Digital holographic reconstructions of ultrasound from laser vibrometry recordings

Sammanfattning: If sound and noise from an engine, say a lawn mower, is to be reduced, it is of advantage to know from which part of the machine the noise is coming. An interesting medical application for localization of sound sources is optoacoustic imaging of tissue. A localization of the sound source in this case could be an useful tool for an early detection of high blood concentrations around tumors. Besides localization of primary sources, reconstruction of scattered or diffusive reflected fields could be a useful tool for studying opaque materials and objects. This could reveal abnormalities such as cavities or regions of different materials or densities in an object. In this thesis a scanning laser vibrometer is used to make quantitative two-dimensional measurements of ultrasound (US) fields in air. The laser light traverses the measurement volume to and from a rigid reflector and determines the velocity of the change in optical path length, which at constant geometry only depends on the changes in index of refraction. Assuming adiabatic conditions, the refractive index rate is proportional to the sound pressure rate and quantitative measures of the sound field are possible to achieve. These recordings are used for digital reconstruction of the acoustic fields by use of phase conjugation. Both recordings and reconstructions of the sound fields are performed for several wavelengths by tuning the primary US transducer around its resonance frequency. Combining several reconstructions for different wavelengths enhances the depth resolution. This resolution is improved even further by applying a phase filter. It is shown that this reconstruction technique makes possible a very thorough localization of the primary source even for highly scattered US-fields.