In vivo photoacoustic imaging to detect the growth of mouse conjunctival melanoma

Abstract

Purpose : To image and monitor the growth of conjunctival melanoma using intrinsic photoacoustic tomography (PAT).

Methods : A mouse model of conjunctival melanoma was created by subconjunctival injection of B16F10 melanotic cells (5 μl; 1.4 x 10³ cells/μl) into the right eye in B6 albino mice (n=4) under general anesthesia. Mice were scanned at days 5, 7 (n=4), 9 (n=4), 10 (n=1), 13 (n=3), 15 (n=1) and 18 (n=1) after injection using PAT (MSOT inVision 128, iThera Medical). Cross-sectional images through the head, with spatial resolution of 150μm, were captured in 0.5 mm steps with 10 averages per scan at wavelengths (680, 700, 715, 730, 760, 800, 850 and 900nm) spanning the near-infrared absorption spectra of Hb, HbO₂ and melanin. A model linear algorithm was used to reconstruct images. Spectral unmixing was performed using linear regression of ViewMSOT software with the known absorption spectra for melanin, Hb and HbO₂ as input. Following sacrifice on days 10 (n=1), 13 (n=2) and 18 (n=1), whole-head specimens were fixed and frozen-sectioned.

Results : PAT detected a strong melanin signal in the temporal quadrant of the right anterior orbit of all tumor-bearing mice (n=4) as seen in Fig. 1A. A progressive increase in the size and intensity of the melanin signal was observed here on days 7 (n=4), 9 (n=4), 10 (n=1), 13 (n=3), 15 (n=1) and 18 (n=1), (Fig. 1B). Melanin absorption dominated the photoacoustic signal in the tumor, while Hb and HbO₂ signals were visible in other cranial structures. Melanin signal was consistently absent in other cranial structures (Figs. 1A-B). Whole-head frozen sections, as seen in Fig. 1C, confirmed in vivo photoacoustic results.

Conclusions : Conjunctival melanoma was detected in vivo by PAT with high resolution and without exogenous contrast agent. Tumor growth was consistent with the observed broadening and intensification of photoacoustic signature of melanin. To our knowledge this is the first report of in vivo PAT of ocular melanoma. Further studies to quantify the photoacoustic signals are in progress to assess its utility to monitor ocular tumor growth and response to treatments.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

Figure 1. Photoacoustic images of mouse head on day 5 (A) and before sacrifice at day 10 (B). The melanin signal is shown in green (white arrows), HbO₂ in red and Hb in blue. Frozen section of mouse head shows the tumor in black (black arrow) (C): D=Dorsal, V=Ventral, R=Right, L=Left.