Non-invasive measurements of breast tissue optical properties using frequency-domain photon migration

AAPM Task Group Report 311: Guidance for performance evaluation of fluorescence-guided surgery systems

The last decade has seen a large growth in fluorescence-guided surgery (FGS) imaging and interventions. With the increasing number of clinical specialties implementing FGS, the range of systems with radically different physical designs, image processing approaches, and performance requirements is expanding. This variety of systems makes it nearly impossible to specify uniform performance goals, yet at the same time, utilization of different devices in new clinical procedures and trials indicates some need for common knowledge bases and a quality assessment paradigm to ensure that effective translation and use occurs. It is feasible to identify key fundamental image quality characteristics and corresponding objective test methods that should be determined such that there are consistent conventions across a variety of FGS devices. This report outlines test methods, tissue simulating phantoms and suggested guidelines, as well as personnel needs and professional knowledge bases that can be established. This report frames the issues with guidance and feedback from related societies and agencies having vested interest in the outcome, coming from an independent scientific group formed from academics and international federal agencies for the establishment of these professional guidelines.

Analytical solution of the vector radiative transfer equation for single scattered radiance

In this paper, derivation of the analytical solution of the vector radiative transfer equation for the single scattered radiance of three-dimensional semi-infinite media with a refractive index mismatch at the boundary is presented. In particular, the solution is obtained in the spatial domain and spatial frequency domain. Besides the general derivation, determination of the amplitude scattering matrix, which is required for the analytical solution, is given in detail. Furthermore, the incorporation of Fresnel equations due to a refractive index mismatch at the boundary is presented. Finally, verification of the derived formulas is performed using a self-implemented electrical field Monte Carlo method based on Jones formalism. For this purpose, the solution based on Jones formalism is converted to Stokes-Mueller formalism. For the verification, spherical particles are assumed as scatterers, whereby arbitrary size distributions can be considered.

Distance insensitive reflectance setup for the spectrally resolved determination of the optical properties of highly turbid media

A measurement system for a distance insensitive acquisition of the reflectance from turbid media is presented. The geometric relationships of the detection unit are discussed theoretically and subsequently verified using Monte Carlo simulations. In addition, an experimental setup is presented to prove the theoretical considerations and simulations. The use of the presented measurement system allows measurements of the reflectance in a distance range of approximately 2.5cm with a deviation of less than ±0.5% for highly scattering media. This contrasts with the use of a fiber in a classical detection unit placed at a defined angle and position relative to the sample surface, which results in deviations of ±30% in the measured reflectance over the same distance range.

Multichannel Wireless Neurosensing System for battery-free monitoring of neuronal activity

Electrical activity recordings are critical for evaluating and understanding brain function. We present a novel wireless, implantable, and battery-free device, namely the Wireless Neurosensing System (WiNS), and for the first time, we evaluate multichannel recording capabilities in vivo. For a preliminary evaluation, we performed a benchtop experiment with emulated sinusoidal signals of varying amplitude and frequency, representative of neuronal activity. We later performed and analyzed electrocortical recordings in rats of evoked somatosensory activity in response to three paradigms: hind/fore limb and whisker stimulation. Wired recordings were used for comparison and validation of WiNS. We found that through the channel multiplexing element of WiNS, it is possible to perform multichannel recordings with a maximum sampling rate of ∼10 kHz for a total of eight channels. This sampling rate is appropriate for monitoring the full range of neuronal signals of interest, from low-frequency population recordings of electrocorticography and local field potentials to high-frequency individual neuronal spike recordings. These in vivo experiments demonstrated that the evoked neuronal activity recorded with WiNS is comparable to that recorded with a wired system under identical circumstances. Analysis of critical parameters for interpreting the somatosensory evoked activity showed no statistically significant difference between the parameters obtained by a wired system versus those obtained using WiNS. Therefore, WiNS can match the performance of more invasive recording systems. WiNS is a groundbreaking technology with potential applications throughout neuroscience as it offers a simple alternative to address the pitfalls of battery-powered neuronal implants.

Accurately calibrated frequency domain diffuse optical spectroscopy compared against chemical analysis of porcine adipose tissue

Frequency domain diffuse optical spectroscopy (fdDOS) is a noninvasive technique to estimate tissue composition and hemodynamics. While fdDOS has been established as a valuable modality for clinical research, comparison of fdDOS with direct chemical analysis (CA) methods has yet to be reported. To compare the two approaches, we propose a procedure to confirm accurate calibration by use of liquid emulsion and solid silicone phantoms. Tissue fat (FAT) and water (H₂ O) content of two ex vivo porcine tissue samples were optically measured by fdDOS and compared to CA values. We show an average H₂ O error (fdDOS minus CA) and SD of 1.9 ± 0.2% and -0.9 ± 0.2% for the two samples. For FAT, we report a mean error of -9.3 ± 1.3% and 0.8 ± 1.3%. We also measured various body sites of a healthy human subject using fdDOS with results suggesting that accurate calibration may improve device sensitivity.

Three-compartment-breast (3CB) prior-guided diffuse optical tomography based on dual-energy digital breast tomosynthesis (DBT)

Diffuse optical tomography (DOT) is a non-invasive functional imaging modality that uses near-infrared (NIR) light to measure the oxygenation state and the concentration of hemoglobin. By complementarily using DOT with other anatomical imaging modalities, physicians can diagnose more accurately through additional functional image information. In breast imaging, diagnosis of dense breasts is often challenging because the bulky fibrous tissues may hinder the correct tumor characterization. In this work, we proposed a three-compartment-breast (3CB) decomposition-based prior-guided optical tomography for enhancing DOT image quality. We conjectured that the 3CB prior would lead to improvement of the spatial resolution and also of the contrast of the reconstructed tumor image, particularly for the dense breasts. We conducted a Monte-Carlo simulation to acquire dual-energy X-ray projections of a realistic 3D numerical breast phantom and performed digital breast tomosynthesis (DBT) for setting up a 3CB model. The 3CB prior was then used as a structural guide in DOT image reconstruction. The proposed method resulted in the higher spatial resolution of the recovered tumor even when the tumor is surrounded by the fibroglandular tissues compared with the typical two-composition-prior method or the standard Tikhonov regularization method.

