Imaging optically scattering objects with ultrasound-modulated optical tomography

Comparison of contrast-to-noise ratios of different detection methods in ultrasound optical tomography

Ultrasound optical tomography (UOT) is a hybrid imaging modality based on interaction between ultrasound and light, with a potential to extend optical imaging capabilities in biological tissues to depths of several centimeters. Several methods have been developed to detect the UOT signal. To better understand their potential for deep tissue imaging, we present a theoretical contrast-to-noise comparison between the spectral hole burning, single-shot off-axis holography, speckle contrast, and photorefractive detection methods for UOT. Our results indicate that spectral hole burning filters have the potential to reach the largest imaging depths. We find that digital off-axis holography and photorefractive detection can have good contrast-to-noise ratio at significant depths. The speckle contrast method has a smaller penetration depth comparatively.

Integrated Neurophotonics: Toward Dense Volumetric Interrogation of Brain Circuit Activity-at Depth and in Real Time

We propose a new paradigm for dense functional imaging of brain activity to surmount the limitations of present methodologies. We term this approach "integrated neurophotonics"; it combines recent advances in microchip-based integrated photonic and electronic circuitry with those from optogenetics. This approach has the potential to enable lens-less functional imaging from within the brain itself to achieve dense, large-scale stimulation and recording of brain activity with cellular resolution at arbitrary depths. We perform a computational study of several prototype 3D architectures for implantable probe-array modules that are designed to provide fast and dense single-cell resolution (e.g., within a 1-mm3 volume of mouse cortex comprising ∼100,000 neurons). We describe progress toward realizing integrated neurophotonic imaging modules, which can be produced en masse with current semiconductor foundry protocols for chip manufacturing. Implantation of multiple modules can cover extended brain regions.

Ultrasound-modulated optical glucose sensing using a 1645 nm laser

Regular and frequent blood glucose monitoring is vital in managing diabetes treatment plans and preventing severe complications. Because current invasive techniques impede patient compliance and are not infection-free, many noninvasive methods have been proposed. Among them, optical methods have drawn much attention for their rich optical contrast, but their resolution is degraded in deep tissue. Here, we present an ultrasound-modulated optical sensing (UOS) technique to noninvasively monitor glucose that uses an infrared laser (1645 nm) and a single-element focused ultrasound transducer. Focused ultrasound waves can acoustically localize diffused photons in scattering media, and thus optical contrast can be represented with much enhanced spatial resolution. To maximize the signal-to-noise ratio, we compared the modulation depths of UOS signals in both continuous and burst ultrasound transmission modes. Finally, UOS measurements of various glucose concentrations are presented and compared with those acquired in phantoms with a conventional diffuse optical sensing method. The UOS measurements in a 20 mm thick tissue-mimicking phantom show 26.6% accuracy in terms of mean absolute relative difference (MARD), which indicates the great potential of the proposed technique as a noninvasive glucose sensor.

Imaging through highly scattering human skulls with ultrasound-modulated optical tomography

Advances in human brain imaging technologies are critical to understanding how the brain works and the diagnosis of brain disorders. Existing technologies have different drawbacks, and the human skull poses a great challenge for pure optical and ultrasound imaging technologies. Here we demonstrate the feasibility of using ultrasound-modulated optical tomography, a hybrid technology that combines both light and sound, to image through human skulls. Single-shot off-axis holography was used to measure the field of the ultrasonically tagged light. This Letter paves the way for imaging the brain noninvasively through the skull, with optical contrast and a higher spatial resolution than that of diffuse optical tomography.

Investigating ultrasound-light interaction in scattering media

SIGNIFICANCE

Ultrasound-assisted optical imaging techniques, such as ultrasound-modulated optical tomography, allow for imaging deep inside scattering media. In these modalities, a fraction of the photons passing through the ultrasound beam is modulated. The efficiency by which the photons are converted is typically referred to as the ultrasound modulation's "tagging efficiency." Interestingly, this efficiency has been defined in varied and discrepant fashion throughout the scientific literature.

