Thermoacoustic Effect

The thermoacoustic effect describes the generation of acoustic waves when a short electromagnetic pulse (microwave or optical) is absorbed by a medium, causing rapid thermoelastic expansion. The absorbed energy density H(r) raises the local temperature by ΔT = H/(ρ·Cp), where ρ≈1000 kg/m³ and Cp≈4 J/(g·K). The resulting thermoelastic stress launches pressure waves described by
p(r,t) = (β/Cp) ∂H(r)/∂t ⊗ G(r,t),
with β the thermal expansion coefficient (~3×10⁻⁴ K⁻¹) and G the acoustic Green’s function (v_s≈1500 m/s). For microwave fluences of 1–10 mJ/cm², ΔT~10⁻³ K yields pressures on the order of 10–100 Pa.

These broadband acoustic emissions (1–10 MHz) enable high-resolution imaging of electromagnetic absorption and thermal properties. Thermoacoustic imaging exploits tissue dielectric contrasts (e.g., blood vs. lipid) and functional changes such as hemoglobin oxygenation or volume dynamics, making it a versatile modality in preclinical and emerging clinical research.