Regional myocardial function.

Methods for noninvasive tissue tracking and quantification of myocardial shortening with magnetic resonance imaging have been developed using presaturation tagging and velocity encoded phase maps. The concurrent development of rapid scanning techniques, such that a complete cine loop of a slice can be obtained in a breath-hold, has made the measurement of myocardial wall motion in patients a simple and reproducible examination. These methods make it possible to quantify the severity and extent of regional heart wall motion abnormalities both at rest and during stress.

In vivo measurement of T*2 and field inhomogeneity maps in the human heart at 1.5 T.

Cardiac echo-planar imaging suffers invariably from regions of severe distortion and T*2 decay in the myocardium. The purpose of this work was to perform local measurements of T*2 and field inhomogeneities in the myocardium and to identify the sources of focal signal loss and distortion. Field inhomogeneity maps and T*2 were measured in five normal volunteers in short-axis slices spanning from base to apex.

Model studies of the role of mechano-sensitive currents in the generation of cardiac arrhythmias.

Mechano-electrical feedback is studied by incorporating linear, instantaneously activating mechano-sensitive conductances into single cardiac cell models, as well as one- and two-dimensional cardiac network models. The models qualitatively reproduce effects of maintained mechanical stretch on experimentally measured action potential characteristics such as amplitude, maximum diastolic potential, peak upstroke velocity, and conduction velocity. Models are also used to simulate stretch-induced depolarizations, action potentials, and arrhythmias produced by pulsatile volume changes in left ventricle of dog.

Advanced MR imaging techniques for evaluation of the heart and great vessels.

A "one-stop shop" for evaluating cardiac disease with magnetic resonance (MR) imaging is progressing toward clinical reality and promises to have a major effect on the care of patients with cardiac disease. T1-weighted conventional spin-echo imaging gated to the cardiac cycle yields good anatomic detail but requires long imaging times and provides only static images of a single cardiac phase. Fast MR imaging with electrocardiographically (ECG) gated, low-flip-angle, segmented k-space gradient-recalled-echo (GRE) sequences provides excellent image quality with sufficiently high temporal resolution to "freeze" cardiac motion.

Improved sampling of myocardial motion with variable separation tagging.

A novel tagging method is introduced that increases the spatial resolution of estimates of myocardial radial thickening. The separation of adjacent parallel tag planes is customized to match the expected motion of specific regions of the heart wall. In regions in which the tags increase their separation over systole (radial thickening), the tag planes are placed close together at end diastole.

Imaging asynchronous mechanical activation of the paced heart with tagged MRI.

A method for imaging the rapid temporal-spatial evolution of myocardial deformations in the paced heart is proposed. High time resolution-tagged MR images were obtained after stimulation of the myocardium with an MR-compatible pacing system. The images were analyzed to reconstruct dynamic models of local 3D strains over the entire left ventricle during systole.

Precision of myocardial contour estimation from tagged MR images with a "black-blood" technique.

Rationale And Objectives: The authors determined whether blood presaturation of tagged magnetic resonance (MR) images affects identification of left ventricular endocardial borders.

Materials And Methods: Three healthy volunteers underwent MR imaging performed with a breath-hold segmented spoiled gradient-recalled-echo sequence with tissue tagging. Two saturation pulses (in the basal and apical regions) were used to generate black-blood images.

Signal-to-noise measurements in magnitude images from NMR phased arrays.

A method is proposed to estimate signal-to-noise ratio (SNR) values in phased array magnitude images, based on a region-of-interest (ROI) analysis. It is shown that the SNR can be found by correcting the measured signal intensity for the noise bias effects and by evaluating the noise variance as the mean square value of all the pixel intensities in a chosen background ROI, divided by twice the number of receivers used. Estimated SNR values are shown to vary spatially within a bound of 20% with respect to the true SNR values as a result of noise correlations between receivers.

Quantification and reduction of ghosting artifacts in interleaved echo-planar imaging.

A mathematical analysis of ghosting artifacts often seen in interleaved echo-planar images (EPI) is presented. These artifacts result from phase and amplitude discontinuities between lines of k-space in the phase-encoding direction, and timing misregistrations from system filter delays. Phase offsets and time delays are often measured using "reference" scans, to reduce ghosting through postprocessing.

Model-free reconstruction of three-dimensional myocardial strain from planar tagged MR images.

A technique is presented for reconstructing a three-dimensional myocardial strain map from a set of parallel-tagged MR images. Radial strains were reconstructed from in vivo data from an anesthetized dog with values between .05 and .

