Frequency-swept ultrasound-modulated optical tomography of scattering media

A novel frequency-swept ultrasound-modulated optical tomography technique was developed to image scattering media. A frequency-swept ultrasonic wave was used to modulate the laser light passing through a scattering medium. The modulated light was received by an optical detector and was heterodyned with a reference frequency sweep. The heterodyned signal was recorded in the time domain and was then analyzed in the frequency domain to yield a one-dimensional image along the ultrasonic axis. Multiple one-dimensional images obtained at various positions perpendicular to the ultrasonic axis were combined to yield a two-dimensional tomographic image of the medium.     

High-resolution ultrasound-modulated optical tomography in biological tissues

We present a novel implementation of high-resolution ultrasound-modulated optical tomography that, based on optical contrast, can image several millimeters deep into soft biological tissues. A long-cavity confocal Fabry-Perot interferometer, which provides a large etendue and a short response time, was used to detect the ultrasound-modulated coherent light that traversed the scattering biological tissue. Using 15-MHz ultrasound, we imaged with high-contrast light-absorbing structures placed >3 mm below the surface of chicken breast tissue. The resolution along the axial and the lateral directions with respect to the ultrasound propagation direction was better than 70 and 120 microm, respectively. The resolution can be scaled down further by use of higher ultrasound frequencies. This technology is complementary to other imaging technologies, such as confocal microscopy and optical-coherence tomography, and has the potential for broad biomedical applications.     

Birefringence characterization of biological tissue by use of optical coherence tomography

An improved polarization-sensitive optical coherence tomography (OCT) system is developed and used to measure birefringence in porcine myocardium tissue and produce two-dimensional birefringence mapping of the tissue. Signal-to-noise issues that cause systematic measurement errors are analyzed to determine the regime in which such measurements are accurate. The advantage of polarization-sensitive OCT systems over standard OCT systems in avoiding image artifacts caused by birefringence is also demonstrated.     

Multi-optical-wavelength ultrasound-modulated optical tomography: a phantom study

We used multiple optical wavelengths to study ultrasound-modulated optical tomography (UOT) in tissue phantoms. By using intense acoustic bursts and a CCD camera-based speckle contrast detection technique, we observed variations of the ultrasound-modulated signal at various optical absorptions. The experimental variations were found to be highly correlated with predictions from Monte Carlo simulations. By irradiating the sample at two optical wavelengths, we quantitatively estimated the total concentration and the concentration ratio of double dyes in objects embedded in tissue phantoms. The results suggest that UOT can potentially provide noninvasive functional imaging of the total concentration and oxygen saturation of hemoglobin in biological tissue.     

Imaging optically scattering objects with ultrasound-modulated optical tomography

We show the feasibility of imaging objects having different optical scattering coefficients relative to the surrounding scattering medium using ultrasound-modulated optical tomography (UOT). While the spatial resolution depends on ultrasound parameters, the image contrast depends on the difference in scattering coefficient between the object and the surrounding medium. Experimental measurements obtained with a CCD-based speckle contrast detection scheme are in agreement with Monte Carlo simulations and analytical calculations. This study complements previous UOT experiments that demonstrated optical absorption contrast.     

Intense acoustic bursts as a signal-enhancement mechanism in ultrasound-modulated optical tomography

Biophotonic imaging with ultrasound-modulated optical tomography (UOT) promises ultrasonically resolved imaging in biological tissues. A key challenge in this imaging technique is a low signal-to-noise ratio (SNR). We show significant UOT signal enhancement by using intense time-gated acoustic bursts. A CCD camera captured the speckle pattern from a laser-illuminated tissue phantom. Differences in speckle contrast were observed when ultrasonic bursts were applied, compared with when no ultrasound was applied. When CCD triggering was synchronized with burst initiation, acoustic-radiation-force-induced displacements were detected. To avoid mechanical contrast in UOT images, the CCD camera acquisition was delayed several milliseconds until transient effects of acoustic radiation force attenuated to a satisfactory level. The SNR of our system was sufficiently high to provide an image pixel per acoustic burst without signal averaging. Because of the substantially improved SNR, the use of intense acoustic bursts is a promising signal enhancement strategy for UOT.     

Shot-noise detection of ultrasound-tagged photons in ultrasound-modulated optical imaging

We propose a new detection method for ultrasound-modulated optical tomography that allows us to perform parallel speckle detection with optimum shot-noise sensitivity, using a CCD camera. Moreover, we show that making use of a spatial filter system allows us to fully filter out speckle decorrelation noise. This method is confirmed by a test experiment.     

Two-dimensional birefringence imaging in biological tissue by polarization-sensitive optical coherence tomography

Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.     

Cross-polarized backscatter in optical coherence tomography of biological tissue

We have observed that cross-polarized backscatter measured by optical coherence tomography of human skin in vivo is surprisingly strong. We identify and give evidence of its main origins: single scattering from nonspherical particles and multiple scattering by particles with sizes much larger than a wavelength. Our findings show that depolarized light scattered by dense large-diameter particles maintains a high degree of temporal coherence and that differential-polarization imaging improves contrast between particles of different sizes.     

