Guest lecture: Dynamic Speckle Images: Laser and Ultrasound
Dr. Héctor Rabal
Ph. D. in Physics, Optics Research Center (CIOp), La Plata, Argentina
When an optically rough surface is illuminated with coherent light it appears to be covered with small bright points and dark regions. This phenomenon is called speckle. It degrades the quality of the optical image and large efforts were initially devoted to eliminate it or blur its texture appearance.
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If the optical path of the employed light changes along time the speckled surface seems to boil. It is called boiling speckle or dynamic speckle. This can be due to movements of the scattering centers in the sample and/or changes in the refractive index of the medium in what can be described as a generalized Doppler effect.
The dynamics of the speckle are related to its physical causes and by measuring the detected activity some information can be obtained on what is happening in the illuminated sample.
The activity of the speckle is measured by applying several different descriptors to their digital images. As speckle appears as a texture, the descriptors usually used to characterize textures are sometimes used.
This approach has been found to be useful in different fields: to measure blood perfusion, seeds vigor and composition, plants roots growing, bruising in fruits, bacterial chemotaxis, blood clotting time measurement, drying of polymers, etc.
The choice of the adequate descriptors for the description of each phenomenon, usually poorly known, is not easy to determine a priori. So, in practice, sets of descriptors can be combined by using neural networks, both supervised and not supervised.
Besides, optical singularities called vortices are present in the speckled images. The spatial location of these singularities permits very precise measurement of rigid motions in the sample as well as activity measurements.
Ultrasound images also show this speckle patterns and its dynamics. Usual medical equipment filters out the dynamics to improve the images.
We propose that the descriptors used for laser dynamic speckle can be used to improve the diagnosis of pathologies in ultrasound images by using a similar approach.
To intend such research, sets of ultrasound images of healthy and pathologic cases with confirmed diagnosis should be provided and the results of their processing evaluated by the corresponding experts. We intend to join efforts between several speckle groups and interested US experts to explore this possibility.