Optical coherence tomography (OCT) is a powerful interferometric technology used to obtain cross-sectional tissue images noninvasively with micrometer resolution, millimeter penetration depth, and a video-rate imaging speed [1
]. Due to its non-contact and high resolution nature, OCT has become a valuable tool in a number of medical fields. Recently, OCT technique has been increasingly used to perform functional imaging as well. Doppler optical coherence tomography (DOCT) or optical Doppler tomography (ODT) is one kind of functional extension of OCT, which combines the Doppler principle with OCT. ODT has been widely used for in-vivo
imaging of blood flow in live animals and human beings [2
Clinically, vocal fold vibration has been widely imaged using laryngeal videostroboscopy and high speed video, as these methods provide clinically relevant important information on vocal fold behavior in health and pathology. Lohscheller et al. obtained functional information regarding the vibration vocal folds, such as vibrating frequency, velocity, and acceleration [8
]. Although videostroboscopy provides an excellent method to dynamically assess the vocal folds, it only provides information on the surface of the vocal folds; therefore, the condition of the vocal folds underneath the surface remains unknown using these systems. There is a wide spectrum of diseases that can occur in the vocal folds, including benign polyps, premalignant and malignant lesions. Differentiating these afflictions using only direct visualization can be difficult and a biopsy is often required. The key element to differentiate these lesions has to do with visualizing the integrity of the basement membrane. A loss of the basement membrane integrity is a hallmark of cancers of the vocal fold. Currently, there is no reliable noninvasive method to diagnose laryngeal cancer without introducing a biopsy. However, doing a biopsy in the vocal folds can come with its own risk of creating permanent damage to the vocal folds; therefore, the importance of using a noninvasive imaging method that can visualize below the surface of the vocal cords, such as Ultrasound, and OCT, is highly practical. Ultrasound has also been used to image the vibrating vocal folds [11
]. Although functional information can be obtained from an ultrasound, color Doppler ultrasound images suffer from low resolution and low frame rate. Hence, there is immense value in being able to image dynamically and in real-time image the structure and characteristics of the vibrating vocal folds, as much pathology is below the thin subsurface of this organ. Recently, imaging vibration vocal folds using OCT has been demonstrated by several groups [14
]. Lüerßen et al. demonstrated the vibration vocal fold OCT image at an imaging speed of 10 frames per second [14
]. Our group has demonstrated in-vivo
imaging of human vibrating vocal folds with a 1.3 µm, 20 kHz swept source OCT system and a hand-held probe [15
]. Functional information, such as vibrating frequency, was obtained by analysis of the OCT structure images. Kober et al. used a triggered 10 kHz swept source OCT system to image the excised half calf larynx [16
]. With the help of the particle image analysis method, the authors obtained the velocity vector in the cross section images from the OCT structure images.
For humans, the actual vocal fundamental frequencies vary by sex. In females it is approximately 200 Hz and in males it is approximately 120 Hz. For imaging such high frequency movement, a high speed imaging system is essential to provide high frame rate images for the analysis. In addition, Doppler OCT requires much more dense scanning between A-lines. In order to cover a large enough field of view and obtain high quality Doppler images at the same time, a fast system is essential to provide high frame rate.
In this paper, we demonstrate functional imaging of vibrating vocal folds ex-vivo with a high speed swept source OCT and ODT system. The system has a maximum imaging speed of 100 kHz A-line per second, a central wavelength of 1.05 µm, and a depth resolution of 7 µm. The functional information regarding the vibrating vocal folds, such as vibrating frequency, vibrating amplitude, and speed was obtained by fitting the surface curve of the vibrating vocal fold. To the best of our knowledge, this is the first time high quality and cross-sectional velocity distribution images of the vibrating vocal fold were obtained with an OCT and ODT system at a frame rate of 100 frames per second.