AIM

The aim of this study is the ultrasound tagging efficiency in a manner consistent with its definition and experimentally verify the contributive (or noncontributive) relationship between the mechanisms involved in the ultrasound optical modulation process.

APPROACH

We adopt a general description of the tagging efficiency as the fraction of photons traversing an ultrasound beam that is frequency shifted (inclusion of all frequency-shifted components). We then systematically studied the impact of ultrasound pressure and frequency on the tagging efficiency through a balanced detection measurement system that measured the power of each order of the ultrasound tagged light, as well as the power of the unmodulated light component.

RESULTS

Through our experiments, we showed that the tagging efficiency can reach 70% in a scattering phantom with a scattering anisotropy of 0.9 and a scattering coefficient of 4  mm  -  1 for a 1-MHz ultrasound with a relatively low (and biomedically acceptable) peak pressure of 0.47 MPa. Furthermore, we experimentally confirmed that the two ultrasound-induced light modulation mechanisms, particle displacement and refractive index change, act in opposition to each other.

CONCLUSION

Tagging efficiency was quantified via simulation and experiments. These findings reveal avenues of investigation that may help improve ultrasound-assisted optical imaging techniques.

Ultrasound-mediation of self-illuminating reporters improves imaging resolution in optically scattering media

In vivo imaging of self-illuminating bio-and chemiluminescent reporters is used to observe the physiology of small animals. However, strong light scattering by biological tissues results in poor spatial resolution of the optical imaging, which also degrades the quantitative accuracy. To overcome this challenging problem, focused ultrasound is used to modulate the light from the reporter at the ultrasound frequency. This produces an ultrasound switchable light 'beacon' that reduces the influence of light scattering in order to improve spatial resolution. The experimental results demonstrate that apart from light modulation at the ultrasound frequency (AC signal at 3.5 MHz), ultrasound also increases the DC intensity of the reporters. This is shown to be due to a temperature rise caused by insonification that was minimized to be within acceptable mammalian tissue safety thresholds by adjusting the duty cycle of the ultrasound. Line scans of bio-and chemiluminescent objects embedded within a scattering medium were obtained using ultrasound modulated (AC) and ultrasound enhanced (DC) signals. Lateral resolution is improved by a factor of 12 and 7 respectively, as compared to conventional CCD imaging. Two chemiluminescent sources separated by ~10 mm at ~20 mm deep inside a 50 mm thick chicken breast have been successfully resolved with an average signal-to-noise ratio of approximately 8-10 dB.

Lock-in camera based heterodyne holography for ultrasound-modulated optical tomography inside dynamic scattering media

Ultrasound-modulated optical tomography (UOT) images optical contrast deep inside scattering media. Heterodyne holography based UOT is a promising technique that uses a camera for parallel speckle detection. In previous works, the speed of data acquisition was limited by the low frame rates of conventional cameras. In addition, when the signal-to-background ratio was low, these cameras wasted most of their bits representing an informationless background, resulting in extremely low efficiencies in the use of bits. Here, using a lock-in camera, we increase the bit efficiency and reduce the data transfer load by digitizing only the signal after rejecting the background. Moreover, compared with the conventional four-frame based amplitude measurement method, our single-frame method is more immune to speckle decorrelation. Using lock-in camera based UOT with an integration time of 286 μs, we imaged an absorptive object buried inside a dynamic scattering medium exhibiting a speckle correlation time ([Formula: see text]) as short as 26 μs. Since our method can tolerate speckle decorrelation faster than that found in living biological tissue ([Formula: see text] ∼ 100-1000 μs), it is promising for in vivo deep tissue non-invasive imaging.

