SEARCH · PubMed Health

Results for “Models, Cardiovascular”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 253 records · Page 14Linked to original sources

PubMed1994

Real time blood flow imaging by spiral scan phase velocity mapping.

The work describes the development of a novel sequence that uses rapid spiral k-space sampling, combined with phase velocity mapping, for real time flow velocity imaging. The performance of the technique is assessed on phantoms for both through-plane and in-plane flows. The flow measurements compared well with those measured using a bucket and stopwatch. One advantage of the technique is that flow related signal loss is minimal due to the early acquisition of the center of k-space data. Flow artifacts were observed for in-plane flow and these were understood with the aid of computer simulations. In vivo studies involved cine velocity mapping in normal volunteers; aortic blood flow waveforms acquired by spiral scanning in two cardiac cycles compared well with data from a conventional gradient-echo sequence. Potential applications of the method are demonstrated by studying the response of aortic flow to physical exercise and the real time monitoring of aortic flow during a valsalver maneuver.

Algorithms↗

PubMed1994

Tag contrast in breath-hold CINE cardiac MRI.

Contrast between tagged and nontagged myocardium was investigated using rapid gradient echo segmented k-space CINE MRI. The transient behavior of magnetization was measured in stationary and moving phantoms using gradient recalled acquisition in steady-state GRASS and spoiled GRASS (SPGR) sequences with TR approximately 7 ms and TE approximately 2.5 ms. Bloch equation simulations were used to compute theoretical results. Understanding the transient behavior of magnetization is important because tags only persist in the myocardium during the nonequilibrium transition to steady state. The transition to steady state for both SPGR and GRASS is reproducible after one heartbeat, and including unprocessed data from the first heartbeat leads to image artifacts. In a moving phantom, simulations and experimental results showed that GRASS and SPGR are essentially equivalent. Tag-tissue contrast in SPGR was very sensitive to imaging tip angle. The optimum tip angle for the scanning parameters used in this study was 11 degrees.

Algorithms↗

PubMed1994

The application of breath hold phase velocity mapping techniques to the measurement of coronary artery blood flow velocity: phantom data and initial in vivo results.

A segmented kappa-space breath hold phase velocity mapping technique has been developed for the study of coronary artery blood flow velocity. In vitro validation has been performed using a number of pulsatile flow phantoms and the accuracy of the technique for determining the velocity increase at the site of a stenosis demonstrated in several phantom models. Examples of both in-plane and through-plane velocity maps of the left anterior descending and right coronary arteries of normal subjects in early diastole are presented. In one subject, through-plane velocity maps were obtained in the right and left anterior descending arteries throughout the cardiac cycle in order to generate flow velocity time curves. The problems associated with coronary artery velocity mapping are discussed.

Artifacts↗

PubMed1994

Surface coil cardiac tagging and 31P spectroscopic localization with B1-insensitive adiabatic pulses.

A technique is presented for MRI tagging in the presence of inhomogeneous B1 fields. A rectangular tagging grid is produced with B1-insensitive adiabatic pulses in a magnetization preparation period that precedes image acquisition. Phantom results demonstrate that the method is well-suited to surface coil experiments. The technique is applied to a canine model of myocardial ischemia to track the spatially dependent wall motion of the left ventricle during the cardiac cycle. Transmural 31P spectra are acquired from the same double-tuned surface coil, with tagging and spectroscopy performed for the first time, during normal, ischemic, and recovery conditions for the same animal.

Adenosine Triphosphate↗

PubMed1994

Development and evaluation of tracking algorithms for cardiac wall motion analysis using phase velocity MR imaging.

Phase velocity magnetic resonance imaging (MRI) has shown considerable potential for tracking distinct regions of the myocardium throughout the cardiac cycle. Phase contrast MR imaging produces multiple images, each phase encoded for velocity in a different direction, in which individual pixels depict the local motion of the tissue. In this work we present in detail three algorithms for tracking motion based on these images. Both simulated and phantom data are used to examine some of the problems encountered in practice in tracking points based on velocity maps. Solutions to these problems are offered when possible. The impact of noise and low order phase errors in the data on each of the three tracking approaches is examined. In addition, problems due to tissue expansion and contraction, to 2D versus 3D tracking, and to round off errors from motion which is small relative to pixel size or slice thickness, are considered. An example using data obtained in vivo is included to demonstrate the efficacy of the best of the three tracking algorithms in measuring left ventricular circumferential shortening preinfarct and postinfarct in a canine model.

