Optical measurement of arterial mechanical properties: from atherosclerotic plaque initiation to rupture

Molecular Contrast Optical Coherence Tomography and Its Applications in Medicine

The growing need to understand the molecular mechanisms of diseases has prompted the revolution in molecular imaging techniques along with nanomedicine development. Conventional optical coherence tomography (OCT) is a low-cost in vivo imaging modality that provides unique high spatial and temporal resolution anatomic images but little molecular information. However, given the widespread adoption of OCT in research and clinical practice, its robust molecular imaging extensions are strongly desired to combine with anatomical images. A range of relevant approaches has been reported already. In this article, we review the recent advances of molecular contrast OCT imaging techniques, the corresponding contrast agents, especially the nanoparticle-based ones, and their applications. We also summarize the properties, design criteria, merit, and demerit of those contrast agents. In the end, the prospects and challenges for further research and development in this field are outlined.

In-vivo mechanical characterization of coronary atherosclerotic plaques in living swine using intravascular laser speckle imaging

The ability to evaluate the viscoelastic properties of coronary arteries is crucial for identifying mechanically unstable atherosclerotic plaques. Here, we demonstrate for the first time in living swine, the capability of intravascular laser speckle imaging (ILSI) to measure an index of coronary plaque viscoelasticity, τ, using a human coronary to swine xenograft model. Cardiac motion effects are evaluated by comparing the EKG-non-gated τ ¯ N G , and EKG-gated τ ¯ G among different plaque types. Results show that both τ ¯ N G and τ ¯ G are significantly lower in necrotic-core plaques compared with stable lesions. Discrete-point pullback measurements demonstrate the capability of ILSI for rapid mechanical characterization of coronary segments under physiological conditions, in-vivo.

Evaluation Through the Optical Coherence Tomography Analysis of the Influence of Non-Alcoholic Fatty Liver Disease on the Gingival Inflammation in Periodontal Patients

PURPOSE

The purpose of this ex vivo study is to exhibit the inflammatory changes that occur within the gingival tissue by using optical coherence tomography (OCT) in periodontal patients with non-alcoholic fatty liver disease (NAFLD) and if NAFLD could influence the local periodontal inflammation.

PATIENTS AND METHODS

Gingival tissue samples obtained from patients were divided into three groups - P (periodontitis), NAFLD+P (NAFLD+periodontitis) and H (healthy) groups - and were scanned using an OCT light beam, in order to perform a qualitative and quantitative analysis of images. The value of average pixel density has been associated with the degree of inflammation.

RESULTS

The highest average pixel density was found in patients from the H group, while the lowest value of average pixel density was recorded in gingival tissue samples collected from patients with NAFLD+P. The image assessments from NAFLD+P group delivered lower values of average pixel density than those of P group, suggesting a possible influence of this disease on the inflammatory tissular changes produced by periodontal disease.

CONCLUSION

After comparing the OCT analysis results obtained for the three groups of patients, we can consider that NAFLD may be an aggravating factor for the inflammation of periodontal disease.

Tutorial on laser speckle rheology: technology, applications, and opportunities

SIGNIFICANCE

The onset of several diseases is frequently marked with anomalous mechanical alteration of the affected tissue at the intersection of cells and their microenvironment. Therefore, mapping the micromechanical attributes of the tissues could enhance our understanding of the etiology of human disease, improve the diagnosis, and help stratify therapies that target these mechanical aberrations.

AIM

We review the tremendous opportunities offered through using optics for imaging the micromechanical properties, at length scales inaccessible to other modalities, in both basic research and clinical medicine. We specifically focus on laser speckle rheology (LSR), a technology that quantifies the mechanical properties of tissues in a rapid, noncontact manner.

APPROACH

In LSR, the shear viscoelastic modulus is measured from the time-variant speckle intensity fluctuations reflected off the tissue. The LSR technology is engineered and configured into several embodiments, including bench-top optical systems, endoscopes for minimally invasive procedures, portable point-of-care devices, and microscopes.

RESULTS

These technological nuances have primed the LSR for widespread applications in diagnosis and therapeutic monitoring, as demonstrated here, in cardiovascular disease, coagulation disorders, and tumor malignancies.

CONCLUSION

The fast-paced technological advancements, elaborated here, position the LSR as a competent candidate for many more exciting opportunities in basic research and medicine.

