Optimization of spectral-domain optical coherence tomography with a supercontinuum source for in vivo motion detection of low reflective outer hair cells in guinea pig cochleae

Abstract

Sound evokes sub-nanoscale vibration within the sensory epithelium. The epithelium contains not only immotile cells but also contractile outer hair cells (OHCs) that actively shrink and elongate synchronously with the sound. However, the in vivo motion of OHCs has remained undetermined. The aim of this work is to perform high-resolution and -accuracy vibrometry in live guinea pigs with an SC-introduced spectral-domain optical coherence tomography system (SD-OCT). In this study, to reveal the effective contribution of SC source in the recording of the low reflective materials with the short total acquisition time, we compare the performances of the SC-introduced SD-OCT (SCSD-OCT) to that of the conventional SD-OCT. As inanimate comparison objects, we record a mirror, a piezo actuator, and glass windows. For the measurements in biological materials, we use in/ex vivo guinea pig cochleae. Our study achieved the optimization of a SD-OCT system for high-resolution in vivo vibrometry in the cochlear sensory epithelium, termed the organ of Corti, in mammalian cochlea. By introducing a supercontinuum (SC) light source and reducing the total acquisition time, we improve the axial resolution and overcome the difficulty in recording the low reflective material in the presence of biological noise. The high power of the SC source enables the system to achieve a spatial resolution of 1.72 ± 0.00 μm on a mirror and reducing the total acquisition time contributes to the high spatial accuracy of sub-nanoscale vibrometry. Our findings reveal the vibrations at the apical/basal region of OHCs and the extracellular matrix, basilar membrane.

The cochlear hook region detects harmonics beyond the canonical hearing range

“…In contrast, the amplitudes increased more linearly for sub-3hBF stimulation. Although guinea pigs do not seem to hear these ultrasounds under ordinary conditions, the result was comparable to the vibration response in the epithelium of other cochlear turns during stimulation with audible sounds (F. Nin, Choi, Ota, Qi, & Hibino, 2021;Strimbu & Olson, 2022).…”

“…Nevertheless, the number of samples was almost equivalent to that in previous studies (F. Nin et al, 2021;F. Nin et al, 2012;Ota et al, 2020;Strimbu & Olson, 2022).…”

“…We used a customized OCT system based on a commercial SD-OCT (Ganymede GAN621; Thorlabs, US) and a supercontinuum (SC) light source (SuperK FIU-15; NKT Photonics, Denmark). This modification was similar to that used in a previous OCT system termed the SCSD-OCT system (F. Nin et al, 2021), in which the light source was connected to an optical bandpass filter (SuperK SPLIT; NKT Photonics, Denmark), a fiber delivery system (SuperK CONNECT; NKT Photonics, Denmark), and a single-mode broadband fiber (FD7; NKT Photonics, Denmark). An objective lens (M Plan Apo NIR 20×, Mitutoyo, Japan) with a focal length of 10 mm and a depth of focus of 1.7 μm was used.…”

“…After tracheostomy, the animals were artificially respirated with room air using a respirator (SN-408-7; Shinano Manufacturing, Japan). The cochlea was exposed using a method similar to that previously described for biophysical measurements (F. Nin et al, 2021; F. Nin, Reichenbach, Fisher, & Hudspeth, 2012; Ota et al, 2020). The head was fixed using a made-to-order stereotactic apparatus.…”

“…For optical recordings, we performed artificial ventilation in urethane-anesthetized guinea pigs as described above. Using a ventrolateral approach, the cochlea was exposed by opening the bulla as previously described (F. Nin et al, 2021; F. Nin et al, 2012; Ota et al, 2020). An acrylic plate connected to a flexible base stage (SL20/M; Thorlabs, USA) was fixed to the head of the animal.…”