Flow quantitation by contrast echocardiography

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Abstract
The combination of a standardized echographic contrast agent with the analysis of the ultrasonic radio frequency (RF) signal allowedin vitro flow quantitation in a circulation model. The purpose of this study was to investigate both the effects of biological tissues, intervening between probe and insonated structure, and the effects of the angle of incidence between flow and ultrasonic beam on RF flow quantitation. Thus, the contrast agent SHU 454 was intravenously injected (0.4 ml) as a bolus into a circulation model, at variable flow rates, while keeping the pressure and volume of the vessel constant. Injections were performed with saline interposed between probe and vessel and after the addition of the subcutaneous tissue of a pig; injections were also performed using the probe normal to the flow and with an angle of incidence of 45°. Echographic data were recorded by a mechanical sector scanner, capable of sampling the RF signal from a region of interest positioned in the center of the vein. Contrast echo time-intensity curves were generated. As expected, both peak intensity and the area under the curves decreased with intervening tissue (− 58 and − 70% of baseline values, respectively, p<0.001). Surprisingly, mean transit time also decreased with intervening tissue (from 1.12±0.25 seconds with saline, to 0.92±0.13 seconds with tissue, p<0.001), thus producing a systematic overestimation of flow (21% on the average). To compensate for signal attenuation, contrast injections were repeated in the presence of tissue after increasing the electronic signal amplification (10 dB), and transit time did not significantly differ from control. Moreover, mean transit time was slightly shorter with an angle of 45° (1.03 ° 0.19 seconds) than with an angle of 90° (1.12±0.25 seconds, p<0.05). However, when the data collected with both angles of incidence were plotted together, the correlation with flow remained very close (r=0.94). In conclusion: intervening tissue influences flow quantitation by contrast echocardiography: these modifications can be compensated for by increasing the electronic signal amplification; the angle of incidence between flow and ultrasonic beam also influences flow quantitation: however, this influence is trivial, and can be neglected for practical purposes.