Photorefractive detection of tissue optical and mechanical properties by ultrasound modulated optical tomography

Ultrasound-modulated optical tomography is a developing hybrid imaging modality that combines high optical contrast and good ultrasonic resolution for imaging soft biological tissue. We developed a photorefractive-crystal-based, time-resolved detection scheme with the use of a millisecond long ultrasound burst to image both the optical and the mechanical properties of biological tissues, with improved detection efficiency of ultrasound-tagged photons.     

Two-dimensional depth-resolved Mueller matrix characterization of biological tissue by optical coherence tomography

We built a polarization-sensitive optical coherence tomographic system and measured the two-dimensional depth-resolved full 4 x 4 Mueller matrix of biological tissue for what is believed to be the first time. The Mueller matrix measurements, which we made by varying the polarization states of the light source and the detector, yielded a complete characterization of the polarization property of the tissue sample. The initial experimental results indicated that this new approach reveals some tissue structures that are not perceptible in standard optical coherence tomography.     

Ultrasound-enhanced chemiluminescence tomography in biological tissue

This paper reports ultrasound-assisted optical imaging of chemiluminescent probes in biological tissue. A focused low power ultrasound sonochemically enhances a peroxyoxalate chemiluminescence (CL) that involves indocyanine green (ICG) as luminescent pigments. By scanning the focus, it produces tomographic images of CL in scattering media. The authors demonstrate imaging using a slab of porcine muscle measuring 50 × 50 × 75 mm, in which a capsuled CL reagent is embedded at 25 mm depth. Spatial resolution of imaging and concentration characteristics of CL reagents to enhanced CL intensity are also studied to evaluate the potential for use in bio-imaging applications with exploring the CL enhancement mechanisms. CL enhancement ratio, defined as the ratio of ultrasonically enhanced CL intensity to the base intensity without ultrasound irradiation, was found to be constant even in varying ICG and oxidizer concentrations, implying to be applicable for quantitative determination of these molecules.     

Effects of modulation phase of ultrasound-modulated light on the ultrasound-modulated optical image in turbid media

In this paper, our investigations suggest that the modulation phase of ultrasound-modulated light escaping from the different locations in the ultrasonic field is different. In turbid media, the modulation phase causes the ultrasound-modulated light intensity collected outside the media to fluctuate. However, the ultrasound-modulated optical technology uses the ultrasound-modulated light signals to image. Consequently, the modulation phase affects the quality of ultrasound-modulated optical imaging.     

Two-dimensional depth-resolved Mueller matrix of biological tissue measured with double-beam polarization-sensitive optical coherence tomography

A double-beam polarization-sensitive system based on optical coherence tomography was built to measure the Mueller matrix of scattering biological tissue with high spatial resolution. The Jones matrix of a sample can be determined with a single scan and subsequently converted into an equivalent nondepolarizing Mueller matrix. As a result, the system can be used to measure the Mueller matrix of an unstable sample, such as soft tissue. The polarization parameters of a porcine tendon, including magnitude and orientation of birefringence and diattenuation, were extracted by decomposition of the measured Mueller matrix.     

Real-time phase-resolved functional optical coherence tomography by use of optical Hilbert transformation

We have developed a novel real-time phase-resolved functional optical coherence tomography system that uses optical Hilbert transformation. When we use a resonant scanner in the reference arm of the interferometer, with an axial scanning speed of 4 kHz, the frame rate of both structural and Doppler blood-flow imaging with a size of 100 by 100 pixels is 10 Hz. The system has high sensitivity and a larger dynamic range for measuring the Doppler frequency shift that is due to moving red blood cells. Real-time images of in vivo blood flow in human skin obtained with this interferometer are presented.     

Determination of local polarization properties of biological samples in the presence of diattenuation by use of Mueller optical coherence tomography

A unique feature of polarization-sensitive Mueller optical coherence tomography is that, by measuring Jones or Mueller matrices, it can reveal the complete polarization properties of biological samples, even in the presence of diattenuation. We map local polarization properties for the first time to our knowledge by using polar decomposition in combination with least-squares fitting to differentiate measured integrated Jones matrices with respect to depth. We also introduce the new concept of dual attenuation coefficients to characterize diattenuation per unit infinitesimal length in tissues. We experimentally verify the algorithm using measurements of a section of porcine tendon and the septum of a rat heart.     

Complete-angle projection diffuse optical tomography by use of early photons

We present the first, to our knowledge, experimental images of complex-shaped phantoms embedded in diffuse media by use of optical tomography. Imaging is based on a complete-angle projection tomographic technique that utilizes transmitted early photons. Results are contrasted with measurements obtained at later gates as well as pseudocontinuous-wave data. The scanning system developed employs noncontact illumination and detection technologies that allow for high spatial sampling of transmitted photons. Combining this system with complete-angle illumination is found to be an important strategy toward improved imaging performance, resulting in a better-posed inversion problem. The appropriateness of reconstruction algorithms similar to those employed in x-ray computed tomography are showcased, and suggestions for model improvements are provided.     

Subsurface defect detection in ceramics by high-speed high-resolution optical coherent tomography

We use optical coherence tomography with a new configuration to determine the size and location of subsurface defects in solid ceramic and composite ceramic materials. Cross-sectional subsurface regions either parallel or perpendicular to the surface were examined. We present experimental results showing that the size and distribution of small subsurface defects can be determined with depth and lateral resolutions of 10 and 4 microm, respectively.