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Acoustic-Resolution Photoacoustic Microscopy
Acoustic-resolution photoacoustic microscopy (AR-PAM) is an imaging method based on the photoacoustic effect and is a subset of photoacoustic microscopy. Unlike the other subset of photoacoustic microscopy, optical resolution photoacoustic microscopy (OR-PAM), AR-PAM has an acoustic focus that is tighter than the diffused optical beam. AR-PAM specializes in the quasi-diffusive regime between 1 and 3mm of penetration depth, and may go up to 10mm, beyond which high frequency photoacoustic attenuation sets in. OR-PAM typically images up to 1mm and photoacoustic computed tomography (PACT) typically images up to 10cm. High laser pulse repetition rate is important in AR-PAM, resulting in its low pulse energy of less than 1mJ. This is higher than OR-PAM, which typically uses less than 1µJ and lower than PACT, which typically uses greater than 10mJ.

Resolution
A typical AR-PAM setup has around 50 µm lateral and 15µm axial spatial resolution, which is better than PACT but worse than OR-PAM. A wider ultrasonic bandwidth leads to better axial resolution and a higher central ultrasonic frequency leads to better lateral resolution. Tighter acoustic focusing improves lateral resolution but this is at the cost of focal zone.

Brightfield AR-PAM
Unlike darkfield imaging that uses scattered light, brightfield imaging uses directly transmitted light. For brightfield imaging, high photon delivery efficiency allows high-repetition rate low-energy pulsed lasers. In Wang et.al.'s 2012 study, this efficiency allowed video rate 30Hz 2-D B-scan imaging that measured oxygen dynamics of mouse cardiovasculature. Nevertheless, most AR-PAM setups use darkfield imaging since brightfield illumination creates strong interference signals from tissue surfaces; surface photoacoustic waves prevent detection of deep photoacoustic waves.

Deep penetration AR-PAM
Song et.al.'s deep penetration darkfield AR-PAM system used near-infrared pulses of 804nm and a 5MHz ultrasonic transducer to achieve deep penetration. A 5MHz ultrasonic trasducer is low for AR-PAM since most transducers for AR-PAM are around 50MHz. They were able to achieve up to 38mm of penetration in chicken breast tissue.

Applications
AR-PAM has applications in anatomical imaging, including microvascular networks and internal organs, functional imaging, including oxygen saturation, hemodynamic response, and vasomotion, as well as molecular imaging, including contrast agent-aided monitoring. AR-PAM is useful in the fields of cardiology, neurology, dermatology, and oncology.

Anatomical imaging
Anatomical imaging focuses on revealing anatomical structures of a tissue. One of the popular AR-PAM anatomical imaging systems today includes a system that has both an acoustic focus and an optical focus, essentially combining AR-PAM and OR-PAM in one. Estrada et.al. used a system with an optical focus at 1mm and an acoustic focus at 3mm to achieve 18 2-dimensional cross sections per second over an imaging range of 3mm in a mouse brain.

Functional imaging
Functional imaging focuses on revealing functions of a tissue. When the first functional AR-PAM was developed by Zhang et.al. in 2006, many of their examples showed functional imaging. They performed in-vivo imaging of angiogenesis and melanoma as well as hemoglobin oxygen saturation monitoring of single blood vessels.

Molecular imaging
Molecular imaging focuses on revealing molecules inside a tissue. Pu et.al. developed semiconducting polymer nanoparticles as photoacoustic molecular imaging probes. Their nanoparticles acted as contrast agents for reactive oxygen species and allowed their real-time in-vivo monitoring.