Concentration requirements for subharmonic quantitative contrast enhanced high frequency ultrasound flow studies

The ability to perform quantitative haemodynamic measurements in the microcirculation using high frequency ultrasound and microbubbles is dependent on the ability to suppress tissue signals and the concentration of the agent. While tissue suppression has been accomplished by exploiting nonlinear properties of microbubbles, it is still unclear as to the optimal dose to facilitate flow studies. A dose response study of the contrast Agent MicroMarker was performed in the renal cortex of a mouse to get an indication of the optimal range of doses. A transmit pulse of 30 MHz, 20% bandwidth and peak negative pressure (PNP) of 1 MPa was used to elicit a 15 MHz subharmonic response from the agent. Doses up to 300 mu L, kg(-1), corresponding to 10 million bubbles per mL of blood, were investigated. Experiments showed a wash-in phase which took approximately 3 to 5 seconds, followed by a plateau in signal which lasted for approximately 40 to 50 seconds and a slow washout phase. The average enhancement calculated over the plateau peaked at 4.5 times (6.5 dB) over the noise floor. The duration of enhancement increased with dose up to 230 seconds. The integrated enhancement (area under the time intensity curves) was linear over the range of doses investigated. The variability between mice was 10% to 20% for doses between 20 and 100 pl, kg-1. A dose of 40 % was observed for a dose of 300 mu L kg(-1) and is believed to be due to microbubbles-induced attenuation. This study has established the utility of microbubble contrast agents for quantitative flow imaging in the microcirculation using high frequency ultrasound for doses up to 300 mu L kg(-1) dose range. However, larger doses should be investigated in order to improve enhancement.