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Perspective on optical imaging for functional assessment in musculoskeletal extremity trauma surgery
The potential to image subsurface fluorescent contrast agents at high spatial resolution has facilitated growing interest in short-wave infrared (SWIR) imaging for biomedical applications. The early but growing literature showing improvements in resolution in small animal models suggests this is indeed the case, yet to date, images from larger animal models that more closely recapitulate humans have not been reported. We report the first imaging of SWIR fluorescence in a large animal model. Specifically, we imaged the vascular kinetics of an indocyanine green (ICG) bolus injection during open craniotomy of a mini-pig using a custom SWIR imaging instrument and a clinical-grade surgical microscope that images ICG in the near-infrared-I (NIR-I) window. Fluorescence images in the SWIR were observed to have higher spatial and contrast resolutions throughout the dynamic sequence, particularly in the smallest vessels. Additionally, vessels beneath a surface pool of blood were readily visualized in the SWIR images yet were obscured in the NIR-I channel. These first-in-large-animal observations represent an important translational step and suggest that SWIR imaging may provide higher spatial and contrast resolution images that are robust to the influence of blood.
HSF and CP imaging methods are both known to alter the reflectance image sensitivity to diffuse multiply- scattered and superficially backscattered photons. This results in enhanced contrast, compared to standard wide-field imaging, based on tissue surface microstructure and composition. Measurements in tissue-simulating optical phantoms show that CP images display contrast based on both scattering and absorption, while HSF is specifically sensitive to scatter-only contrast, strongly suppressing absorption-based contrast. By altering the frequency used, the degree of contrast suppression or enhancement can be tuned.1 This suggests that an inexpensive HSF imaging system could have potential to aid diagnostic procedures, where CP is the current state-of-the-art imaging modality.
Tumor implantation model for rapid testing of lymphatic dye uptake from paw to node in small animals
The goal of this work was to successfully deploy and test an intra-nodal cancer-cell injection model to enable planar fluorescence imaging of a clinically relevant blue dye, specifically methylene blue – used in the sentinel lymph node procedure – in normal and tumor-bearing animals, and subsequently segregate tumor-bearing from normal lymph nodes. This direct-injection based tumor model was employed in athymic rats (6 normal, 4 controls, 6 cancer-bearing), where luciferase-expressing breast cancer cells were injected into axillary lymph nodes. Tumor presence in nodes was confirmed by bioluminescence imaging before and after fluorescence imaging. Lymphatic uptake from the injection site (intradermal on forepaw) to lymph node was imaged at approximately 2 frames/minute. Large variability was observed within each cohort.
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