Science Research Info: Use of Photoacoustic Tomography(PAT) for Clinical vascular imaging

Process:

In study, Researchers used an optical parametric oscillator (OPO- a laser device that converts a high-energy laser into two lower-energy beams through a nonlinear optical process) to generate the photoacoustic waves and an Fabry-Pérot FP ultrasound scanner to map them over the surface of the skin.

In photoacoustic tomography (PAT), the OPO is used to generate light pulses that are absorbed by tissues, generating ultrasound waves for high-resolution imaging.

Nanosecond laser pulses in the 700–900 nm spectral range (where tissue attenuation is low) are emitted by the OPO. The light beam incident on the acoustically sensitive thin film polymer. Fabry-Pérot interferometer(FPI) consists of two parallel, partially reflective mirrors separated by a small distance,. Light that enters the cavity undergoes multiple reflections between the mirrors.

The FPI is transparent in the 560–1,300 nm wavelength range, the beam passes through the sensor, illuminates the tissue, which generates ultrasound waves. These waves propagate to the FPI, interfere between the light reflected from the two FPI mirrors, resulting in a modulation in the reflected optical power.

Here, scan times can be reduced to a few seconds and even hundreds of milliseconds by parallelizing the optical architecture of the sensor readout, by using excitation lasers with high pulse-repetition frequencies and by exploiting compressed sensing.

Equations :

Photoacoustic Wave Equation

p(r,t) is the acoustic pressure at position r and time t.
c is the speed of sound in the medium.
β is the thermal expansion coefficient of the tissue.
Cp is the specific heat capacity of the tissue.
H(r,t) is the heat source term, which depends on the light absorption in the tissue, function of the light fluence (power per unit area) and optical absorption coefficient.

heat source H(r,t) is proportional to the absorbed optical energy: