The invention is a fast-scanning ODT system that uses phase information derived from a Hilbert transformation to increase the sensitivity of flow velocity measurements while maintaining high spatial resolution. The significant increases in scanning speed and velocity sensitivity realized by the invention make it possible to image in vivo blood flow in human skin. The method of the invention overcomes the inherent limitations of the prior art ODT by using a phase change between sequential line scans for velocity image reconstruction. The ODT signal phase or phase shifts at each pixel can be determined from the complex function, {tilde over (Γ)}ODT(t), which is determined through analytic continuation of the measured interference fringes function, ΓODT(t), by use of a Hilbert transformation, by electronic phase demodulation, by optical means, or a fast Fourier transformation. The phase change in each pixel between axial-line scans is then used to calculate the Doppler frequency shift. Sequential measurements of a single line scan, measurements of sequential line scans or measurements of line scans in sequential frames may be used. Because the time between line scans is much longer than the pixel time window, very small Doppler shifts can be detected with this technique. In addition, spatial resolution and velocity sensitivity are decoupled. Furthermore, because two axial-line scans are compared at the same location, speckle modulations in the fringe signal cancel each other and, therefore, will not affect the phase-difference calculation.

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Phase-resolved optical coherence tomography and optical doppler tomography for imaging fluid flow in tissue with fast scanning speed and high velocity sensitivity

Many mathematical formulations of the heat transfer in living tissues'-' have been for the purposes of studying thermal regulation, comfort, or other phenomenon where significant localized (as opposed to whole-body or regional) variations in temperature and heat flux were of little interest. The advent of intensified interest in hyperthermia as a cancer therapy and the safety associated with ultrasound and microwave radiation, as well as attempts at a quantitative interpretation of thermographic measurements, however, have made it highly desirable to have formulations that are valid also for small-scale temperature variations. Living tissues differ from nonbiological materials primarily because of the presence of the vasculature. The large number and the architectural and dimensional variety of blood vessels clearly make it impractical to account for their individual contribution to heat transfer processes in the tissue with the exception, of course, of the larger arteries and veins. In the fields of heat transfer and fluid flow, when one encounters problems with a large number of structures whose individual dimensions are small relative to the macroscopic phenomenon under study, a common practice is to adopt the so-called continuum description. In this description, only the collective behavior of the small structures is taken into consideration in a certain statistical manner. Usually the influence of the small structures are ultimately expressed in terms of continuum properties of the medium, in our case, the thermal conductivity, specific heat, and blood perfusion rate of the tissue. It is the purpose of this report to explore the theoretical basis for the relationship between these properties and the architecture and function of the vasculature. It will be shown that because of the vasculature, and the large rate of blood perfusion, living biological tissues are fundamentally different from inert materials. Consequently, the familiar thermal properties can no longer be assumed to be independent of the parameters of the temperature field. In other words, these properties may vary, depending on the nature of the application. In view of the fact that existing formulations of the bio-heat transfer problem have been found to be more or less satisfactory for the description of heat transfer involving large-scale tempera-

Microvascular contributions in tissue heat transfer.

The mechanical and morphological properties of segments of certain precisely defined resistance vessels (approximately 150 micrometer lumen diameter) in the mesenteric bed of spontaneously hypertensive (SHR) and normotensive (WKY) rats have been compared in vitro under carefully controlled conditions and also after fixation. At a given transmural pressure, the relaxed SHR vessels (compared with the WKY vessels) would have had a 16% smaller lumen diameter (P less than 0.01) and a 49% thicker media (P less than 0.005), so that the media volume per unit segment length was 31% greater (P less than 0.05). The smooth muscle cells were arranged circumferentially in about four layers in the SHR vessels and in about three layers in the WKY vessels. The SHR active wall tension in response to potassium was 53% greater (P less than 0.02) and to norepinephrine was 50% greater (P less than 0.01) than for WKY. However, the ED50 values for the norepinephrine dose-response curves were similar (approximately 5 micrometer). Activation with potassium plus norepinephrine gave greater responses in both vessel types, than with either agent alone, but the SHR responses were on average only 19% greater than the WKY (P less than 0.10). However, under these conditions, the SHR vessels would have been able to contract against 45% greater transmural pressures (P less than 0.001) because of their smaller lumen. On maximal activation, the mean force developed by each cell (approximately 3.85 micro N) was the same in both vessel types, even though on average (P = 0.10) the SHR cells had a 21% greater cross-sectional area. The results support the Folkow hypothesis that in genetic hypertension the increased peripheral resistance is associated with structural changes in the resistance vessels.

Direct evidence that the greater contractility of resistance vessels in spontaneously hypertensive rats is associated with a narrowed lumen, a thickened media, and an increased number of smooth muscle cell layers.

The objective of this study is to develop a model of the cardiovascular system capable of simulating the short-term (< or = 5 min) transient and steady-state hemodynamic responses to head-up tilt and lower body negative pressure. The model consists of a closed-loop lumped-parameter representation of the circulation connected to set-point models of the arterial and cardiopulmonary baroreflexes. Model parameters are largely based on literature values. Model verification was performed by comparing the simulation output under baseline conditions and at different levels of orthostatic stress to sets of population-averaged hemodynamic data reported in the literature. On the basis of experimental evidence, we adjusted some model parameters to simulate experimental data. Orthostatic stress simulations are not statistically different from experimental data (two-sided test of significance with Bonferroni adjustment for multiple comparisons). Transient response characteristics of heart rate to tilt also compare well with reported data. A case study is presented on how the model is intended to be used in the future to investigate the effects of post-spaceflight orthostatic intolerance.

https://www.physiology.org/doi/pdf/10.1152/japplphysiol.00241.2001

Computational modeling of cardiovascular response to orthostatic stress

The diameter, area, and volume of individual human erythrocytes (of 8 subjects, newborn to age 71) were determined by photographing the cells hanging on edge. Measurements from high magnification prints were processed by computer. The distributions of diameter, area, and volume are described statistically, with the unexpectedly linear regression equations for their interrelations. The plot of area vs. volume for the 1016 normal cells from seven subjects (newborn excluded) was remarkably linear with a "straight-line" boundary restricting the distribution. Shape was characterized by a dimensionless "sphericity index" (4.84.volume 2/3 /area). Cells of larger volume tended to be thinner than the smaller cells. The red cell can easily be deformed at constant volume, but an increase in membrane area results in hemolysis. A theoretical geometric parameter, the "minimum cylindrical diameter" (MCDiam), in microns, the thinnest cylindrical channel through which each individual cell could pass, predicts the linear boundary of the plot of area vs. volume. The MCDiam value of 3.66 µ ± 0.04 SEM accurately represents the thinnest channel through which 95% of the cells can pass. In two splenectomized patients with hereditary spherocytosis the MCDiam was increased to approximately 4.0 µ, suggesting that the severest restriction is located in the spleen.

J P Shillingford

Papers

Physiology and Biophysics of the Circulation