RESUMO
Recent optical and acoustical studies have shown a threshold behaviour in the response of phospholipid-coated contrast agents, for a certain range of sizes. Below the acoustic pressure threshold, the microbubbles' scattering efficacy is significantly reduced compared to that above the threshold. Here we investigate the clinical relevance of the observed threshold behaviour. A cardiac ultrasound scanner system was used to analyse the pressure-dependence of the scatter intensity. The scattering of a native suspension of a phospholipid-coated contrast agent was compared to that of a suspension in which microbubbles with a size larger than 3.0 microm in diameter were extracted. A power modulation scheme at the fundamental frequency was applied. After linearly scaling and subtracting the B-mode images recorded at 70 and 200 kPa, the contrast-to-tissue ratio (CTR) of the native suspension was 3.2dB, whereas the CTR of the filtered suspension was 20 dB. The 17 dB difference is attributed to the threshold behaviour. Well-established ultrasound imaging techniques such as fundamental power modulation imaging could benefit from the pressure-dependent scattering properties of this type of contrast microbubbles.
Assuntos
Meios de Contraste , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Microbolhas , Ultrassonografia/métodos , Imagens de Fantasmas , Pressão , Reprodutibilidade dos Testes , Espalhamento de Radiação , Sensibilidade e Especificidade , Ultrassonografia/instrumentaçãoRESUMO
A linear relationship between the relative expansion of an off-resonance ultrasound contrast microbubble and low acoustic pressures is expected. In this study, high-speed optical recordings of individual phospholipid-coated microbubbles were used to investigate this relationship for microbubbles ranging from 2 to 11 microm and for acoustic pressures ranging from 20 to 250 kPa at a driving frequency of 1.7 MHz. For microbubbles larger than 5 microm, the relative expansion (DeltaD/D0) increased linearly with applied acoustic pressure, starting at the origin. The response of smaller microbubbles (<5 microm) also increased linearly with the applied acoustic pressure. However, linearity started at an acoustic pressure threshold value of 30 to 120 kPa for the different individual microbubbles. Below these pressure values, little or no oscillation was observed. The results may be explained by size-dependent mechanical properties of the phospholipid shells. An imaging technique such as power modulation imaging could profit from the presence of an acoustic pressure threshold in the microbubble response.
