Colour-coded echographic flow imaging and spectral analysis of cerebrospinal fluid (CSF)

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Abstract
An in-vitro steady flow system was designed to determine the lowest flow velocities that can be detected by echographic colour flow imaging and spectral analysis. The flow detection level was determined hydrostatically by reducing the fluid level to below the point at which a flow signal was visible, then increasing the height until the colour flow reappeared. This was confirmed in all instances by spectral analysis. The height (angle-corrected velocity) of the spectral envelope was also determined. Mean volume flow was then obtained by using a graduated cylinder and a stop watch. The lowest hematocrit detectable was identified using a stepwise dilution of packed human red blood cells with 0.9% sodium chloride. Three different PVC-tubes with inner diameters of 2.1 mm, 1.1 mm and 0.51 mm were used. It was found that: 1) echographic flow imaging is remarkably sensitive to low concentrations of scattering particles (lowest hematocrit detected by colour flow and spectrum was 0.003% using the 2.1 mm diameter tube); 2) lowest hematocrit values which allowed detection of true mean flow velocities below 2 cm/sec increased with decreasing lumen: 0.006% for the 2.1 mm tube; 0.22% for the 1.1 mm tube and 1.6% for the 0.51 mm tube; 3) higher velocities and/or greater lumina were necessary to detect hematocrit values below 1%; 4) for a hematocrit between 0.1% and 44% measured mean flow velocities were less than half the values obtained from spectral envelope; 5) the error of spectral flow velocity determination was relatively constant for this hematocrit range (0.1%–44%), but varied with tube size; 6) minimal concentrations of red blood cells could be differentiated from air bubbles by signal intensity at constant receiver gain settings; 7) there was no difference in the height of the velocity envelope between air and red blood cells. The diagnostic, clinical and scientific implications of these findings are discussed.