepsy will result from understanding not only how populations of neurons interact but also how the physiological processes in individual neurons are altered in epileptogenesis. Because invertebrate neurons have been so accessible to experimentation, it has been possible to explore in detail the basic mechanisms controlling neuronal excitability using these cells and to make some useful predictions about electrophysiological mechanisms that may be present in central neurons. This chapter deals with two electrophysiological processes that have been observed in invertebrate neurons and that may have some relevance to understanding the basic mechanisms of epilepsy. We review first the past and current studies of invertebrate burst firing neurons. It appears that the electrophysiological mechanisms producing burst firing may be present in CNS neurons participating in epileptogenesis. With caution, the information gleaned from invertebrate studies may be applicable to higher systems. The second process we consider is the phenomenon of spike frequency adaptation seen in invertebrates. Spike frequency adaptation is the process by which the firing rate of the neuron declines despite the maintenance of a constant stimulus. This process is not so thoroughly studied as burst firing, but it appears to represent a cellular mechanism designed to suppress prolonged periods of repetitive firing. Clearly, the suppression of such a process would produce excessive neuronal excitability, while its enhancement might have some anticonvulsant effects. The extreme sensitivity of spike frequency adaptation to barbiturates suggests such a possibility. These two electrophysiological processes are interesting in themselves and also because they may underlie the genesis or control of seizures. However, the greater significance is that, to understand the basic mechanisms of epilepsy, we may be well advised to examine neuronal processes in systems not considered to have seizure susceptibility.

Action Potentials↗
PubMed1989

A realistic dynamic cardiac phantom for evaluating radionuclide ventriculography: description and initial studies with the left ventricular chamber.

A phantom was devised to validate scintigraphically determined left ventricular ejection fractions (LVEFs) and cardiac chamber volumes in the following simulated cardiac situations: normal contraction, moderately impaired left ventricular contraction, severely impaired left ventricular contraction, mitral regurgitation, and cardiomyopathy. The phantom, assembled from anatomically realistic cardiac chambers, simulated contraction and expansion using individual chamber pumps coordinated by a microcomputer. Scintigraphic studies were performed by sequential imaging of [99mTc]pertechnetate introduced into each chamber. The images were analyzed like conventional clinical studies, using both automatic and manual techniques. Scintigraphic techniques correlated with chamber volumes that were determined by weight to yield the following regression formulae: LVEF (by automatic method 1) = 1.08 x LVEF (by weight) -5.11; LVEF (by automatic method 2) = 1.00 x LVEF (by weight) -3.15; and LVEF (by manual method) = 1.04 x LVEF (by weight) -5.08 ml (Correlation coefficients greater than 0.98). The absolute left ventricular volumes (LVVs), determined by scintigraphy, correlated well with LVVs determined by weight. These correlations were performed with separations between the center of the left ventricle and the collimator varying from 5 cm to 9 cm. The regression formulae for 5, 7, and 9 cm distances were: LVV (by counts) = 0.99 x LVV (by weight) + 0.13, LVV (by counts) = 1.04 x LVV (by weight) + 9.08, LVV (by counts) = 0.88 x LVV (by weight) + 15.25, respectively. At 9 cm, slight volumetric underestimation occurred, as predicted from the work of Fearnow et al., possibly because of oversubtraction of background. Thus, this phantom provides a useful tool for validating scintigraphic cardiac blood-pool studies simulating a wide range of clinically relevant situations.

Heart↗
PubMed1989

Estimation of hematocrit profile symmetry recovery length downstream from a bifurcation.

Downstream from a microvascular bifurcation the distribution of blood cells in the vessel lumen is not symmetric. A diffusion process is used to model the rearrangement of red cells as blood flows between junctions in the microcirculation. A Fourier series approach is used to solve the model diffusion convection equation in slit geometry. Both flat and parabolic velocity profiles are considered. The eigenvalues, found using the Rayleigh-Ritz method, are used to find an upper bound on distance required for a symmetric red cell distribution to be obtained. The method has also been applied to cylindrical geometry and the computed symmetry recovery lengths are compared to distances between bifurcations measured in vivo. These estimates indicate that red cell distributions are frequently asymmetric in the microcirculation. Such asymmetries can have a strong effect on plasma skimming and material balance calculations.

Animals↗
PubMed1989

Effects of non-Newtonian blood and metabolic states of the blood and vessel wall on the optimum design of single vessels and the vascular bifurcation.

Employing the optimality principle, we attempted to predict the effects of non-Newtonian blood and the metabolic states of individual vessel segments on the optimum vascular design. Our results implied that irrespective of the vessel caliber, the optimum flow rate of non-Newtonian blood through a cylindrical vessel is less than that of Newtonian blood by not more than some 12-13%, even though the non-Newtonian nature is within the pathologically-realistic highest range. Non-Newtonian blood does not exert the slightest degree of influence on the optimum geometry of the vascular bifurcation. In contrast, as the metabolic state of the vessel wall overwhelms that of the blood, the optimum flow through the cylindrical vessel becomes markedly increased: the optimum relative caliber of the branch of the bifurcation decreases and the optimum branching angle increases.

Blood↗