Fiber-optic system for dual-modality imaging of glucose probes 18F-FDG and 6-NBDG in atherosclerotic plaques

Abstract

Background: Atherosclerosis is a progressive inflammatory condition that underlies coronary artery disease (CAD)-the leading cause of death in the United States. Thus, the ultimate goal of this research is to advance our understanding of human CAD by improving the characterization of metabolically active vulnerable plaques within the coronary arteries using a novel catheter-based imaging system. The aims of this study include (1) developing a novel fiber-optic imaging system with a scintillator to detect both 18F and fluorescent glucose probes, and (2) validating the system on ex vivo murine plaques.

Methods: A novel design implements a flexible fiber-optic catheter consisting of both a radio-luminescence and a fluorescence imaging system to detect radionuclide 18F-fluorodeoxyglucose (18F-FDG) and the fluorescent analog 6-(N-(7-Nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-6-Deoxyglucose (6-NBDG), respectively. Murine macrophage-rich atherosclerotic carotid plaques were imaged ex vivo after intravenous delivery of 18F-FDG or 6-NBDG. Confirmatory optical imaging by IVIS-200 and autoradiography were also performed.

Results: Our fiber-optic imaging system successfully visualized both 18F-FDG and 6-NBDG probes in atherosclerotic plaques. For 18F-FDG, the ligated left carotid arteries (LCs) exhibited 4.9-fold higher radioluminescence signal intensity compared to the non-ligated right carotid arteries (RCs) (2.6 × 10(4) ± 1.4 × 10(3) vs. 5.4 × 10(3) ± 1.3 × 10(3) A.U., P = 0.008). Similarly, for 6-NBDG, the ligated LCs emitted 4.3-fold brighter fluorescent signals than the control RCs (1.6 × 10(2) ± 2.7 × 10(1) vs. 3.8 × 10(1) ± 5.9 A.U., P = 0.002). The higher uptake of both 18F-FDG and 6-NBDG in ligated LCs were confirmed with the IVIS-200 system. Autoradiography further verified the higher uptake of 18F-FDG by the LCs.

Conclusions: This novel fiber-optic imaging system was sensitive to both radionuclide and fluorescent glucose probes taken up by murine atherosclerotic plaques. In addition, 6-NBDG is a promising novel fluorescent probe for detecting macrophage-rich atherosclerotic plaques.

Figure 1. Schematic diagram of the dual-modality fiber-optic imaging system.

The catheter fluorescence imaging (CFI) system combines components shown in (a) and (b) including a 540 nm filter placed in a filter wheel to detect glucose probes such as 6-NBDG fluorophore. Scintillating screen is removed during the fluorescence measurement. The catheter radionuclide imaging (CRI) system is shown in (b) to detect ¹⁸F-FDG radionuclide when the CFI light is turned off, no emission filter is in the optical path, and a scintillating screen is used in front of the wide-angle lens. The novel system was placed inside a light-tight black box to prevent any ambient light.

Figure 2. ROI selection on IVIS-200 images.

Three images–background, luminescent, and photograph were collected using IVIS-200 to create an overlay image. Then, a region of interest (ROI, shown with circle) was selected on the left carotid (LC) and right carotid (RC) arteries. The overlay image was corrected for field flatness, which was specific to the camera used in the IVIS-200 imaging system.

Figure 3. Radioluminescence imaging of ex vivo murine carotid arteries.

Radioluminescence imaging was taken one hour after ¹⁸F-FDG intravenuous injection (0.964 mCi) with the CRI system: (a–c) and with the IVIS-200 system: (d–e). (a) LC: ligated left carotid artery, (b) RC: non-ligated right carotid artery artery (negative control), (c) H: heart (positive control). (d) both LC and RC artery under scintillator, (e) left: heart under scintillator, right: heart without scintillator; (g) average signal intensity of tissue samples from the CRI system images (h) average radiance of the samples from the IVIS-200 system images. High radioluminescence signal from LCs was detected by the CRI system and confirmed by IVIS-200.

Figure 4. Autoradiography imaging for confirmatory purpose.

Ex vivo autoradiography from a mouse injected with ¹⁸F-FDG. The ligated left carotid artery (LC) showed high amount of ¹⁸F-FDG uptake compared to the non-ligated right carotid artery (RC).

Figure 5. Fluorescence imaging of ex vivo murine carotids arteries.

Fluorescence imaging was taken one hour after 6-NBDG intravenous injection (177 µL) with the CFI imaging system: (a–c) and with the IVIS-200 imaging system: (d, d-1). (a) LC: ligated left carotid artery with white area (green arrow) in the middle representing the ligation location, (b) RC: non-ligated right carotid artery artery (negative control), (c) H: heart (positive control). Dark line in the heart represents a gap in signal due the ventricular cavity (green arrow). (d) IVIS-200 images with GFP and GFP-background filters at 15 seconds exposure. (d)-1 processed image of (d) after a CF applied and background noise subtracted. (e) Average signal intensity of all samples (n = 3) was calculated based on the CFI system images. (f) Average radiant efficiency from the same samples is shown based on the IVIS-200 system images. High fluorescence signal from LCs was detected by the CFI system and confirmed by IVIS-200.