Acoustic Loudness Factor: An Experimental Parameter for Benchmarking Small Molecule Photoacoustic Probes

Photoacoustic imaging (PAI) is an emerging biomedical imaging modality with promise as a point-of-care diagnostic. This imaging modality relies on optical excitation of an absorber followed by production of ultrasound through the photoacoustic effect, resulting in high spatial resolution with imaging depths in the cm range. Although fluorescence brightness has guided the design and optimization of improved fluorophores for fluorescence imaging, no such benchmarking parameter exists for PAI. Consequently, there is no consensus on rational design strategies to improve photoacoustic signal from small molecule dyes. Herein, we disclose the discovery of the first benchmarking parameter for small molecule dye performance in PAI, which we term the acoustic loudness factor (ALF). Importantly, ALF can predict PAI performance without the need for access to photoacoustic instrumentation. ALF enables the prediction of PAI performance for common dye classes and can be used to guide the systematic evaluation of design strategies to enhance PAI performance. Lastly, we demonstrate that enhancements in ALF are directly translatable to in vivo PAI. Akin to the use of fluorescence brightness in fluorophore design and evaluation for fluorescence imaging, we anticipate that ALF will guide the design and evaluation of improved probes for PAI.