Maximizing the Reliability and Precision of Measures of Prefrontal Cortical Oxygenation Using Frequency-Domain Near-Infrared Spectroscopy

Frequency-domain near-infrared spectroscopy (FD-NIRS) has been used for non-invasive assessment of cortical oxygenation since the late 1990s. However, there is limited research demonstrating clinical validity and general reproducibility. To address this limitation, recording duration for adequate validity and within- and between-day reproducibility of prefrontal cortical oxygenation was evaluated. To assess validity, a reverse analysis of 10-min-long measurements (n = 52) at different recording durations (1-10-min) was quantified via coefficients of variation and Bland-Altman plots. To assess within- and between-day within-subject reproducibility, participants (n = 15) completed 2-min measurements twice a day (morning/afternoon) for five consecutive days. While 1-min recordings demonstrated sufficient validity for the assessment of oxygen saturation (StO2) and total hemoglobin concentration (THb), recordings ≥4 min revealed greater clinical utility for oxy- (HbO) and deoxyhemoglobin (HHb) concentration. Females had lower StO2, THb, HbO, and HHb values than males, but variability was approximately equal between sexes. Intraclass correlation coefficients ranged from 0.50-0.96. The minimal detectable change for StO2 was 1.15% (95% CI: 0.336-1.96%) and 3.12 µM for THb (95% CI: 0.915-5.33 µM) for females and 2.75% (95%CI: 0.807-4.70%) for StO2 and 5.51 µM (95%CI: 1.62-9.42 µM) for THb in males. Overall, FD-NIRS demonstrated good levels of between-day reliability. These findings support the application of FD-NIRS in field-based settings and indicate a recording duration of 1 min allows for valid measures; however, data recordings of ≥4 min are recommended when feasible.

Comparison of Frequency-Domain and Time-Domain Baseline Correction Approaches for Infrared Absorption Spectroscopy of Mixtures Containing Up to 464 Components

Many baseline correction approaches have been developed to address baseline artifacts observed in measured infrared (IR) absorption spectra during post-processing. These approaches offer distinct advantages and disadvantages, and the choice of which one to employ depends on the complexity of baseline artifacts present in a particular application. In this paper, we compare the performance of two baseline correction approaches: a frequency-domain polynomial fitting approach and a time-domain modified free induction decay approach, under various baseline scenarios, spectral resolutions, and noise levels for mixtures containing up to 464 species. Our results showed that the frequency-domain approach outperformed the time-domain approach by a factor of up to 16 when the baseline was represented by a sine wave with fewer than two cycles over the full spectral range. On the other hand, the time-domain approach performed up to 12 times better when the baseline featured two cycles of a sine wave. Additionally, we observed that the time-domain approach exhibited higher sensitivity to spectral resolution and underperformed when the noise level was high. The findings of this study emphasize the importance of numerically testing a few candidate approaches for a given application, taking into consideration baseline characteristics, as well as the spectral resolution and noise constraints of the application.

Single-distance and dual-slope frequency-domain near-infrared spectroscopy to assess skeletal muscle hemodynamics

Significance

Non-invasive optical measurements of deep tissue (e.g., muscle) need to take into account confounding contributions from baseline and dynamic optical properties of superficial tissue (adipose tissue).

Aim

Discriminate superficial and deep tissue hemodynamics using data collected with frequency-domain (FD) near-infrared spectroscopy (NIRS) in a dual-slope (DS) configuration.

Approach

Experimental data were collected in vivo on the forearm of three human subjects during a 3-min arterial occlusion or 1-min venous occlusion. Theoretical data were generated using diffusion theory for two-layered media with varying values of the reduced scattering coefficient (μ s ' ) (range: 0.5 to 1.1    mm - 1 ) and absorption coefficient (μ a ) (range: 0.005 - 0.015    mm - 1 ) of the two layers, and top layer thickness (range: 2 to 8 mm). Data were analyzed using diffusion theory for a homogeneous semi-infinite medium.

Results

Experimental data in vivo were consistent with simulated data for a two-layered medium with a larger μ s ' in the top layer, comparable absorption changes in the top and bottom layers during venous occlusion, and smaller absorption changes in the top vs. bottom layers during arterial occlusion.

