BIODISTRIBUTION AND TOXICITY OF MR IMAGING CONTRAST-MEDIA

SIGNAL INTENSITY on a magnetic resonance image is mainly determined by proton density and T1 and T2. Image contrast, described as the difference in signal intensity between two tissues, is dependent on a number of physical parameters and can be altered by changing these parameters. Contrast agents are pharmaceuticals that can accentuate the difference in signal intensity between two adjacent tissue compartments by directly interfering with the proton relaxation times in one more than in the other. Such agents are designed with the purpose of improving image contrast between normal and abnormal tissues and may enhance the contrast positively or negatively, depending on their relative T1, T2, or T2* effects. Effective MR imaging contrast agents include paramagnetic (metal ions such as gadolinium. manganese, dysprosium, iron; free radicals; nitrous oxides), magnetic (superparamagnetic or ferromagnetic iron oxide particles), or diamagnetic (substances that displace hydrogen nuclei such as gases, deuterated water, perfluorocarbons) materials. The clinical effect of an MR imaging contrast medium is determined not only by the contrast-enhancing properties of the compound but also by its pharmacokinetic properties, which determine where the agent is distributed in the body and the length of time adequate concentrations are maintained in target organs. Furthermore, it is of crucial importance that the drug be well tolerated when given in doses necessary to achieve the clinical effect.