Gradient-Based Quantitative Image Reconstruction in Ultrasound-Modulated Optical Tomography: First Harmonic Measurement Type in a Linearised Diffusion Formulation

Ultrasound-modulated optical tomography is an emerging biomedical imaging modality which uses the spatially localised acoustically-driven modulation of coherent light as a probe of the structure and optical properties of biological tissues. In this work we begin by providing an overview of forward modelling methods, before deriving a linearised diffusion-style model which calculates the first-harmonic modulated flux measured on the boundary of a given domain. We derive and examine the correlation measurement density functions of the model which describe the sensitivity of the modality to perturbations in the optical parameters of interest. Finally, we employ said functions in the development of an adjoint-assisted gradient based image reconstruction method, which ameliorates the computational burden and memory requirements of a traditional Newton-based optimisation approach. We validate our work by performing reconstructions of optical absorption and scattering in two- and three-dimensions using simulated measurements with 1% proportional Gaussian noise, and demonstrate the successful recovery of the parameters to within ±5% of their true values when the resolution of the ultrasound raster probing the domain is sufficient to delineate perturbing inclusions.

Quantitative estimation of mechanical and optical properties from ultrasound assisted optical tomography data

We demonstrate quantitative optical property and elastic property imaging from ultrasound assisted optical tomography data. The measurements, which are modulation depth M and phase ϕ of the speckle pattern, are shown to be sensitively dependent on these properties of the object in the insonified focal region of the ultrasound (US) transducer. We demonstrate that Young's modulus (E) can be recovered from the resonance observed in M versus ω (the US frequency) plots and optical absorption (μ(a)) and scattering (μ(s)) coefficients from the measured differential phase changes. All experimental observations are verified also using Monte Carlo simulations.

State-of-the art of acousto-optic sensing and imaging of turbid media

Acousto-optic (AO) is an emerging hybrid technique for measuring optical contrast in turbid media using coherent light and ultrasound (US). A turbid object is illuminated with a coherent light source leading to speckle formation in the remitted light. With the use of US, a small volume is selected,which is commonly referred to as the "tagging" volume. This volume acts as a source of modulated light, where modulation might involve phase and intensity change. The tagging volume is created by focusing ultrasound for good lateral resolution; the axial resolution is accomplished by making either the US frequency, amplitude, or phase time-dependent. Typical resolutions are in the order of 1 mm. We will concentrate on the progress in the field since 2003. Different schemes will be discussed to detect the modulated photons based on speckle detection, heterodyne detection, photorefractive crystal (PRC) assisted detection, and spectral hole burning (SHB) as well as Fabry-Perot interferometers. The SHB and Fabry-Perot interferometer techniques are insensitive to speckle decorrelation and therefore suitable for in vivo imaging. However, heterodyne and PRC methods also have potential for in vivo measurements. Besides measuring optical properties such as scattering and absorption, AO can be applied in fluorescence and elastography applications.

Application of a maximum likelihood algorithm to ultrasound modulated optical tomography

In pulsed ultrasound modulated optical tomography (USMOT), an ultrasound (US) pulse performs as a scanning probe within the sample as it propagates, modulating the scattered light spatially distributed along its propagation axis. Detecting and processing the modulated signal can provide a 1-dimensional image along the US axis. A simple model is developed wherein the detected signal is modelled as a convolution of the US pulse and the properties (ultrasonic/optical) of the medium along the US axis. Based upon this model, a maximum likelihood (ML) method for image reconstruction is established. For the first time to our knowledge, the ML technique for an USMOT signal is investigated both theoretically and experimentally. The ML method inverts the data to retrieve the spatially varying properties of the sample along the US axis, and a signal proportional to the optical properties can be acquired. Simulated results show that the ML method can serve as a useful reconstruction tool for a pulsed USMOT signal even when the signal-to-noise ratio (SNR) is close to unity. Experimental data using 5 cm thick tissue phantoms (scattering coefficient μ(s) = 6.5 cm(-1), anisotropy factor g=0.93) demonstrate that the axial resolution is 160 μm and the lateral resolution is 600 μm using a 10 MHz transducer.

