Targeted, multimodality PET-CT and optical imaging platform for visualizing biological function.

In this paper, Nahrendorf et al. demonstrate a multi-modality imaging platform that combines the use of nuclear (positron emission tomography, PET), anatomic (computed tomography, CT), and optical (fluorescence-mediated tomography, FMT). The performance of this integrated imaging system was demonstrated with detection and localization of implanted colon cancer lesions in a mouse model. This study was performed using an imaging probe based on a dextran-coated (cross-linked iron oxide, CLIO) nanoparticle that accumulates preferentially in macrophages. These nanoparticles are dual-labeled with either fluorine-18 (18F) or copper-64 (64Cu), short half-life positron emitting radioisotopes, and with a far red fluorescent dye (VT680). First, linearity and the minimum detection thresholds of PET (Siemens Inveon) and optical (VisEn Medical FMT-2500) imaging over a range of concentrations for the imaging agents, 18F-CLIO-VT680 and 64Cu-CLIO-VT680, were evaluated using a phantom that consisted of serial dilutions of the probes in agar. Image z-stacks were collected and volume rendered to produce 3-dimensional images, and excellent correlation (r2 > 0.99) between the PET and optical signals was found. The PET and optical images were then converted into a standard (DICOM) format, and 3-dimensional surface rendered reconstructions of the PET and optical images were co-registered using imaging landmarks and fiducial markers. Congruence between these images in the xy-plane was demonstrated and quantified with a cross-correlation function (CCF) based on Pearson's coefficient. A maximum correlation coefficient of the 2-dimensional signal of 0.85 was found. The in vivo performance of this imaging platform was demonstrated by imaging the distribution of the 64Cu-CLIO-VT680 probe in a BALB/c mouse model using an autologous CT26 colon cancer implanted subcutaneously in the mouse flank. The mice were injected with the dual-labeled probe, and then studied with PET-CT and optical imaging 24 hours later. The tumors were clearly seen in the flank of the mice on both imaging modalities. After image co-registration, the fluorescence concentrations were shown to correlate with PET activity. In addition, co-localization of the PET and optical signals in the xy-plane was demonstrated in vivo. The functional capability of this approach was then further demonstrated by visualizing multiple molecular markers in vivo simultaneously. Tumor-bearing mice were injected intravenously with 3 optical probes: 1) an RGD (arginine–glycine–aspartic acid) peptide that binds to αVβ3 integrin (excitation 635 nm); 2) a nanoparticle, described above, that targets tumor associated macrophages (excitation 680 nm); and 3) a cathepsin that is sensitive to over expressed proteases (excitation 750 nm). The tumor from the 3 spectrally resolved datasets collected from separate optical channels was co-localized with the PET-CT images. However, on higher magnification, each imaging agent was found to have a unique distribution within the immediate vicinity of the tumor. The integrin signal was found to be highest at the margins and the cathepsin and macrophage activity was greater within the bulk lesion. These findings were validated on excised tumor in vitro with microscopy and autoradiography.