Pulsed laser diode excitation for transcranial photoacoustics

Photoacoustic (PA) imaging of deep tissue tends to employ Q-switched lasers with high pulse energy to generate high optical fluence and therefore high PA signal. Compared to Q-switched lasers, pulsed laser diodes (PLDs) typically generate low pulse energies. In PA applications with strong acoustic attenuation, such as through human skull bone, the broadband PA waves generated by nanosecond laser pulses are significantly reduced in bandwidth during their propagation to a detector. As high-frequency PA signal components are not transmitted through the skull, we propose not to generate them by increasing the excitation pulse duration. Because PLDs are mainly limited in their peak power output, an increase in pulse duration linearly increases pulse energy and, therefore, PA signal amplitude. Here, we show that the optimal pulse duration for deep PA sensing through thick human skull bone is far higher than in typical PA applications. Counterintuitively, this makes PLD excitation well-suited for transcranial photoacoustics. We show this in PA sensing experiments on ex vivo human skull bone.