Algorithms↗

PubMed1994

MRI signal void due to in-plane motion is all-or-none.

The process of MRI signal attenuation due to in-plane intravoxel velocity inhomogeneity is described. Given rigid rotation or linear shear, velocity phase-sensitivity will induce a phase distribution that varies linearly with position, which is exactly equivalent to the effect of a spatial phase encoding gradient pulse. It follows that the effect of such motion on the raw MRI signal is to displace it a fixed distance in kappa-space. Attenuation becomes marked when the center of the spin-echo reaches an edge of kappa-space, which happens when intravoxel phase shifts reach pi radian/voxel. Because spin echoes are typically peaked sharply at center, this attenuation usually is abrupt. Analytic and numerical simulations of linear and nonlinear velocity fields confirm abrupt MRI attenuation where phase dispersion exceeds pi radian/voxel. Examples of this phenomenon include the abrupt loss of blood signal adjacent the vessel wall in laminar flow, abrupt loss of subendocardial signal in early diastole, and sudden disappearance due to rotation of a kidney during a measurement of diffusion.

Artifacts↗

PubMed1994

MR gradient echo imaging of intravascular blood oxygenation: T2* determination in the presence of flow.

The T2* relaxation time of blood varies with its oxygen saturation. To evaluate the feasibility of imaging intravascular blood oxygenation in humans using a conventional 1.5T MR system, we have implemented a method to measure T2* of blood despite the presence of pulsatile flow. The method was tested in a) stationary and flow phantoms, b) blood samples at different levels of oxygen saturation, and c) a human hypoxia model. Our results demonstrate the ability of cardiac-triggered, flow compensated gradient echo imaging to obtain reproducible T2* measurements of flowing blood in vivo.

Artifacts↗

PubMed1994

High speed bolus tagging: time resolved velocity quantification of pulsatile flow in a single breath hold.

We have implemented a high speed method for cardiac-triggered blood velocity quantification within a single breath hold on a conventional MR system. The method, based on bolus tagging, was tested using a pulsatile flow phantom and evaluated in vivo. The image acquisition time was reduced by a factor of N by acquiring N phase encode lines per bolus tag application. The clarity of the flow tag was found to vary with how k-space was covered during data collection. The technique was optimized and multiple bolus tag images were obtained throughout the cardiac cycle within a single breathold.

Aorta, Thoracic↗

PubMed1995

Three-point phase-contrast velocity measurements with increased velocity-to-noise ratio.

We describe a technique to increase the velocity-to-noise ratio (VNR) of phase-contrast magnetic-resonance velocity images based on making three measurements/flow encoding axis rather than the usual two. A phase-aliased high first moment data set and a nonaliased low first moment data set are acquired, and the high-moment data are phase unwrapped using the low-moment data. The VNR of the resulting measurement is given by that of the high-moment measurement and increases linearly with the first moment. A factor of 4 gain in VNR was observed with only a 50% increase in scan time. Thus, this method is a much more efficient way to increase VNR than simple averaging.

Cerebrospinal Fluid↗

PubMed1995

Limitations of stimulated echo acquisition mode (STEAM) techniques in cardiac applications.

Stimulated echoes are widely used for imaging functional tissue parameters such as diffusion coefficient, perfusion, and flow rates. They are potentially interesting for the assessment of various cardiac functions. However, severe limitations of the stimulated echo acquisition mode occur, which are related to the special dynamic properties of the beating heart and flowing blood. To the well-known signal decay due to longitudinal relaxation and through-plane motion between the preparation and the read-out period of the stimulated echoes, additional signal loss is often observed. As the prepared magnetization is fixed with respect to the tissue, this signal loss is caused by the tissue deformation during the cardiac cycle, which leads to a modification of the modulation frequency of the magnetization. These effects are theoretically derived and corroborated by phantom and in vivo experiments.

Blood Flow Velocity↗

PubMed1995

A phased array coil for human cardiac imaging.

A prototype cardiac phased array receiver coil was constructed that comprised a cylindrical array and a separate planar array. Both arrays had two coil loops with the same coil dimensions. Data acquisition with the cylindrical array placed on the human chest, and the planar array placed under the back, yielded an overall enhancement of the signal-to-noise ratio (SNR) over the entire heart by a factor of 1.1-2.85 over a commercially available flexible coil and a commercially available four-loop planar phased array coil. This improvement in SNR can be exploited in cardiac imaging to increase the spatial resolution and reduce the image acquisition time.

Heart↗

PubMed1995

Three-dimensional spatial EPR imaging of the rat heart.

