Thermoacoustic Tomography

The thermoacoustic effect converts pulsed microwave energy into broadband acoustic waves via rapid thermoelastic expansion of tissue. In Thermoacoustic Tomography (TAT), short (<100 ns) microwave pulses (frequency ~1 GHz, energy fluence ~1–10 mJ/cm²) illuminate the head. Absorbed energy raises local temperature (ΔT~10⁻³ K), generating pressure waves detected by an array of ultrasound transducers (bandwidth 1–10 MHz). The measured pressure p(r,t) relates to the deposited energy density H(r) by

$$p(r,t) = \frac{\beta}{C_p}\,\frac{\partial H(r)}{\partial t}\;*\;G(r,t),$$

where β≈3×10⁻⁴ K⁻¹, Cp≈4 J/(g·K), and G is the Green’s function for sound speed vs≈1500 m/s. Typical signal amplitudes are 10–100 Pa.

Reconstruction employs time-of-flight backprojection or iterative model‐based inversion accounting for acoustic heterogeneities (e.g., skull). Spatial resolution (100 μm–1 mm) depends on detector bandwidth and aperture; penetration depth reaches several centimeters. With 10–50 Hz pulse repetition, TAT achieves 20–100 ms temporal resolution suitable for mapping hemodynamic and dielectric changes correlated with neural activity.