The photoacoustic effect was first discovered by Alexander Graham Bell in the late 19th century. During his research in wireless communication, Bell surprisingly found that sound wave could be induced in solids if it was exposed to rapidly interrupted light. The same effect was observed in gases and liquids by other researchers in their following experiments. The invention of laser in the 1970s revived the application of photoacoustic effect since high intensity light of tunable frequency could be generated to introduce sound with increased amplitude and sensitivity.

Fig 2 Mechanism of photoacoustic imaging

As shown in the figure above, the mechanism of the photoacoustic effect can be illustrated in a series of steps. First an electromagnetic pulse, such as microwave or laser, is used to irradiate the biological tissues. The electromagnetic energy is absorbed and causes the constituent molecules become thermally exited. Periodic thermal expansion from the exited area generates acoustic waves that are detected by ultrasonic transducer to form images.

Fig 3 Absorption spectrum of biological tissues

The laser induced ultrasonic signals depend on the optical absorption properties in the biological sample and thus reveal the structure and function of the tissues based on optical contrast.