Photoacoustic Imaging for Differential Diagnosis of Benign Polyps versus Malignant Polyps of the Gallbladder: A Preliminary Study

Abstract

Objective: To investigate the feasibility of ex vivo multispectral photoacoustic (PA) imaging in differentiating cholesterol versus neoplastic polyps, and benign versus malignant polyps, of the gallbladder.

Materials and methods: A total of 38 surgically confirmed gallbladder polyps (24 cholesterol polyps, 4 adenomas, and 10 adenocarcinomas) from 38 patients were prospectively included in this study. The surgical specimens were set on a gel pad immersed in a saline-filled container. The PA intensities of polyps were then measured, using two separate wavelength intervals (421-647 nm and 692-917 nm). Mann-Whitney U test was performed for the comparison of normalized PA intensities between the cholesterol and neoplastic polyps, and between the benign and malignant polyps. Kruskal-Wallis test was conducted for the comparison of normalized PA intensities among the cholesterol polyps, adenomas, and adenocarcinomas.

Results: A significant difference was observed in the normalized PA intensities between the cholesterol and neoplastic polyps at 459 nm (median, 1.00 vs. 0.73; p = 0.032). Comparing the benign and malignant polyps, there were significant differences in the normalized PA intensities at 765 nm (median, 0.67 vs. 0.78; p = 0.013), 787 nm (median, 0.65 vs. 0.77; p = 0.034), and 853 nm (median, 0.59 vs. 0.85; p = 0.028). The comparison of the normalized PA intensities among cholesterol polyps, adenomas, and adenocarcinomas demonstrated marginally significant differences at 765 nm (median, 0.67 vs. 0.66 vs. 0.78, respectively; p = 0.049).

Conclusion: These preliminary results indicate that benign versus malignant gallbladder polyps might exhibit different spectral patterns on multispectral PA imaging.

Keywords: Gallbladder neoplasm; Gallbladder polyp; Neoplastic polyp; Photoacoustic technique.

Fig. 1. Normalized PA intensities of cholesterol and neoplastic polyps.

Mean normalized PA intensities of cholesterol and neoplastic polyps (A) at shorter wavelengths between 421 and 647 nm, and (B) at longer wavelengths between 692 and 917 nm. Error bars denote standard deviation. Cholesterol polyps had significantly higher PA intensities at 459 nm compared to neoplastic polyps. PA = photoacoustic

Fig. 2. Normalized PA intensities of benign and malignant polyps.

Mean normalized PA intensities of benign and malignant polyps (A) at shorter wavelengths between 421 and 647 nm, and (B) at longer wavelengths between 692 and 917 nm. Error bars denote standard deviation. Malignant polyps had significantly higher PA intensities at 765, 787, and 853 nm compared to benign polyps.

Fig. 3. Normalized PA intensities of cholesterol polyps, adenomas, and adenocarcinomas.

Mean normalized PA intensities of cholesterol polyps, adenomas, and adenocarcinomas (A) at shorter wavelengths between 421 and 647 nm, and (B) at longer wavelengths between 692 and 917 nm. Normalized PA intensities of adenocarcinomas were significantly higher than those of cholesterol polyps at 765 nm.

Fig. 4. Gray-scale ultrasound images and normalized PA intensities of cholesterol polyp and adenocarcinoma.

A. GB cholesterol polyp (arrow) in 53-year-old woman. Gray-scale US exhibits hyperechoic polypoid mass (2.1 cm) in GB body. B. GB adenocarcinoma (arrow) in 45-year-old woman. Gray-scale US shows iso- to hyperechoic mass (2.3 cm) in GB body. C, D. Mean normalized PA intensities of two polyps (C) at shorter wavelengths between 421 and 647 nm, and (D) at longer wavelengths between 692 and 917 nm. Two polyps show distinct PA spectral patterns: PA intensity of cholesterol polyp continuously decreases at 692–917 nm, whereas adenocarcinoma exhibits absorption peak at 875 nm. GB = gallbladder, US = ultrasonography