Lanthanide doped luminescence nanothermometers in the biological windows: strategies and applications

The development of lanthanide-doped non-contact luminescent nanothermometers with accuracy, efficiency and fast diagnostic tools attributed to their versatility, stability and narrow emission band profiles has spurred the replacement of conventional contact thermal probes. The application of lanthanide-doped materials as temperature nanosensors, excited by ultraviolet, visible or near infrared light, and the generation of emissions lying in the biological window regions, I-BW (650 nm-950 nm), II-BW (1000 nm-1350 nm), III-BW (1400 nm-2000 nm) and IV-BW (centered at 2200 nm), are notably growing due to the advantages they present, including reduced phototoxicity and photobleaching, better image contrast and deeper penetration depths into biological tissues. Here, the different mechanisms used in lanthanide ion-doped nanomaterials to sense temperature in these biological windows for biomedical and other applications are summarized, focusing on factors that affect their thermal sensitivity, and consequently their temperature resolution. Comparing the thermometric performance of these nanomaterials in each biological window, we identified the strategies that allow boosting of their sensing properties.

Quo Vadis, Nanoparticle-Enabled In Vivo Fluorescence Imaging?

The exciting advancements that we are currently witnessing in terms of novel materials and synthesis approaches are leading to the development of colloidal nanoparticles (NPs) with increasingly greater tunable properties. We have now reached a point where it is possible to synthesize colloidal NPs with functionalities tailored to specific societal demands. The impact of this new wave of colloidal NPs has been especially important in the field of biomedicine. In that vein, luminescent NPs with improved brightness and near-infrared working capabilities have turned out to be optimal optical probes that are capable of fast and high-resolution in vivo imaging. However, luminescent NPs have thus far only reached a limited portion of their potential. Although we believe that the best is yet to come, the future might not be as bright as some of us think (and have hoped!). In particular, translation of NP-based fluorescence imaging from preclinical studies to clinics is not straightforward. In this Perspective, we provide a critical assessment and highlight promising research avenues based on the latest advances in the fields of luminescent NPs and imaging technologies. The disillusioned outlook we proffer herein might sound pessimistic at first, but we consider it necessary to avoid pursuing "pipe dreams" and redirect the efforts toward achievable-yet ambitious-goals.

Tissue Phantoms for Biomedical Applications in Raman Spectroscopy: A Review

Raman spectroscopy is a group of analytical techniques, currently applied in several research fields, including clinical diagnostics. Tissue-mimicking optical phantoms have been established as an essential intermediate stage for medical applications with their employment from spectroscopic techniques to be constantly growing. This review outlines the types of tissue phantoms currently employed in different biomedical applications of Raman spectroscopy, focusing on their composition and optical properties. It is therefore an attempt to present an informed range of options for potential use to the researchers.

Optics Based Label-Free Techniques and Applications in Brain Monitoring

Functional near-infrared spectroscopy (fNIRS) has been utilized already around three decades for monitoring the brain, in particular, oxygenation changes in the cerebral cortex. In addition, other optical techniques are currently developed for in vivo imaging and in the near future can be potentially used more in human brain research. This paper reviews the most common label-free optical technologies exploited in brain monitoring and their current and potential clinical applications. Label-free tissue monitoring techniques do not require the addition of dyes or molecular contrast agents. The following optical techniques are considered: fNIRS, diffuse correlations spectroscopy (DCS), photoacoustic imaging (PAI) and optical coherence tomography (OCT). Furthermore, wearable optical brain monitoring with the most common applications is discussed.

An all-optical technique enables instantaneous single-shot demodulation of images at high frequency

High-frequency demodulation of wide area optical signals in a snapshot manner remains a technological challenge. If solved, it could open tremendous perspectives in 3D imaging, vibrometry, free-space communications, automated vision, or ballistic photon imaging in scattering media with numerous applications in smart autonomous vehicles and medical diagnosis. We present here a snapshot quadrature demodulation imaging technique, capable of estimating the amplitude and phase from a single acquisition, without synchronization of emitter and receiver, and with the added capability of continuous frequency tuning. This all-optical optimized setup comprises an electro-optic crystal acting as a fast sinusoidal optical transmission gate, and allows four quadrature image channels to be recorded simultaneously with any conventional camera. We report the design, experimental validation and examples of applications of such wide-field quadrature demodulating system that allowed snapshot demodulation of images with good spatial resolution and continuous frequency selectivity up to a few 100s of kilohertz.

Traditional lock-in detection methods have been limited for wide-field imaging. Here, the authors present an all-optical design which enables four quadrature image channels to be recorded simultaneously, and show demodulation of wide-field images based on a single frame acquisition.

Determination of optical properties of human tissues obtained from parotidectomy in the spectral range of 250 to 800 nm

The optical properties of human tissues are an important parameter in medical diagnostics and therapy. The knowledge of these parameters can encourage the development of automated, computer-driven optical tissue analysis methods. We determine the absorption coefficient μa and scattering coefficient μ s ' of different tissue types obtained during parotidectomy in the wavelength range of 250 to 800 nm. These values are determined by high precision integrating sphere measurements in combination with an optimized inverse Monte Carlo simulation. To conserve the optical behavior of living tissues, the optical spectroscopy measurements are performed immediately after tissue removal. Our study includes fresh samples of the ear, nose, and throat (ENT) region, as muscle tissue, nervous tissue, white adipose tissue, stromal tissue, parotid gland, and tumorous tissue of five patients. The measured behavior of adipose corresponds well with the literature, which sustains the applied method. It is shown that muscle is well supplied with blood as it features the same characteristic peaks at 430 and 555 nm in the absorption curve. The parameter μ s ' decreases for all tissue types above 570 nm. The accuracy is adequate for the purposes of providing μa and μ s ' of different human tissue types as muscle, fat, nerve, or gland tissue, which are embedded in large complex structures such as in the ENT area. It becomes possible for the first time to present reasonable results for the optical behavior of human soft tissue located in the ENT area and in the near-UV, visual, and near-infrared areas.

