Fig. 1. Photoacoustic endoscope imaging system

Fig. 2. Imaging system and imaging targets. (a) Photo of the system; (b) photo of the stent; (c) in vivo imaging photo of rat rectum; (d) enlarged photo of the boxed area in Fig.2(c)

Fig. 3. Overall flowchart of three-dimensional vessel enhancement algorithm

Fig. 4. Resolution of the photoacoustic system. (a) Transverse resolution of photoacoustic system at 2.09 mm depth; (b) axial resolution of photoacoustic system at 2.09 mm depth; (c) MAP image of rat rectum

Fig. 5. Stent and rat rectum images. (a) Three-dimensional image of the stent; (b) RMAP image of the stent; (c) three- dimensional image of the rat rectum; (d) RMAP image of the rat rectum; (e)--(h) cross section images of arrow positions ①--④

Fig. 6. Comparison of the intensity profiles along the dashed lines in the original and binary images. (a)(d) Original images; (b)(e) enhanced images; (c)(f) comparison of the intensity profiles

Fig. 7. RMAP images of rat-1 rectum processed by different algorithms. (a) Original RMAP image; (b) RMAP image processed by the algorithm in this paper; (c) RMAP image processed by the algorithm in Ref. [26]; (d)-(f) RMAP images processed by single-scale Hessian matrix, the scales are 1, 3, and 5, respectively

Fig. 8. Three-dimensional photoacoustic images of rat rectum before and after enhancement. (a)(c)(e) Original three- dimensional photoacoustic images of rats-1, rat-2 and rat-3; (b)(d)(f) enhanced three-dimensional photoacoustic images

Fig. 9. Abnormal blood vessel structure caused by limited detection angle^[29]. (a) Abnormal vessel structure; (b) vessel image after compensation