Frequency Domain Dynamic Light Scattering (FD-DLS) and Fourier Transform Angular Light Scattering (FT-ALS)

 

The Frequency Domain Dynamic Light Scattering (FD-DLS) is a new methodology that facilitates study of dynamics in arbitrarily scattering samples. Dynamic light scattering (DLS) measures the intensity of light scattered by particles in the sample over a broad range of temporal scales. DLS instruments obtain a correlation factor from the intensity versus time profile for situations where light scatters no more than once in the medium. The principle of DLS is extended to Diffusing Wave Spectroscopy (DWS), for highly scattering media, by taking into account the path-length distribution P(s) of the scattered light. However, this requires a priori knowledge of P(s), significantly restricting the applicability of dynamic measurements in highly scattering media. FD-DLS, thus overcomes the limitations of both DLS and DWS.

The Fourier Transform Angular Light Scattering (FTALS) is the spatial equivalent of Fourier Transform Infrared Spectroscopy (FTIR). Light scattering is combined with a typical inverted microscope and all the spatial frequencies are simultaneously detected. It provides ultrasensitive detection of angular scattering as well as fast measurement from live cells and tissues. It combines the high spatial resolution associated with optical microscopy and intrinsic averaging of light scattering techniques. The enhanced sensitivity is granted by the image-plane measurement of the optical phase and amplitude and the spatial resolution of the scatterer positions is well preserved.

Description/Details

The Frequency Domain Dynamic Light Scattering (FD-DLS) technique employs a typical Fourier domain optical coherence tomography system to measure the temporal correlation (between the scattering wave vector and the diffusion coefficient D) as well as the path length distribution of the scattered light, P(s). It addresses the limitations of both the DLS and the DWS since it does not require the investigators to make assumptions about the sample under observation and it can be used in complex media. The underlying principle of Fourier Transform Angular Light Scattering (FT-ALS) is to retrieve the phase and amplitude associated with a coherent microscope image and numerically propagate this field to the scattering plane. Since all the spatial frequencies are simultaneously detected it provides high throughput along with much higher sensitivity to angular scattering than the conventional FTIR. The experimental setup combines light scattering with an inverted microscope. A fast Fourier transform is applied to obtain the angular scattering in the spatial domain. Accurate phase retrieval for elastic light scattering (ELS) measurements and fast acquisition speed for dynamic light scattering (DLS) studies are achieved by diffraction phase microscopy (DPM), which provides the phase shift associated with transparent structures from a single interferogram measurement. The phase measurement is performed in the image plane of a microscope rather than the Fourier plane which offers important advantages in the case of the thin samples.

Applications 

FD-DLS In-vivo blood viscometry, which is critical for patients with heart disease. Quantifying the dynamics of blood perfusion. Potential diagnostic capability in certain types of cancer where cancer tissue experiences increased blood perfusion. FT-ALS It provides ultrasensitive measurements of light scattering from live cells and tissues Cell sorting Stain-free histology

Benefits

FD-DLS technique has broad application to characterizing scattering and dynamic media. It is an absolute measurement, where knowledge of the composition of the suspended particles is not needed. It is especially helpful if the optical properties of the suspended particles are not known or if the suspension is made up of particles with different optical properties. The Frequency Domain Dynamic Light Scattering (FD-DLS) is not limited by the complexity of the scattering medium. No a priori information about the path length distribution function is needed. Based on commercially available Optical Coherence Tomography (OCT) system.

FT-ALS provides significantly higher sensitivity to angular scattering than the traditional goniometer based measurement. Capable of measuring single particles. Measurements on live cells over a broad range of angles High throughput technique with exhaustive information in a single CCD exposure. In the image plane of a thin and transparent sample, such as live cells, the intensity is evenly distributed, which efficiently utilizes the limited dynamic range of the CCD. Ability to detect weak scattering signals over broad temporal (milliseconds to hours) and spatial (fraction of microns to centimeters) scales.