Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Models, Cardiovascular”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

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

At least 235 records · Page 13Linked to original sources
PubMed1990

Epicardial and endocardial localized 31P magnetic resonance spectroscopy: evidence for metabolic heterogeneity during regional ischemia.

Previous studies have noted that myocardial blood flow and high energy phosphates are heterogeneous across the myocardial wall during ischemia. In order to determine whether differences in metabolites between the subendocardium and subepicardium could be detected using 31P magnetic resonance spectroscopy, the Fourier series window (FSW) experiment was implemented on a porcine model of graded regional ischemia. FSW experiments using a planar phantom showed a 46% improvement in localization to the subendocardium compared to a one-pulse experiment. Animal studies of graded ischemia demonstrated a gradient in the phosphocreatine to inorganic phosphate ratio in the myocardium that paralleled the gradient in blood flow. These studies demonstrate the ability of spatially localized 31P magnetic resonance spectroscopy to detect regional changes in myocardial high energy phosphates localized to the subepicardium and subendocardium.

Animals↗
PubMed1990

Bulk magnetic susceptibility shifts in NMR studies of compartmentalized samples: use of paramagnetic reagents.

The bulk magnetic susceptibility (BMS) shift of a nuclear resonance frequency caused by a paramagnetic compound is of importance in vivo NMR, both magnetic resonance spectroscopy and magnetic resonance imaging. However, since it is a rather complicated phenomenon, it has been the source of many misinterpretations in the literature. We have reworked and organized the theory of the BMS shift. This includes accounting for the important effects of local susceptibility. We have conducted experiments on phantom samples in order to illustrate the principles involved. Our phantoms consist of capillaries and coaxial cylinders. They simulate the situations of blood vessels oriented parallel and perpendicular to the magnetic field and the interstitial spaces surrounding them. In most of our experiments, the paramagnetic compound was one of several different hyperfine shift reagents for cation resonances. These were chosen to cover a range of potencies, in both magnitude and sign, of the shifts they produce. However, we also used a reagent which was incapable of inducing a hyperfine shift and thus could cause only a BMS shift. Although we report only 23Na spectra in this paper, the latter samples simulate the cases where one observes the water 1H resonance in experiments employing hyperfine shift reagents for cations. There have been a number of such investigations recently reported in the literature. The principles considered in this paper allow us to offer new interpretations for the results of several experiments published in the last few years.

Electron Spin Resonance Spectroscopy↗
PubMed1990

The application of phase shifts in NMR for flow measurement.

A brief overview of the history of the application of phase shifts in NMR, and in particular NMR imaging, is presented. The imaging methods include direct phase mapping, Fourier flow imaging (where the flow data are Fourier transformed into one dimension of an image), and alternative methods, where flow-related phase shifts are utilized for flow measurement from the magnitude of the signal. A discussion then follows of the principal errors that can affect the accuracy of the various flow imaging techniques, with particular reference to the phase mapping methods that have been used extensively in our institution. The results from a number of experiments are included to illustrate the extent of the errors and methods of removing or minimizing these effects are suggested.

Blood Circulation↗
PubMed1990

MRI signal loss due to microcirculation: phantom studies.

In order to study perfusion effects in MRI under different conditions we developed two different kinds of phantoms. The first phantom exhibits linear capillary flow with several capillaries within one voxel. The second consists of an anion exchange resin with beads forcing the spins to change directions such that they undergo accelerations and decelerations. Both phantoms were imaged with standard spin-echo sequences and signal intensities were quantified at various echo times. Qualitative and quantitative agreements of the data with the results obtained by computations of signal loss due to spin-phase phenomena are excellent, thus suggesting that perfusion effects can be fully understood using these phenomena. It is argued that the phantoms used in conjunction with conventional spin-echo sequences represent a realistic model for studying true capillary networks in conjunction with specialized perfusion sequences.

Humans↗
PubMed1990

Flow velocity quantitation using inversion tagging.

A method for quantitating flow velocities is presented. The technique tags multiple boli of magnetization in transit across a thick selection slab using rf inversion pulses. Results in phantoms and in vivo demonstrate that the method is robust and can provide velocity determinations in tortuous vessels.

Blood Flow Velocity↗
PubMed1991

NMR determination of myocardial pH in vivo: separation of tissue inorganic phosphate from blood 2,3-DPG.

Phosphorus NMR can measure myocardial tissue pH from the chemical shift of inorganic phosphate (Pi) in isolated buffer-perfused hearts, but in vivo the Pi peak originating from the myocardium is obscured by the resonance of 2,3-diphosphoglycerate (DPG) in the blood, making pH difficult to determine. Taking advantage of the fact that most of the interfering DPG is within the cardiac chambers and is rapidly flowing out of the sensitive volume of our coil, we developed a pulse sequence which would separate myocardial Pi signal from interfering DPG. We tested this method on a flow phantom and in living rat heart, using exogenous glycerol phosphate as a blood-pool marker. The results indicated that signal from moving and nonmoving substances could be separated, and derived values for myocardial pH and PCr/Pi ratio were consistent with previous estimates. This method should be useful for studying myocardial acid-base physiology with NMR.

