| Compared with other medical imaging modalities, ultrasound imaging has been widely used in diagnosis and clinical applications owning to its merits of low-cost, real-time, high safety, and no documented side effects. By using proper contrast agents, the contrast and sensitivity of ultrasound imaging have been greatly improved. However, ultrasound imaging is still limited because of the following reasons. Firstly, ultrasound is reflected very strongly when passing from tissue to gas, and vice versa. Secondly, the method is of limited use in diagnosing fractures because ultrasound does not pass well through bones. Finally, the quality of ultrasound imaging is mediocre when its contrast is lower than that of MRI or computed tomography (CT).Magnetic resonance imaging is another imaging tool which is non-invasive and capable of providing functional information with high spatial resolution and excellent soft-tissue contrast. In particular, MRI can provide information about blood flow and vessel morphology and identify stenotic arteries for early treatment. Magnetic iron oxide nanoparticles with superparamagnetic property can be used as a powerful contrast agent for MRI to further increase its brightness and contrast. One shortcoming of MRI is that it can not provide real-time motion-related images. MRI can be affected by movement, making it unsuitable for investigating problems such as mouth tumors because coughing or swallowing can make the images less clear.To sum up, different imaging modalities have their respective advantages and disadvantages in the spatial resolution, and no single imaging modality possesses all the advantages satisfying the need of all clinical applications. In many cases, US image and MRI are complimentary, and both modalities are needed to discern possible pathological changes in tissue. Therefore, it is extremely desirable to fuse the image information of different modes. To fuse US and MRI together, US-MRI registration is required. Due to US image’s strong noise, it is a great challenge to register US image with any other modality images. Some studies focused on three-dimensional US-MRI registration or three-dimensional US-CT registration in the field of operation navigation. Whether feature-based or voxel-based registration is used, segmentation of multimodality medical images is required. The registration result depends directly on the segmentation results. It is fair to say that US image segmentation is a difficult issue at present.(1) With the help of dual modality contrast agent, a new registration scheme between ultrasound image and magnetic resonance image is presented in this paper. Dual modality contrast agent is used in both ultrasound and MRI imaging process to enhance and preserve most important information of interest. The spatial transformation model between ultrasound image and magnetic resonance image is built by algorithm. And both physical spatial transformation and intensity variation between the two modality images have been taken into consideration. With the use of multi-scale pyramid method, the proposed algorithm performs well not only for global deformation but for local deformation. Based on the above-mentioned scheme, the experiment also demonstrates that ultrasound-MRI registration may be a promising method for obtaining more accurate image information.(2) MMBs were used in both ultrasound and magnetic resonance imaging process to enhance the most important information of interest. In order to reduce the influence of the speckle noise to registration, semi-automatic segmentations of US image and MRI were carried out by using active contour model. After that, a robust optical flow model between US image segmentation (floating image) and MRI segmentation (reference image) was built, and the vector flow field was estimated by using the Coarse-to-fine Gaussian pyramid and graduated non-convexity (GNC) schemes. Qualitative and quantitative analyses of multiple group comparison experiments showed that registration results using all methods tested in this paper without MMBs were unsatisfactory. On the contrary, the robust optical flow model combined with MMBs led to the best registration results.(3) With the help of magnetic microbubble contrast agent, a new fusion scheme between ultrasound image and magnetic resonance image is presented. Magnetic microbubble contrast agent is used in both ultrasound and MRI imaging process to enhance and preserve most important information of interest. By using nonsubsampled contourlet transform (NSCT), source images are decomposed. Then, an self-adaptive algorithm of dynamic weighted non-negative matrix factorization(DWNMF) is utilized to complete the fusion of low-frequency subband coefficients, the fusion course of bandpass directional subband coefficients can be realized by use of the model of spatial frequency-motivated pulse coupled neural networks. Finally, the ultimate fused image can be obtained with inverse NSCT. Based on the above-mentioned scheme, the experiment demonstrates that ultrasound-MRI fusion may be a promising method for obtaining high quality fusion image.(4) In order to apply the registration and fusion algorithm for clinical tumor diagnosis, the paper carries out registration and fusion based on US imaging and MRI of nude mouse. Firstly, nude mouse is used for animal model, and human tumor model is established in the subcutaneous of the nude mouse. Secondly, US imaging and MRI of the tumor model of nude mouse are carries out with and without MMBs. The experiment demonstrates that with MMBs US-MRI registration and fusion of nude mouse is feasible.For the first time, in the paper the dual modality contrast agent is introduced into the field of image registration and fusion. Using the same contrast agent for both US image and MRI would not only bring convenience to medical professionals, but also reduced side effects and health care cost. Our algorithm implementation was intensity-based and was independent of the metric used. Therefore, it can be adapted to different image modalities. |
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