Spatial sensitivity of acousto-optic and optical near-infrared spectroscopy sensing measurements

Near-infrared spectroscopy (NIRS) is a popular sensing technique to measure tissue oxygenation noninvasively. However, the region of interest (ROI) is often beneath a superficial layer, which affects its accuracy. By applying focused ultrasound in the ROI, acousto-optic (AO) techniques can potentially minimize the effect of physiological changes in the superficial layer. Using absorption perturbation experiments in both transmission and reflection modes, we investigated the spatial sensitivity distributions and mean penetration depths of an AO system based on a digital correlator and two popular NIRS systems based on i. intensity measurements using a single source and detector configuration, and ii. spatially resolved spectroscopy. Our results show that for both transmission and reflection modes, the peak relative sensitivities of the two NIRS systems are near to the superficial regions, whereas those of the AO technique are near to the ROIs. In the reflection mode, when the ROI is deeper than 14 mm, the AO technique has a higher absolute mean sensitivity than the two NIRS techniques. As the focused ultrasound is moved deeper into the turbid medium, the mean penetration depth increases accordingly. The focused ultrasound can shift the peak relative sensitivity of the AO measurement toward its focused region.

Measuring tissue properties and monitoring therapeutic responses using acousto-optic imaging

Acousto-optic imaging is a hybrid imaging technique that exploits the interaction between light and sound to image optical contrast at depth in optically turbid media with the high spatial resolution of ultrasound. Quantitative measurement of optical properties using this technique is confounded by multiple parameters that influence the detected acousto-optic signal. In this article, we describe the origin of the acousto-optic response and review techniques that have been proposed to relate this response to the optical properties of turbid media. We present an overview of two acousto-optic sensing approaches. In the first, we demonstrate that the local transport mean free path within turbid media can be obtained by varying the pressure of the ultrasound field and processing the resulting acousto-optic signals. In the second, we demonstrate that the acousto-optic response elicited by a high-intensity ultrasound field during thermal therapy can be used to monitor the onset of lesion formation, ascertain lesion volume, and provide real-time control of exposure duration.

Ultrasound-mediated optical tomography: a review of current methods

Ultrasound-mediated optical tomography (UOT) is a hybrid technique that is able to combine the high penetration depth and high spatial resolution of ultrasound imaging to overcome the limits imposed by optical scattering for deep tissue optical sensing and imaging. It has been proposed as a method to detect blood concentrations, oxygenation and metabolism at depth in tissue for the detection of vascularized tumours or the presence of absorbing or scattering contrast agents. In this paper, the basic principles of the method are outlined and methods for simulating the UOT signal are described. The main detection methods are then summarized with a discussion of the advantages and disadvantages of each. The recent focus on increasing the weak UOT signal through the use of the acoustic radiation force is explained, together with a summary of our results showing sensitivity to the mechanical shear stiffness and optical absorption properties of tissue-mimicking phantoms.

Real-time monitoring of high-intensity focused ultrasound lesion formation using acousto-optic sensing

High-intensity focused ultrasound (HIFU) is a promising modality that is used to noninvasively ablate soft tissue tumors. Nevertheless, real-time treatment monitoring with diagnostic ultrasound still poses a significant challenge since tissue necrosis, in the absence of cavitation or boiling, provides little acoustic contrast with normal tissue. In comparison, the optical properties of tissue are significantly altered accompanying lesion formation. A photorefractive crystal-based acousto-optic (AO) sensing system that uses a single HIFU transducer to simultaneously generate tissue necrosis and pump the AO interaction is used to monitor the real-time optical changes associated with thermal lesions induced in chicken breast ex vivo. It is found that the normalized change in AO response increases proportionally with the volume of necrosis. This study demonstrates AO sensing can identify the onset and growth of lesion formation in real time and, when used as feedback to guide exposures, results in more predictable lesion formation.

Spectral hole burning for ultrasound-modulated optical tomography of thick tissue

We apply spectral hole burning (SHB)-aided detection in ultrasound-modulated optical tomography (UOT) to image optical heterogeneities in thick tissue-mimicking phantom samples and chicken breast tissue. The efficiency of SHB is improved by using a Tm(3+):YAG crystal of higher doping concentration (2.0-atomic%) and a double-pass pumping configuration, in which the pump beam is transmitted through the crystal twice to burn a deeper spectral hole with the available optical intensity. With the improved SHB-UOT system, we image absorbing, scattering, and phase objects that are embedded in the middle plane of a 30-mm-thick phantom sample. The imaging resolution was 0.5 mm in the lateral direction, as defined by the focal width of the ultrasonic transducer, and 1.5 mm in the axial direction, as determined by the ultrasonic burst length. We also image two absorbing objects embedded in a 32-mm-thick chicken breast sample. The results suggest that the improved SHB-UOT system is one step closer to the practical optical imaging application in biological and clinical studies.

