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978-3-8439-2605-8, Reihe Elektrotechnik
Optimizing a semiconductor laser based photoacoustic imaging system
172 Seiten, Dissertation Ruhr-Universität Bochum (2016), Softcover, A5
Photoacoustic imaging is a growing research topic. It combines optical- and acoustic imaging and hereby benefits from optical contrast and acoustical resolution and penetration depth. Many imaging systems use large high power solid state laser systems to generate the necessary light pulses. Instead, it is desirable to use semiconductor lasers, which are much smaller, can be integrated into the ultrasound transducer housing and are typically also cheaper. However, these suffer from lower pulse power and longer light pulses. In the past, the resulting low signal to noise ratio (SNR) was improved by applying photoacoustic coded excitation, which utilizes the high pulse repetition frequencies that are possible with semiconductor lasers. While the available pulse power of semiconductor lasers is growing as new technology becomes available, it becomes necessary to consider laser safety aspects in the optimization.
In this dissertation, a high power semiconductor laser based photoacoustic imaging system is optimized considering laser safety restrictions. Laser safety restrictions are reviewed specifically for application to photoacoustic imaging. Optical and acoustic simulations are employed to analyze the effect of geometric changes in the experimental settings on the imaging. Some of these simulations are verified in experiments. Optimizations based upon the laser safety restrictions, such as adjusting the light profile on the skin or light pulse amplitudes are discussed. To form images with high SNR, a new “fused” image type is introduced that contains the response to multiple wavelengths at once. Compared to single wavelength images, the SNR can rise by a factor of up to the number of wavelengths used, depending on the imaged object and the lasers employed. Additionally, it is possible to reuse the acquisitions to reconstruct the single wavelength images at improved SNR compared to purely sequential acquisition of fused and single wavelength images. The optimization of fused and single wavelength images is discussed in detail and the improvement is demonstrated in experiments.