In vivo imaging of human oral hard and soft tissues by polarization-sensitive optical coherence tomography

Since optical coherence tomography (OCT) provides three-dimensional high-resolution images of biological tissue, the benefit of polarization contrast in the field of dentistry is highlighted in this study. Polarization-sensitive OCT (PS OCT) with phase-sensitive recording is used for imaging dental and mucosal tissues in the human oral cavity in vivo. An enhanced polarization contrast of oral structures is reached by analyzing the signals of the co- and crosspolarized channels of the swept source PS OCT system quantitatively with respect to reflectivity, retardation, optic axis orientation, and depolarization. The calculation of these polarization parameters enables a high tissue-specific contrast imaging for the detailed physical interpretation of human oral hard and soft tissues. For the proof-of-principle, imaging of composite restorations and mineralization defects at premolars as well as gingival, lingual, and labial oral mucosa was performed in vivo within the anterior oral cavity. The achieved contrast-enhanced results of the investigated human oral tissues by means of polarization-sensitive imaging are evaluated by the comparison with conventional intensity-based OCT.

Optical coherence elastography - OCT at work in tissue biomechanics [Invited]

Optical coherence elastography (OCE), as the use of OCT to perform elastography has come to be known, began in 1998, around ten years after the rest of the field of elastography - the use of imaging to deduce mechanical properties of tissues. After a slow start, the maturation of OCT technology in the early to mid 2000s has underpinned a recent acceleration in the field. With more than 20 papers published in 2015, and more than 25 in 2016, OCE is growing fast, but still small compared to the companion fields of cell mechanics research methods, and medical elastography. In this review, we describe the early developments in OCE, and the factors that led to the current acceleration. Much of our attention is on the key recent advances, with a strong emphasis on future prospects, which are exceptionally bright.

Intraluminal laser speckle rheology using an omni-directional viewing catheter

A number of disease conditions in luminal organs are associated with alterations in tissue mechanical properties. Here, we report a new omni-directional viewing Laser Speckle Rheology (LSR) catheter for mapping the mechanical properties of luminal organs without the need for rotational motion. The LSR catheter incorporates multiple illumination fibers, an optical fiber bundle and a multi-faceted mirror to permit omni-directional viewing of the luminal wall. By retracting the catheter using a motor-drive assembly, cylindrical maps of tissue mechanical properties are reconstructed. Evaluation conducted in a test phantom with circumferentially-varying mechanical properties demonstrates the capability of the LSR catheter for the accurate mechanical assessment of luminal organs.

Multi-modal optical imaging characterization of atherosclerotic plaques

We combined cross-polarization optical coherence tomography (CP OCT) and non-linear microscopy based on second harmonic generation (SHG) and two-photon-excited fluorescence (2PEF) to assess collagen and elastin fibers and other vascular structures in the development of atherosclerosis, including identification of vulnerable plaques, which remains an important clinical problem and imaging application. CP OCT's ability to visualize tissue birefringence and cross-scattering adds new information about the microstructure and composition of the plaque. However its interpretation can be ambiguous, because backscattering contrast may have a similar appearance to the birefringence related fringes. Our results represent a step towards minimally invasive characterization and monitoring of different stages of atherosclerosis, including vulnerable plaques. CP OCT image of intimal thickening in the human coronary artery. The dark stripe in the cross-polarization channel (arrow) is a polarization fringe related to the phase retardation between two eigen polarization states. It is histologically located in the area of the lipid pool, however this stripe is a polarization artifact, rather than direct visualization of the lipid pool.

Bayesian maximum likelihood estimator of phase retardation for quantitative polarization-sensitive optical coherence tomography

This paper presents the theory and numerical implementation of a maximum likelihood estimator for local phase retardation (i.e., birefringence) measured using Jones-matrix-based polarization sensitive optical coherence tomography. Previous studies have shown conventional mean estimations of phase retardation and birefringence are significantly biased in the presence of system noise. Our estimator design is based on a Bayes' rule that relates the distributions of the measured birefringence under a particular true birefringence and the true birefringence under a particular measured birefringence. We used a Monte-Carlo method to calculate the likelihood function that describes the relationship between the distributions and numerically implement the estimator. Our numerical and experimental results show that the proposed estimator was asymptotically unbiased even with low signal-to-noise ratio and/or for the true phase retardations close to the edge of the measurement range. The estimator revealed detailed clinical features when applied to the in vivo anterior human eye.