2,3-Diphosphoglycerate↗
PubMed1991

Visualization of altered myocardial lipids by 1H NMR chemical-shift imaging following ischemic insult.

Acute myocardial infarction is associated with an accumulation of lipids. Spectroscopic and chemical-shift imaging strategies which can depict the spatial distribution of these chemical species are evolving. The present study was undertaken to test whether the Dixon method could detect spatially lipids known to accumulate in myocardium after an ischemic insult. Seven dogs underwent a 24-h coronary artery occlusion (LAD = 4, Cx = 3). Post mortem, hearts were removed and imaged ex vivo. Myocardial samples were also evaluated by high-resolution 1H NMR spectroscopy. Lipid images revealed regions of increased signal intensity, in the regions corresponding to the myocardial infarction, particularly in the periphery of the infarction. An increase in mobile lipids was observed by 1H NMR spectroscopy of myocardial samples with moderately reduced blood flow and corresponding to regions with increased signal intensity on the lipid image. This study shows that chemical-shift imaging may be useful for detecting alterations in myocardial lipid levels following an ischemic insult.

Animals↗
PubMed1991

Magnetic resonance diffusion/perfusion phantom experiments.

Recently, several models for determining microcirculatory parameters using magnetic resonance imaging have been proposed. These include the intravoxel incoherent motion (IVIM) model, the intravoxel coherent motion (IVCM) model, and various tracer models. In order to evaluate these models before extension into physiological systems, phantom studies were used to assess model assumptions, measurement uncertainties, and sensitivity to changes in perfusion. Emphasis is placed on the IVIM model, but the techniques discussed could be extended to evaluation of other models as well. An overview of considerations in pulse sequence development, phantom design, and data interpretation is presented for a variety of phantoms ranging in complexity from stationary volumes of fluid and mechanically pumped phantoms to isolated animal kidneys and finally to an in vivo animal model.

Animals↗
PubMed1991

MR angiography without subtraction.

A new NMR method for producing angiograms with strong suppression of the static spin signal and without the need for subtraction is described and demonstrated. A velocity-selective pulse sequence was implemented whereby the magnetization of all stationary spins is driven to the -z axis and is not detected, while maximizing the signal intensity of the moving spins. A theory of the method is presented and gives good agreement with experimental results obtained on a flow phantom. It is shown theoretically and experimentally that high-quality velocity-independent angiograms of the head and neck can be obtained with strong suppression of static spin signal when TR approximately T1. The method can be extended to produce three-dimensional angiograms.

Angiography↗
PubMed1992

The design of pulse sequences employing spatial presaturation for the suppression of flow artifacts.

The use of spatial presaturation to suppress the signal, and therefore also the artifacts, from flowing blood has become an important tool in the arsenal of techniques to suppress pulsatile flow artifacts in magnetic resonance images. However, a detailed theoretical analysis of the behavior of these flow artifact suppression pulses and of the important aspects of implementing suppression pulses in combination with particular imaging sequences has yet to be presented. In this paper we present a general theoretical framework to describe the flow artifact suppression technique. This analysis addresses the following four major issues: (1) the spin washout characteristics of the imaging sequence, (2) the interference between the flow signal suppression pulses and the imaging sequence, (3) the flow velocity range for a single application of the suppression pulse, and (4) the total flow velocity range for a suppression pulse repeated with a constant time interval between applications of the pulse. The predictions of our theoretical model are confirmed by experimental measurements made with stationary and flow phantoms. The results of this investigation provide guidelines for the design of flow artifact suppression pulse sequences and, in addition, should aid in the future development and refinement of the spatial presaturation technique as applied to flow signal suppression.

Artifacts↗
PubMed1992

Quantification of myocardial blood flow and extracellular volumes using a bolus injection of Gd-DTPA: kinetic modeling in canine ischemic disease.

In order to clarify the relationship between coronary artery disease (including myocardial infarction) and image contrast in gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA)-enhanced MRI it was decided to model the myocardial tissue distribution and clearance of Gd-DTPA using the modified Kety equation. Using a canine model, myocardial tissue Gd-DTPA concentrations ([Gd-DTPA]m) were measured 1 or 5 min after a bolus injection of Gd-DTPA or immediately after the end of a constant infusion of Gd-DTPA in a total of 35 dogs. It was found that within 5 min of a bolus injection [Gd-DTPA]m is determined primarily by myocardial blood flow (MBF) and after about 10 min primarily by myocardial extracellular volumes (MECV). This study suggests that repeat, rapid (every 2-4 s) measurements of myocardial T1 relaxation rates following the bolus injection of Gd-DTPA are required to calculate MBF (i.e., myocardial tissue perfusion) and MECV.

Animals↗
PubMed1992

Intravascular (catheter) NMR receiver probe: preliminary design analysis and application to canine iliofemoral imaging.