Assessment of ultrasound modulation of near infrared light on the quantification of scattering coefficient

PURPOSE

To assess the effect of ultrasound modulation of near infrared (NIR) light on the quantification of scattering coefficient in tissue-mimicking biological phantoms.

METHODS

A unique method to estimate the phase of the modulated NIR light making use of only time averaged intensity measurements using a charge coupled device camera is used in this investigation. These experimental measurements from tissue-mimicking biological phantoms are used to estimate the differential pathlength, in turn leading to estimation of optical scattering coefficient. A Monte-Carlo model based numerical estimation of phase in lieu of ultrasound modulation is performed to verify the experimental results.

RESULTS

The results indicate that the ultrasound modulation of NIR light enhances the effective scattering coefficient. The observed effective scattering coefficient enhancement in tissue-mimicking viscoelastic phantoms increases with increasing ultrasound drive voltage. The same trend is noticed as the ultrasound modulation frequency approaches the natural vibration frequency of the phantom material. The contrast enhancement is less for the stiffer (larger storage modulus) tissue, mimicking tumor necrotic core, compared to the normal tissue.

CONCLUSIONS

The ultrasound modulation of the insonified region leads to an increase in the effective number of scattering events experienced by NIR light, increasing the measured phase, causing the enhancement in the effective scattering coefficient. The ultrasound modulation of NIR light could provide better estimation of scattering coefficient. The observed local enhancement of the effective scattering coefficient, in the ultrasound focal region, is validated using both experimental measurements and Monte-Carlo simulations.

Fast Monte Carlo simulations of ultrasound-modulated light using a graphics processing unit

Ultrasound-modulated optical tomography (UOT) is based on "tagging" light in turbid media with focused ultrasound. In comparison to diffuse optical imaging, UOT can potentially offer a better spatial resolution. The existing Monte Carlo (MC) model for simulating ultrasound-modulated light is central processing unit (CPU) based and has been employed in several UOT related studies. We reimplemented the MC model with a graphics processing unit [(GPU), Nvidia GeForce 9800] that can execute the algorithm up to 125 times faster than its CPU (Intel Core Quad) counterpart for a particular set of optical and acoustic parameters. We also show that the incorporation of ultrasound propagation in photon migration modeling increases the computational time considerably, by a factor of at least 6, in one case, even with a GPU. With slight adjustment to the code, MC simulations were also performed to demonstrate the effect of ultrasonic modulation on the speckle pattern generated by the light model (available as animation). This was computed in 4 s with our GPU implementation as compared to 290 s using the CPU.

Ultrasound-modulated fluorescence from rhodamine B aqueous solution

We report experimentally observed ultrasound-modulated fluorescence (UMF) from a submillimeter tube filled with rhodamine B aqueous solution. The tube was submerged in water and a scattering medium. Based on the measured data, we find that the UMF signals might be generated from three mechanisms: modulation of the excitation light, modulation of the emission light, and modulation of the properties of fluorophore. In addition, a linear relationship between the UMF and the drive voltage applied to the ultrasound transducer is found.

Ultrasound-modulated optical imaging using a high-power pulsed laser and a double-pass confocal Fabry-Perot interferometer

We report the use of short ultrasonic bursts and high-peak-power laser pulses to detect absorbing objects in thick scattering media (SMs). The detection of ultrasound-tagged photons is performed with a double-pass confocal Fabry-Perot interferometer. Photons shifted by the fundamental and harmonic frequencies of the ultrasonic bursts were observed. Absorbing objects were detected in 30- and 60-mm-thick SMs including a sample of biological tissue.