In vivo characterization of light scattering properties of human skin in the 475- to 850-nm wavelength range in a Swedish cohort

We have determined in vivo optical scattering properties of normal human skin in 1734 subjects, mostly with fair skin type, within the Swedish CArdioPulmonary bioImage Study. The measurements were performed with a noninvasive system, integrating spatially resolved diffuse reflectance spectroscopy and laser Doppler flowmetry. Data were analyzed with an inverse Monte Carlo algorithm, accounting for both scattering, geometrical, and absorbing properties of the tissue. The reduced scattering coefficient was found to decrease from 3.16 ± 0.72 to 1.13 ± 0.27 mm-1 (mean ± SD) in the 475- to 850-nm wavelength range. There was a negative correlation between the reduced scattering coefficient and age, and a significant difference between men and women in the reduced scattering coefficient as well as in the fraction of small scattering particles. This large study on tissue scattering with mean values and normal variation can serve as a reference when designing diagnostic techniques or when evaluating the effect of therapeutic optical systems.

Rapid scanning wide-field clutter elimination in epi-optoacoustic imaging using comb LOVIT

Epi-style optoacoustic (OA) imaging provides flexibility by integrating the irradiation optics and ultrasound receiver, yet clutter generated by optical absorption near the probe obscures deep OA sources. Localised vibration tagging (LOVIT) retrieves OA signal from images that are acquired with and without a preceding ultrasonic pushing beam: Radiation force leads to a phase shift of signals coming from the focal area resulting in their visibility in a difference image, whereas clutter from outside the pushing beam is eliminated. Disadvantages of a single-focus approach are residual clutter from inside the pushing beam above the focus, and time-intensive scanning of the focus to retrieve a large field-of-view. To speed up acquisition, we propose to create multiple foci in parallel, forming comb-shaped ARF patterns. By subtracting OA images obtained with interleaved combs, this technique moreover results in greatly improved clutter reduction in phantoms mimicking optical, acoustic and elastic properties of breast tissue.

Rapid scanning wide-field clutter elimination in epi-optoacoustic imaging using comb LOVIT

Epi-style optoacoustic (OA) imaging provides flexibility by integrating the irradiation optics and ultrasound receiver, yet clutter generated by optical absorption near the probe obscures deep OA sources. Localised vibration tagging (LOVIT) retrieves OA signal from images that are acquired with and without a preceding ultrasonic pushing beam: Radiation force leads to a phase shift of signals coming from the focal area resulting in their visibility in a difference image, whereas clutter from outside the pushing beam is eliminated. Disadvantages of a single-focus approach are residual clutter from inside the pushing beam above the focus, and time-intensive scanning of the focus to retrieve a large field-of-view. To speed up acquisition, we propose to create multiple foci in parallel, forming comb-shaped ARF patterns. By subtracting OA images obtained with interleaved combs, this technique moreover results in greatly improved clutter reduction in phantoms mimicking optical, acoustic and elastic properties of breast tissue.

Spatially Resolved Lateral Transmission Measurements to Characterize Changes in the Scattering Coefficient and the Anisotropy Factor

A new setup is described to characterize the scattering coefficient and the scattering phase function of liquid media. The setup utilizes the basic idea of a spatially resolved reflectance measurement combined with a sophisticated illumination geometry. The sample is illuminated parallel and close to the interface of the sample and a glass window to get information from single scattered and multiple scattered light. By illuminating the sample with a fiber orientated with the axis parallel to the glass surface, small distances to the source can be examined unimpeded by the illumination beam. The derived information is, for example, not only sensitive to the concentration of the scatterers but also to the size of the scattering particles. We present the setup including the theory to describe the light propagation in the whole configuration using Monte Carlo simulations. The validation has been done with polystyrene microsphere dispersions with different scattering coefficients. As application for the developed setup, we show measurements of different milk samples which vary in concentration of fat, protein, and in fat droplet size during homogenization process. By measuring milk, we show the ability of the sensor to determine information about the scattering phase function without diluting the sample. For sensors in the dairy industry, a measurement with no pre-processing and no diluting of the sample is worthwhile, because this can be used to determine the fat and protein concentration on-line.

Quantitative real-time optical imaging of the tissue metabolic rate of oxygen consumption

The tissue metabolic rate of oxygen consumption (tMRO2) is a clinically relevant marker for a number of pathologies including cancer and arterial occlusive disease. We present and validate a noncontact method for quantitatively mapping tMRO2 over a wide, scalable field of view at 16  frames  /  s. We achieve this by developing a dual-wavelength, near-infrared coherent spatial frequency-domain imaging (cSFDI) system to calculate tissue optical properties (i.e., absorption, μa, and reduced scattering, μs', parameters) as well as the speckle flow index (SFI) at every pixel. Images of tissue oxy- and deoxyhemoglobin concentration (  [  HbO2  ]   and [HHb]) are calculated from optical properties and combined with SFI to calculate tMRO2. We validate the system using a series of yeast-hemoglobin tissue-simulating phantoms and conduct in vivo tests in humans using arterial occlusions that demonstrate sensitivity to tissue metabolic oxygen debt and its repayment. Finally, we image the impact of cyanide exposure and toxicity reversal in an in vivo rabbit model showing clear instances of mitochondrial uncoupling and significantly diminished tMRO2. We conclude that dual-wavelength cSFDI provides rapid, quantitative, wide-field mapping of tMRO2 that can reveal unique spatial and temporal dynamics relevant to tissue pathology and viability.

