Abstract
Tendons are tough, flexible, and ubiquitous tissues that connect muscle to bone. Tendon injuries are a common musculoskeletal injury, which affect 7% of all patients and are involved in up to 50% of sports-related injuries in the United States. Various imaging modalities are used to evaluate tendons, and both magnetic resonance imaging and sonography are used clinically to evaluate tendons with non-invasive and non-ionizing radiation. However, these modalities cannot provide 3-dimensional (3D) structural images and are limited by angle dependency. In addition, anisotropy is an artifact that is unique to the musculoskeletal system. Thus, great care should be taken during tendon imaging. The present study evaluated a functional photoacoustic microscopy system for in-vivo tendon imaging without labeling. Tendons have a higher density of type 1 collagen in a cross-linked triple-helical formation (65–80% dry-weight collagen and 1–2% elastin in a proteoglycan-water matrix) than other tissues, which provides clear endogenous absorption contrast in the near-infrared spectrum. Therefore, photoacoustic imaging with a high sensitivity to absorption contrast is a powerful tool for label-free imaging of tendons. A pulsed near-infrared fiber-based laser with a centered wavelength of 780 nm was used for the imaging, and this system successfully provided a 3D image of mouse tendons with a wide field of view (5 × 5 mm2).
Highlights
Tendons are tough, flexible, and ubiquitous tissues that connect muscle to bone and transfer the force that is generated by muscle movement to the bone[1]
We described the label-free PA microscopy (PAM) system for in-vivo tendon imaging using a 780 nm fiber-based pulsed laser
The PA image of the in vivo study demonstrated that the structural details in the region of a tendon could be clearly classified from other structures
Summary
Flexible, and ubiquitous tissues that connect muscle to bone and transfer the force that is generated by muscle movement to the bone[1]. The orientation change of tendon fibers can cause signal degradation and lead to a misinterpreted diagnosis This problem can be reduced using a long echo time and stronger magnetic field, which provides a better signal-to-noise ratio (SNR) and resolution[19]. The ultrasound beam should be perpendicular to the collagen fibers, and even a 2° deviation can eliminate the sonographic image and potentially lead to a misinterpreted diagnosis[20]. This artifact is called anisotropy and is unique to the musculoskeletal system. The boundaries of the tendons are not clearly shown in an OCT image of the musculoskeletal system[10]
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