Conclusions

The dataset generated by DS FD-NIRS may allow for discrimination of superficial and deep absorption changes in two-layered media, thus lending itself to individual measurements of hemodynamics in adipose and muscle tissue.

The Validity of Ultra-Short-Term Heart Rate Variability during Cycling Exercise

Ultra-short-term heart rate variability (HRV) has been validated in the resting state, but its validity during exercise is unclear. This study aimed to examine the validity in ultra-short-term HRV during exercise considering the different exercise intensities. HRVs of twenty-nine healthy adults were measured during incremental cycle exercise tests. HRV parameters (Time-, frequency-domain and non-linear) corresponding to each of the 20% (low), 50% (moderate), and 80% (high) peak oxygen uptakes were compared between the different time segments of HRV analysis (180 s (sec) segment vs. 30, 60, 90, and 120-sec segments). Overall, the differences (bias) between ultra-short-term HRVs increased as the time segment became shorter. In moderate- and high-intensity exercises, the differences in ultra-short-term HRV were more significant than in low intensity exercise. Thus, we discovered that the validity of ultra-short-term HRV differed with the duration of the time segment and exercise intensities. However, the ultra-short-term HRV is feasible in the cycling exercise, and we determined some optimal time duration for HRV analysis for across exercise intensities during the incremental cycling exercise.

Comprehensive Investigation of Parameters Influencing Fluorescence Lifetime Imaging Microscopy in Frequency- and Time-Domain Illustrated by Phasor Plot Analysis

Having access to fluorescence lifetime, researchers can reveal in-depth details about the microenvironment as well as the physico-chemical state of the molecule under investigation. However, the high number of influencing factors might be an explanation for the strongly deviating values of fluorescent lifetimes for the same fluorophore reported in the literature. This could be the reason for the impression that inconsistent results are obtained depending on which detection and excitation scheme is used. To clarify this controversy, the two most common techniques for measuring fluorescence lifetimes in the time-domain and in the frequency-domain were implemented in one single microscopy setup and applied to a variety of fluorophores under different environmental conditions such as pH-value, temperature, solvent polarity, etc., along with distinct state forms that depend, for example, on the concentration. From a vast amount of measurement results, both setup- and sample-dependent parameters were extracted and represented using a single display form, the phasor-plot. The measurements showed consistent results between the two techniques and revealed which of the tested parameters has the strongest influence on the fluorescence lifetime. In addition, quantitative guidance as to which technique is most suitable for which research task and how to perform the experiment properly to obtain consistent fluorescence lifetimes is discussed.

RMSSD Is More Sensitive to Artifacts Than Frequency-Domain Parameters: Implication in Athletes' Monitoring

Easy-to-use and accurate heart rate variability (HRV) assessments are essential in athletes' follow-up, but artifacts may lead to erroneous analysis. Artifact detection and correction are the purpose of extensive literature and implemented in dedicated analysis programs. However, the effects of number and/or magnitude of artifacts on various time- or frequency-domain parameters remain unclear. The purpose of this study was to assess the effects of artifacts on HRV parameters. Root mean square of the successive differences (RMSSD), standard deviation of the normal to normal inter beat intervals (SDNN), power in the low- (LF) and high-frequency band (HF) were computed from two 4-min RR recordings in 178 participants in both supine and standing positions, respectively. RRs were modified by (1) randomly adding or subtracting 10, 30, 50 or 100 ms to the successive RRs; (2) a single artifact was manually inserted; (3) artifacts were automatically corrected from signal naturally containing artifacts. Finally, RR recordings were analyzed before and after automatic detection-correction of artifacts. Modifying each RR by 10, 30, 50 and 100 ms randomly did not significantly change HRV parameters (range -6%, +6%, supine). In contrast, by adding a single artifact, RMSSD increased by 413% and 269%, SDNN by 54% and 47% in supine and standing positions, respectively. LF and HF changed only between -3% and +8% (supine and standing) in the artifact condition. When more than 0.9% of the signal contained artifacts, RMSSD was significantly biased, whilst when more than 1.4% of the signal contained artifacts LF and HF were significantly biased. RMSSD and SDNN were more sensitive to a single artifact than LF and HF. This indicates that, when using RMSSD only, a single artifact may induce erroneous interpretation of HRV. Therefore, we recommend using both time- and frequency-domain parameters to minimize the errors in the diagnoses of health status or fatigue in athletes.