Performance assessment of diffuse optical spectroscopic imaging instruments in a 2-year multicenter breast cancer trial

We present a framework for characterizing the performance of an experimental imaging technology, diffuse optical spectroscopic imaging (DOSI), in a 2-year multicenter American College of Radiology Imaging Network (ACRIN) breast cancer study (ACRIN-6691). DOSI instruments combine broadband frequency-domain photon migration with time-independent near-infrared (650 to 1000 nm) spectroscopy to measure tissue absorption and reduced scattering spectra and tissue hemoglobin, water, and lipid composition. The goal of ACRIN-6691 was to test the effectiveness of optically derived imaging endpoints in predicting the final pathologic response of neoadjuvant chemotherapy (NAC). Sixty patients were enrolled over a 2-year period at participating sites and received multiple DOSI scans prior to and during 3- to 6-month NAC. The impact of three sources of error on accuracy and precision, including different operators, instruments, and calibration standards, was evaluated using a broadband reflectance standard and two different solid tissue-simulating optical phantoms. Instruments showed <0.0010 mm−1 (10.3%) and 0.06 mm−1 (4.7%) deviation in broadband absorption and reduced scattering, respectively, over the 2-year duration of ACRIN-6691. These variations establish a useful performance criterion for assessing instrument stability. The proposed procedures and tests are not limited to DOSI; rather, they are intended to provide methods to characterize performance of any instrument used in translational optical imaging.

Characterization of the Optical Properties of Turbid Media by Supervised Learning of Scattering Patterns

Fabricated tissue phantoms are instrumental in optical in-vitro investigations concerning cancer diagnosis, therapeutic applications, and drug efficacy tests. We present a simple non-invasive computational technique that, when coupled with experiments, has the potential for characterization of a wide range of biological tissues. The fundamental idea of our approach is to find a supervised learner that links the scattering pattern of a turbid sample to its thickness and scattering parameters. Once found, this supervised learner is employed in an inverse optimization problem for estimating the scattering parameters of a sample given its thickness and scattering pattern. Multi-response Gaussian processes are used for the supervised learning task and a simple setup is introduced to obtain the scattering pattern of a tissue sample. To increase the predictive power of the supervised learner, the scattering patterns are filtered, enriched by a regressor, and finally characterized with two parameters, namely, transmitted power and scaled Gaussian width. We computationally illustrate that our approach achieves errors of roughly 5% in predicting the scattering properties of many biological tissues. Our method has the potential to facilitate the characterization of tissues and fabrication of phantoms used for diagnostic and therapeutic purposes over a wide range of optical spectrum.

Vertical-cavity surface-emitting laser sources for gigahertz-bandwidth, multiwavelength frequency-domain photon migration

Frequency-domain photon migration (FDPM) uses modulated laser light to measure the bulk optical properties of turbid media and is increasingly applied for noninvasive functional medical imaging in the near-infrared. Although semiconductor edge-emitting laser diodes have been traditionally used as miniature light sources for this application, we show that vertical-cavity surface-emitting lasers (VCSELs) exhibit output power and modulation performance characteristics suitable for FDPM measurements of tissue optical properties at modulation frequencies exceeding 1 GHz. We also show that an array of multiple VCSEL devices can be coherently modulated at frequencies suitable for FDPM and can improve optical power. In addition, their small size and simple packaging make them an attractive choice as components in wearable sensors and clinical FDPM-based optical spectroscopy systems. We demonstrate the benefits of VCSEL technology by fabricating and testing a unique, compact VCSEL-based optical probe with an integrated avalanche photodiode. We demonstrate sensitivity of the VCSEL-based probe to subcutaneous tissue hemodynamics that was induced during an arterial cuff occlusion of the upper arm in a human subject.

Combined multi-distance frequency domain and diffuse correlation spectroscopy system with simultaneous data acquisition and real-time analysis

Frequency domain near infrared spectroscopy (FD-NIRS) and diffuse correlation spectroscopy (DCS) have emerged as synergistic techniques for the non-invasive assessment of tissue health. Combining FD-NIRS oximetry with DCS measures of blood flow, the tissue oxygen metabolic rate can be quantified, a parameter more closely linked to underlying physiology and pathology than either NIRS or DCS estimates alone. Here we describe the first commercially available integrated instrument, called the "MetaOx", designed to enable simultaneous FD-NIRS and DCS measurements at rates of 10 + Hz, and offering real-time data evaluation. We show simultaneously acquired characterization data demonstrating performance equivalent to individual devices and sample in vivo measurements of pulsation resolved blood flow, forearm occlusion hemodynamic changes and muscle oxygen metabolic rate monitoring during stationary bike exercise.

Ultrafast wavelength multiplexed broad bandwidth digital diffuse optical spectroscopy for in vivo extraction of tissue optical properties

Frequency-domain diffuse optical spectroscopy (FD-DOS) utilizes intensity-modulated light to characterize optical scattering and absorption in thick tissue. Previous FD-DOS systems have been limited by large device footprints, complex electronics, high costs, and limited acquisition speeds, all of which complicate access to patients in the clinical setting. We have developed a new digital DOS (dDOS) system, which is relatively compact and inexpensive, allowing for simplified clinical use, while providing unprecedented measurement speeds. The dDOS system utilizes hardware-integrated custom board-level direct digital synthesizers and an analog-to-digital converter to generate frequency sweeps and directly measure signals utilizing undersampling at six wavelengths modulated at discrete frequencies from 50 to 400 MHz. Wavelength multiplexing is utilized to achieve broadband frequency sweep measurements acquired at over 97 Hz. When compared to a gold-standard DOS system, the accuracy of optical properties recovered with the dDOS system was within 5.3% and 5.5% for absorption and reduced scattering coefficient extractions, respectively. When tested in vivo, the dDOS system was able to detect physiological changes throughout the cardiac cycle. The new FD-dDOS system is fast, inexpensive, and compact without compromising measurement quality.

Miniature spectral imaging device for wide-field quantitative functional imaging of the morphological landscape of breast tumor margins

We have developed a portable, breast margin assessment probe leveraging diffuse optical spectroscopy to quantify the morphological landscape of breast tumor margins during breast conserving surgery. The approach presented here leverages a custom-made 16-channel annular photodiode imaging array (arranged in a 4 × 4 grid), a raster-scanning imaging platform with precision pressure control, and compressive sensing with an optimized set of eight wavelengths in the visible spectral range. A scalable Monte-Carlo-based inverse model is used to generate optical property [ ? s ? ( ? ) and ? a ( ? ) ] measures for each of the 16 simultaneously captured diffuse reflectance spectra. Subpixel sampling (0.75 mm) is achieved through incremental x , y raster scanning of the imaging probe, providing detailed optical parameter maps of breast margins over a 2 × 2 ?? cm 2 area in ? 9 ?? min . The morphological landscape of a tumor margin is characterized using optical surrogates for the fat to fibroglandular content ratio, which has demonstrated diagnostic utility in delineating tissue subtypes in the breast.

