Abstract
.Dynamic optical coherence elastography (OCE) tracks elastic wave propagation speed within tissue, enabling quantitative three-dimensional imaging of the elastic modulus. We show that propagating mechanical waves are mode converted at interfaces, creating a finite region on the order of an acoustic wavelength where there is not a simple one-to-one correspondence between wave speed and elastic modulus. Depending on the details of a boundary’s geometry and elasticity contrast, highly complex propagating fields produced near the boundary can substantially affect both the spatial resolution and contrast of the elasticity image. We demonstrate boundary effects on Rayleigh waves incident on a vertical boundary between media of different shear moduli. Lateral resolution is defined by the width of the transition zone between two media and is the limit at which a physical inclusion can be detected with full contrast. We experimentally demonstrate results using a spectral-domain OCT system on tissue-mimicking phantoms, which are replicated using numerical simulations. It is shown that the spatial resolution in dynamic OCE is determined by the temporal and spatial characteristics (i.e., bandwidth and spatial pulse width) of the propagating mechanical wave. Thus, mechanical resolution in dynamic OCE inherently differs from the optical resolution of the OCT imaging system.
Highlights
Optical coherence elastography (OCE) uses optical coherence tomography (OCT) to detect local tissue displacement following an applied force, enabling the quantification of biomechanical properties within a region of interest
For sampling typically used in OCT, we show that the spatial resolution in dynamic OCE is primarily determined by a characteristic wavelength of the mechanical wave pulse and further complicated by complex wave-interface interactions
Even with typical limitations imposed by a practical measurement system, we have shown using numerical simulations and experimental studies in tissue-mimicking phantoms that OCE spatial resolution is fundamentally limited by the properties of propagating mechanical waves
Summary
Optical coherence elastography (OCE) uses optical coherence tomography (OCT) to detect local tissue displacement following an applied force, enabling the quantification of biomechanical properties within a region of interest. Detecting spatial heterogeneities in a mechanical structure can help monitor and treat disease progression in numerous fields, including ophthalmology,[1,2,3,4,5] dermatology,[6] cardiology,[7] gastroenterology,[8] and oncology.[9,10,11] While it is a general belief that dynamic OCE is capable of high-resolution (on the order of the imaging modality, OCT) mapping of tissue mechanical properties, the spatial resolution in wave-based optical elastography has not yet been characterized. Preservation of temporal wave shape is exploited in traditional time-of-flight reconstruction to locally estimate propagation speed and, thereby, infer the elastic modulus.[33] In the spatial domain, the original waveform can stretch or compress as a function of local propagation speed This scaling has been exploited to locally estimate propagation speed based on the wavenumber, thereby inferring elastic modulus.[24,34,35]
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