Optical coherence tomography

Optical coherence tomography (OCT) is a non-contact method for imaging the topological and internal microstructure of samples in three dimensions. OCT can be configured as a conventional microscope, as an ophthalmic scanner, or using endoscopes and small diameter catheters for accessing internal biological organs. In this Primer, we describe the principles underpinning the different instrument configurations that are tailored to distinct imaging applications and explain the origin of signal, based on light scattering and propagation. Although OCT has been used for imaging inanimate objects, we focus our discussion on biological and medical imaging. We examine the signal processing methods and algorithms that make OCT exquisitely sensitive to reflections as weak as just a few photons and that reveal functional information in addition to structure. Image processing, display and interpretation, which are all critical for effective biomedical imaging, are discussed in the context of specific applications. Finally, we consider image artifacts and limitations that commonly arise and reflect on future advances and opportunities.

Noninvasive Optical Measurements of Dynamic Cerebral Autoregulation by Inducing Oscillatory Cerebral Hemodynamics

Objective: Cerebral autoregulation limits the variability of cerebral blood flow (CBF) in the presence of systemic arterial blood pressure (ABP) changes. Monitoring cerebral autoregulation is important in the Neurocritical Care Unit (NCCU) to assess cerebral health. Here, our goal is to identify optimal frequency-domain near-infrared spectroscopy (FD-NIRS) parameters and apply a hemodynamic model of coherent hemodynamics spectroscopy (CHS) to assess cerebral autoregulation in healthy adult subjects and NCCU patients. Methods: In five healthy subjects and three NCCU patients, ABP oscillations at a frequency around 0.065 Hz were induced by cyclic inflation-deflation of pneumatic thigh cuffs. Transfer function analysis based on wavelet transform was performed to measure dynamic relationships between ABP and oscillations in oxy- (O), deoxy- (D), and total- (T) hemoglobin concentrations measured with different FD-NIRS methods. In healthy subjects, we also obtained the dynamic CBF-ABP relationship by using FD-NIRS measurements and the CHS model. In healthy subjects, an interval of hypercapnia was performed to induce cerebral autoregulation impairment. In NCCU patients, the optical measurements of autoregulation were linked to individual clinical diagnoses. Results: In healthy subjects, hypercapnia leads to a more negative phase difference of both O and D oscillations vs. ABP oscillations, which are consistent across different FD-NIRS methods and are highly correlated with a more negative phase difference CBF vs. ABP. In the NCCU, a less negative phase difference of D vs. ABP was observed in one patient as compared to two others, indicating a better autoregulation in that patient. Conclusions: Non-invasive optical measurements of induced phase difference between D and ABP show the strongest sensitivity to cerebral autoregulation. The results from healthy subjects also show that the CHS model, in combination with FD-NIRS, can be applied to measure the CBF-ABP dynamics for a better direct measurement of cerebral autoregulation.

Investigation of heart rate variability of patients undergoing coronary artery bypass grafting using the statistical signal characterization method

BACKGROUND

Autonomic function can be estimated non-invasively using heart rate variability (HRV). HRV of patients undergoing coronary artery bypass grafting (CABG) is investigated in time-domain and frequency-domain before and after CABG to study the effect of operation on the status of patients.

OBJECTIVE

The main purpose of this work is to evaluate the effect of CABG surgery on patients with ischemic heart disease (IHD) before operation, and to monitor the status of patients on day 6 and day 30 after the CABG operation.

METHODS

The statistical signal characterization (SSC) technique is used in this work in order to derive different morphology-based parameters to indirectly describe time-domain and frequency-domain HRV parameters in 24 patients undergoing CABG operation, before the operation (Group 1: G1), 6 days after operation (Group 2: G2) and 30 days after operation (Group 3: G3). The data is obtained from the Sultan Qaboos University Hospital in Oman.

