Fiber-optic endoscopes, particularly those using OCT, are crucial for visualizing internal organs. However, current miniaturized probes suffer limitations in imaging performance due to fabrication challenges, including spherical aberration and a shallow depth of focus. These limitations hinder high-resolution imaging of delicate structures in small animals and narrow lumens, such as mouse models commonly used for cardiovascular research. Existing techniques, such as assembling discrete micro-optical elements or splicing fiber lenses, fail to adequately correct for aberrations, particularly astigmatism introduced by the protective catheter sheath. Ultrafast laser nanostructuring offers a potential solution but has not been fully exploited for aberration correction in endoscopic OCT. This research aimed to develop a novel fabrication technique to overcome these limitations and create a high-resolution, ultrathin OCT probe.

The literature highlights the need for miniaturized high-resolution endoscopic probes for imaging delicate organs and small animals, minimizing trauma during procedures. Current limitations in fabrication techniques lead to spherical aberration and shallow depth of focus, particularly in miniaturized probes. Traditional methods like assembling discrete elements or splicing fiber lenses don't effectively correct for aberrations. Ultrafast laser nanostructuring shows promise but has not been fully applied to create aberration-corrected probes for endoscopic OCT.

The researchers developed an ultrathin OCT probe using 3D microprinting with two-photon lithography. A 450 µm length of no-core fiber was spliced onto a single-mode fiber to expand the light beam before it reached the 3D-printed freeform optic. The freeform micro-optic, directly printed onto the no-core fiber, redirects and focuses the beam via total internal reflection (TIR) and compensates for astigmatism caused by the catheter sheath. The fiber assembly was integrated into a torque coil enabling 3D scanning. The probe's surface was profiled using a noncontact confocal surface profiler, and the beam profile was measured in water to simulate in vivo conditions. Ex vivo imaging of a human carotid artery and in situ imaging of mouse thoracic aortas were performed to evaluate probe performance. Histological sections were used for validation.

The 3D-printed probe achieved a measured full width at half maximum (FWHM) focal spot size of 12.4 µm and effective depths of focus of 760 µm (x-axis) and 1100 µm (y-axis). The deviation between the design and the printed micro-optic was less than 34 ± 12 nm RMS for the TIR mirror and less than 71 ± 52 nm for the planar surface. Ex vivo imaging of a severely narrowed human carotid artery successfully visualized an internal thrombus and fibroatheroma, validated by histology. In situ imaging of mouse aortas demonstrated high-quality images without rotational distortion, revealing microstructural details not previously achievable with such small probes. The probe's overall diameter, including the catheter sheath, was 0.457 mm.

The results demonstrate the successful fabrication of an ultrathin, aberration-corrected OCT probe using 3D microprinting. The probe's high resolution and large depth of focus enable visualization of microstructural details in both human and mouse arteries, offering significant advancements in minimally invasive imaging. The ability to image features such as thrombi, necrotic cores, and fibrous caps has critical implications for cardiovascular disease diagnosis. The small size and flexibility of the probe minimize tissue trauma and expand the potential applications of OCT endoscopy.

This study presents a significant advancement in endoscopic OCT technology. The fabrication of the smallest reported freeform 3D imaging probe with high resolution and depth of focus opens new avenues for minimally invasive diagnostics and interventions in various medical fields. Future research could explore the application of this technology to other imaging modalities and expand its clinical applications.

The study focused on cardiovascular applications. Further research is needed to evaluate the probe's performance in other anatomical locations and disease contexts. The in situ mouse imaging was performed after blood depletion; future studies should evaluate the probe's capabilities in the presence of flowing blood.