SPECT-OPT multimodal imaging enables accurate evaluation of radiotracers for β-cell mass assessments

Abstract

Single Photon Emission Computed Tomography (SPECT) has become a promising experimental approach to monitor changes in β-cell mass (BCM) during diabetes progression. SPECT imaging of pancreatic islets is most commonly cross-validated by stereological analysis of histological pancreatic sections after insulin staining. Typically, stereological methods do not accurately determine the total β-cell volume, which is inconvenient when correlating total pancreatic tracer uptake with BCM. Alternative methods are therefore warranted to cross-validate β-cell imaging using radiotracers. In this study, we introduce multimodal SPECT - optical projection tomography (OPT) imaging as an accurate approach to cross-validate radionuclide-based imaging of β-cells. Uptake of a promising radiotracer for β-cell imaging by SPECT, (111)In-exendin-3, was measured by ex vivo-SPECT and cross evaluated by 3D quantitative OPT imaging as well as with histology within healthy and alloxan-treated Brown Norway rat pancreata. SPECT signal was in excellent linear correlation with OPT data as compared to histology. While histological determination of islet spatial distribution was challenging, SPECT and OPT revealed similar distribution patterns of (111)In-exendin-3 and insulin positive β-cell volumes between different pancreatic lobes, both visually and quantitatively. We propose ex vivo SPECT-OPT multimodal imaging as a highly accurate strategy for validating the performance of β-cell radiotracers.

Figure 1. Histological quantitative analysis of BCM correlates with ¹¹¹In-exendin-3 uptake.

(A,B) Representative images of insulin positive areas in control (A) and alloxan treated (B) rats. (C) Bar chart displaying BCM (mg) (n = 5). (D) Graph displaying quantitative analysis of β-cell inter-lobular distribution (n = 4–5). (E) BCM (mg) was plotted against ¹¹¹In-exendin-3 uptake (kBq, kilobecquerels) as determined by SPECT (r² = 0.52, p = 8.13 × 10⁻⁰⁶). In (A–E) untreated rats are shown in black and alloxan-treated rats are shown in white. Data are shown as means ± SEM, where *p < 0.05, **p < 0.01 and ***p < 0.001 compared with the corresponding healthy group. Scale bar represents 100 μm.

Figure 2. Multimodal imaging of pancreatic β-cells with SPECT and OPT.

(A,B) Ex vivo SPECT scans of representative splenic lobes from a healthy (A) and an alloxan treated animal (B) respectively. (C,D) OPT generated iso-surface images of the same lobes as visualized in (A,B). Alloxan-treated rats exhibit lower ¹¹¹In-exendin-3 uptake and β-cell volume when compared to the control group. Scale bar represents 3 mm.

Figure 3. SPECT based radioactive quantification of ¹¹¹In-exendin-3 uptake correlates with OPT based quantification of insulin positive β-cell volume.

(A,B) Quantification of absolute ¹¹¹In-exendin-3 uptake (kBq) by SPECT (A) and total β-cell volume (μm³) by OPT (B) (n = 5). (C,D) Graphs illustrating the heterogeneous distribution of β-cells between the splenic, gastric (GL) and duodenal lobes (DL) as determined by SPECT (C) and OPT (D) respectively (n = 4–5). (E) Graph showing pancreatic uptake of ¹¹¹In-exendin-3 by separate lobes plotted against β-cell volume shows a strong correlation (r² = 0.77, p = 2.07 × 10⁻¹⁰). In (A–E) untreated rats are shown in black and alloxan-treated rats are shown in white. Data are shown as means ± SEM, where *p < 0.05, **p < 0.01 and ***p < 0.001 compared with the corresponding healthy group.