  In photoacoustic computed tomography (PACT) with short-pulsed laser
excitation, wideband acoustic signals are generated in biological tissues with
frequencies related to the effective shapes and sizes of the optically
absorbing targets. Low-frequency photoacoustic signal components correspond to
slowly varying spatial features and are often omitted during imaging due to the
limited detection bandwidth of the ultrasound transducer, or during image
reconstruction as undesired background that degrades image contrast. Here we
demonstrate that low-frequency photoacoustic signals, in fact, contain
functional and molecular information, and can be used to enhance structural
visibility, improve quantitative accuracy, and reduce spare-sampling artifacts.
We provide an in-depth theoretical analysis of low-frequency signals in PACT,
and experimentally evaluate their impact on several representative PACT
applications, such as mapping temperature in photothermal treatment, measuring
blood oxygenation in a hypoxia challenge, and detecting photoswitchable
molecular probes in deep organs. Our results strongly suggest that
low-frequency signals are important for functional and molecular PACT.
