Synchronous high-speed OCT imaging with sensor less brushless DC motor and FDML laser in a phase-locked loop

Abstract

High-speed endoscopic optical coherence tomography (OCT) imaging in the MHz range has shown great potential in various medical applications ranging from cancer screening to vascular disease monitoring. High-speed imaging always suffers from non-uniform rotational distortion (NURD) due to asynchronous motor rotation with the OCT system. Several research groups have previously attempted to solve this problem, using either an expensive motor with a sensor or numerical correction after data acquisition. However, both techniques pose challenges for practical use. Therefore, in this study, we use an inexpensive sensorless brushless DC motor with a Fourier domain mode-locked (FDML) laser-based MHz OCT system and try to resolve the problem of synchronization using three different modalities, (i) Slave-mode: The FDML frequency serves as a master frequency for the motor, which is phase-locked to the FDML frequency, (ii) Master-mode: The revolution trigger obtained from the motor’s back electromotive force (BEMF) signal serves as a trigger signal for the OCT imaging system, (iii) Both: Fully synchronized setup, where the motor rotation is synchronized with the laser and the imaging system is synchronized with the motor to achieve phase-stable OCT imaging. The first case slightly fluctuates in live preview and imaging due to the absence of a revolution trigger, while the second has varying motor speeds. Therefore, we use the third case to phase-lock the motor with FDML and get a distortion-free live preview and image acquisition. Finally, we demonstrate high-speed SS-OCT structural imaging (at 3.3 MHz A-scan rates) of a finger with a 16 mm diameter probe (at 40,000 rpm).

High-resolution rectoscopy using MHz optical coherence tomography: a step towards real time 3D endoscopy

“…The use of two probes was deemed necessary for this study due to the limitations of the initial probe in achieving the required lateral resolution and stability 33 , which were crucial for the accurate visualization of colonic structures. The implementation of a second-generation probe was found to significantly improve imaging accuracy and reduce overall jitter, resulting in clearer and more reliable OCT images 34 .…”

“…This first version (RV1) featured a 16 mm diameter, incorporating a brushless DC motor with bush bearing (BETAFPV, 0703, China) for rotational scanning. The optical assembly comprised a fiber-collimator (Thorlabs, F260APC-C, USA) and a 20 mm focusing lens (Thorlabs, AC080-020-C, USA), which together achieved a spot size of 19 µm in air, despite a theoretical design targeting 12 µm 33 . This probe employed a Polymethyl methacrylate (PMMA) window with a 1 mm thickness for circumferential scanning, ensuring both strength and maintaining imaging clarity without compromise.…”

“…This probe employed a Polymethyl methacrylate (PMMA) window with a 1 mm thickness for circumferential scanning, ensuring both strength and maintaining imaging clarity without compromise. The complete technical specifications and performance characteristics of this RV1 probe are detailed in the respective reference 33 .…”

“…To synchronize the motor speed with the OCT acquisition, the motor's back electromotive force (BEMF) signal was used as OCT frame trigger. This virtual Hall sensing approach could reduce non-uniform rotational distortion (NURD) artifacts to less than 4 mrad 33 , which enabled quasi phase-stable OCT imaging and instantaneous real-time visualization without the need for post-processing corrections 33 . A rotation speed of 40,000 rpm was chosen to enable a frame rate of ~ 667 Hz in line with the acquisition rate of the OCT-system.…”