In vivo low-coherence spectroscopic measurements of local hemoglobin absorption spectra in human skin

Optical density based quantification of total haemoglobin concentrations with spectroscopic optical coherence tomography

Spectroscopic optical coherence tomography (sOCT) has emerged as a new possibility for non-invasive quantification of total haemoglobin concentrations [tHb]. Recently, we demonstrated that [tHb] measured in ex-vivo human whole-blood with a conventional sOCT system achieves a precision of 9.10 g/dL with a bias of 1.50 g/dL. This precision improved by acquiring data with a combination of focus tracking and zero-delay acquisition (FZA) that compensated for experimental limitations, increasing to 3.80 g/dL with a bias of 1.50 g/dL. Nevertheless, sOCT precision should improve at least to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim 2$$\end{document}∼2 g/dL to be clinically relevant. Therefore, sOCT-based [tHb] determinations require the development of new analysis methods that reduce the variability of [tHb] estimations. In this work, we aim to increase sOCT precision by retrieving the [tHb] content from a numerical optimisation of the optical density (OD), while considering the blood absorption flattening effect. The OD-based approach simplifies previous two-step Lambert–Beer fitting approaches to a single step, thereby reducing errors during the fitting procedure. We validated our model with ex-vivo [tHb] measurements on flowing whole-blood samples in the clinical range (7–23 g/dL). Our results show that, with the new model, conventional sOCT can determine [tHb] with a precision of 3.09 g/dL and a bias of 0.86 g/dL compared to a commercial blood analyser. We present further precision improvement by combining the OD methodology with FZA, leading to a precision of 2.08 g/dL with a bias of 0.46 g/dL.

Parametric imaging of attenuation by optical coherence tomography: review of models, methods, and clinical translation

SIGNIFICANCE

Optical coherence tomography (OCT) provides cross-sectional and volumetric images of backscattering from biological tissue that reveal the tissue morphology. The strength of the scattering, characterized by an attenuation coefficient, represents an alternative and complementary tissue optical property, which can be characterized by parametric imaging of the OCT attenuation coefficient. Over the last 15 years, a multitude of studies have been reported seeking to advance methods to determine the OCT attenuation coefficient and developing them toward clinical applications.

AIM

Our review provides an overview of the main models and methods, their assumptions and applicability, together with a survey of preclinical and clinical demonstrations and their translation potential.

RESULTS

The use of the attenuation coefficient, particularly when presented in the form of parametric en face images, is shown to be applicable in various medical fields. Most studies show the promise of the OCT attenuation coefficient in differentiating between tissues of clinical interest but vary widely in approach.

CONCLUSIONS

As a future step, a consensus on the model and method used for the determination of the attenuation coefficient is an important precursor to large-scale studies. With our review, we hope to provide a basis for discussion toward establishing this consensus.

Quantification of total haemoglobin concentrations in human whole blood by spectroscopic visible-light optical coherence tomography

The non-invasive quantification of total haemoglobin concentrations [tHb] is highly desired for the assessment of haematologic disorders in vulnerable patient groups, but invasive blood sampling is still the gold standard in current clinical practice. This work demonstrates the potential of visible-light spectroscopic optical coherence tomography (sOCT) for quantifying the [tHb] in human whole blood. To accurately quantify the [tHb] from the substantial optical attenuation by blood in the visible wavelength range, we used a combination of zero-delay acquisition and focus tracking that ensures optimal system sensitivity at any depth inside the sample. Subsequently, we developed an analysis model to adequately correct for the high scattering contribution by red blood cells to the sOCT signal. We validate our method and compare it to conventional sOCT (without focus tracking and zero-delay acquisition) through ex-vivo measurements on flowing human whole blood, with [tHb] values in the clinical range of 7-23 g/dL. For our method with optimized sensitivity, the measured and expected values correlate well (Pearson correlation coefficient = 0.89, p < 0.01), with a precision of 3.8 g/dL. This is a considerable improvement compared to conventional sOCT (Pearson correlation coefficient = 0.59, p = 0.16; precision of 9.1 g/dL).

