Individual in vivo imaging modalities have relative strengths (i.e. sensitivity, resolution, quantitative accuracy, throughput capabilities) for molecular, anatomical and physiological detections in preclinical and clinical oncology (Keunen et al., 2014, James & Gambhir, 2012). Fluorescence (FLI) and bioluminescence imaging (BLI), Positron Emission Tomography (PET), and Single-Photon Emission Computed Tomography (SPECT) provide functional information while X-ray Computed Tomography (CT), Magnetic Resonance Imaging (MRI), and UltraSound (US) provide mostly anatomical information. BLI offers high throughput and highly sensitive detections (Müller et al., 2013). PET and SPECT are quantitative and can allow for dynamic imaging. Imaging techniques can be combined to leverage the strengths of multiple modalities. Most frequently, a functional imaging modality is performed in sequence with an anatomical modality to obtain an anatomical reference. Combined and system-integrated SPECT/CT and PET/CT is now common in the preclinical imaging field. There is currently a growing interest in combining functional imaging with MRI (Sauter et al., 2010). This is driven in part by a desire to avoid the specimen dose associated with CT imaging. Additionally, MRI provides superior soft tissue contrast and the ability to perform advanced MRI techniques such as diffusion weighted imaging (DWI) that can be leveraged in oncology studies (Preuss et al., 2014). Multiple functional imaging modalities may also be performed on the same sample. For example, BLI is highly sensitive and may be employed to monitor tumor development from the early stages of development. This may be combined with highly quantitative PET imaging in studies of therapeutic response. A full discussion of the merits of the various imaging modalities is available in the recent reviews by de Jong et al. (2014), Albanese et al. (2013), and James & Gambhir (2012).
For SPECT imaging, Foxn1nu mice (Jackson Laboratories; Bar Harbor, ME) were grafted subcutaneous with 700,000 HCT 116-hNIS-NEO (Imanis Life Science; Rochester, MN) tumor cells expressing the human sodium iodide symporter. Imaging was performed at 2-3 weeks post implantation when tumors reached approximately 1-3 mm diameter. Foxn1nu mice receiving no cell grafts or treatments were used for this initial validation of cross-platform 18FDG-PET/MR imaging.
PET and SPECT acquisitions were collected using the Bruker Albira PET/SPECT/CT imaging system. For PET and SPECT imaging mice received between 100 and 200 μCi 18F-FDG and 500 μCi and 1 mCi 99mTc respectively, one hour prior to imaging. MR imaging was performed using the Bruker ICON 1T MR system using a 3D MR T2-weighted RARE sequence.
Multimodal Animal Bed and Fiducial Marker Lid
The Bruker Multimodal Animal Bed (MMAB) was used for transferring animals between the Bruker Albira PET/SPECT/CT imaging system and the Bruker ICON 1T MR imaging system. The fiducial markers lid (FML) was designed with a 1 mL circuitous fill line. The line extends the length of the bed. This design provides multiple reference points for fiducial matching. The fill line was fit with threaded inlet ports to facilitate filling. The FML was printed using the Projet printer using Somos WaterClear Ultra 10122.
Image registration was performed using PMOD Technologies (Zurich, Switzerland) Fuse It module, provided standard with the Albira PET/SPECT/CT imaging systems. This module allows for dual image plane adjustments including rotations, X, Y, Z movements, and image re-scaling. We used manual registration of the FML line between cross-platform images.
During initial cross-platform studies made in the absence of fiducial marker solutions we found post imaging registration to be subjective, relying largely on anatomical/physiological landmarks. Fiducial
capillary tubes filled with mixed contrast agents are frequently used to facilitate cross-platform imaging and we later employed capillary tubes filled with contrast agents. Filling and placement of such tubes and secure placement within the confines of an immobilization bed add additional complications to the study preparation process. To facilitate more streamlined fiducial matching we designed a FML (Figure 1) that is compatible with the Bruker MMABs. The FML line can be filled easily using a fill syringe and Luer adapter.
Additionally, the FML lid conforms to the general mold of the Bruker MMAB factory lid and connects similarly to the MMAB bed base (Figure 2). The FML line extends the length of the bed so it can be
utilized for any anatomical region.