MRI is emerging as a particularly advantageous modality for MI because of its high spatial resolution (when compared with PET and SPECT), very good sample penetration (when compared with optical imaging methods), it's widespread clinical availability, and lack of ionizing radiation. A tremendous drawback, however, is its low sensitivity compared with these other methods, which necessitates the development ofpowerful amplification strategies. MR contrast agents that could serve as potential bases for molecular imaging probes are in clinical use but specific derivatives for molecular imaging have not yet been approved (23). In general, MR contrast agents are designed to shorten the relaxation times of the tissue of interest and therefore increase the relaxation rates (37). There are two major classes of MR contrast agents; paramagnetic contrast agents which are designed to predominantly affect T1 and thus tend to provide increased MR signal and superparamagnetic agents and agents designed to predominantly shorten T2 and T2* relaxation times and thereby decrease signal on appropriately weighted MR imaging sequences. In both cases, the potency of a contrast agent is commonly expressed as relaxivity and is typically measured relative to the paramagnetic or superparamagnetic ion concentration, such as gadolinium or iron (with units of mmol/ s). For targeted contrast agents, however, the relaxivity per particle is more useful for comparing the contrast agent effect per binding site. In current development, there are two main strategies for MRI-suitable contrast media for molecular imaging; those based on iron oxide and those using nanoparticles or emulsions which incorporate gadolinium ions. These will be briefly elaborated explained below.
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