Diffuse optics often results in ill-posed inverse problems. Here we investigate a time-of-flight (TOF) informed interferometric diffuse optical approach, which applies a series of TOF filters to multiply scattered light collected via the human head. Rather than 1D data sets which are typical of diffuse optics, we obtain a 2D data set at a single source-collector separation, where the dimensions are TOF filter width and correlation time lag. We investigate recovery of optical properties, layer thicknesses, and blood flow indices by fitting a 3-layer model of the human head. We recover reasonable brain-to-scalp blood flow index ratios.
Diffuse correlation spectroscopy is a widely used optical technique for recovery of blood flow. Its applications have included monitoring of ischemia, blood flow in tumors, and cerebral blood flow. Recently, several variants on this technology have been developed with potential to enhance sensitivity to deep tissues, increase signal-to-noise ratio, and lower costs. Here, we employ diffuse correlation spectroscopy, speckle contrast optical spectroscopy, and interferometric diffusing wave spectroscopy concurrently to measure in vivo and in vitro flow. The results elucidate the advantages and disadvantages of each modality and will aid researchers in selecting a blood flow monitoring method for specific applications.
The Siegert relationship is an important tool in biomedical optics to infer statistics of temporal field fluctuations from observations of intensity fluctuations. The Siegert relationship derivation assumes dynamic Gaussian fields with zero mean. To accommodate a non-ergodic or static field, a modification of the Siegert relationship is often invoked. We show that conventional forms of the modified Siegert relationship, which assume that the coherence factor of dynamic fields also determines mutual coherence between dynamic and static fields, are incorrect in general. We propose a more general form of the modified Siegert relationship and validate it experimentally.
Interferometric DOS (iDOS) is a new class of approaches that promises to improve the quantitative accuracy and depth specificity of blood flow index (BFI). iDOS techniques have alternatively achieved either time-of-flight (TOF) discrimination or highly parallel detection, but not both at once. Here, we break this barrier with a single iDOS instrument. Specifically, we show that rapid tuning of a temporally coherent laser during the sensor integration time increases the effective linewidth seen by a highly parallel interferometer. With a deep TOF filter applied to a high throughput interferometer, we demonstrate brain-specific BFI imaging.
In this work, we demonstrate a fiber-based interferometric Diffuse Optical Spectroscopy (iDOS) approach to obtain quasi-concurrent information at early and late times of flight via a simple fiber optic switch. Time-of-flight (TOF) filtering is enabled by reducing the effective temporal coherence of the laser source, here achieved through rapid wavelength tuning. Early and late TOFs are alternatively interrogated by optical switches that select between reference paths with short and long time delays, respectively. This approach is used on a human forearm to obtain quasi-concurrent deep and superficial blood flow index at baseline and during occlusion.
KEYWORDS: Interferometry, Heterodyning, Signal to noise ratio, Brain, Diffuse optical imaging, Homodyne detection, Sensors, Cameras, Signal detection, Near infrared spectroscopy
The field of diffuse optics has provided a rich set of neurophotonic tools to measure the human brain noninvasively. Interferometric detection is a recent, exciting methodological development in this field. The approach is especially promising for the measurement of diffuse fluctuation signals related to blood flow. Benefitting from inexpensive sensor arrays, the interferometric approach has already dramatically improved throughput, enabling the measurement of brain blood flow faster and deeper. The interferometric approach can also achieve time-of-flight resolution, improving the accuracy of acquired signals. We provide a historical perspective and summary of recent work in the nascent area of interferometric diffuse optics. We predict that the convergence of interferometric technology with existing economies of scale will propel many advances in the years to come.
