Multimodality Imaging of Tumor Xenografts and Metastases in Mice with Combined Small-Animal PET, Small-Animal CT, and Bioluminescence Imaging

Systemic metastasis is the primary cause of mortality for most cancer types and represents a major therapeutic challenge in oncology. Determination of the presence and extent of metastasis is a cornerstone of diagnosis in oncology. PET has been playing an increasingly important role in this staging process, largely because of improved detection of lymph node and systemic metastases with 18F-FDG (1), and the introduction of combined clinical PET/CT scanners has further strengthened this role. The biologic mechanism of metastasis has become better understood through the study of the migration and seeding of tumoral cells, tumor–stroma interactions, vascularization of tumors, genetic mouse models, and gene expression and proteomic patterns that correlate with metastasis. These insights have highlighted the need for more realistic models of tumor xenografts that suffer from altered vascularization, have no relevant tumor–stroma interactions, and often rely on immunodeficient mice (2). Recently, our group and others have made progress in adapting whole-body imaging techniques to small animals so that tumor burden can be visualized and quantified in a single animal over time. For this purpose, we have developed reporter gene systems for PET (3) and for bioluminescence imaging (BLI) (4). We have used these systems to study the role of specific genes in cancer progression (5), to study viral vector targeting of metastasis (6), and to noninvasively monitor novel treatments for melanoma metastases (7,8). We have also developed a trimodality fusion reporter gene that allows detection of gene expression by fluorescence, BLI, and PET (9). For the latter, the reporter probe used is 9-[4-18F-fluoro-3-(hydroxymethyl)butyl]guanine (18F-FHBG), which is phosphorylated by the reporter gene product, a mutant herpes simplex virus type 1 thymidine kinase (HSV1-tk) enzyme. Recently developed small-animal CT systems (10,11) allow imaging of the anatomy of living mice with a high spatial resolution of 50–200 μm based on differential x-ray absorption by different tissues. Tissue contrast depends mainly on differences in tissue density and on the presence of mass amounts of contrast agents, providing limited if any molecular information. Hence, most current applications of small-animal CT have focused on specific tissues, such as bone (11), that can be well depicted because of their favorable contrast properties or tissues that have required the use of specific contrast agents (12). We have previously used small-animal CT to study a localized model of prostate cancer bone metastasis in the tibia of mice (13). The high specificity of PET tracers for their molecular targets provides few anatomic landmarks—a disadvantage in studies in which the origin of the signal is not known, such as cell and metastasis trafficking studies. To improve this weakness of PET, we investigated the use of small-animal PET and small-animal CT as a multimodality imaging method allowing visualization of molecular information within an adequate anatomic framework. We used BLI to validate our findings and to compare combined small-animal PET/CT with BLI within the same animal. Our study was a preliminary assessment of combined PET/CT with the following specific goals: establishment of a methodology to obtain coregistered small-animal PET and small-animal CT images, validation of these fused PET/CT images in tumor xenografts, evaluation of fused PET/CT in the detection of pulmonary melanoma metastases in a mouse model with 18F-FDG and 18F-FHBG (intravenously administered), evaluation of fused PET/CT in the detection of pulmonary melanoma metastases in a mouse model with 18F-FDG and 18F-FHBG, and comparison of this result to BLI metastasis detection.