The stability, size, and ultrasonic properties of several ultrasonic microbubble contrast agents such as Albunex, SHU-454, 1% hydrogen peroxide, and sonicated solutions such as 70% and 50% dextrose, 70% sorbitol, 5% albumin, Renografin-76, and others were evaluated. Albunex was the only tested agent that was sufficiently stable over an extended period of time for in vitro ultrasonic characterization. The attenuation and backscatter coefficients of Albunex at 5 and 7.5 MHz were found to be linearly proportional to microsphere concentration at low concentrations (less than 0.01% for attenuation and less than 0.002% for backscatter). Also, Albunex was found to be more echogenic than soft tissues at 5 MHz even after being diluted to 0.0003% of the original concentration. Next, the feasibility of using this acoustic information for contrast blood flowmetry was investigated. In vitro flow estimates in a mock flow loop were made using only the ultrasonic properties of the contrast agent. Bolus injections of Albunex and indicator-dilution curves inferred from ultrasonic measurements were used to estimate calibrated flow rate ranging from 400 to 5000 mL/min. The flow estimates from attenuation measurements showed a good correlation with those from an independent method (r = 0.97), but the results from backscatter studies did not correlate well. These results demonstrate that attenuation measurement may be a feasible alternative for in vivo blood flow measurement in conjunction with the indicator dilution principle or estimation of tissue perfusion such as myocardial perfusion using a time-activity approach.
The biliary clearances of [14C]erythritol (Cery) and [3H]mannitol (Cmann) were measured simultaneously in dogs during cholereses induced by sodium taurocholate and by secretin. Cery increased equally with the increase in bile flow induced by taurocholate, whereas mannitol entry into bile was partially restricted; deltaCery/deltabile flow averaged 0.96; deltaCmann/deltaCery averaged 0.81. Values for erythritol clearance exceeded bile flow by a constant volume over a wide range of bile flows, a result that suggests distal reabsorption of a fixed amount of fluid, independent of canalicular bile production. During secretin-induced choleresis both Cery and Cmann accompanied 30-40% of the increase in bile flow, and the ratio of Cmann/Cery was 1.02. Thus the secretin-responsive region is permeable to both erythritol and mannitol. This affects the extent to which measured erythritol clearance accurately reflects canalicular bile formation; Cery may underestimate or overestimate canalicular bile flow. The electrolyte composition of bile remained relatively constant over a broad range of bile flows although the characteristics of taurocholate- and secretin-induced biles differed from each other. Taurocholate-stimulated bile was virtually isotonic. Secretin-induced bile had a high total concentration of electrolyte (mean concentration 367 meq/liter) rich in chloride and bicarbonate and was hypertonic.
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