Photoacoustic Imaging for Monitoring Calcium Dynamics in the Heart.

Photoacoustic imaging (PAI) is a hybrid imaging modality that captures ultrasound signals produced by thermoelastic expansion when pulsed laser light is absorbed by optical absorbers, enabling visualization of biological tissues at greater depths than those of conventional fluorescence imaging. While PAI has been conventionally used for biological, structural, and morphological studies, showing promise for imaging calcium dynamics in cardiac tissue, its practical implementation remains undemonstrated. In this study, we implemented a sectional excitation strategy using a thin, sheet-shaped laser beam to confine optical excitation to defined tissue planes. We applied this approach to photoacoustic imaging of a perfused bullfrog heart loaded with liposome-encapsulated calcium-sensitive dye. We successfully achieved real-time visualization of calcium dynamics within the atrial cross-section, while simultaneous electrocardiographic recordings enabled temporal correlation between photoacoustic signal fluctuations and cardiac electrical activity. This method provides a less-invasive approach to assess calcium transients in deep tissue, and broadens the application of PAI from morphology to physiological function. These findings highlight the potential of this optical-acoustic hybrid modality as a powerful tool for calcium imaging in physiological and pharmacological studies involving deep-tissue organs-particularly in applications such as heart imaging, where conventional optical techniques are limited by shallow penetration depth.