Identifying intestinal fibrosis and inflammation by spectroscopic photoacoustic imaging: an animal study in vivo

Abstract

Crohn's disease (CD) is a chronic autoimmune disease characterized by obstructing intestinal strictures. Conventional imaging modalities can identify the strictures but cannot characterize whether a stricture is predominantly inflammatory or fibrotic. The purpose of this study is to examine the capability of photoacoustic (PA) imaging to characterize intestinal fibrosis and inflammation in vivo. Intestinal strictures in a rat model of CD were imaged with a PA-ultrasound parallel imaging system. Internal and external illuminations were attempted, both with transcutaneous PA signal reception. The PA signal magnitudes acquired at wavelengths targeting individual molecular components and the derived functional information showed significant differences between the inflammatory and fibrotic strictures, consistent with histological inflammation and fibrosis.

Keywords: (170.3880) Medical and biological imaging; (170.5120) Photoacoustic imaging; (170.6510) Spectroscopy, tissue diagnostics; (170.7170) Ultrasound.

Fig. 1

Optical absorption spectra of relevant molecular components in this study. The shaded areas are the wavelengths ranges that are targeting the hemoglobin and collagen contents, respectively. Hb: Hemoglobin.

Fig. 2

The experiment setups. (a) The internal illumination delivery was achieved using a fiber optic diffuser. (b) The external illumination was delivered by integrating fiber optics to the US probe with a fiber holder. The details of the fiber holder are shown in Fig. 3.

Fig. 3

PA-US parallel imaging probe (a) Side view; (b) Bottom view; (c) Front view.

Fig. 4

Gross pathology of the cecum and colon from acute vs. chronic TNBS rats. Ce = cecum, PC = proximal colon (unaffected), DC = affected distal colon. Upper panel inset illustrates the gross luminal differences between the acute and chronic models with hemorrhagic and necrotic regions in the acute model and the thickened, contracted features characteristic of the chronic fibrosis. Histological features of the acute model (A-D). H&E staining (A) identifies increased inflammatory cells (dark blue nuclei, detailed in inset C), and large regions of extravascular red blood cells (B, arrowheads) in the acute model with red blood cell extravasation from blood vessels (asterisks, detailed in inset D). Trichrome staining (B,D) reveled structural collagen only. Large collagen deposition was not observed. In contrast, histological features of the chronic fibrosis model (E-I) demonstrated massive transmural collagen deposition without red blood cell extravasation. H & E staining revealed a healed ulcer (E, arrow) with few inflammatory cells present (inset, G). Trichrome staining (F) illustrates massive disorganized collagen deposition (blue staining, detailed in H) and architectural disruption by collagen extending into the deeper muscle layers (inset I).

Fig. 5

PA-US parallel imaging with internal illumination. (a) shows the example images of the PA-US parallel images acquired in a chronic TNBS animal. The skin and muscle layers in the US images are shaded in green and purple. (b) shows the statistics with the quantitative values from each animal.

Fig. 6

The representative US and PA images within the ROIs. The top and bottom rows are acquired in acute and chronic animals, respectively. Column 1 present the US images. The green and purple shaded areas in the first column are skin and muscle layers, respectively. Columns 2-4 are PA images acquired in parallel with the US imaging at 1310, 750 and 850 respectively. Columns 5-6 are the US images with coregistered relative oxygenation and collagen/Hb ratio within the ROIs. M: Membrane supporting the acoustic stand-off. AW: Abdominal wall. BW: Back wall. ROI: Region-of-interest. ISC: Intestinal stricture cross-section. PB: Pelvic bone. Hb: Hemoglobin.

Fig. 7

Statistical results of the averaged factors derived from all the animals.