PubMed · 7845081
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T Leuthner. 1994. Development system for pulse oximetry.. https://doi.org/10.1007/bf02515323
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Methods of assessing the imaging characteristics of intravascular ultrasound systems are described and applied to an HP Sonos 3000 system imaging at 12, 20 and 30 MHz. The presence of geometrical image distortion associated with bending of the catheter was detected by imaging silk filaments positioned at 45 degrees around a circle. Distortion was clearly present indicating that it is likely to occur in clinical studies. Resolution measurements (-12 dB) were made from images of 30 microns Nylon. The axial resolution did not change with depth and was in the range 150-400 microns. The lateral resolution increased linearly with depth for all frequencies, with values in the range 0.9-1.7 mm at a depth of 5 mm. High frequency catheter transducers therefore generate ultrasound with beamshapes similar to those expected from the familiar performance of devices in the 2-10 MHz range. Penetration measurements were made with material known to be tissue equivalent at 3-7 MHz. The penetration depth varied from 4.5 to 14 mm depending on catheter and frequency. There is a need for the manufacture of tissue equivalent materials for use in the frequency range 20-30 MHz, and also for the design of phantoms based on these materials for the measurement of contrast-detail performance and penetration depth.
Contractile function of the left ventricle is often measured regionally, particularly in intact animals. In this paper I dispute the validity of the common assumption that regional shortening accurately reflects whole-heart mechanics, even when contractility is homogeneous, by using an ellipsoidal model of the left ventricle to examine the relationship between measures of regional function and whole-heart function. Segment shortening and wall thickening are the most common measures of regional function, while the end-systolic pressure-volume relationship, or Emax, is a well-validated index of contractility in the whole heart. Assuming that the arterial end-systolic pressure vs stroke volume relationship is constant, segment shortening shows a monotonically increasing, convex relation to Emax. If, in addition, myocardium is considered an incompressible material, then wall thickening also shows a monotonically increasing, convex relation to Emax. Thus, segment shortening and wall thickening are quite insensitive when measuring high contractility and overly sensitive when measuring low-contractility. Finally, I consider the non-linear relationship between regional measures of function and global contractility, and thereby resolve apparently contradictory findings regarding the inotropic responsiveness of stunned myocardium.
This paper examines mainly oscillatory behavior of a fluid-conveying collapsible tube using a two-dimensional flexible channel made of a pair of membranes. The equation of equilibrium of the membrane in a large deflection theory is coupled with the equations of continuity and momentum of an incompressible flow in a one-dimensional flow theory accounting for flow separation. An explicit finite difference method was used to solve the governing equations numerically. According to numerical results, the fluids in the inlet and outlet rigid channels have strong effects on the oscillation of the system. Depending on initial values for the numerical integration, there may exist both a stable static equilibrium and an oscillatory solution for the same parameter values, but only if the external pressure is sufficiently large.