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Paper Information

Journal:   IRANIAN JOURNAL OF MEDICAL PHYSICS   WINTER 2006 , Volume 2 , Number 9; Page(s) 43 To 54.
 
Paper: 

MODELING OF ULTRASOUND GENERATION BY LASER AND INVESTIGATION OF ACOUSTIC DIFFRACTION EFFECT IN ULTRASOUND WAVEFORM

 
 
Author(s):  DEHGHANI BID GOLI Z., MIRAN BEYGI M.H., ABOU ALHASANI MOHAMMAD JAVAD
 
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Abstract: 

Introduction: Photo-acoustic that idiomatically is called optoacoustic or laser-ultrasound is a technique that uses optical energy for the generation of ultrasound waves in various media. The waveform of the generated ultrasound contains useful information about the optical properties of the absorbing media and is influenced by laser pulse specifications. The obtained pressure signal undergoes changes prior to reaching the transducer due to the propagation in the media. In this research, the effect of laser parameters, the properties of the absorbent media and acoustic diffraction in generated ultrasound wave, have been investigated.
Materials and Methods: In this research, a frequency domain has been introduced for the generation of acoustic pressure as a result of laser energy reception transfer functions in two modes of forward and backward. By the use of this model, the effect of the pulse duration, the profile and wavelength of laser and also the absorption coefficient of the sample in the generated ultrasound properties have been investigated. A transfer function for the acoustic diffraction has been introduced and the effect of diffraction in ultrasound waveform has been simulated. Also the effect of the longitudinal and the radial distance of ultrasound transducer from the irradiated surface and also the ultrasonic source dimension that is influenced by laser beam diameter in ultrasound waveform, have been surveyed. All the simulations have been accomplished in MATLAB7 software platform.
Results: Increasing laser pulse duration causes an increase in the amplitude of ultrasound and a decrease in its frequency content. The increase in the amplitude causes an increase in the absorption coefficient of the sample. The generated optoacoustic waves in response to lasers with Gaussian and asymmetric Gaussian profile are not significantly different, consequently Gaussian pulse can be used in modeling with good approximation. As a result of acoustic diffraction while propagating in media, a backward single pole wave changes to a bipolar one. An increase in ultrasonic source dimension (laser beam diameter) causes an increase in the amplitude and the width of the optoacoustic waves.
Discussion and Conclusion: To design a laser-ultrasound system with maximum efficiency, one has to choose a laser wavelength in which tissue has a high absorption coefficient. The pulse duration and the beam diameter of the laser should be chosen correctly with respect to the ultrasonic transducer properties.

 
Keyword(s): LASER ULTRASOUND, OPTOACOUSTIC, ACOUSTIC DIFFRACTION, MODELING, SOFTWARE SIMULATION
 
References: 
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