Spatially confined quantification of bilirubin concentrations by spectroscopic visible-light optical coherence tomography

Spatially confined measurements of bilirubin in tissue can be of great value for noninvasive bilirubin estimations during neonatal jaundice, as well as our understanding of the physiology behind bilirubin extravasation. This work shows the potential of spectroscopic visible-light optical coherence tomography (sOCT) for this purpose. At the bilirubin absorption peak around 460 nm, sOCT suffers from a strong signal decay with depth, which we overcome by optimizing our system sensitivity through a combination of zero-delay acquisition and focus tracking. In a phantom study, we demonstrate the quantification of bilirubin concentrations between 0 and 650 µM with only a 10% difference to the expected value, thereby covering the entire clinical pathophysiological range.

Dual-band optical coherence tomography using a single supercontinuum laser source

We developed a simultaneous visible-light (Vis) and near-infrared (NIR) dual-band optical coherence tomography (OCT) system using a single supercontinuum laser source. The goal was to benchmark our newly developed Vis-OCT against the well-developed NIR-OCT. The Vis-OCT subsystem operated at 91 nm full-width-at-half-maximum (FWHM) bandwidth centered at 566 nm; the NIR-OCT subsystem operated at 93 nm FWHM bandwidth centered at 841 nm. The axial resolutions were 1.8 and 4.4  μm in air for the Vis- and NIR-OCT subsystems, respectively. We compared the respective performances, including anatomical imaging, angiography, absolute retinal blood flow measurements, and spectroscopic analysis for retinal blood oxygen saturation (sO2), between the two subsystems in rodents in vivo. While demonstrating minor discrepancies related to operation wavelengths, both subsystems showed comparable performances in the first three tests. However, we were only able to retrieve sO2 using the Vis-OCT subsystem.

Transmission optical coherence tomography based measurement of optical material properties

We present transmission optical coherence tomography (transmission OCT) as a versatile tool to measure optical material properties of turbid media. The transmission OCT signal is described in detail and it is demonstrated how the group refractive index (n(g)), group velocity dispersion (GVD) and optical attenuation can be determined from this signal. We experimentally validate the refractive index properties of glasses, liquids and glucose water solutions in terms of n(g) and GVD. Measurements of scattering coefficients are determined using transmission OCT for suspensions of silica particles. Quantitative agreement is obtained with a dependent scattering model, both for the average as well as the wavenumber resolved optical attenuation coefficient. Good agreement is observed between our measurements and literature values.

Oxygen saturation monitoring using resonance Raman spectroscopy

BACKGROUND

The knowledge of hemoglobin oxygen saturation (SO2) and tissue oxygenation is critical to identify the presence of shock and therapeutic options. The resonance vibrational enhancement of hemoglobin allows measurement of oxy- and deoxy species of hemoglobin and resonance Raman spectroscopy (RRS-StO2) has been successfully used to measure aggregate microvascular oxygenation. We tested the hypothesis that noninvasive oxygen saturation measured by RRS-StO2 could serve as surrogate of systemic central venous SO2.

METHODS

In anesthetized rats, measurements of RRS-StO2 made in oral mucosa, skin, muscle, and liver were compared with measurements of central venous SO2 using traditional multi-wavelength oximetry. Various oxygenation levels were obtained using a stepwise hemorrhage while over 100 paired blood samples and Raman-based measurements were performed. The relationships between RRS-StO2 and clinically important systemic blood parameters were also evaluated. RRS-StO2 measurements were made in 3-mm diameter tissue areas using a microvascular oximeter and a handheld probe.

RESULTS

Significant correlations were found between venous SO2 and RRS-StO2 measurements made in the oral mucosa (r = 0.913, P < 0.001), skin (r = 0.499, P < 0.01), and liver (r = 0.611, P < 0.05). The mean difference between sublingual RRS-StO2 and blood sample SO2 values was 5.4 ± 1.6%. Sublingual RRS-StO2 also correlated with lactate (r = 0.909, P < 0.01), potassium (r = 0.757, P < 0.01), and pH (r = 0.703, P < 0.05).

CONCLUSIONS

Raman-based oxygen saturation is a promising technique for the noninvasive evaluation of oxygenation in skin, thin tissues, and solid organs. Under certain conditions, sublingual RRS-StO2 measurements correlate with central venous SO2.