This investigation explores the feasibility of a catheter-based receiver probe for NMR study of arterial walls. Simulations and phantom experiments demonstrate the spatial response of several "inside-out" probe coil designs, including loop, "birdcage," "multipole," "center return," and opposed solenoids. For a target defined by an annulus in a plane perpendicular to B0, the opposed solenoid design provides substantially superior homogeneity to other designs considered. Canine iliofemoral artery images were acquired using a catheter probe in a whole-body, 1.5-T clinical imaging system. In situ (cadaver) images acquired with TE 70, TR 2400, 2-mm slice thickness, and 78 x 78-microns in-plane voxel size in 10-min acquisition times show vessel wall structures identified as intima, internal elastic lamina, media, and adventitia. In vivo images from similar acquistion conditions are much more poorly resolved, presumably due to motion, despite the use of cardiac gating and gradient moment nulling, so the feasibility of obtaining high-resolution in vivo MR images of the arterial wall remains in doubt.

Animals↗
PubMed1992

Nuclear magnetic resonance velocity spectra of pulsatile flow in a rigid tube.

Velocity spectra can be derived from velocity-encoded nuclear magnetic resonance (NMR) images. Velocity spectra are histograms showing the amounts of fluid flowing at different velocities in the sensitive volume of the measurement. Velocity spectra may prove to be useful in characterizing the flow of blood in small vessels, for example, in detecting the presence of stenoses and in evaluating their severity. NMR velocity spectra acquired in vivo are sufficiently complicated that a model system was designed and tested to investigate the velocity spectra of pulsatile flow. This study measured the NMR velocity spectra of pulsatile flow in a rigid tube and compared them to velocity spectra derived from Doppler ultrasound measurements and to velocity spectra inferred from a theoretical model driven by the measured pressure difference function. The experimental results from each technique agree.

Blood Flow Velocity↗
PubMed1993

Aortic ghost artifact in ultrashort TE multislice gradient echo MR images is not increased by paramagnetic enhancement.

Pulsation artifact on gradient echo images with ultrashort TE (i.e., < 3 msec) and intermediate TR is primarily from view-to-view amplitude modulation. Paramagnetic contrast agents increase the signal from blood during diastole without increasing the intensity of unsaturated systolic blood, decreasing signal modulation between systole and diastole. In a phantom and in humans, artifact decreased or remained the same following contrast enhancement.

Aorta, Abdominal↗
PubMed1993

Pitfalls in MR measurement of tissue blood flow with intravascular tracers: which mean transit time?

Measuring tissue blood flow with NMR imaging of intravascular tracers is more difficult than measurements of tissue blood volume. One major obstacle to the application of the Central Volume Principle is the direct measurement of the mean transit time. In this note, we demonstrate that mean transit time (MTT), which relates tissue blood volume to blood flow via the Central Volume Principle, is not the first moment of the concentration-time curve for MR or CT imaging of purely intravascular tracers. However, while first moment methods cannot be used by themselves to determine absolute flow, we show that transit curves may provide a useful relative measure of flow, for example, by considering ratios of the first moments.

Blood Flow Velocity↗
PubMed1993

An asymmetric slice profile: spatial alteration of flow signal response in 3D time-of-flight NMR angiography.

We introduce an asymmetric slice profile technique, which alters the spatial response of the flow signal in 3D time-of-flight NMR angiography. By gradually increasing the flip angle from the inflow to the outflow portions of the slab, the inflow refreshment effect is distributed over a wide slab thickness. The asymmetric slice profile is simply produced by using a Gaussian RF excitation with an overlapping presaturation. The spatial distribution of steady flow signal in a phantom study demonstrated an essential agreement with a numerical simulation. 3D time-of-flight NMR angiography of volunteers' heads using this technique provided a smooth vascular depiction over a wide slab thickness.

Blood Vessels↗
PubMed1993

Blood flow measurement using variable velocity encoding in the RR interval.

Velocity-encoded phase imaging using asynchronous gating requires input of a velocity encoding value to set the velocity sensitivity of the pulse sequence. The raw data interpolation and reconstruction scheme that the pulse sequence uses forces the encoding value to be constant throughout the RR interval. The sequence and the raw data interpolation scheme were modified to allow two velocity encodings during the RR interval. Two-hundred cm/s encoding was used in systole, and 30 cm/s in diastole. Changing the encoding in diastole significantly improved the accuracy and precision of ascending aorta flow measurements.

Aorta↗
PubMed1993

Localized real-time velocity spectra determination.

The accurate measurement of flow velocity has long been a subject of NMR research. In the field of medical imaging, a variety of techniques primarily based on the principle of Fourier encoding have been described. Due to time constraints, necessary trade-offs exist between spatial versus velocity spectral resolution. In general, either the average velocity of individual pixels is displayed or velocity spectral determinations are made at the cost of spatial localization. The recent development of multidimensional excitation pulses makes spatial localization possible during the excitation phase of the pulse sequences. This approach, coupled with time varying gradient readout, can be used to obtain single-shot localized velocity spectra. Using these concepts, we have obtained in vivo real-time measurements of localized velocity spectra on our clinical imager.

Aorta↗