Abstract
Transparent “Window to the Brain” (WttB) cranial implants made from a biocompatible ceramic, nanocrystalline Yttria-Stabilized Zirconia (nc-YSZ), were recently reported. These reports demonstrated chronic brain imaging across the implants in mice using optical coherence tomography (OCT) and laser speckle imaging. However, optical properties of these transparent cranial implants are neither completely characterized nor completely understood. In this study, we measure optical properties of the implant using a swept source OCT system with a spectral range of 136 nm centered at 1,300 nm to characterize the group refractive index of the nc-YSZ window, over a narrow range of temperatures at which the implant may be used during imaging or therapy (20–43°C). Group refractive index was found to be 2.1–2.2 for OCT imaging over this temperature range. Chromatic dispersion for this spectral range was observed to vary over the sample, sometimes flipping signs between normal and anomalous dispersion. These properties of nc-YSZ should be considered when designing optical systems and procedures that propagate light through the window, and when interpreting OCT brain images acquired across the window.
Highlights
Neurosurgeries often involve craniectomy to gain access to the brain for therapy, followed by the placement of a cranial implant to replace the excised bone
optical coherence tomography (OCT) is an imaging technique based on broadband nearinfrared light which can penetrate into scattering media such as brain tissue underlying the Window to the Brain” (WttB) implant
This method allowed for determination of real sample thickness dnc−YSZ at each location and temperature, using the optical thickness T divided by the group refractive index ng (Equation 2) along with a calibration factor of 6.19 microns per pixel in the OCT image (Table 2)
Summary
Neurosurgeries often involve craniectomy (removal of a portion of the cranial bone) to gain access to the brain for therapy, followed by the placement of a cranial implant to replace the excised bone. Transparent Cranial Implant Group Index optical coherence tomography (OCT) (Halaney et al, 2020). OCT is an imaging technique based on broadband nearinfrared light which can penetrate into scattering media such as brain tissue underlying the WttB implant. Optical properties of these transparent cranial implants are neither completely characterized nor completely understood. Fundamental optical properties of the implant such as the group refractive index and chromatic dispersion are important to consider when planning or designing timebased and/or multispectral imaging strategies across the window, and for correct interpretation of recorded brain images. Characterizing variation of group refractive index and chromatic dispersion will allow development of dispersion compensation techniques for obtaining higher quality OCT images. Design of other multispectral imaging strategies that utilize cranial implant windows will be impacted by the group refractive index and chromatic dispersion
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