Investigation of a dependence of a registered signal on an extinction parameter μ of a scattering media (the water solution of milk, thickness L) at an acoustooptic tomography showed the signal values could be essentially differed at the same μL in various charges of milk. It was essentially noticeable at the medium parameter μL <18. In the present work the influence of medium scattering anisotropy parameter g on the registered signal value S was experimentally studied. As the scattering media the water solutions of the mixtures of a skimmed milk and cream in some proportions were used. The result of the study is the necessity to take into consideration the possible g-factor change in the scattering media including biological ones at a decision of the acoustooptic tomography inverse problem.
By analyzing spatial distribution of the laser radiation modulated by ultrasound (the ac photocurrent at ultrasonic frequency) in scattering medium, images of some optical inhomogeneities have been obtained. The signal registration conditions corresponded to the optimal ones to measure the ballistic component of scattered laser light. The changes of the acousto-optic contrast and sharpness were investigated in dependence on the form and size of the absorbing light objects and on the scaftering medium parameters. The image quality of plane and volume buried objects with an equal section in plane orthogonal to the laser radiation axis was similar, quite high, and didn't depend on the object position along this axis.
KEYWORDS: Laser scattering, Scattering, Ultrasonics, Acoustics, Light scattering, Liquids, Acousto-optics, Photodetectors, Signal detection, Signal to noise ratio
The acoustooptic images of volume and plane light-absorbing objects imbedded in a rectangular cuvette with a scattering liquid were obtained under the intermediate scattering conditions when μLC less than 35 where μ is an extinction coefficient and LC is the length of the cuvette along the laser beam axis. Photodetector collected the scattered and ballistic components of laser radiation, transmitted through the scattering medium traversed by focused ultrasonic beam. The alternating current at ultrasonic frequency was used as an imaging parameter. The characteristics of the alternating current were experimentally studied to optimize a registration of the component corresponding to the sound-modulated scattered light. The quality of the acoustooptic imaging was analyzed in dependence on the registration conditions, on the value of the scattering parameter and on the form and size of objects imbedded into the scattering liquid. It was obtained that under the used conditions the location of imbedded ultrasound-transparent objects in direction perpendicular to the laser axis was quite exact with the satisfactory contrast and sharpness as opposed to that along the direction of laser beam. An image contrast and sharpness in lateral flat of the volume objects were higher than that of plane figure.
On the basis of the Bragg diffraction model of coherent- optical interaction of laser radiation with ultrasonic wave, the influence of ultrasound axis position and ultrasound frequency value on the amplitude characteristics of ac photodetector current at the ultrasound frequency was theoretically analyzed. For intermediate scattering regime where the ballistic and low-step scattered light components are comparable in their values, the images of the absorbing squares were reconstructed on the base of the ac current amplitude at 3 MHz frequency and dc current distributions. The possibilities of the acousto-optical imaging of scattering medium were discussed on the basis of the results obtained after comparison with traditional measurements of dc photocurrent distributions.
On the basis of the suggested model of coherent-optical interaction of laser radiation with ultrasonic wave, the influence of geometry of experiment on the amplitude and phase characteristics of ac photo-detector current at the ultrasound frequency was experimentally and theoretically analyzed. For intermediate scattering regime where the ballistic and low-step scattered light components are measurable their contributions to ac photocurrent were separated by experimental data processing. With a use of theoretically predicted and experimentally confirmed optimal conditions of the measurement, the images of the absorbing half-plane were reconstructed in weak scattering media with use of the distributions of the ac amplitude of the current at 3 MHz ultrasound frequency and dc current. The possibilities of the acousto-optical imaging of multiply scattering medium were discussed on the basis of the results obtained after comparison with traditional measurements of dc photocurrent distributions.
Ultrasound modulation of scattered laser light was studied to develop the technique of acoustooptical visualization of the objects imbedded in scattering media. The focused ultrasound wave intersected the laser radiation in a medium. They passed through the medium laser light was collected by a photo-detector. The ac current characterized the optical properties of the zone of intersection of laser and sound beams. On the basis of the suggested model of coherent- optical interaction of laser radiation with ultrasonic wave, the expression for ac current was derived taking account of finite size of the laser beam diameter and the Gaussian distribution of the laser beam intensity, when the Bragg angle was much less than 1. The influence of medium scattering properties and geometry of experiment on the ac photo-detector current and signal to noise ratio was analyzed. With a use of theoretically predicted and experimentally confirmed results, the images of the absorbing half-plane using distributions of the ac amplitude of the current at 3 MHz ultrasound frequency and dc signal were reconstructed in weak scattering media. The possibilities of the acoustooptical imaging were discussed on the basis of the results obtained after comparison with traditional measurements of dc photocurrent distributions.
The theoretical analyze of coherent acousto-optical interaction in the turbid medium resulting in the appearance of ultrasound modulation of the photo-detector current has been fulfilled. Analysis of the influence of tissue scattering properties and geometry of experiment on spatial resolution of technique of acousto-optical imaging by using of considered model is made. The relations between amplitude and phase of the ac current and the receiver system parameters are derived. The optimal conditions for measurements of the ac current on ultrasound frequency are determined. The experimental testing of these conditions have been accomplished with focused ultrasound wave at 3 MHz. The noise characteristics of detection system were studied as well.
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