Tomographic fluorescence imaging of tumor vascular volume in mice

Purpose: To prospectively determine the feasibility of imaging vascular volume fraction ( VVF) and its therapeutic inhibition in mouse models of cancer with three-dimensional fluorescence molecular tomography ( FMT). Materials and Methods: All studies were approved by the institutional animal review committee and were in accordance with National Institutes of Health guidelines. CT26 colon tumor-bearing mice were imaged with FMT after intravenous administration of long-circulating near-infrared fluorescent bloodpool agents optimized for two nonoverlapping excitation wavelengths ( 680 and 750 nm). A total of 58 mice were used for imaging VVF to evaluate the following: ( a) differences in ectopically and orthotopically implanted tumors ( n = 10), ( b) cohorts of mice ( n = 24) treated with anti-vascular endothelial growth factor ( VEGF) antibody, ( c) serial imaging in same animal to determine natural course of angiogenesis ( n = 4), and ( d) dose response to anti-VEGF therapy ( n = 20). To compare groups receiving antiangiogenic chemotherapy, analysis of variance was used. Results: Fluorochrome concentrations derived from FMT measurements were reconstructed with an accuracy of +/- 10% at 680 nm and +/- 7% at 750 nm and in a depth-independent manner, unlike at reflectance imaging. FMT measurements of vascular fluorescent probes were linear, with concentration over several orders of magnitude ( r > 0.98). VVFs of colonic tumors, which varied considerably among animals ( 3.5% +/- 1.5 [ standard deviation]), could be depicted with in vivo imaging in three dimensions with less than 5 minutes of imaging and less than 3 minutes of analysis. The natural course of angiogenesis and its inhibition could be reliably imaged and depicted serially in different experimental setups. Conclusion: FMT is a tomographic optical imaging technique that, in conjunction with appropriate fluorescent probes, allows quantitative visualization of biologic processes.