Rapid vibration of vocal folds in the larynx generates sound waves, an essential process in human speech. Injury and disease can alter the microstructure and mechanical properties of the tissues in vocal folds, which can degrade vocal quality and lead to voice loss1
. Therefore, there is a great deal of interest in imaging the motion of vocal folds for diagnosis, measuring their mechanical properties in situ
, and developing novel treatments to restore their function. Laryngeal stroboscopy and high-speed videoscopy have proven useful to visualize the vocal folds during phonation2
. However, these methods are limited to surface view. For the larynx, elastic layers deep underneath the epithelium are key to normal function and to various disease states, but they are neither visible nor readily assessed by the current examination methods. Subsurface or cross-sectional imaging may provide new information about the physiological function and pathophysiology of these organs in both clinical and preclinical settings.
Optical coherence tomography (OCT) has the ability to image cross-sections of soft tissues up to about 2 mm in depth with spatial resolution of about 10 μm. Current imaging speed of OCT is, however, insufficient to directly capture the motion of vocal folds in the audio frequency range. In laryngeal imaging, studies with high-speed camera suggested that frame rates of about 4,000 frames per second (fps) are needed to optimally capture the surface motion of vocal folds with high-speed cameras2
. To achieve this frame rate with 1,000 axial lines (A-lines) per frame, the A-line rate should be about 4 MHz or higher. Current OCT systems are operated at A-line rates far less than several hundreds of kHz3,4,5,6
; Although higher A-line rates can be reachable with advanced laser technology7
, the increased speed inevitably involves a reduction of signal-to-noise ratio (SNR), limiting the maximum practical A-line rate. Furthermore, the speed requirement would be even more demanding if one wishes to capture motion in volume over time, i.e. 4D imaging8
In the case of periodic or quasi-periodic motion, such as vocal vibration, gated imaging is an established, effective technique. A traditional method called prospective gating acquires data at a specific phase over many motion cycles9
, as used in laryngeal stroboscopy and cardiac MRI. Because the data is acquired during a small fraction of time in each cycle, the time required to acquire the full data set can take orders of magnitude longer than simple static anatomical imaging10
. In contrast, another technique called retrospective gating employs continuous image acquisition and realignment of the data according to the acquired images or simultaneously acquired physiological signal11
. The application of retrospective gating to OCT has been demonstrated for visualizing cardiac motion of Xenopus laevis12
, which was further extended to imaging embryonic hearts of a chicken and a mouse at beating frequencies of about 10 Hz13
In this paper, we describe a modified scheme of dynamic OCT capable of producing “snapshots” of periodic tissue motion at frequencies over 100 Hz by employing motion-triggered laser scanning. At each transverse location, multiple A-line images are continuously acquired over a single cycle, and the probe beam is moved to a next spot at the end of the period of oscillation. Subsequent A-line registration in post processing synthesizes phase-aligned snapshots of tissue oscillation over the entire vibratory cycle. Compared to the previous gated imaging techniques, the triggered data acquisition facilitates precise temporal and spatial registration of A-lines, minimizing artifacts associated with asynchrony between the periods of sample motion and A-line acquisition. The frequency range that can be captured with dynamic OCT is determined by the A-line rate rather than its frame rate of the system and could be easily extended to the entire audible frequency range up to over 20 kHz with an A-line rate of approximately 200 kHz or higher. Since the data are acquired continuously, it offers distinct advantages of faster image acquisition compared to the prospective gated techniques and more robust time synchronization compared to the conventional retrospective gating methods. We demonstrate the proof-of-concept and an initial application of this technique for imaging aerodynamically driven vocal folds in an ex vivo calf larynx model.