Use of ultrasound for noninvasive study of blood vessel responsiveness

To provide a noninvasive means for studying individual macroscopic blood vessels, an ultrasound scanner was modified to provide a recording of blood vessel diameter. The instrument has A- and B-modes of signal display. The B-mode is used to position the probe and the A-mode to measure vessel diameter. The A-mode has two electronic gates for tracking the echo from each of two structures, i.e., near and far wall of vessel. The front gate was modified to pick up the falling rather than rising edge of a peak generated by the vessel wall. An analog signal proportional to the distance between gates was derived for recording with a strip-chart recorder. Probe holders were constructed to optimize positioning and holding of probe. Stability was excellent (reading varied 0.05 mm over 1 h). Axial resolution was between 0.3 and 0.73 mm. Discrepancy of measurements of plastic tubing made by ocular and ultrasound varied from 1.1 to 4.6%. Discrepancy with lightly fixed vessels was 2.7-8.2%. Ex vivo measurements on vessels with viable smooth muscle were more variable, perhaps because of actual change during measurements. Changes in vessel diameter induced by change in hydrostatic pressure and exposure to histamine were recorded.