Pressure (mechanical) effects in infrared tissue ablation

We experimentally demonstrate that the acoustic transients propagating as a result Free-Electron Laser (FEL) ablation in brain tissue exhibit a strong FEL wavelength dependence. These acoustic transients were measured with a time-resolved, polarization quadrature laser interferometer. The transients are multiphased, with displacements of tens of microns and durations of tens of milliseconds. We calculated the Fourier transforms, power spectra, and pressure transients based on these displacement data sets. For 3.0 μm irradiation, the bandwidth of the Fourier components extends to ~20 kHz, while for 6.45 μm irradiation the bandwidth of the Fourier components extend to ~8 kHz. For the 3.0 μm irradiation, the power spectra indicate acoustic energy propagates in the bandwidth up to ~12 kHz, with structure in the 1-4 kHz range. For the 6.45 μm radiation, the mechanical power spectra indicate the acoustic energy propagates in the bandwidth up to ~7 kHz, with structure throughout. The pressure transients resulting from 3.0 μm irradiation have a leading phase with a faster onset, shorter duration, and more than ten times the peak pressure compared to that observed in pressure transients resulting from 6.45 μm irradiation. For 3.0 μm irradiation, the observed pressure transients have peak pressures in the MPa range and durations of ~1 ms, while for 6.45 μm irradiation the pressure transients have peak pressures in the 0.1 MPa range and durations of about ~3 ms.