Diffuse optical tomography for breast cancer imaging guided by computed tomography: A feasibility study

Diffuse optical tomography (DOT) has attracted attentions in the last two decades due to its intrinsic sensitivity in imaging chromophores of tissues such as hemoglobin, water, and lipid. However, DOT has not been clinically accepted yet due to its low spatial resolution caused by strong optical scattering in tissues. Structural guidance provided by an anatomical imaging modality enhances the DOT imaging substantially. Here, we propose a computed tomography (CT) guided multispectral DOT imaging system for breast cancer imaging. To validate its feasibility, we have built a prototype DOT imaging system which consists of a laser at the wavelength of 650 nm and an electron multiplying charge coupled device (EMCCD) camera. We have validated the CT guided DOT reconstruction algorithms with numerical simulations and phantom experiments, in which different imaging setup parameters, such as projection number of measurements and width of measurement patch, have been investigated. Our results indicate that an air-cooling EMCCD camera is good enough for the transmission mode DOT imaging. We have also found that measurements at six angular projections are sufficient for DOT to reconstruct the optical targets with 2 and 4 times absorption contrast when the CT guidance is applied. Finally, we have described our future research plan on integration of a multispectral DOT imaging system into a breast CT scanner.

Spatial and Spectral Information in Optical Mammography

This article reviews our research activities in the area of optical mammography and relates them to the historical developments and the current state and trends in the field. The guiding threads for this article are the roles played in optical mammography by spatial and spectral information. The first feature, spatial information, is limited by the diffusive nature of light propagation but can take advantage of the exceptionally high optical contrast featured by blood vessels and blood-rich areas in the breast. We describe a method to correct for edge effects, a spatial second-derivative algorithm, and a two-dimensional phased-array approach that enhance the image contrast, the spatial resolution, and the depth discrimination in optical mammograms. The second feature, spectral information, is the most powerful and unique capability of optical mammography, and allows for functional measurements associated with hemoglobin concentration and oxygenation, water concentration, lipids content, and the wavelength dependence of tissue scattering. We present oxygenation-index images obtained from multi-wavelength optical data that point to the diagnostic potential of oxygenation information in optical mammography. The optimization of the spatial and spectral information in optical mammography has the potential to create a role for this imaging modality in the detection and monitoring of breast cancer.

Unveiling in Vivo Subcutaneous Thermal Dynamics by Infrared Luminescent Nanothermometers

The recent development of core/shell engineering of rare earth doped luminescent nanoparticles has ushered a new era in fluorescence thermal biosensing, allowing for the performance of minimally invasive experiments, not only in living cells but also in more challenging small animal models. Here, the potential use of active-core/active-shell Nd(3+)- and Yb(3+)-doped nanoparticles as subcutaneous thermal probes has been evaluated. These temperature nanoprobes operate in the infrared transparency window of biological tissues, enabling deep temperature sensing into animal bodies thanks to the temperature dependence of their emission spectra that leads to a ratiometric temperature readout. The ability of active-core/active-shell Nd(3+)- and Yb(3+)-doped nanoparticles for unveiling fundamental tissue properties in in vivo conditions was demonstrated by subcutaneous thermal relaxation monitoring through the injected core/shell nanoparticles. The reported results evidence the potential of infrared luminescence nanothermometry as a diagnosis tool at the small animal level.

Real-time simultaneous single snapshot of optical properties and blood flow using coherent spatial frequency domain imaging (cSFDI)

In this work we present and validate a wide-field method for the real-time mapping of tissue absorption, scattering and blood flow properties over wide regions of tissue (15 cm x 15 cm) with high temporal resolution (50 frames per second). We achieve this by applying Fourier Domain demodulation techniques to coherent spatial frequency domain imaging to extract optical properties and speckle flow index from a single snapshot. Applying this technique to forearm reactive hyperemia protocols demonstrates the ability to resolve intrinsic physiological signals such as the heart beat waveform and the buildup of deoxyhemoglobin associated with oxygen consumption.

Dual-color photoacoustic lymph node imaging using nanoformulated naphthalocyanines

Demarking lymph node networks is important for cancer staging in clinical practice. Here, we demonstrate in vivo dual-color photoacoustic lymphangiography using all-organic nanoformulated naphthalocyanines (referred to as nanonaps). Nanonap frozen micelles were self-assembled from two different naphthalocyanine dyes with near-infrared absorption at 707 nm or 860 nm. These allowed for noninvasive, nonionizing, high resolution photoacoustic identification of separate lymphatic drainage systems in vivo. With both types of nanonaps, rat lymph nodes buried deeply below an exogenously-placed 10 mm thick layer of chicken breast were clearly visualized in vivo. These results show the potential of multispectral photoacoustic imaging with nanonaps for detailed mapping of lymphatic drainage systems.

Cherenkov radiation fluence estimates in tissue for molecular imaging and therapy applications

Cherenkov radiation has recently emerged as an interesting phenomenon for a number of applications in the biomedical sciences. Its unique properties, including broadband emission spectrum, spectral weight in the ultraviolet and blue wavebands, and local generation of light within a given tissue, have made it an attractive new source of light within tissue for molecular imaging and phototherapy applications. While several studies have investigated the total Cherenkov light yield from radionuclides in units of [photons/decay], further consideration of the light propagation in tissue is necessary to fully consider the utility of this signal in vivo. Therefore, to help further guide the development of this novel field, quantitative estimates of the light fluence rate of Cherenkov radiation from both radionuclides and radiotherapy beams in a biological tissue are presented for the first time. Using Monte Carlo simulations, these values were found to be on the order of 0.01-1 nW cm(-2) per MBq g(-1) for radionuclides, and 1-100 μW cm(-2) per Gy s(-1) for external radiotherapy beams, dependent on the given waveband, optical properties, and radiation source. For phototherapy applications, the total light fluence was found to be on the order of nJ cm(-2) for radionuclides, and mJ cm(-2) for radiotherapy beams. The results indicate that diagnostic potential is reasonable for Cherenkov excitation of molecular probes, but phototherapy may remain elusive at such exceedingly low fluence values. The results of this study are publicly available for distribution online at www.dartmouth.edu/optmed/.

