Scanning acoustic microscopy (SAM) using optical transducers

Absorption of short laser pulses in a material gives rise to broadband ultrasound waves that are suitable for high resolution imaging. Two imaging modes are possible depending on whether the ultrasound is generated at the object surface or within the object. The former mode is usually called the laser ultrasound technique and yields images due to reflection of the wave at structures with acoustic mismatch to their surroundings. The latter mode is called photoacoustic (or optoacoustic, thermoacosutic) imaging and is based on optical contrast because of the proportionality between optical absorption and generated pressure. In many materials the imaging information obtained with the two methods is complementary. The general objective of the proposed project is therefore to combine the two imaging modes in one single device. Ideally, the designed scanning acoustic microscope (SAM) will generate two exactly overlapping images of a single object, one showing structures with acoustic and the other one with optical contrast. This will be achieved by constructing a single transducer that is capable of either generating an ultrasound field by laser irradiation of an absorbing target or passing the incident laser pulse directly to the sample where it is absorbed and generates a photoacoustic wave. In both cases the received ultrasound wave is measured with a broadband optical ultrasound receiver. Both, the target and the detector, are designed following the principles of limited diffraction beam generation for optimal lateral resolution and depth of field. The former by using a conical shape and the latter by building an optical fiber based ring sensor. The strategy to reach the objective consists of the following steps: Design of a ring detector using a fiber optic interferometer bent to a circle. Design of an absorbing target optimized for laser generation of a limited diffraction ultrasound beam. Develop methods for signal processing, including a compensation for frequency-dependent attenuation and synthetic aperture focusing techniques as well as deconvolution procedures for reduction of imaging artifacts. Design and testing of a scanning microscope based on the detector, ultrasound generation and signal processing technologies investigated in the previous steps. The project has a solid foundation in previous basic research carried out by the applicants in the fields of photoacoustic tomography, laser-ultrasound materials characterization, simulations and high resolution measurements of photoacoustically generated sound fields. The microscope developed in this project is expected to have applications in medical diagnostics, preclinical and biological research and materials science.

Absorption of short laser pulses in a material gives rise to broadband ultrasound waves that are suitable for high resolution imaging. Two imaging modes are possible depending on whether the ultrasound is generated at the object surface or within the object. The former mode is usually called the laser ultrasound technique and yields images due to reflection of the wave at structures with acoustic mismatch to their surroundings. The latter mode is called photoacoustic (or optoacoustic, thermoacoustic) imaging and is based on optical contrast because of the proportionality between optical absorption and generated pressure. In many materials the imaging information obtained with the two methods is complementary. The general objective of this project was to combine the two imaging modes in one single device. Therefore the following steps were performed: Design of a ring detector using a fiber optic interferometer bent to a circle. Design of an absorbing target optimized for laser generation of a limited diffraction ultrasound beam. Develop methods for signal processing, including a compensation for frequency-dependent attenuation and synthetic aperture focusing techniques as well as deconvolution procedures for reduction of imaging artifacts. Design and testing of a scanning microscope based on the detector, ultrasound generation and signal processing technologies investigated in the previous steps. With approximately 15 scientific publications the scientific output and the fruitful cooperation within this project of the partners from Graz and Linz was the cornerstone for a follow-up project, which could be already started and is funded by the FWF.