In vitro analysis of ultrasound second generation contrast agent diluted in saline solution

Purpose. To evaluate the changes in response of a second-generation sonographic contrast agent diluted in saline solution at different concentrations when different scanning techniques, and saline solution temperatures and pH were used. Materials and methods. A series of tests was devised to analyse the behaviour of the sonographic contrast agent at different concentrations, temperatures, pH and scanning techniques. Latex balloons were used as phantoms. These were filled with 0.9% Sodium Chloride solution mixed with a suspension of sulphur hexafluoride microbubbles stabilised with phospholipids (Sono Vue®) and scanned with the harmonic imaging technique at low acoustic pressure. For each image obtained, we calculated the mean grey-scale level values and the standard deviations of grey-scale level were calculated within a ROI on the US images obtained; the resulting data were used to create echogenicity curves of UCA the echo-enhancer over time at the different conditions tested. Results. We noted that Sono Vue® maintains adequate backscatter properties even at low concentration (0.15%) in N/S solution. The amount of echogenicity can be considered adequate for the average duration of an ultrasound examination. These properties are not affected by the scanning technique used on the phantom containing the UCA contrast agent/NaCl solution. The pH of the saline solution does not affect the survival of the microbubbles. Temperature becomes a significant value beyondover 33-37°C; this condition favours rupture of the microbubbles with subsequent reduction of the echogenicity after 10-15 min, and complete disappearance after 30 min. US beam attenuation related to the concentration of the microbubbles in the saline solution volume has a non-linear behaviour; at low amplification levels, attenuation becomes more significant when using Sono Vue® concentrations higher than 0.4%. Unexpected greater improvement in US beam transmission was experienced for each of the three Sono Vue® concentrations tested, and especially for 0.2% and 0.4%, compared to saline solution alone. Conclusions. Our results show that a second-generation contrast agent may be used even diluted at low concentration in saline solution. This characteristic opens up new diagnostic perspectives possibilities for the use of contrast-enhanced harmonic imaging; in particular, its can be suggested used in hollow organs and functional studies. These in vitro results require by confirmation by clinical applications which are under evaluation and experimentation.