Multiple scattering in optical coherence tomography signal: Monte Carlo modeling and experimental study

The angle biased Monte Carlo technique is applied to simulate the OCT signal from homogeneous turbid medium. The OCT signal is divided into two categories: one is from a specific imaging target layer in the turbid medium; the other is from the other background medium. The Class II signal has wider spatial and angular distribution than the Class I signal. And it experiences more scattering events. The multiple scattered photons will decrease the contrast of the OCT image and their contributions become dominant at larger depths. The average number of scattering events increases with the probing depth for both Class I and II lights. Experimental study is conducted by measuring the depth-resolved degree of polarization (DOP) of the back- scattered signal from the turbid media. The DOP is derived form the Stokes vector measurements. The incident light is linear polarized and could be depolarize by the multiple scattering. The DOP decreases to 0.5 when Class I signal is equal to the Class II signal. Experiments in the intralipid solution with different scattering coefficient show the imaging depth is limited to 3-4 optical depths

Two-dimensional tissue imaging by use of parallel detection of ultrasound-modulated laser speckles

Ultrasound-modulated optical tomography in biological tissue was studied. An ultrasonic beam was focused into a biological tissue sample to modulate the laser light passing through the ultrasonic beam inside the tissue. The speckle field formed by the transmitted laser light was detected by a CCD camera with the source-synchronous-illumination lock- in technique. The ultrasound-modulated laser light reflects the local optical and mechanical properties within the ultrasonic beam and can be used for tomographic imaging of the tissue. We implemented frequency-swept modulation to obtain spatial resolution along the ultrasonic axis. 2D images of biological tissue were successfully obtained with both single frequency modulation and frequency-swept modulation. 3D images could be acquired as well in principle

Propagation of polarized light in turbid media: simulated animation sequences

A time-resolved Monte Carlo technique was used to simulate the propagation of polarized light in turbid media. Calculated quantities include the reflection Mueller matrices, the transmission Mueller matrices, and the degree of polarization (DOP). The effects of the polarization state of the incident light and of the size of scatterers on the propagation of DOP were studied. Results are shown in animation sequences

Sensitivity of photoacoustic microscopy

Building on its high spatial resolution, deep penetration depth and excellent image contrast, 3D photoacoustic microscopy (PAM) has grown tremendously since its first publication in 2005. Integrating optical excitation and acoustic detection, PAM has broken through both the optical diffusion and optical diffraction limits. PAM has 100% relative sensitivity to optical absorption (i.e., a given percentage change in the optical absorption coefficient yields the same percentage change in the photoacoustic amplitude), and its ultimate detection sensitivity is limited only by thermal noise. Focusing on the engineering aspects of PAM, this Review discusses the detection sensitivity of PAM, compares the detection efficiency of different PAM designs, and summarizes the imaging performance of various endogenous and exogenous contrast agents. It then describes representative PAM applications with high detection sensitivity, and outlines paths to further improvement

Photoacoustic microscopy

Photoacoustic microscopy (PAM) is a hybrid in vivo imaging technique that acoustically detects optical contrast via the photoacoustic effect. Unlike pure optical microscopic techniques, PAM takes advantage of the weak acoustic scattering in tissue and thus breaks through the optical diffusion limit (∼1 mm in soft tissue). With its excellent scalability, PAM can provide high-resolution images at desired maximum imaging depths up to a few millimeters. Compared with backscattering-based confocal microscopy and optical coherence tomography, PAM provides absorption contrast instead of scattering contrast. Furthermore, PAM can image more molecules, endogenous or exogenous, at their absorbing wavelengths than fluorescence-based methods, such as wide-field, confocal, and multi-photon microscopy. Most importantly, PAM can simultaneously image anatomical, functional, molecular, flow dynamic and metabolic contrasts in vivo. Focusing on state-of-the-art developments in PAM, this Review discusses the key features of PAM implementations and their applications in biomedical studies

Simulation study of ultrasound-modulated optical tomography

Monte-Carlo modeling technique was used to simulate the ultrasound-modulated optical tomography. The difference between absorption and scattering objects was compared. Simulation result indicated that this technique is sensitive to object absorption property, while the scattering properties have less effect on the output AC/DC signal intensity. It was also demonstrated that inhomogeneity and the background optical properties of the scattering medium could change the AC/DC value. The signal-to-noise ratio problem in the experiment is carefully analyzed. The major noise source is the speckle noise caused by the small particle movement within the tissue sample. The decorrelation time of the speckle pattern was measured in the tissue sample. In order to reduce the speckle noise, the data acquisition time must be less than the speckle decorrelation time

Photoacoustic brain imaging: from microscopic to macroscopic scales

Human brain mapping has become one of the most exciting contemporary research areas, with major breakthroughs expected in the coming decades. Modern brain imaging techniques have allowed neuroscientists to gather a wealth of anatomic and functional information about the brain. Among these techniques, by virtue of its rich optical absorption contrast, high spatial and temporal resolutions, and deep penetration, photoacoustic tomography (PAT) has attracted more and more attention, and is playing an increasingly important role in brain studies. In particular, PAT complements other brain imaging modalities by providing high-resolution functional and metabolic imaging. More importantly, PAT’s unique scalability enables scrutinizing the brain at both microscopic and macroscopic scales, using the same imaging contrast. In this review, we present the state-of-the-art PAT techniques for brain imaging, summarize representative neuroscience applications, outline the technical challenges in translating PAT to human brain imaging, and envision potential technological deliverables

Time-resolved polarization imaging: Monte Carlo simulation

Monte Carlo method was used to simulate time resolved polarization imaging in turbid media. Mie theory was used to calculate the Meuller matrix of a single scattering event. In the simulation, the Stokes vector of each incident photon package was traced. The summation of the Stokes vectors of the traced photon packages gave the total output Stokes vector. The time integrated Mueller matrix of transmittance and reflectance light of a turbid media were calculated. The transmittance Mueller matrix and reflectance Mueller matrix have very different patterns. The time resolved 2D images of degree of polarization (DOP) for transmitted light and reflected light were calculated. The patterns showed different features for linearly polarized incident light and for circularly polarized light. The DOP patterns were also related to the scattering properties of the sample. The time resolved 2D DOP of the internal optical flux was also calculated. The DOP evolution was demonstrated vividly by the simulation results. The different patterns for linearly/circularly polarized light were compared. Linearly polarized light survived longer in turbid media with a small particle size. Circularly polarized light survived longer in turbid media with a larger particle size