Compositional prior information in computed infrared spectroscopic imaging

Compositional prior information is used to bridge a gap in the theory between optical coherence tomography (OCT), which provides high-resolution structural images by neglecting spectral variation, and imaging spectroscopy, which provides only spectral information without significant regard to structure. A constraint is proposed in which it is assumed that a sample is composed of N distinct materials with known spectra, allowing the structural and spectral composition of the sample to be determined with a number of measurements on the order of N. We present a forward model for a sample with heterogeneities along the optical axis and show through simulation that the N-species constraint allows unambiguous inversion of Fourier transform interferometric data within the spatial frequency passband of the optical system. We then explore the stability and limitations of this model and extend it to a general 3D heterogeneous sample.

A literature review and novel theoretical approach on the optical properties of whole blood

Optical property measurements on blood are influenced by a large variety of factors of both physical and methodological origin. The aim of this review is to list these factors of influence and to provide the reader with optical property spectra (250–2,500 nm) for whole blood that can be used in the practice of biomedical optics (tabulated in the appendix). Hereto, we perform a critical examination and selection of the available optical property spectra of blood in literature, from which we compile average spectra for the absorption coefficient (μ_a), scattering coefficient (μ_s) and scattering anisotropy (g). From this, we calculate the reduced scattering coefficient (μ_s′) and the effective attenuation coefficient (μ_eff). In the compilation of μ_a and μ_s, we incorporate the influences of absorption flattening and dependent scattering (i.e. spatial correlations between positions of red blood cells), respectively. For the influence of dependent scattering on μ_s, we present a novel, theoretically derived formula that can be used for practical rescaling of μ_s to other haematocrits. Since the measurement of the scattering properties of blood has been proven to be challenging, we apply an alternative, theoretical approach to calculate spectra for μ_s and g. Hereto, we combine Kramers–Kronig analysis with analytical scattering theory, extended with Percus–Yevick structure factors that take into account the effect of dependent scattering in whole blood. We argue that our calculated spectra may provide a better estimation for μ_s and g (and hence μ_s′ and μ_eff) than the compiled spectra from literature for wavelengths between 300 and 600 nm.

Molecular Imaging in Optical Coherence Tomography

Optical coherence tomography (OCT) is a medical imaging technique that provides tomographic images at micron scales in three dimensions and high speeds. The addition of molecular contrast to the available morphological image holds great promise for extending OCT's impact in clinical practice and beyond. Fundamental limitations prevent OCT from directly taking advantage of powerful molecular processes such as fluorescence emission and incoherent Raman scattering. A wide range of approaches is being researched to provide molecular contrast to OCT. Here we review those approaches with particular attention to those that derive their molecular contrast directly from modulation of the OCT signal. We also provide a brief overview of the multimodal approaches to gaining molecular contrast coincident with OCT.

Volumetric in vivo visualization of upper urinary tract tumors using optical coherence tomography: a pilot study

PURPOSE

Knowledge of tumor stage and grade is paramount for treatment decision making in cases of upper urinary tract urothelial carcinoma but this condition cannot be accurately assessed by current techniques. Optical coherence tomography can hypothetically provide the urologist with real-time intraoperative information on tumor grade and stage. In this pilot study we report what are to our knowledge the first results of optical coherence tomography for grading and staging upper urinary tract urothelial carcinoma.

MATERIALS AND METHODS

Eight consecutive patients underwent ureterorenoscopy for suspicion or followup of upper urinary tract urothelial carcinoma. Optical coherence tomography data sets were intraoperatively obtained from the ureter and renal pelvis. All patients eventually underwent nephroureterectomy. Optical coherence tomography staging was done by visual inspection of lesions found on optical coherence tomography images. Optical coherence tomography grading was done by quantifying optical coherence tomography signal attenuation in mm(-1) on lesions and comparing results with the histopathological diagnosis. The Wilcoxon rank sum test was used for statistical analysis.

RESULTS

For 7 in vivo optical coherence tomography diagnoses staging was in accordance with histology. In patient 8 tumor thickness transcended optical coherence tomography imaging depth range and, therefore, invasiveness findings were inconclusive. For grading the median attenuation coefficient for grade 2 and 3 lesions was 1.97 (IQR 1.57-2.30) and 3.53 mm(-1) (IQR 2.74-3.94), respectively (p<0.001). Healthy urothelium was too thin to reliably determine the attenuation coefficient.

CONCLUSIONS

Optical coherence tomography is a promising, minimally invasive tool for real-time intraoperative optical diagnosis of tumors in the upper urinary tract. Our results warrant future research in a larger sample size to determine the accuracy of grading and staging by optical coherence tomography, and its possible implementation in the diagnostic algorithm for upper urinary tract urothelial carcinoma.

