Techniques and Applications of Multi-modality Small Animal Imaging in Cancer Research

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Small-animal models are used extensively in disease research, genomics research, drug development, and developmental biology. The development of noninvasive small-animal imaging techniques with adequate spatial resolution and sensitivity is therefore of prime importance. In particular, multimodality small-animal imaging can provide complementary information. This paper presents a method for registering high-frequency ultrasonic (microUS) images with small-animal positron-emission tomography (microPET) images. Registration is performed using six external multimodality markers, each being a glass bead with a diameter of 0.43–0.60 mm, with 0.1 μl of [18F]FDG placed in each marker holder. A small-animal holder is used to transfer mice between the microPET and microUS systems. Multimodality imaging was performed on C57BL/6J black mice bearing WF-3 ovary cancer cells in the second week after tumor implantation, and rigid-body image registration of the six markers was also performed. The average registration error was 0.31 mm when all six markers were used, and increased as the number of markers decreased. After image registration, image segmentation and fusion are performed on the tumor. Our multimodality small-animal imaging method allows structural information from microUS to be combined with functional information from microPET, with the preliminary results showing it to be an effective tool for cancer research. In this study, we used a microUS system that we developed in-house as an alternative method for tumor growth calipers. In addition, microUS was combined with small-animal positron-emission tomography (microPET) for tumor metastatic assessment. MicroUS provides anatomical information that can be used for tumor volume measurements while microPET is a functional imaging method with positron-emitting radiophamaceuticals, such as 18F-labeled deoxyglucose, [18F]FDG. In this study, microUS and microPET were performed in a mouse tumor longitudinal study (2-8 weeks), both with 3D tumor segmentation and volume measurements. The average tumor volume doubling time as determined during the exponential phase was 7.46 days by microUS. MicroUS and microPET are complementary to each other as microUS has superior spatial resolution and microPET provides functional information such as hypoxia or necrosis in the progression of the tumor. With image registration and fusion, the combination can be a valuable tool for cancer research. To investigate the feasibility of the functional information which provided from microUS, we used the contrast enhanced ultrasound (CEUS) techniques to characterize liver focal lesions and detect three vascular contrast phases in Hepatitis B virus X (HBx) transgenic mice. Specifically, high-frequency ultrasound liver imaging with albumin-shelled microbubbles was employed to detect three vascular contrast phases and characterize focal liver lesions that developed in thirteen HBx transgenic mice at around 14 to 16 months of age. In the thirteen mice, the arterial phase ranges from 2 to 60 seconds post contrast injection. The time period from 10 to 30 minutes post contrast injection was defined as the parenchyma phase in this study. Comparing the imaging and the pathology results, the sensitivity, specificity and accuracy of CEUS for the detection of malignant focal liver lesion in HBx transgenic mice were 91%, 100% and 92%. To characterize the features of the focal liver lesion and detect the three vascular contrast phases of malignant focal liver lesions, the results were arranged according to the guidelines of European Federation of Societies for Ultrasound in Medicne and Biology. Histopathology investigations confirmed the development of the lesion in these thirteen mice. Finally, we propose to use a novel technique, called the ensemble empirical mode decomposition (EEMD) for contrast nonlinear imaging, to improve the contrast in CEUS imaging. Compared with the results based on the traditional nonlinear imaging technique, the new approach obtains improved performance for tissue components removal from the mixed signals effectively and objectively, and provides us with more accurate contrast nonlinear signals. These results demonstrated that high-frequency CEUS imaging is potentially for characterizing malignant focal liver lesions in mice and is valuable to provide functional information for preclinical study. The CEUS technique can combine with microPET imaging in the future. The combing methods of microUS and microPET multimodality imaging systems could be extended and other imaging modalities (ex: MRI, in vivo bioluminescent imaging, in vivo fluorescent imaging, autoradiography) integrate into these new techniques. The homemade microbubbles could be constructed as a multimodality contrast agent. A multiplicity of ligands may be coupled to microbubbles directly via covalent bonds or indirectly through avidin-biotin interactions. Ultrasonically reflective particles can be complexed to paramagnetics for MR or radionuclide for nuclear or D-luciferin for bioluminescent or fluorescence for microscope multimodal imaging. The new technique provides an alternative method for cancer research in small animal.