Effect of 18F-FDG uptake time on lesion detectability in PET imaging of early stage breast cancer

Prior reports have suggested that delayed FDG-PET oncology imaging can improve the contrast-to-noise ratio (CNR) for known lesions. Our goal was to estimate realistic bounds for lesion detectability for static measurements with one to four hours between FDG injection and image acquisition. Tumor and normal tissue kinetic model parameters were estimated from dynamic PET studies of patients with early stage breast cancer. These were used to generate time-activity curves (TACs) out to four hours, for which we assumed both nonreversible and reversible models with different rates of FDG dephosphorylation (k4). For each pair of tumor and normal tissue TACs, 600 PET sinogram realizations were generated, and images were reconstructed using OSEM. Test statistics for each tumor and normal tissue region of interest were output from the computer model observers and evaluated using an ROC analysis with the calculated AUC providing a measure of lesion detectability. For the nonreversible model (k4 = 0), the AUC increased in 11/23 (48%) of patients for one to two hours after the current standard post-radiotracer injection imaging window of one hour. This improvement was driven by increased tumor/normal tissue contrast before the impact of increased noise due to radiotracer decay began to dominate the imaging signal. As k4 was increased from 0 to 0.01 min-1, the time of maximum detectability shifted earlier, as the decreasing FDG concentration in the tumor lowered the CNR. These results imply that delayed PET imaging may reveal low-conspicuity lesions that would have otherwise gone undetected.

Optical imaging for breast cancer prescreening

Breast cancer prescreening is carried out prior to the gold standard screening using X-ray mammography and/or ultrasound. Prescreening is typically carried out using clinical breast examination (CBE) or self-breast examinations (SBEs). Since CBE and SBE have high false-positive rates, there is a need for a low-cost, noninvasive, non-radiative, and portable imaging modality that can be used as a prescreening tool to complement CBE/SBE. This review focuses on the various hand-held optical imaging devices that have been developed and applied toward early-stage breast cancer detection or as a prescreening tool via phantom, in vivo, and breast cancer imaging studies. Apart from the various optical devices developed by different research groups, a wide-field fiber-free near-infrared optical scanner has been developed for transillumination-based breast imaging in our Optical Imaging Laboratory. Preliminary in vivo studies on normal breast tissues, with absorption-contrasted targets placed in the intramammary fold, detected targets as deep as 8.8 cm. Future work involves in vivo imaging studies on breast cancer subjects and comparison with the gold standard X-ray mammography approach.

The association between breast tissue optical content and mammographic density in pre- and post-menopausal women

Mammographic density (MD), associated with higher water and lower fat content in the breast, is strongly related to breast cancer risk. Optical attenuation spectroscopy (OS) is a non-imaging method of evaluating breast tissue composition by red and near-infrared light transmitted through the breast that, unlike mammography, does not involve radiation. OS provides information on wavelength dependent light scattering of tissue and on absorption by water, lipid, oxy-, deoxy-hemoglobin. We propose that OS could be an alternative marker of breast cancer risk and that OS breast tissue measures will be associated with MD. In the present analysis, we developed an algorithm to estimate breast tissue composition and light scattering parameters using a spectrally constrained global fitting procedure employing a diffuse light transport model. OS measurements were obtained from 202 pre- and post-menopausal women with normal mammograms. Percent density (PD) and dense area (DA) were measured using Cumulus. The association between OS tissue composition and PD and DA was analyzed using linear regression adjusted for body mass index. Among pre-menopausal women, lipid content was significantly inversely associated with square root transformed PD (β = -0.05, p = 0.0002) and DA (β = -0.05, p = 0.019); water content was significantly positively associated with PD (β = 0.06, p = 0.008). Tissue oxygen saturation was marginally inversely associated with PD (β = -0.03, p = 0.057) but significantly inversely associated with DA (β = -0.10, p = 0.002). Among post-menopausal women lipid and water content were significantly associated (negatively and positively, respectively) with PD (β_lipid = -0.08, β_water = 0.14, both p<0.0001) and DA (β_lipid = -0.10, p<0.0001; β_water = 0.11, p = 0.001). The association between OS breast content and PD and DA is consistent with more proliferation in dense tissue of younger women, greater lipid content in low density tissue and higher water content in high density tissue. OS may be useful for assessing physiologic tissue differences related to breast cancer risk, particularly when mammography is not feasible or easily accessible.

Frequency-modulated light scattering interferometry employed for optical properties and dynamics studies of turbid media

In the present work, fiber-based frequency-modulated light scattering interferometry (FMLSI) is developed and employed for studies of optical properties and dynamics in liquid phantoms made from Intralipid(®). The fiber-based FMLSI system retrieves the optical properties by examining the intensity fluctuations through the turbid medium in a heterodyne detection scheme using a continuous-wave frequency-modulated coherent light source. A time resolution of 21 ps is obtained, and the experimental results for the diluted Intralipid phantoms show good agreement with the predicted results based on published data. The present system shows great potential for assessment of optical properties as well as dynamic studies in liquid phantoms, dairy products, and human tissues.

Modeling time resolved light propagation inside a realistic human head model

BACKGROUND

Near infrared spectroscopy imaging is one of the new techniques used for investigating structural and functionality of different body tissues. This is done by injecting light into the medium and measuring the photon intensity at the surface of the tissue.

METHODS

In this paper the different medical applications, various imaging and simulation techniques of NIRS imaging is described. Each method is introduced and discussed. Then, the optimized model is prepared for numerical simulations. In this paper, the finite element method is used for solving the diffusion equation numerically.

