A number of approaches employ optical coherence tomography (OCT) to obtain the mechanical properties of biological tissue. These are generally referred to as optical coherence elastography (OCE), and have demonstrated promising applications with studies in cornea, breast, muscle, skin, and other soft tissues. A particular application of interest is the brain, in which changes in local and global elastic properties may correlate with the onset and progression of degenerative brain diseases. In this preliminary study, mice brains are studied ex vivo and in situ with preservation of the brain/skull anatomical architecture. A small 6 mm diameter portion of the skull is replaced with a glass cap to allow for OCT imaging. Various permutations of source placement for generating shear waves and modes of excitation are evaluated to optimize the experimental setup. The use of reverberant shear wave fields, which takes advantage of inevitable reflections from boundaries and tissue inhomogeneities, allow for estimation of the shear wave speed, which is directly related to the elastic modulus of soft tissues. Preliminary estimates for the shear wave speed in brains of recently deceased mice are obtained. This study demonstrates potential applications in brain OCE ex vivo and in vivo.
In medicine, both pathological (e.g. cirrhosis) and non-pathological states (e.g. aging) can be characterized by changes in the mechanical properties of biological tissue. The use of optical techniques to measure and map the elastic properties of soft tissue, known as optical elastography, is an emergent field with applications in various clinical disciplines, including ophthalmology and dermatology. In this paper, a brief overview of optical elastography will be provided with a short taxonomy. Categories include appropriate types of tissue models (semi-infinite, single thin layer, composite stacks), clinical tasks (classification or estimation), and excitation modes (transient, continuous, quasi-static, or molecular shift). We will then discuss examples of current advances, including optical coherence elastography using reverberant shear wave fields and Brillouin microscopy. The examples will demonstrate how current and future techniques may address clinical needs. Advantages and disadvantages of these techniques will be presented, augmenting the framework of the categorization system. With emerging applications, the taxonomy may be expanded providing a roadmap to future techniques.
The H-scan analysis of ultrasound images is a matched-filter approach derived from analysis of scattering from incident pulses in the form of Gaussian-weighted Hermite polynomial functions. This framework is applied in a preliminary study of thyroid lesions to examine the H-scan outputs for three categories: normal thyroid, benign lesions, and cancerous lesions within a total group size of 46 patients. In addition, phantoms comprised of spherical scatterers are analyzed to establish independent reference values for comparison. The results demonstrate a small but significant difference in some measures of the H-scan channel outputs between the different groups.