8 March 2006 Development of a pulsed NIR multiwavelength laser diode excitation system for biomedical photoacoustic applications
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An important capability of photoacoustic methods is the ability to make spatially resolved spectroscopic measurements of blood oxygenation by imaging at multiple NIR excitation wavelengths, usually sourced from Q-switched Nd:YAG pumped OPO based systems. These excitation sources are usually bulky, expensive and with limited scope for varying the pulse repetition rate and pulse width. An alternative would be to use pulsed laser diodes as excitation sources. To evaluate the possibility of developing a multiwavelength excitation system composed of three wavelengths 810, 850 and 905nm, a single wavelength (905nm) system was built. To achieve a sufficient SNR, four high peak power pulsed laser diodes were combined. The design of the laser drivers provided variable pulse duration (65-500ns) and repetition rates of up to 2.5KHz. This allowed the pulse duration to be optimised in order to (a) maximise the energy in the generated photoacoustic signal and (b) reduce the effects of frequency dependent acoustic attenuation of tissue on the propagating acoustic wave by avoiding the generation of excessively high frequency components. It also enabled the high repetition rate of laser diodes to be exploited in order to average a large number of acquisitions over a short time period to increase SNR. Preliminary measurements of SNR were made in phantoms using the single wavelength excitation system, to demonstrate the practical biomedical utility of the system. A tissue phantom consisting of two capillaries (null set 460μm)filled with an absorbing dye of similar optical properties to blood (μa ≈ 1mm-1), immersed at different depths in a 1% solution of intralipid (μs ≈ 1mm-1) was used. To further demonstrate the capability of the system it was combined with a cylindrical scanning system to image a strongly absorbing cylinder immersed to a depth of 1cm in 1% solution of intralipid (μs ≈ 1mm-1). This study demonstrated the potential for using laser diodes as excitation sources for pulsed photoacoustic spectroscopic biomedical applications.
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Thomas J. Allen, Thomas J. Allen, Paul C. Beard, Paul C. Beard, } "Development of a pulsed NIR multiwavelength laser diode excitation system for biomedical photoacoustic applications", Proc. SPIE 6086, Photons Plus Ultrasound: Imaging and Sensing 2006: The Seventh Conference on Biomedical Thermoacoustics, Optoacoustics, and Acousto-optics, 60861P (8 March 2006); doi: 10.1117/12.655537; https://doi.org/10.1117/12.655537

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