RESULTS

Diffusion equation was solved for realistic human head model using finite element approach for a point light source and time resolved case. The photon intensity distribution in different head layers has been shown and the intensity orientation via the CSF layer has been illustrated.

CONCLUSION

Simulating the photon transformation inside the tissue is essential for investigating the NIRS imaging technique. The finite element approach is a fast and accurate method for simulating this fact. The time resolved approach of this technique could illustrate the photon migration and intensity orientation in the tissue for time dependent light sources in tissues.

Reflection-mode multiple-illumination photoacoustic sensing to estimate optical properties

OBJECTIVES

We analyze a reflection-mode multiple-illumination photoacoustic method which allows us to estimate optical scattering properties of turbid media based on fitting light-transport models and explore its limits in optical property estimation and depth-dependent fluence compensation.

BACKGROUND

Recent simulation results show significant promise for a technique called multiple-illumination photoacoustic tomography (MI-PAT) to quantitatively reconstruct both absorption and scattering heterogeneities in turbid medium. Prior to experiments, it is essential to develop and analyze a measurement technique and probe capabilities of quantitative measurements that focus on sensing rather than imaging.

METHODS

This technique involved translation of a 532 nm pulsed-laser light spot while focusing an ultrasound receiver on a sub-surface optical absorber immersed in a scattering medium at 3, 4 and 5 mm below the surface. Measured photoacoustic amplitudes for media with different reduced scattering coefficients are fitted with a light propagation model to estimate optical properties.

RESULTS

When the absorber was located at 5 mm below the membrane in media with a reduced scattering coefficient of 4.4 and 5.5 cm(-1), the true values were predicted with an error of 5.7% and 12.7%, respectively. We observe accuracy and the ability of estimating optical scattering properties decreased with the increased reduced scattering coefficient. Nevertheless, the estimated parameters were sufficient for demonstrating depth-dependent fluence compensation for improved quantitation in photoacoustic imaging.

Clutter elimination for deep clinical optoacoustic imaging using localised vibration tagging (LOVIT)

This paper investigates a novel method which allows clutter elimination in deep optoacoustic imaging. Clutter significantly limits imaging depth in clinical optoacoustic imaging, when irradiation optics and ultrasound detector are integrated in a handheld probe for flexible imaging of the human body. Strong optoacoustic transients generated at the irradiation site obscure weak signals from deep inside the tissue, either directly by propagating towards the probe, or via acoustic scattering. In this study we demonstrate that signals of interest can be distinguished from clutter by tagging them at the place of origin with localised tissue vibration induced by the acoustic radiation force in a focused ultrasonic beam. We show phantom results where this technique allowed almost full clutter elimination and thus strongly improved contrast for deep imaging. Localised vibration tagging by means of acoustic radiation force is especially promising for integration into ultrasound systems that already have implemented radiation force elastography.

Study of the effect of mechanical pressure on determination of position and size of tumor in biological phantoms

Diffuse optical tomography (DOT) is an emerging oncological imaging modality that is based on a near-infrared optical technique. DOT provides the spatial volume and depth of tumors by determination of optical properties of biological tissues, such as the absorption and scattering coefficients. During a DOT, the optical fibers are kept in contact with biological tissues that introduce a certain amount of pressure on the local biological tissue. Due to this pressure, the shape of the organ, for instance a breast, deforms. Moreover, this pressure could influence the intrinsic characteristics of the biological tissue. Therefore, pressure can be an important parameter in DOT. In this paper, the effects of pressure on the determination of the size and position of a tumor in biological phantoms are studied. To do so, tissue-like phantoms that are made of intralipid, Indian ink, and agar are constructed. Defects with optical properties similar to those of tumors are placed inside the phantoms. Then various values of pressure are applied to the phantoms. Subsequently, the optical properties of phantoms as well as the position and size of the tumor are reconstructed by inverse models based on the boundary integral method. The variations of reconstructed data induced by pressure are studied. The results demonstrate that pressure causes an increase in the scattering coefficient.

Three-dimensional fluorescence tomography of human breast tissues in vivo using a hand-held optical imager

Diffuse optical imaging using non-ionizing radiation is a non-invasive method that shows promise towards breast cancer diagnosis. Hand-held optical imagers show potential for clinical translation of the technology, yet they have not been used towards 3D tomography. Herein, 3D tomography of human breast tissue in vivo is demonstrated for the first time using a hand-held optical imager with automated coregistration facilities. Simulation studies are performed on breast geometries to demonstrate the feasibility of 3D tomographic imaging using a hand-held imager under perfect (1:0) and imperfect (100:1, 50:1) fluorescence absorption contrast ratios. Experimental studies are performed in vivo using a 1 µM ICG filled phantom target placed non-invasively underneath the flap of the breast tissue. Results show the ability to perform automated tracking and coregistered imaging of human breast tissue (with tracking accuracy on the order of ∼1 cm). Three-dimensional tomography results demonstrated the ability to recover a single target placed at a depth of 2.5 cm, from both the simulated (at 1:0, 100:1 and 50:1 contrasts) and experimental cases on actual breast tissues. Ongoing efforts to improve target depth recovery are carried out via implementation of transmittance imaging in the hand-held imager.

The effect of temporal impulse response on experimental reduction of photon scatter in time-resolved diffuse optical tomography

New fast detector technology has driven significant renewed interest in time-resolved measurement of early photons in improving imaging resolution in diffuse optical tomography and fluorescence mediated tomography in recent years. In practice, selection of early photons results in significantly narrower instrument photon density sensitivity functions (PDSFs) than the continuous wave case, resulting in a better conditioned reconstruction problem. In this work, we studied the quantitative impact of the instrument temporal impulse response function (TIRF) on experimental PDSFs in tissue mimicking optical phantoms. We used a multimode fiber dispersion method to vary the system TIRF over a range of representative literature values. Substantial disagreement in PDSF width--by up to 40%--was observed between experimental measurements and Monte Carlo (MC) models of photon propagation over the range of TIRFs studied. On average, PDSFs were broadened by about 0.3 mm at the center plane of the 2 cm wide imaging chamber per 100 ps of the instrument TIRF at early times. Further, this broadening was comparable on both the source and detector sides. Results were confirmed by convolution of instrument TIRFs with MC simulations. These data also underscore the importance of correcting imaging PDSFs for the instrument TIRF when performing tomographic image reconstruction to ensure accurate data-model agreement.

