5.5.1　Principles of photoacoustic imaging

Photoacoustic imaging（PAI）is a new non-invasive biological imaging technique.When pulsed or intensity-modulated continuous-wave laser irradiates biological tissue，tumor，melanin，hemoglobin and other substances in the biological tissue absorb photon energy.After the absorption of the light，an initial temperature rise leads to a pressure rise，which propagates as a photoacoustic wave（Figure 5-9）.The ultrasonic detector located on the surface of targets is utilized to receive these ultrasonic signals.The collected signals are processed and the corresponding image is reconstructed by computer，through inversion of the optical features of the light-absorbing substances in the tissue.

Photoacoustic imaging has the advantages of high resolution optical imaging and deep penetration of ultrasound imaging.On the one hand，photoacoustic imaging is based on the difference in optical absorption of substances inside tissues with the features of high resolution and high contrast，similar with optical imaging.On the other hand，photoacoustic imaging detects ultrasonic signals instead of optical signals，which overcomes the penetrating depth limitation of optical imaging due to the high light scattering of tissues.Therefore，photoacoustic imaging is the most promising imaging technique in recent years，which can provide high-resolution and high-contrast tissue images for deep tissues（50-70 mm）.For superficial organs like skin，photoacoustic imaging can almost achieve imaging in the entire depth of skin.(https://www.daowen.com)

The occurrence and development of dermatosis are often accompanied by changes in the physicochemical properties of skin tissue，such as morphology and composition，metabolism，blood oxygen content and pigment changes.Although histopathological analysis is the clinical gold standard，which can accurately detect above physicochemical properties，the drawbacks of long time consumption and invasion limit its wide application.Photoacoustic imaging can quickly and non-invasively display information such as blood oxygen saturation and blood flow in intravital tissues，chemical composition and functions of tissues（e.g.，fat，tumor，hemoglobin，collagen，melanin and water content，and their locations）.In addition，as a superficial organ，skin can fully absorb light and the effects by sound diffraction and attenuation can be negligible.Therefore，photoacoustic imaging has developed rapidly in dermatology.