We report on a multimodal imaging system comprising optical coherence tomography (OCT), pulse echo ultrasound imaging (USI), and acoustic radiation force optical coherence elastography (ARF OCE), capable of volumetric structural and mechanical imaging with micrometer-to-centimeter- scale spatial coverage. A spectral-domain OCT setup (1300 nm central wavelength, with transverse and axial resolutions of 6-8 μm and 3.5 μm, respectively) imaged the sample from above, and a 10 MHz immersion ultrasound transducer provided a counter-propagating co-aligned beam for both USI and ARF excitation from below.
Although typical ARF elastography systems report acoustic focal spot sizes greater than 300 μm, studies indicate that reducing the region of excitation (ROE) improves mechanical contrast. To decrease the ROE diameter, we designed and fabricated acoustic lenses made of silicone and agar of various curvatures to increase the numerical aperture of the acoustic beam. We achieved a spot size of 240 μm – a 28% decrease from an initial spot size of 330 μm.
We characterized mechanical resolution of the ARF-OCE elastograms using a gelatin-agar co-gel phantom exhibiting a sharp ‘step’ in mechanical properties. Differentiating the mechanical step response, we obtained the mechanical impulse response with FWHM of 165±2 μm using ARF excitation with ROE diameter of 700 μm at the sample surface. Our results suggest that mechanical resolution (width of the impulse response function) cannot be described by just the ROE or OCT resolution alone. Future work will aim to further reduce the ROE, and will further investigate the effects of ROE and ARF excitation frequency on mechanical resolution.
Mechanical properties of cells and tissues play an important role in governing both normal and diseased biological processes. Recent findings in mechanobiology have demonstrated that viscosity, independent of elasticity, of extracellular matrix (ECM) can alter cellular behaviors. To obtain a comprehensive understanding of the mechanical properties of viscoelastic biological tissues for biomedical applications and mechanobiology research, both the elasticity and the viscosity must be characterized. Although optical coherence elastography (OCE) has emerged as a promising tool for probing the mechanical properties of biological tissues, quantitative OCE methods have mostly been limited to elasticity reconstruction or relied on the use of a presumed mechanical model, which may or may not adequately describe the response of a given tissue type. We present the first experimental demonstration of a mechanical model-independent reconstruction of complex shear modulus from direct measurement of surface wave propagation in viscoelastic media with dynamic acoustic radiation force (ARF)-OCE. Our results suggest that elasticity imaging based on shear wave speed alone could overlook potentially significant variations in the viscoelastic properties of biological tissues.