Maintenance of cerebral blood flow (CBF) is required for normal brain function. Yet, measuring CBF in adult humans requires high-end medical instrumentation. Here, leveraging two-dimensional (2D) complementary metal-oxide-semiconductor (CMOS) technology, we present and validate an optical approach, called multi-exposure interferometric Diffusing Wave Spectroscopy (MiDWS), to monitor blood flow index (BFI), a proxy for CBF, via the adult human forehead. MiDWS employs interferometry to detect low light levels, and probes the optical field autocorrelation by varying the sensor exposure time. This approach may enable human brain optical BFI monitoring with 2D CMOS sensors, with associated economies of scale.
Significance: There is an essential need to develop wearable multimodality technologies that can continuously measure both blood flow and oxygenation in deep tissues to investigate and manage various vascular/cellular diseases.
Aim: To develop a wearable dual-wavelength diffuse speckle contrast flow oximetry (DSCFO) for simultaneous measurements of blood flow and oxygenation variations in deep tissues.
Approach: A wearable fiber-free DSCFO probe was fabricated using 3D printing to confine two small near-infrared laser diodes and a tiny CMOS camera in positions for DSCFO measurements. The spatial diffuse speckle contrast and light intensity measurements at the two different wavelengths enable quantification of tissue blood flow and oxygenation, respectively. The DSCFO was first calibrated using tissue phantoms and then tested in adult forearms during artery cuff occlusion.
Results: Phantom tests determined the largest effective source–detector distance (15 mm) and optimal camera exposure time (10 ms) and verified the accuracy of DSCFO in measuring absorption coefficient variations. The DSCFO detected substantial changes in forearm blood flow and oxygenation resulting from the artery occlusion, which meet physiological expectations and are consistent with previous study results.
Conclusions: The wearable DSCFO may be used for continuous and simultaneous monitoring of blood flow and oxygenation variations in freely behaving subjects.
A noncontact electron multiplying charge-coupled-device (EMCCD)-based speckle contrast diffuse correlation tomography (scDCT) technology has been recently developed in our laboratory, allowing for noninvasive three-dimensional measurement of tissue blood flow distributions. One major remaining constraint in the scDCT is the assumption of a semi-infinite tissue volume with a flat surface, which affects the image reconstruction accuracy for tissues with irregular geometries. An advanced photometric stereo technique (PST) was integrated into the scDCT system to obtain the surface geometry in real time for image reconstruction. Computer simulations demonstrated that a priori knowledge of tissue surface geometry is crucial for precisely reconstructing the anomaly with blood flow contrast. Importantly, the innovative integration design with one single-EMCCD camera for both PST and scDCT data collection obviates the need for offline alignment of sources and detectors on the tissue boundary. The in vivo imaging capability of the updated scDCT is demonstrated by imaging dynamic changes in forearm blood flow distribution during a cuff-occlusion procedure. The feasibility and safety in clinical use are evidenced by intraoperative imaging of mastectomy skin flaps and comparison with fluorescence angiography.
Occlusion calibrations and gating techniques have been recently applied by our laboratory for continuous and absolute diffuse optical measurements of forearm muscle hemodynamics during handgrip exercises. The translation of these techniques from the forearm to the lower limb is the goal of this study as various diseases preferentially affect muscles in the lower extremity. This study adapted a hybrid near-infrared spectroscopy and diffuse correlation spectroscopy system with a gating algorithm to continuously quantify hemodynamic responses of medial gastrocnemius during plantar flexion exercises in 10 healthy subjects. The outcomes from optical measurement include oxy-, deoxy-, and total hemoglobin concentrations, blood oxygen saturation, and relative changes in blood flow (rBF) and oxygen consumption rate (rV˙O2). We calibrated rBF and rV˙O2 profiles with absolute baseline values of BF and V˙O2 obtained by venous and arterial occlusions, respectively. Results from this investigation were comparable to values from similar studies. Additionally, significant correlation was observed between resting local muscle BF measured by the optical technique and whole limb BF measured concurrently by a strain gauge venous plethysmography. The extensive hemodynamic and metabolic profiles during exercise will allow for future comparison studies to investigate the diagnostic value of hybrid technologies in muscles affected by disease.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.