RESULTS

The SSC parameters Mean(mt) and Mean(dt) are reduced in all 24 patients and in 23 out of 24 patients in G2 compared to G1 (6-days after operation compared with before operation), respectively. Comparing G3 to G1 the reduction in Mean(mt) and Mean(dt) is noted in 18 of the 24 patients.

CONCLUSIONS

The parameters Mean(mt) and Mean(dt) are successful parameters to follow the HRV for patients undergoing CABG surgery. A relation between those SSC parameters and the HRV time-domain and frequency-domain parameters is investigated in this paper to understand the physiological behavior of the patients.

Plane wave elastography: a frequency-domain ultrasound shear wave elastography approach

In this paper, we propose plane wave elastography (PWE), a novel ultrasound shear wave elastography (SWE) approach. Currently, commercial methods for SWE rely on directional filtering based on the prior knowledge of the wave propagation direction, to remove complicated wave patterns formed due to reflection and refraction. The result is a set of decomposed directional waves that are separately analyzed to construct shear modulus fields that are then combined through compounding. Instead, PWE relies on a rigorous representation of the wave propagation using the frequency-domain scalar wave equation to automatically select appropriate propagation directions and simultaneously reconstruct shear modulus fields. Specifically, assuming a homogeneous, isotropic, incompressible, linear-elastic medium, we represent the solution of the wave equation using a linear combination of plane waves propagating in arbitrary directions. Given this closed-form solution, we formulate the SWE problem as a nonlinear least-squares optimization problem which can be solved very efficiently. Through numerous phantom studies, we show that PWE can handle complicated waveforms without prior filtering and is competitive with state-of-the-art that requires prior filtering based on the knowledge of propagation directions.

Mapping of Agricultural Subsurface Drainage Systems Using a Frequency-Domain Ground Penetrating Radar and Evaluating Its Performance Using a Single-Frequency Multi-Receiver Electromagnetic Induction Instrument

Subsurface drainage systems are commonly used to remove surplus water from the soil profile of a poorly drained farmland. Traditional methods for drainage mapping involve the use of tile probes and trenching equipment that are time-consuming, labor-intensive, and invasive, thereby entailing an inherent risk of damaging the drainpipes. Effective and efficient methods are needed in order to map the buried drain lines: (1) to comprehend the processes of leaching and offsite release of nutrients and pesticides and (2) for the installation of a new set of drain lines between the old ones to enhance the soil water removal. Non-invasive geophysical soil sensors provide a potential alternative solution. Previous research has mainly showcased the use of time-domain ground penetrating radar, with variable success, depending on local soil and hydrological conditions and the central frequency of the specific equipment used. The objectives of this study were: (1) to test the use of a stepped-frequency continuous wave three-dimensional ground penetrating radar (3D-GPR) with a wide antenna array for subsurface drainage mapping and (2) to evaluate its performance with the use of a single-frequency multi-receiver electromagnetic induction (EMI) sensor in-combination. This sensor combination was evaluated on twelve different study sites with various soil types with textures ranging from sand to clay till. While the 3D-GPR showed a high success rate in finding the drainpipes at five sites (sandy, sandy loam, loamy sand, and organic topsoils), the results at the other seven sites were less successful due to the limited penetration depth of the 3D-GPR signal. The results suggest that the electrical conductivity estimates produced by the inversion of apparent electrical conductivity data measured by the EMI sensor could be a useful proxy for explaining the success achieved by the 3D-GPR in finding the drain lines.

Review of the state of the art in cardiovascular endoscopy imaging of atherosclerosis using photoacoustic techniques with pulsed and continuous-wave optical excitations

Intravascular photoacoustics (IV-PA) is an emerging atherosclerosis imaging modality that provides chemical-specific optical information of arterial walls with acoustic depth penetration and resolution. As lipid composition of atherosclerotic plaques is considered to be one of the primary indicators for plaque vulnerability, many IV-PA applications are calibrated so as to target plaque necrotic cores. Based on the mode of optical excitation and the corresponding signal processing technique, IV-PA is categorized into two different modalities. The pulse-based IV-PA has been the universal IV-PA imaging mode with its high peak power and straightforward time-domain signal processing technique. As an alternative, the low power continuous-wave (CW)-based IV-PA has been under intense development as a radar-like frequency-domain signal processing modality. The two state-of-the-art types of IV-PA are reviewed in terms of their physics and imaging capabilities, with major emphasis on frequency-swept CW-based IV-PA that has been recently introduced in the field.