Quantitative characterization of turbid media using pressure contrast acousto-optic imaging

An acousto-optic imaging technique suitable for the local and quantitative determination of subsurface optical properties in turbid media is presented. Acousto-optic signals elicited by ultrasound pulses at two different peak pressures in turbid media are detected by using photorefractive-crystal-based interferometry. The ratio of the measured signals, once calibrated for a particular set of pressure pulses, is found to give a direct measure of the reduced scattering coefficient of the interaction region between the light and sound. Measurements of the reduced scattering coefficient of inclusions buried in diffuse tissue phantoms are demonstrated.

Ultrasound-modulated optical tomography in reflection mode with ring-shaped light illumination

We have succeeded in implementing ring-shaped light illumination ultrasound-modulated optical tomography (UOT) in reflection mode. The system used intense acoustic bursts and a charge-coupled device (CCD) camera-based speckle contrast detection method. In addition, the implementation allows placing the tissue sample below (not within) an acoustic coupling water tank and scanning the tissue without moving the sample. Thus, the UOT system is more clinically applicable than previous transmission-mode systems. Furthermore, we have successfully imaged an ex vivo methylene-blue-dyed sentinel lymph node (SLN) embedded at a depth of 13 mm in chicken breast tissue. This UOT system offers several advantages: noninvasiveness, nonionizing radiation, portability, cost effectiveness, and the possibility of combination with ultrasound pulse-echo imaging and photoacoustic imaging. One potential application of the UOT system is mapping SLNs in axillary staging for breast cancer patients.

Ex vivo blood vessel imaging using ultrasound-modulated optical microscopy

Recently we developed ultrasound-modulated optical microscopy (UOM) based on a long-cavity confocal Fabry-Perot interferometer (CFPI). This interferometer is used for real-time detection of multiply scattered light modulated by high frequency (30 to 75 MHz) ultrasound pulses propagating in an optically, strongly scattering medium. In this work, we use this microscope to study the dependence of ultrasound-modulated optical signals on the optical absorption and scattering properties of objects embedded about 3 mm deep in tissue mimicking phantoms. These results demonstrate that UOM has the potential to map both optical absorption and scattering contrast. Most importantly, for the first time in the field of ultrasound-modulated optical imaging, we image blood vasculature in highly scattering tissue samples from a mouse and a rat. Therefore, UOM could be a promising tool to study the morphology of blood vasculature and blood-associated functional parameters, such as oxygen saturation.

Ultrasound-modulated optical microscopy

We demonstrate that microscopic imaging is feasible in ultrasound-modulated optical tomography (UOT) of soft biological tissues, using a high-frequency focused ultrasound transducer with a 75-MHz central frequency. Our experiments in tissue mimicking phantoms show that at an imaging depth of about 2 mm, an axial resolution better than 30 microm can be achieved, whereas the lateral resolution is 38 microm. A long-cavity scanning confocal Fabry-Perot interferometer (CFPI) is used for real-time detection of multiply scattered light modulated by high-frequency ultrasound pulses propagating in an optically scattering medium. We also compare the performances of various high-frequency focused ultrasound transducers with central frequencies of 15 MHz, 30 MHz, 50 MHz, and 75 MHz. The comparison is based on two-dimensional (2-D) images of optically absorbing objects positioned at a few millimeters depth below the surface of both optically scattering phantoms and soft biological tissue samples. Our experimental results show that modulation depth and image contrast decrease with an increase in ultrasound frequency. In addition, we use analytical calculations to show that modulation depth decreases with increasing ultrasound frequency.

Ultrasound-modulated optical imaging using a powerful long pulse laser

Ultrasound-modulated optical imaging (or tomography) is an emerging biodiagnostic technique which provides the optical spectroscopic signature and the localization of an absorbing object embedded in a strongly scattering medium. We propose to improve the sensitivity of the technique by using a pulsed single-frequency laser to raise the optical peak power applied to the scattering medium and thereby collect more ultrasonically tagged photons. Moreover, when the detection of tagged photons is done with a photorefractive interferometer, the high optical peak power reduces the response time of the photorefractive crystal below the speckle field decorrelation time. Results obtained with a GaAs photorefractive interferometer are presented for 30- and 60-mm thick scattering media.