Extremity veins: evaluation with fast-spin-echo MR venography.

The authors evaluated a flow-independent magnetic resonance (MR) imaging technique in extremity veins with slow flow that relies on the prolonged T2 of blood to create angiographic contrast. A commercially available heavily T2-weighted fast-spin-echo sequence was optimized for MR venography in volunteer and phantom studies. Good depiction of venous anatomy was routinely obtained with the optimized method.

Measurement of ovarian volume by transvaginal sonography before ovulation induction with human menopausal gonadotrophin for in-vitro fertilization can predict poor response.

The study tests the hypothesis that small ovaries measured on transvaginal sonography (TVS) are associated with a poor response to ovulation induction by human menopausal gonadotrophin (HMG) for in-vitro fertilization (IVF). A total of 140 infertile patients with morphologically normal ovaries undergoing IVF was studied. The mean ovarian volume of each patient was measured on TVS before starting HMG.

Techniques for high-speed cardiac magnetic resonance imaging in rats and rabbits.

Progress in research on hypertension, heart failure, aging, post-infarct remodeling, and the molecular basis of cardiovascular diseases in general has been greatly facilitated in recent years by the development of specialized small-mammal models by selective breeding and/or genetic alteration. Routine noninvasive evaluation of cardiac function and perfusion in these animals models, however, is difficult using existing methods. In principle, MRI can be used for this purpose, but in practice this is difficult because of problems related to RF coils, cardiac gating, and imaging pulse sequences.

Quantitative cardiac perfusion: a noninvasive spin-labeling method that exploits coronary vessel geometry.

Purpose: To quantitate myocardial arterial perfusion with a noninvasive magnetic resonance (MR) imaging technique that exploits the geometry of coronary vessel anatomy.

Materials And Methods: MR imaging was performed with a spin-labeling method in six arrested rabbit hearts at 4.7 T.

Pencil excitation with interleaved fourier velocity encoding: NMR measurement of aortic distensibility.

A technique is presented for rapidly and noninvasively determining aortic distensibility, by NMR measurement of pulse-wave velocity in the aorta. A cylinder of magnetization is excited along the aorta, with Fourier-velocity encoding and readout gradients applied along the cylinder axis. Cardiac gating and data interleaving improve the effective time resolution to as high as 3 ms.

MRI of myocardial function: motion tracking techniques.

Methods for the noninvasive measurement of three-dimensional myocardial motion with MRI have recently been developed using presaturation tagging and velocity-encoded phase maps. The quality of clinical cardiac MRI studies has also recently improved with the advent of breath-hold scanning. The combination of breath-hold imaging with tagging and velocity-encoding sequences has made the measurement of myocardial wall motion in patients a simple and reproducible exam.

A magnetization-driven gradient echo pulse sequence for the study of myocardial perfusion.

A T1-weighted imaging pulse sequence for contrast-based studies of myocardial perfusion is presented and evaluated in phantoms and in vivo. The sequence is similar to spoiled gradient-recalled echo sequences except that nonselective preparatory RF pulses drive magnetization to steady state prior to image acquisition. Steady state is thus obtained in both tissue and blood resulting in a stable, homogeneous, and dark pre-contrast baseline.

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.

Three-dimensional myocardial deformations: calculation with displacement field fitting to tagged MR images.

Purpose: To reconstruct three-dimensional (3D) myocardial deformations from orthogonal sets of parallel-tagged magnetic resonance (MR) images.

Materials And Methods: Displacement information in the direction normal to the undeformed tag planes was obtained at points along tag lines. Three independent sets of one-dimensional displacement data were used to fit an analytical series expression to describe 3D displacement as a function of deformed position.

Multi-shot EPI for improvement of myocardial tag contrast: comparison with segmented SPGR.

To assess the potential value of multi-shot EPI relative to segmented k-space SPGR for myocardial tagging, we measured tag contrast for both sequences in a phantom and human study and compared it with theoretical predictions. In the human heart, EPI tag contrast was three times that of SPGR at the end of systole. Tag duration was lengthened with EPI to at least 600 ms.

Minimization of dead-periods in MRI pulse sequences for imaging oblique planes.

With the advent of breath-hold MR cardiac imaging techniques, the minimization of TR and TE for oblique planes has become a critical issue. The slew rates and maximum currents of gradient amplifiers limit the minimum possible TR and TE by adding dead-periods to the pulse sequences. We propose a method of designing gradient waveforms that will be applied to the amplifiers instead of the slice, readout, and phase encoding waveforms.