Multi-distance frequency-domain optical measurements of coherent cerebral hemodynamics

We report non-invasive, bilateral optical measurements on the forehead of five healthy human subjects, of 0.1 Hz oscillatory hemodynamics elicited either by cyclic inflation of pneumatic thigh cuffs, or by paced breathing. Optical intensity and the phase of photon-density waves were collected with frequency-domain near-infrared spectroscopy at seven source-detector distances (11-40 mm). Coherent hemodynamic oscillations are represented by phasors of oxyhemoglobin (O) and deoxyhemoglobin (D) concentrations, and by the vector D/O that represents the amplitude ratio and phase difference of D and O. We found that, on average, the amplitude ratio (|D/O|) and the phase difference (∠(D/O)) obtained with single-distance intensity at 11-40 mm increase from 0.1 and -330°, to 0.2 and -200°, respectively. Single-distance phase and the intensity slope featured a weaker dependence on source-detector separation, and yielded |D/O| and ∠(D/O) values of about 0.5 and -200°, respectively, at distances greater than 20 mm. The key findings are: (1) single-distance phase and intensity slope are sensitive to deeper tissue compared to single-distance intensity; (2) deeper tissue hemodynamic oscillations, which more closely represent the brain, feature D and O phasors that are consistent with a greater relative flow-to-volume contributions in brain tissue compared to extracerebral, superficial tissue.

Widefield multifrequency fluorescence lifetime imaging using a two-tap complementary metal-oxide semiconductor camera with lateral electric field charge modulators

Widefield frequency-domain fluorescence lifetime imaging microscopy (FD-FLIM) measures the fluorescence lifetime of entire images in a fast and efficient manner. We report a widefield FD-FLIM system based on a complementary metal-oxide semiconductor camera equipped with two-tap true correlated double sampling lock-in pixels and lateral electric field charge modulators. Owing to the fast intrinsic response and modulation of the camera, our system allows parallel multifrequency FLIM in one measurement via fast Fourier transform. We demonstrate that at a fundamental frequency of 20 MHz, 31-harmonics can be measured with 64 phase images per laser repetition period. As a proof of principle, we analyzed cells transfected with Cerulean and with a construct of Cerulean-Venus that shows Förster Resonance Energy Transfer at different modulation frequencies. We also tracked the temperature change of living cells via the fluorescence lifetime of Rhodamine B at different frequencies. These results indicate that our widefield multifrequency FD-FLIM system is a valuable tool in the biomedical field.

Heart Rate Variability Frequency Domain Alterations among Healthy Nurses Exposed to Prolonged Work Stress

The deregulation of the autonomic nervous system assessed through the heart rate variability (HRV) analysis is a promising pathway linking work stress and cardiovascular diseases. We aim to investigate the associations between HRV High Frequency (HF) and Low Frequency (LF) powers and work stress in a sample of 36 healthy nurses. Perceived work stress was assessed twice one year apart, using the Job Content and Effort Reward Imbalance questionnaires. This allows to classify nurses in three exposure groups: “prolonged high stress” (PHS), “recent high stress” (RHS) and “stable low stress” (SLS). A 24-h ECG monitoring was later performed during a working day (WD) and a subsequent resting day (RD). Statistically significantly lower (p < 0.02) HF and LF means were found in PHS and RHS nurses during the working periods. In the subsequent resting periods, HF means showed increases over time in the RHS (beta = +0.41, p < 0.05), but not in PHS nurses. LF means did not show any substantial increases in the resting periods, in the PHS group with geometric means lower when compared to SLS, in the non-working and resting periods. Our study evidences that both prolonged and recent perceived high work stress were associated with a reduction of HF and LF powers during work. In addition, prolonged stress was associated with a lack of recovery during not-working and resting periods.