While instrumentation capable of performing three-dimensional EPR imaging of free radicals in whole tissues and isolated organs has been developed at L-band, important questions remain regarding the resolution and image quality that can be obtained in practice using the presently available free radical labels. Therefore, studies were performed applying three-dimensional spatial EPR imaging at L-band to image the distribution of free radical labels in the isolated heart and in phantoms of similar size. With nitroxide labels the obtainable resolution is limited by the presence of hyperfine structure in the EPR absorption function that in turn limits the maximum applicable gradient. The authors observed that with the nitroxide labels, resolutions in the range of 1-2 mm are possible, while with a single line glucose char label, resolutions of 0.2 mm are obtained. With the nitroxides, images were of sufficient resolution to resolve the overall global shape of the heart and the location of the left and right ventricular cavities; however, finer structures could not be resolved. With the glucose char much finer resolution could be obtained enabling visualization of the ventricles, aortic root, and proximal coronary arteries.

Animals↗

PubMed1996

Magnetization exchange in capillaries by microcirculation affects diffusion-controlled spin-relaxation: a model which describes the effect of perfusion on relaxation enhancement by intravascular contrast agents.

The effect of perfusion on relaxation time in tissue has only been considered for first-pass kinetics of NMR-signal after application of contrast agents. The importance of perfusion on relaxation has not yet been studied for steady state conditions, i.e., when the intravascular relaxation rate is constant in time. The aim of this study is to develop a model in which T1 relaxation is derived as a function of perfusion and intracapillary volume fraction (regional blood volume). Tissue is considered to be two-compartment system, which consists of intracapillary and extravascular space. Intracapillary relaxation differs from relaxation in the arterial system due to diffusion-exchange of magnetization from extravascular to intracapillary space. Perfusion tends to attenuate this difference and thus counteracts the effect on intracapillary relaxation. Relaxation in the extravascular and intracapillary magnetization are linked by diffusion. This dependence is presented in analytical form and a generic equation is derived. AT1 experiment is considered in which all spins of tissue and blood are inverted at the beginning. Calculations are performed for the fast exchange model of tissue. Perfusion increases relaxation enhancement of intravascular contrast agents. This effect is considerable in highly perfused tissue like myocardium. The dependence of relaxation on perfusion implies an overestimation of the regional blood volume when the calculation of the latter is based on tissue models that neglect perfusion. The model presented here is applied to predict the effect of perfusion on T1 imaging with FLASH-pulse sequences because this technique has been proven to be a powerful method to obtain T1 maps within a short time interval. For the fast exchange model, two algorithms are suggested that determine perfusion and regional blood volume from T1 imaging in the presence and absence of intravascular contrast agents.

Blood Volume↗

PubMed2006

Theoretical and experimental investigation of the VASO contrast mechanism.

Vascular space occupancy (VASO)-dependent functional MRI (fMRI) is a blood-nulling technique capable of generating microvascular cerebral blood volume (CBV)-weighted images. It is shown that at high magnetic field (3.0T) and high spatial resolution (1.89 x 1.89 x 3 mm(3)), the VASO signal changes are too large (6-7%) to originate from CBV effects alone. Additional contributions are investigated theoretically and experimentally as a function of MRI parameters (TR and TE), as well as the signal-to-noise ratio, (SNR) and spatial resolution. First, it is found that an arterial spin labeling (ASL) contribution causes large negative VASO signal changes at short TR. Second, even at high fMRI spatial resolution, CSF volume contributions (7-13%) cause VASO signal changes to become more negative, most noticeably at long TR and TE. Third, white matter (WM) effects reduce signal changes at lower spatial resolution. The VASO technique has been tested using different stimulus paradigms and field strengths (1-3), giving results consistent with comparable tasks investigated using BOLD and cerebral blood flow (CBF)-based techniques. Finally, simulations show that a mixture of fresh and steady-state blood may significantly alter signal changes at short TR (< or =3 s), permitting larger VASO signal changes than expected under pure steady-state conditions. Thus, many competing effects contribute to VASO contrast and care should be taken during interpretation.

Adult↗

PubMed2004

Test of treatment effect in pre-drug and post-drug count data with zero-inflation.

