Near-infrared-II pulsed laser diode array for spectral photoacoustic imaging of deep tissue oxygenation

Spectral photoacoustic imaging (sPAI) has the potential to detect clinically relevant measurements of blood oxygenation, non-invasively in real time. The unique advantage of sPAI is its ability to probe optical chromophores at extended imaging depths compared to purely optical modalities. PA imaging has previously been demonstrated using benchtop nanosecond pulsed solid-state lasers; however, their size and high cost limit the potential for clinical translation. Microfabricated pulsed laser diodes (PLDs) are miniaturized laser sources that can be fabricated and integrated at relatively low cost; however, microfabricated PLDs have been minimally investigated for the generation of photoacoustic signals due to the significantly lower energies they typically provide. In this work, we integrated an array of PLD diodes with optical excitation in the second near-infrared window, demonstrating deep imaging depths that exceed the imaging depths expected based on fluence alone. Using five PLDs integrated in series at a fluence of 61.5 µJ/cm², a maximum imaging depth of 4.26 cm was achieved with a detectable CNR in a porcine tissue phantom. Additionally, spectrally unmixed hemoglobin oxygenation measurements were accurate up to a depth of 3.11 cm with ∼10% accuracy. We validated device performance using an in vivo rat model, demonstrating a detectable SNR up to a depth of 4.13 cm and accurate tissue oxygenation assessment up to 3.85 cm from the kidney under layers of porcine tissue. Our device demonstrates the feasibility of microfabricated PLDs for clinically relevant photoacoustic imaging depths.