Space travel has placed humans in an interesting physiological situation that makes it necessary to secure the health of the astronauts. In space, due to the lack of gravity, there is a fluid shift toward the upper body that results in a decrease in plasma volume. As a result, there is a significant drop in red blood cell mass over the flight period, which could result in space flight anemia. To monitor this change, ultrasound must be used, since it is the most trusted and only flight surgeon approved imaging/detecting modality for space flight. Continued research into the use of ultrasound for monitoring hematocrit levels can improve the lives of humans both in space and on Earth.
A physical means to examine the viability of a cross-correlation detection method for ultrasound (originally demonstrated for optical light scattering) that minimizes multiple scattering effects [23] was demonstrated by conducting a Young’s two-pinhole experiment. This was implemented using a pinhole mask on the receiving transducer to affect cross-correlation. The resulting interference pattern should have a period predicted by the pinhole size, spacing, and frequency of the ultrasound signal. Interference patterns were produced for a series of masks with different pinhole sizes and pinhole separations. The fringe patterns were analyzed, with the measured period compared to the predicted period, and the 300/700(pinhole diameter/separation) mask was determined as the most optimal.
A two-dimensional computer model was developed using the Comsol Multiphysics software package (Comsol AB.). The model was created to analyze the physical cross correlation method and help explain the experimental results, accounting for some of the effects not captured by the analytical model. The simulations showed that the masks with smaller pinholes (~100μm) had periods that were not consistent with the analytic predictions, indicating the presence of effects that were not properly modeled analytically. One of these effects was evanescent wave coupling with the two-pinhole mask. In addition, the signal attenuation and energy were determined for a blood mimicking fluid in a tissue flow phantom. The signal characteristics of the flow model were the first step in determining the optimal spacing of the multiple transducer array prototypes, which will be based on the scattering geometry of the scatterers.
Overall, these methods demonstrated that the two-detector multiple scattering suppression cross-correlation technique is a feasible approach to non-invasive hematocrit determination using ultrasound.