Multidie LED combined with homogenizing optics to improve frequency response and intensity for FLIM applications

Application of light-emitting diodes (LEDs) in frequency-domain fluorescence lifetime imaging microscopy (FLIM) has been limited by the trade-off between modulation frequency and illumination intensity of LEDs, which affects the signal-to-noise ratio in fluorescence lifetime measurements. To increase modulation frequency without sacrificing output power of LEDs, we propose to use LEDs with multiple dice connected in series. The LED capacitance was reduced with series connection; therefore, the frequency response of multidie LED was significantly increased. LEDs in visible light, including blue, green, amber and red, were all applicable in FLIM. We also present a homogenizing optics design, so that multidie LEDs produced uniform illumination on the same focal spot. When the homogenizing optics was combined with multicolour emitters, it provides multiple colour selection in a compact and convenient design.

Toward the measurement of multiple fluorescence lifetimes in flow cytometry: maximizing multi-harmonic content from cells and microspheres

Flow cytometry is a powerful means for in vitro cellular analyses where multi-fluorescence and multi-angle light scattering can indicate unique biochemical or morphological features of single cells. Yet, to date, flow cytometry systems have lacked the ability to capture complex fluorescence dynamics due to the transient nature of flowing cells. In this contribution we introduce a simple approach for measuring multiple fluorescence lifetimes from a single cytometric event. We leverage square wave modulation, Fourier analysis, and high frequency digitization and show the ability to resolve more than one fluorescence lifetime from fluorescently-labelled cells and microspheres. Illustration of a flow cytometer capable of capturing multiple fluorescence lifetime measurements; creating potential for multi-parametric, time-resolved signals to be captured for every color channel.

Modulated CMOS camera for fluorescence lifetime microscopy

Widefield frequency-domain fluorescence lifetime imaging microscopy (FD-FLIM) is a fast and accurate method to measure the fluorescence lifetime of entire images. However, the complexity and high costs involved in construction of such a system limit the extensive use of this technique. PCO AG recently released the first luminescence lifetime imaging camera based on a high frequency modulated CMOS image sensor, QMFLIM2. Here we tested and provide operational procedures to calibrate the camera and to improve the accuracy using corrections necessary for image analysis. With its flexible input/output options, we are able to use a modulated laser diode or a 20 MHz pulsed white supercontinuum laser as the light source. The output of the camera consists of a stack of modulated images that can be analyzed by the SimFCS software using the phasor approach. The nonuniform system response across the image sensor must be calibrated at the pixel level. This pixel calibration is crucial and needed for every camera settings, e.g. modulation frequency and exposure time. A significant dependency of the modulation signal on the intensity was also observed and hence an additional calibration is needed for each pixel depending on the pixel intensity level. These corrections are important not only for the fundamental frequency, but also for the higher harmonics when using the pulsed supercontinuum laser. With these post data acquisition corrections, the PCO CMOS-FLIM camera can be used for various biomedical applications requiring a large frame and high speed acquisition.

A practical method for the detection of freezing of gait in patients with Parkinson's disease

Purpose

Freezing of gait (FOG), increasing the fall risk and limiting the quality of life, is common at the advanced stage of Parkinson’s disease, typically in old ages. A simple and unobtrusive FOG detection system with a small calculation load would make a fast presentation of on-demand cueing possible. The purpose of this study was to find a practical FOG detection system.

Patients and methods

A sole-mounted sensor system was developed for an unobtrusive measurement of acceleration during gait. Twenty patients with Parkinson’s disease participated in this study. A simple and fast time-domain method for the FOG detection was suggested and compared with the conventional frequency-domain method. The parameters used in the FOG detection were optimized for each patient.

Results

The calculation load was 1,154 times less in the time-domain method than the conventional method, and the FOG detection performance was comparable between the two domains (P=0.79) and depended on the window length (P<0.01) and dimension of sensor information (P=0.03).

Conclusion

A minimally constraining sole-mounted sensor system was developed, and the suggested time-domain method showed comparable FOG detection performance to that of the conventional frequency-domain method. Three-dimensional sensor information and 3–4-second window length were desirable. The suggested system is expected to have more practical clinical applications.