A procedure for testing for treatment effect in data similar to the data on premature ventricular contractions (PVC) is presented. We consider a zero-inflated beta-binomial model. Based on this model, we develop score tests to test for treatment effect in the data in which observations in the form of counts are recorded before and after applying a therapy. Results of a small simulation experiment, to study small sample behaviour of a score test and a likelihood ratio test, are reported and the PVC data are analysed. Both the score and the likelihood ratio tests show good level properties. Either the score tests or the likelihood ratio tests can be used for testing the presence of treatment effect. The score tests, however, may be preferable because they use estimates of the parameters only under the null hypothesis and in the important range pi<0.5 power of the score test statistic S1 is slightly better than the likelihood ratio statistic LR1.

Anti-Arrhythmia Agents↗

PubMed2004

Modelling the distribution of ischaemic stroke-specific survival time using an EM-based mixture approach with random effects adjustment.

A two-component survival mixture model is proposed to analyse a set of ischaemic stroke-specific mortality data. The survival experience of stroke patients after index stroke may be described by a subpopulation of patients in the acute condition and another subpopulation of patients in the chronic phase. To adjust for the inherent correlation of observations due to random hospital effects, a mixture model of two survival functions with random effects is formulated. Assuming a Weibull hazard in both components, an EM algorithm is developed for the estimation of fixed effect parameters and variance components. A simulation study is conducted to assess the performance of the two-component survival mixture model estimators. Simulation results confirm the applicability of the proposed model in a small sample setting.

Age Factors↗

PubMed2004

Separating within and between effects in family studies: an application to the study of blood pressure in children.

In epidemiology the analyses of family or twin studies do not always fully exploit the data, as information on differences between siblings is often used while between-families effect are not considered. We show how cross-sectional time-series linear regression analysis can be easily implemented to estimate within- and between-families effects simultaneously and how these effects can then be compared using the Hausman test. We illustrate this approach with data from the Uppsala family study on blood pressure in children with age ranging from 5.5 to 12.3 years for the younger and from 7.5 to 13.8 years for the older siblings. Comparing the effect of differences in birth weight on blood pressure within-family (in full siblings) and between-families (in unrelated children) allows us to study the contributions of fixed and pregnancy-specific maternal effects on birth weight and consequently on blood pressure. Our data showed a 0.88 mmHg decrease (95 per cent confidence interval: -1.7 to -0.03 mmHg) in systolic blood pressure for one standard deviation increase in birth weight between siblings within a family and 0.88 mmHg (95 per cent confidence interval: -1.6 to -0.2 mmHg) decrease in systolic blood pressure for one standard deviation increase in birth weight between unrelated children. These estimates were controlled for sex, age, pubertal stage, body size and pulse rate of the children at examination and for maternal body size and systolic blood pressure. The within- and between-families effects were not significantly different, p = 0.19, suggesting that fixed and pregnancy-specific factors have similar effects on childhood systolic blood pressure.

Adolescent↗

PubMed2004

QT analysis: a complex answer to a 'simple' problem.

Prolongation of the QT interval on a surface electrocardiogram is a biomarker for a potentially life-threatening arrhythmia. It is used by drug developers and regulatory agencies as a measure of drug safety. Heart rate or RR interval (the inverse of heart rate) correction of the QT interval is necessary because of the QT interval shortening that accompanies physiologic decreases in the RR interval. When a drug alters the RR interval, it is important to distinguish a QT change that is due to a drug effect versus an artefact of a heart rate change. A two-step off-drug subject-specific QT correction analysis is discussed. At the first step, a linear mixed model based only on the placebo (off-drug) RR/QT data produces a correction coefficient that can be applied to a generic formula, and QT intervals are corrected for heart rate on both placebo and treatment period data using that formula. At step two, the heart rate corrected QT interval (QTc) is then compared between placebo and treatment groups at a pre-specified heart rate (usually 60 bpm) based on another linear mixed model. This two-step QT analysis implicitly assumes the slope of log(QT) versus log(RR) is unchanged by drug. Practically, it is important to understand how much this assumption can bias the QT prolongation estimates if it is not valid. We propose a one-step off-on-drug subject-specific QT correction analysis that would pool placebo and treatment period RR/QT data and derive different subject specific coefficients for the treatment and placebo data based on a linear mixed model, which can avoid the unchanged slope assumption. It is also a known unbiased and the most efficient method. The applications of both methods are demonstrated through the QT analysis of haloperidol, a neuroleptic known to prolong QTc. Both theoretical and empirical results show that, although the two-step off-drug QT correction analysis is biased, the bias is small in the case of haloperidol (0.1-0.2 ms). The two-step off-drug QT correction analysis is shown to be almost as efficient as our one-step off- and on-drug QT analysis.

Adult↗