Magnetic resonance imaging (MRI) has been applied to many aspects of functional and molecular imaging. Many of the parameters used to produce image contrast in MRI are influenced by the local chemical environment around the atoms being imaged; these parameters can be exploited to probe the molecular content of tissues and this has been shown to have many applications in radiology. Diffusion-weighted imaging is a well-established method for measuring small changes in the molecular movement of water that occurs following the onset of ischaemia and in the presence of tumours. Exogenous contrast agents containing gadolinium or iron oxide have been used to image tissue vascularity, cell migration, and specific biological processes, such as cell death. MR spectroscopy is a technique for measuring the concentrations of tissue metabolites and this has been used to probe metabolic pathways in cancer, in cardiac tissue, and in the brain. Several groups are developing positron-emission tomography (PET)-MRI systems that combine the spatial resolution of MRI with the metabolic sensitivity of PET. However, the application of MRI to functional and molecular imaging is limited by its intrinsic low sensitivity. A number of techniques have been developed to overcome this which utilize a phenomenon termed hyperpolarization; these have been used to image tissue pH, cellular necrosis, and to image the lungs. Although most of these applications have been developed in animal models, they are increasingly being translated into human imaging and some are used routinely in many radiology departments.
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