Application of phasor plot and autofluorescence correction for study of heterogeneous cell population

Protein-protein interactions in cells are often studied using fluorescence resonance energy transfer (FRET) phenomenon by fluorescence lifetime imaging microscopy (FLIM). Here, we demonstrate approaches to the quantitative analysis of FRET in cell population in a case complicated by a highly heterogeneous donor expression, multiexponential donor lifetime, large contribution of cell autofluorescence, and significant presence of unquenched donor molecules that do not interact with the acceptor due to low affinity of donor-acceptor binding. We applied a multifrequency phasor plot to visualize FRET FLIM data, developed a method for lifetime background correction, and performed a detailed time-resolved analysis using a biexponential model. These approaches were applied to study the interaction between the Toll Interleukin-1 receptor (TIR) domain of Toll-like receptor 4 (TLR4) and the decoy peptide 4BB. TLR4 was fused to Cerulean fluorescent protein (Cer) and 4BB peptide was labeled with Bodipy TMRX (BTX). Phasor displays for multifrequency FLIM data are presented. The analytical procedure for lifetime background correction is described and the effect of correction on FLIM data is demonstrated. The absolute FRET efficiency was determined based on the phasor plot display and multifrequency FLIM data analysis. The binding affinity between TLR4-Cer (donor) and decoy peptide 4BB-BTX (acceptor) was estimated in a heterogeneous HeLa cell population.

An endoscopic diffuse optical tomographic method based on the effective detection range

Endoscopic diffuse optical tomography (DOT) is a new medical imaging modality with the potential applications in functional imaging of the internal organs. To cut down the measurement time and the computation burden of image reconstruction, in this paper, we developed the image reconstruction algorithm with the partial measurement in the effective detection range (EDR) of a tubular tissue and the corresponding endoscopic imaging system with a novel endoscopic probe for flexibly selecting the detection sites. For a typical inner size and optical properties of the cervix, it is found that EDR is less than half of the inner circumference. Comparing to the traditional method, the adoption of EDR results in a reduction of more than a factor of two in the time cost for a measurement cycle and for the total iteration reconstruction. Images reconstructed from the simulation data demonstrate that the proposed method achieves equivalent image quality to that obtained from the complete data set. The images reconstructed from the EDR measurements on cervix-like solid phantoms show that both the location and size of the targets are reconstructed correctly. The proposed method will be useful to the development of endoscopic DOT technologies for cancer detection in tubular organs including cervix.

Digital parallel frequency-domain spectroscopy for tissue imaging

Near-infrared (NIR) (650 to 1000 nm) optical properties of turbid media can be quantified accurately and noninvasively using methods based on diffuse reflectance or transmittance, such as frequency-domain photon migration (FDPM). Conventional FDPM techniques based on white-light steady-state (SS) spectral measurements in conjunction with the acquisition of frequency-domain (FD) data at selected wavelengths using laser diodes are used to measure broadband NIR scattering-corrected absorption spectra of turbid media. These techniques are limited by the number of wavelength points used to obtain FD data and by the sweeping technique used to collect FD data over a relatively large range. We have developed a method that introduces several improvements in the acquisition of optical parameters, based on the digital parallel acquisition of a comb of frequencies and on the use of a white laser as a single light source for both FD and SS measurements. The source, due to the high brightness, allows a higher penetration depth with an extremely low power on the sample. The parallel acquisition decreases the time required by standard serial systems that scan through a range of modulation frequencies. Furthermore, all-digital acquisition removes analog noise, avoids the analog mixer, and does not create radiofrequency interference or emission.

Capture of Fluorescence Decay Times by Flow Cytometry

In flow cytometry, the fluorescence decay time of an excitable species has been largely underutilized and is not likely found as a standard parameter on any imaging cytometer, sorting, or analyzing system. Most cytometers lack fluorescence lifetime hardware mainly owing to two central issues. Foremost, research and development with lifetime techniques has lacked proper exploitation of modern laser systems, data acquisition boards, and signal processing techniques. Secondly, a lack of enthusiasm for fluorescence lifetime applications in cells and with bead-based assays has persisted among the greater cytometry community. In this unit, we describe new approaches that address these issues and demonstrate the simplicity of digitally acquiring fluorescence relaxation rates in flow. The unit is divided into protocol and commentary sections in order to provide a most comprehensive discourse on acquiring the fluorescence lifetime with frequency-domain methods. The unit covers (i) standard fluorescence lifetime acquisition (protocol-based) with frequency-modulated laser excitation, (ii) digital frequency-domain cytometry analyses, and (iii) interfacing fluorescence lifetime measurements onto sorting systems. Within the unit is also a discussion on how digital methods are used for aliasing in order to harness higher frequency ranges. Also, a final discussion is provided on heterodyning and processing of waveforms for multi-exponential decay extraction.