Automatic and robust calibration of optical detector arrays for biomedical diffuse optical spectroscopy

The design and testing of a new, fully automated, calibration approach is described. The process was used to calibrate an image-guided diffuse optical spectroscopy system with 16 photomultiplier tubes (PMTs), but can be extended to any large array of optical detectors and associated imaging geometry. The design goals were accomplished by developing a routine for robust automated calibration of the multi-detector array within 45 minutes. Our process was able to characterize individual detectors to a median norm of the residuals of 0.03 V for amplitude and 4.4 degrees in phase and achieved less than 5% variation between all the detectors at the 95% confidence interval for equivalent measurements. Repeatability of the calibrated data from the imaging system was found to be within 0.05 V for amplitude and 0.2 degrees for phase, and was used to evaluate tissue-simulating phantoms in two separate imaging geometries. Spectroscopic imaging of total hemoglobin concentration was recovered to within 5% of the true value in both cases. Future work will focus on streamlining the technology for use in a clinical setting with expectations of achieving accurate quantification of suspicious lesions in the breast.

Broadband absorption spectroscopy of turbid media using a dual step steady-state method

We present a method for the determination of the absorption coefficient of turbid media in a broad wavelength range with high spectral resolution using a dual step method. First, the reduced scattering coefficient is determined for a few wavelengths with spatially resolved reflectance measurements. The reduced scattering coefficient for the intermediate wavelengths is interpolated by fitting a power law. Second, the absorption coefficient is obtained from measurements of the total reflectance using the a priori knowledge of the reduced scattering coefficient. By applying a white light source and a spectrometer to measure the total reflectance, the absorption coefficient is determined with a high spectral resolution. The methodology is verified by comparing the absorption coefficients determined by the spatially resolved reflectance measurements with those obtained by the dual step method. The influence of an unknown refractive index and phase function on the determination of the optical properties is investigated. In addition, the optical properties of Intralipid/ink phantoms and the fat layer of porcine rind were determined. The absorption coefficient of the investigated phantoms varying by four orders of magnitude could be determined with an average error of less than 10%.

Optical tecnology developments in biomedicine: history, current and future

Biomedical optics is a rapidly emerging field for medical imaging and diagnostics. This paper reviews several biomedical optical technologies that have been developed and translated for either clinical or pre-clinical applications. Specifically, we focus on the following technologies: 1) near-infrared spectroscopy and tomography, 2) optical coherence tomography, 3) fluorescence spectroscopy and imaging, and 4) optical molecular imaging. There representative biomedical applications are also discussed here.

Fully automated spatially resolved reflectance spectrometer for the determination of the absorption and scattering in turbid media

We describe a fully automated setup which is based on measurements of the spatially resolved reflectance for the determination of the reduced scattering and absorption coefficients in semi-infinite turbid media. The sample is illuminated with a xenon light source in combination with a monochromator enabling the scan of the wavelength from 450 nm to 950 nm. Reflected light from the sample is detected with a CCD camera providing a high spatial resolution. The essential steps for signal processing including, e.g., the consideration of the optical transfer function and the correct treatment of the background subtraction, are presented. The solutions of the diffusion theory and of the radiative transfer theory are investigated regarding the exact detection and illumination geometry. Systematic errors caused by using the different theories for fitting the optical parameters are characterized. The system was validated using liquid phantoms which contain Intralipid 20% and ink, and the measurement range of the system is specified. Further, we carefully characterized the optical properties of Intralipid 20% in the wavelength range between 450 nm and 950 nm.

Comparative study of tumor hypoxia by diffuse optical spectroscopy and immunohistochemistry in two tumor models

The capabilities of diffuse optical spectroscopy for noninvasive assessing of oxygen status in experimental tumors have been demonstrated. Specific features of the distribution of total hemoglobin, oxygenated hemoglobin, deoxygenated hemoglobin, and blood-oxygen saturation were shown on two tumor models having different histological structure and functional characteristics. The results obtained by the optical technique were verified by immunohistochemical study of tissue samples marked with exogenous marker of hypoxia--pimonidazole.

Multi-projection fluorescence optical tomography using a handheld-probe-based optical imager: phantom studies

A handheld-probe-based optical imager has recently been developed toward three-dimensional tomography. In this study, the improvement of target depth recovery was demonstrated using a multi-projection technique on large slab phantoms using 0.45 cc fluorescing target(s) (with 1:0 contrast ratio) of 1.5 to 2.5 cm deep. Tomographic results using single- and multi- (here dual) projection measurements (with and without a priori information of target location) were compared. In all experimental cases, the use of multi-projection measurements along with a priori information recovered target depth and location closer to their true values, demonstrating its applicability for clinical translation.

Noninvasive in vivo monitoring of cyanide toxicity and treatment using diffuse optical spectroscopy in a rabbit model

Currently, no reliable noninvasive methods exist for monitoring the severity of in vivo cyanide (CN) toxicity, treatment, and resulting physiological changes. We developed a broadband diffuse optical spectroscopy (DOS) system to measure bulk tissue absorption and scattering. DOS was used to optically monitor CN toxicity and treatment with sodium nitrite (NaNO2). To perform experiments, the DOS probe was placed on the hind leg of rabbits. A sodium CN solution was infused intravenously. DOS and concurrent physiologic measurements were obtained. After completion of CN infusion, NaNO2 was infused to induce methemoglobinemia (MetHb). During infusion of CN, blood gas measurements showed an increase in venous partial pressure of oxygen (pO2), and following reversal, venous pO2 values decreased. DOS measurements demonstrated corresponding changes in hemoglobin oxygenation states and redox states of cytochrome-c oxidase (CcO) during CN infusion and NaNO2 treatment. Therefore, DOS enables detection and monitoring of CN toxicity and treatment with NaNO2.