Ultrasound-modulated fluorescent contrast agent for optical imaging through turbid media

Optical imaging in a highly scattering medium is effective only at very shallow depths which limits its use as a diagnostic tool in biomedical imaging. By combining optical and acoustic modalities, high-contrast, physiologicallyrelevant optical information at higher spatial resolutions can be achieved. Hybrid imaging modalities such as acoustooptic and photoacoustic imaging improve resolution over conventional optical imaging, but tissue scattering results in poor signal-to-background ratios especially in deeper tissues. To overcome these challenges, we have developed a novel microbubble (MB) contrast agent surface-loaded with a self-quenching fluorophore. In response to ultrasound, the MB expands and contracts, generating changes in fluorophore surface density. The changes in physical separation between fluorophores modulate the quenching efficiency and produce a fluorescence intensity modulation. To our knowledge, this is the first experimental demonstration of ultrasound modulation of fluorescence using a self-quenching MB scheme. The modulation is spatially localized to the ultrasound focal zone where the pressure is greatest and the largest MB oscillations are induced. The modulated signal can be extracted from a large constant light background, increasing detection sensitivity. This technique can enable sensitive optical imaging with ultrasound-scale sub-millimeter spatial resolution, overcoming significant challenges of optical imaging in deep tissue. The contrast agent MBs were prepared with a shell of phospholipid and lipophilic self-quenching fluorophore. MB ultrasound response was studied in a custom setup which monitored fluorescence emitted from an insonified sample. Fluorescence signals displayed clearly modulated intensity and the fast Fourier transform (FFT) showed a strong component at the ultrasound driving frequency.