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.

Near-infrared frequency-domain optical spectroscopy and magnetic resonance imaging: a combined approach to studying cerebral maturation in neonatal rabbits

The neonatal rabbit brain shows prolonged postnatal development both structurally and physiologically. We use noninvasive near-IR frequency-domain optical spectroscopy (NIRS) and magnetic resonance imaging (MRI) to follow early developmental changes in cerebral oxygenation and anatomy, respectively. Four groups of animals are measured: NIRS in normals, MRI in normals, and both NIRS and MRI with hypoxia-ischemia (HI) (diffusion MRI staging). NIRS and/or MRI are performed from P3 (postnatal day=P) up to P76. NIRS is performed on awake animals with a frequency-domain tissue photometer. Absolute values of oxyhemoglobin concentration ([HbO2]), deoxyhemoglobin concentration ([HbR]), total hemoglobin concentration (HbT), and hemoglobin saturation (StO2) are calculated. The brains of all animals appeared to be maturing as shown in the diffusion tensor MRI. Mean optical coefficients (reduced scattering) remained unchanged in all animals throughout. StO2 increased in all animals (40% at P9 to 65% at P43) and there are no differences between normal, HI controls, and HI brains. The measured increase in StO2 is in agreement with the reported increase in blood flow during the first 2 months of life in rabbits. HbT, which reflects blood volume, peaked at postnatal day P17, as expected since the capillary density increases up to P17 when the microvasculature matures.

Frequency-Domain Models for Nonlinear Microwave Devices Based on Large-Signal Measurements

In this paper, we introduce nonlinear large-signal scattering ( [Formula: see text]) parameters, a new type of frequency-domain mapping that relates incident and reflected signals. We present a general form of nonlinear large-signal [Formula: see text]-parameters and show that they reduce to classic [Formula: see text]-parameters in the absence of nonlinearities. Nonlinear large-signal impedance ( [Formula: see text]) and admittance ( [Formula: see text]) parameters are also introduced, and equations relating the different representations are derived. We illustrate how nonlinear large-signal [Formula: see text]-parameters can be used as a tool in the design process of a nonlinear circuit, specifically a single-diode 1 GHz frequency-doubler. For the case where a nonlinear model is not readily available, we developed a method of extracting nonlinear large-signal [Formula: see text]-parameters obtained with artificial neural network models trained with multiple measurements made by a nonlinear vector network analyzer equipped with two sources. Finally, nonlinear large-signal [Formula: see text]-parameters are compared to another form of nonlinear mapping, known as nonlinear scattering functions. The nonlinear large-signal [Formula: see text]-parameters are shown to be more general.

Near-infrared spiroximetry: noninvasive measurements of venous saturation in piglets and human subjects

We present a noninvasive method to measure the venous oxygen saturation (Sv(O(2))) in tissues using near-infrared spectroscopy (NIRS). This method is based on the respiration-induced oscillations of the near-infrared absorption in tissues, and we call it spiroximetry (the prefix spiro means respiration). We have tested this method in three piglets (hind leg) and in eight human subjects (vastus medialis and vastus lateralis muscles). In the piglet study, we compared our NIRS measurements of the Sv(O(2)) (Sv(O(2))-NIRS(resp)) with the Sv(O(2)) of blood samples. Sv(O(2))-NIRS(resp) and Sv(O(2)) of blood samples agreed well over the whole range of Sv(O(2)) considered (20-95%). The two measurements showed an average difference of 1.0% and a standard deviation of the difference of 5.8%. In the human study, we found a good agreement between Sv(O(2))-NIRS(resp) and the Sv(O(2)) values measured with the NIRS venous occlusion method. Finally, in a preliminary test involving muscle exercise, Sv(O(2))-NIRS(resp) showed an expected postexercise decrease from the initial baseline value and a subsequent recovery to baseline.