![]() ![]() Almost all of these agents are excreted through the kidneys without any significant metabolic change in the body. They are also helpful in dynamic imaging to assess the preferential enhancement of certain pathologic processes, such as hepatocellular carcinoma or renal cell carcinoma. Thus, these agents are suitable for imaging disease processes that disrupt the blood-brain barrier, such as cerebral tumors or cerebral infectious processes. They do not cross the normal blood-brain barrier. These complexes are of low molecular weight and weakly bind to proteins and thus circulate within the intravascular space for a very short period of time before leaking into the extracellular space. The reduction in the T1 relaxation time is greater at low gadolinium concentrations than is routinely achieved clinically this is seen as increased signal intensity on T1-weighted spin-echo or gradient-echo images. This strong paramagnetic effect disturbs the relaxivity of nearby water protons, resulting in a decrease of both T1 and T2 relaxation times. ![]() Gadolinium is a paramagnetic agent because of its seven unpaired electrons. They are classified into four groups depending on the charge (ionic or nonionic) and the biochemical structure of the complex (macrocyclic or linear) (2) (Table 1). Essentially all these agents have gadolinium (a paramagnetic metal in the lanthanide series) as the active component attached to various complexes that determine their charge, pharmacokinetics, biodistribution, and toxicity. These are the first-generation MR contrast agents. Nonspecific extracellular MR contrast agents The non-tissue-specific contrast agents have an initial short intravascular distribution and then distribute in the extracellular space throughout the body without any selective accumulation in an organ. As the name implies, tissue-specific contrast agents are targeted at a specific tissue type (eg, reticuloendothelial cell-specific or hepatocyte-specific) or organ (eg, blood-pool agents for the vascular system or enteral agents for bowel). MR contrast agents are primarily classified into two groups based on their organ specificity-tissue-specific contrast agents and non-tissue-specific contrast agents. In this article, we review various MR contrast agents in clinical use and in development and their current and potential applications. ![]() The resulting capability for shorter MR image acquisition times can allow functional studies, such as MR perfusion, to be performed with better time resolution. Contrast agents may also find further novel applications at high field strengths (>1.5T). In addition, the contrast agents are used for depicting normal and abnormal vasculature or flow-related abnormalities and pathophysiologic processes like perfusion. These contrast agents are used primarily to increase the sensitivity of MR for detecting various pathological processes and also for characterizing various pathologies. (1) MR contrast agents contain paramagnetic or superparamagnetic metal ions that affect the MR signal properties of surrounding tissue. Today, 40% to 50% of all MR examinations use contrast agents. Since the introduction of the first MR contrast agent in 1988, there has been a tremendous increase in the number of contrast-enhanced examinations. Magnetic resonance (MR) contrast agents have made a significant impact in the use of MR imaging for various clinical indications. ![]()
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