Optical biopsy of epithelial cancers by optical coherence tomography (OCT)

Optical coherence tomography (OCT) is an optical technique that measures the backscattering of near-infrared light by tissue. OCT yields in 2D and 3D images at micrometer-scale resolution, thus providing optical biopsies, approaching the resolution of histopathological imaging. The technique has shown to allow in vivo differentiation between benign and malignant epithelial tissue, through qualitative assessment of OCT images, as well as by quantitative evaluation, e.g., functional OCT. This study aims to summarize the principles of OCT and to discuss the current literature on the diagnostic value of OCT in the diagnosis of epithelial (pre)malignant lesions. The authors did a systematic search of the electronic databases PubMed and Embase on OCT in the diagnostic process of (pre)malignant epithelial lesions. OCT is able to differentiate between benign and (pre)malignant lesions of epithelial origin in a wide variety of tissues. In this way, OCT can detect skin cancers, oral, laryngeal, and esophageal cancer as well as genital and bladder cancer. OCT is an innovative technique which enables an optical biopsy of epithelial lesions. The incorporation of OCT in specific tools, like handheld and catheter-based probes, will further improve the implementation of this technology in daily clinical practice.

Quantitative comparison of analysis methods for spectroscopic optical coherence tomography

Spectroscopic optical coherence tomography (sOCT) enables the mapping of chromophore concentrations and image contrast enhancement in tissue. Acquisition of depth resolved spectra by sOCT requires analysis methods with optimal spectral/spatial resolution and spectral recovery. In this article, we quantitatively compare the available methods, i.e. the short time Fourier transform (STFT), wavelet transforms, the Wigner-Ville distribution and the dual window method through simulations in tissue-like media. We conclude that all methods suffer from the trade-off in spectral/spatial resolution, and that the STFT is the optimal method for the specific application of the localized quantification of hemoglobin concentration and oxygen saturation.

Spectral domain detection in low-coherence spectroscopy

Low-coherence spectroscopy (LCS) offers the valuable possibility to measure quantitative and wavelength resolved optical property spectra within a tissue volume of choice that is controllable both in size and in depth. Until now, only time domain detection was investigated for LCS (tdLCS), but spectral domain detection offers a theoretical speed/sensitivity advantage over tdLCS. In this article, we introduce a method for spectral domain detection in LCS (sdLCS), with optimal sensitivity as a function of measurement depth. We validate our method computationally in a simulation and experimentally on a phantom with known optical properties. The attenuation, absorption and scattering coefficient spectra from the phantom that were measured by sdLCS agree well with the expected optical properties and the measured optical properties by tdLCS.

Limitations and opportunities of transcutaneous bilirubin measurements

OBJECTIVE

Although transcutaneous bilirubinometers have existed for over 30 years, the clinical utility of the technique is limited to a screening method for hyperbilirubinemia, rather than a replacement for invasive blood sampling. In this study, we investigate the reason for this limited clinical value and address possibilities for improvement.

METHODS

To obtain better insight into the physiology of bilirubin measurements, we evaluated a transcutaneous bilirubinometer that determines not only the cutaneous bilirubin concentration (TcB) but also the blood volume fraction (BVF) in the investigated skin volume. For 49 neonates (gestational age 30 ± 3.1 weeks, postnatal age 6 [4-10] days) at our NICU, we performed 124 TcB and 55 BVF measurements.

RESULTS

The TcB correlated well with the total serum bilirubin concentration (TSB) (r = 0.88) with an uncertainty of 55 µmol/L. The BVF in the measured skin volume ranged between 0.1% and 0.75%.

CONCLUSIONS

The performance of our bilirubinometer is comparable to existing transcutaneous devices. The limited clinical value of current bilirubinometers can be explained by the low BVF in the skin volume that is probed by these devices. Because the TcB depends for over 99% on the contribution of extravascular bilirubin, it is a physiologically different parameter from the TSB. Hence, the standard method of evaluation that compares the TcB to the TSB is insufficient to fully investigate the clinical value of transcutaneous bilirubinometers, ie, their predictive value for kernicterus. We suggest that the clinical value may be improved considerably by changing either the method of evaluation or the technological design of transcutaneous bilirubinometers.