Photoacoustic Tomography

Photoacoustic tomography (PAT) combines optical contrast with acoustic resolution for deep tissue imaging.

The technique uses pulsed laser light to generate acoustic waves in tissue through the photoacoustic effect. When chromophores (like hemoglobin) absorb light, they undergo rapid thermal expansion, generating ultrasonic waves that can be detected to reconstruct images.

PAT offers excellent spatial resolution at depth and can provide functional information about blood oxygenation, making it particularly useful for small animal neuroimaging and clinical applications.

The initial pressure rise $p_0$ generated by the photoacoustic effect is given by:

$$p_0(\mathbf{r}) \;=\; \Gamma\,\mu_a(\mathbf{r})\,F(\mathbf{r}),$$

where

• $\Gamma$ is the Grüneisen parameter (0.1–0.3),
• $\mu_a$ is the optical absorption coefficient (0.1–100 cm⁻¹),
• $F$ is the laser fluence (10–100 mJ/cm²).

Typical $p_0$ values are on the order of 10–100 kPa.

Ultrasonic propagation in tissue follows the acoustic wave equation:

$$\Bigl[\nabla^2 - \tfrac{1}{c^2}\,\partial^2_t\Bigr]\,p(\mathbf{r},t) = -\tfrac{\beta}{C_p}\,\partial_t H(\mathbf{r},t),$$

where

• $c\approx1500$ m/s (sound speed in soft tissue),
• $\beta\approx3\times10^{-4}$–4×10⁻⁴ K⁻¹ (thermal expansion coefficient),
• $C_p\approx4.18$ J/(g·K) (specific heat),
• $H$ is the absorbed optical energy density.

Detection bandwidths of 1–20 MHz yield axial resolutions of ~75–150 μm. PAT can image to depths of 1–5 cm, achieving lateral spatial resolutions of 50–200 μm and frame rates <1 ms per slice.