The
photo acoustic effect is a process of energy transformation whereby light
absorption in a material produces sound waves. In the imagining of vasculature,
nanosecond pulsed light is applied to the tissue via non-ionising laser pulses
(red) and absorbed by local absorbers (blue). In biological tissues, absorption
can be due to endogenous chromophores such as hemoglobin or melanin, or
exogenously delivered contrast agents. The pulsed radiation is converted into
heat energy. As such, when the light is on it causes heating at the focal spot
and subsequent rapid thermos-elastic expansion of the light-absorbing tissue;
when the light is off, the tissue cools and contracts. This expansion and
contraction results in pressure changes in the region where light was absorbed.

This creates a quantifiable acoustic wave (green) in the ultrasonic range which
can be detected using ultrasonic sensors such as microphones. These signals are
then processed into an image of the underlying tissue through a process of
triangulation. 

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