The embryonic vertebrate heart begins pumping blood long before the development of discernable chambers and valves. At these early stages, the heart tube has been described as a peristaltic pump. Recent advances in confocal laser scanning microscopy and four-dimensional visualization have warranted another look at early cardiac structure and function. We examined the movement of cells in the embryonic zebrafish heart tube and the flow of blood through the heart and obtained results that contradict peristalsis as a pumping mechanism in the embryonic heart. We propose a more likely explanation of early cardiac dynamics in which the pumping action results from suction due to elastic wave propagation in the heart tube.

In this work the effects of noise, resolution, and velocity (flow) on the measurement of intravascular pressure from phase-contrast (PC) MRI are discussed. To elucidate these effects, we employed an axisymmetric geometry that enabled us to calculate pressures in <2 min on a Sun Ultra SPARC 10 workstation. To determine the effects of vascular stenoses, we fabricated several stenotic phantom geometries (with 50%, 75%, and 90% area stenoses), and performed both MRI and computational fluid dynamics (CFD) simulations for various flow rates for these phantom geometries. Noise with Gaussian statistics was added to the velocity field obtained from the CFD simulations. The pressure maps obtained directly from CFD simulations for our phantom geometries were compared with pressure maps derived by our algorithm when 1) the input was noise-corrupted velocity data from CFD, and 2) the input was PC-MRI data collected from the phantoms. The quantitative effects of noise, resolution, and flow rate on the accuracy of pressure measurements were determined. We found that for flow rates below the Reynolds number for turbulent flow, resolution is a more significant determinant of accuracy than SNR. Phase-contrast (PC) MRI methods are widely used to measure blood flow velocities (1). PC-MRI relies on the phase changes of moving spins when the spins are subjected to a magnetic field gradient. In the absence of acceleration and higher-order terms, it can be shown that the accumulated phase offset is directly proportional to the spin velocities, and all three orthogonal components of the velocity vector for points within a prescribed image slice may be determined. The velocity field can then be used to obtain flow patterns, wall shear stress, vascular compliance, and blood pressure. In this work, we concentrate on the blood pressure, and present an analysis of factors that can affect the accuracy of relative intravascular pressure calculations.Previously, Urchuk et al. (7) proposed a singleslice method to measure intracardiac pressures. In the present work, we analyzed the effect of various factors on calculations of pressure from PC-MRI data using the Pressure-Poisson equation. We developed an implementation of the Pressure-Poisson equation that is suitable for measuring pressures in axisymmetric geometries. The resulting algorithm is extremely fast and yields solutions in Ͻ2 min on a Sun SPARC 10 workstation (8).It should be noted that other methods for deriving intravascular pressures are currently used in clinical practice. Invasive methods such as pressure wires and pressure catheters are considered to be the gold standard, and are routinely used in conjunction with X-ray angiography. However, these methods are invasive and involve the use of ionizing radiation. The calculation of intravascular pressure based on velocity mapping by the Bernoulli equation has been standard in Doppler ultrasound (9,10). However, the Bernoulli equation does not consider viscous forces, which can be important factors in calculations of intravascular pr...