New contrast imaging method using double frequency exposure

We present results of a new imaging technique capable of detecting echoes from microbubbles and eliminating those emanating from non-oscillating structures (tissue). The method is based on mixing a low frequency signal and a high frequency signal. The LF signal modulates the microbubble size by making them vibrate between compression and rarefaction phases. During these two phases, an HF signal is transmitted to image the microbubbles, sensing the same bubble, but at two different stages, small and large. Simulations were carried out using the modified Herring equation using 1 cycle at 0.5 MHz and 7 cycles at 3.5 MHz. The results show that the bubbles respond differently compared to single frequency excitation. Optical observations of contrast microbubbles were carried out using a Brandaris high-speed camera. Two transducers were used to transmit 0.5 MHz and 3.5 MHz signals. The optical observations were carried out at a frame rate of 14 MHz and 128 successive frames were recorded. The oscillation of a 4 μm diameter Sonovue microbubble was observed. The optical results show that the bubble response at 3.5 MHz depends on the phase of the LF signal. A larger response is obtained when the bubble is compressed compared to the response at the expansion phase. The decorrelation between these signals is significantly high enough to be used as a parameter to detect contrast microbubbles and discriminate them from tissues. These preliminary results demonstrate the feasibility of this linear approach in improving the contrast detection.