In vivo endoscopic Doppler optical coherence tomography imaging of the colon

Abstract

Background and objective: Colorectal cancer (CRC) remains the second deadliest cancer in the United States. Several screening methods exist; however, detection of small polyps remains a challenge. Optical coherence tomography (OCT) has been demonstrated to be capable of detecting lesions as small as 1 mm in the mouse colon, but detection is based on measuring a doubling of the mucosa thickness. The colon microvasculature may be an attractive biomarker of early tumor development because tumor vessels are characterized by irregular structure and dysfunction. Our goal was to develop an endoscopic method of detecting and segmenting colon vessels using Doppler OCT to enable future studies for improving early detection and development of novel chemopreventive agents.

Method: We conducted in vivo colon imaging in an azoxymethane (AOM)-treated mouse model of colorectal cancer using a miniature endoscope and a swept-source OCT system at 1,040 nm with a 16 kHz sweep rate. We applied the Kasai autocorrelation algorithm to laterally oversampled OCT B-scans to resolve vascular flow in the mucosa and submucosa. Vessels were segmented by applying a series of image processing steps: (i) intensity thresholding; (ii) two-dimensional matched filtering; and (iii) histogram segmentation.

Results: We observed differences in the vessels sizes and spatial distribution in a mature adenoma compared to surrounding undiseased tissue and compared the results with histology. We also imaged flow in four young mice (two AOM-treated and two control) showing no significant differences, which is expected so early after carcinogen exposure. We also present flow images of adenoma in a living mouse and a euthanized mouse to demonstrate that no flow is detected after euthanasia.

Conclusion: We present, to the best of our knowledge, the first Doppler OCT images of in vivo mouse colon collected with a fiber-based endoscope. We also describe a fast and robust image processing method for segmenting vessels in the colon. These results suggest that Doppler OCT is a promising imaging modality for vascular imaging in the colon that requires no exogenous contrast agents. Lasers Surg. Med. 49:249-257, 2017. © 2016 Wiley Periodicals, Inc.

Keywords: adenoma; colorectal cancer; image processing; vascular imaging.

Figure 1

OCT system diagram and endoscope schematic. (a) The x-galvanometer control signal from the OCS1050SS is used to synchronize movement of the endoscope. The red line denotes the beam path in fiber. For the application described in this paper, the linear actuator moves the endoscope along the distal 30 mm of the colon. The rotational motor is used to select the rotational location of the longitudinal B-scan. OCT: OCS1050SS system, SE: synchronization electronics, LA: linear actuator, RM: rotational motor, TS: translation stage, RJ: fiber-optic rotary joint. (b) Schematic of the distal endoscope optics. S: spacer, GL: gradient-index lens, P: 41° rod prism. (c) Photograph of the distal portion of the endoscope.

Figure 2

Comparison of flow map before and after downsampling using bicubic interpolation. (a) A flow map after interpolation, (b) the flow map of the region denoted by the box in (a) prior to downsampling. Note that the repetitive red and blue vertical streaks due to the stepper motor motion have been averaged out in (a). Scale bar is 1 mm. Color bar is – π to π radians.

Figure 3

Comparison of vessel flow profiles before and after applying 2D matched filter. (a) Flow image prior to matched filter; (b) flow image after applying matched filter; (c) profile along black line in (a); profile along black line in (b); (e) histogram segmentation of (a) with upper and lower thresholds set to mean ± 2 standard deviations, excluding black pixels, (f) histogram segmentation of (b) with upper and lower thresholds set to mean ± 2 standard deviations, excluding black pixels. Scale bar is 1 mm. Color bar is – π to π radians (black pixels denote signal-void regions removed during intensity thresholding) and applies to (a) and (b). The blue and red in (e) and (f) indicate positive and negative flow, but not specific flow velocities.

Figure 4

Structural and segmented flow images from two saline-treated mice. (a,d) Structural OCT images, (b,e) result from matched filter operation after computing Kasai autocorrelation, (c,f) result of histogram segmentation on (b,e). Arrows denote portions of vessels parallel to the image plane and arteriole-venule pairs. Scale bars are 1 mm. Images have been stretched vertically by 25% to improve readability. The blue and red in (c) and (f) indicate positive and negative flow, but not specific flow velocities.

Figure 5

Structural and segmented flow images from two AOM-treated mice. (a,d) Structural OCT images, (b,e) result from matched filter operation after computing Kasai autocorrelation, (c,f) result of histogram segmentation on (b,e). Arrow denotes a partially suppressed motion artifact. Scale bars are 1 mm. Images have been stretched vertically by 25% to improve readability. The blue and red in (c) and (f) indicate positive and negative flow, but not specific flow velocities.

Figure 6

Comparison of adenoma tissue from living and euthanized mice. (a) and (b) are OCT images from the living and euthanized mouse, respectively. (c) and (d) are the corresponding flow images after intensity thresholding and matched filtering, (e) and (f) are the corresponding flow images after histogram segmentation. Scale bar is 1 mm. Color bar shows the phase shift in radians and applies to (c) and (d). The blue and red in (e) and (f) indicate positive and negative flow, but not specific flow velocities.

Figure 7

Comparison of adenoma and undiseased tissue. (a) Structural OCT image of adenoma, (b) segmented flow image of adenoma, (c,d) histology cross-section of adenoma at 4× and 10× magnification, (e) structural OCT image of undiseased colon tissue, (f) segmented flow image of undiseased tissue, (g,h) histology cross-section of undiseased tissue at 10× and 20× magnification. Arrows denote blood vessels. Scale bars are: 1 mm (a,b,e,f), 0.5 mm (c), 0.2 mm (d,g), and 0.1 mm (h). The blue and red in (b) and (f) indicate positive and negative flow, but not specific flow velocities.