MRI contrast agent

MRI contrast agents are contrast agents used to improve the visibility of internal body structures in magnetic resonance imaging (MRI). The most commonly used compounds for contrast enhancement are gadolinium-based. Such MRI contrast agents shorten the relaxation times of nuclei within body tissues following oral or intravenous administration. In MRI scanners, sections of the body are exposed to a very strong magnetic field causing primarily the hydrogen nuclei ('spins') of water in tissues to be polarized in the direction of the magnetic field. An intense radiofrequency pulse is applied that tips the magnetization generated by the hydrogen nuclei in the direction of the receiver coil where the spin polarization can be detected. Random molecular rotational oscillations matching the resonance frequency of the nuclear spins provide the 'relaxation' mechanisms that bring the net magnetization back to its equilibrium position in alignment with the applied magnetic field. The magnitude of the spin polarization detected by the receiver is used to form the MR image but decays with a characteristic time constant known as the T1 relaxation time. Water protons in different tissues have different T1 values, which is one of the main sources of contrast in MR images. A contrast agent usually shortens, but in some instances increases, the value of T1 of nearby water protons thereby altering the contrast in the image. MRI contrast agents are contrast agents used to improve the visibility of internal body structures in magnetic resonance imaging (MRI). The most commonly used compounds for contrast enhancement are gadolinium-based. Such MRI contrast agents shorten the relaxation times of nuclei within body tissues following oral or intravenous administration. In MRI scanners, sections of the body are exposed to a very strong magnetic field causing primarily the hydrogen nuclei ('spins') of water in tissues to be polarized in the direction of the magnetic field. An intense radiofrequency pulse is applied that tips the magnetization generated by the hydrogen nuclei in the direction of the receiver coil where the spin polarization can be detected. Random molecular rotational oscillations matching the resonance frequency of the nuclear spins provide the 'relaxation' mechanisms that bring the net magnetization back to its equilibrium position in alignment with the applied magnetic field. The magnitude of the spin polarization detected by the receiver is used to form the MR image but decays with a characteristic time constant known as the T1 relaxation time. Water protons in different tissues have different T1 values, which is one of the main sources of contrast in MR images. A contrast agent usually shortens, but in some instances increases, the value of T1 of nearby water protons thereby altering the contrast in the image. Most clinically used MRI contrast agents work by shortening the T1 relaxation time of protons inside tissues via interactions with the nearby contrast agent. Thermally driven motion of the strongly paramagnetic metal ions in the contrast agent generate the oscillating magnetic fields that provide the relaxation mechanisms that enhance the rate of decay of the induced polarization. The systematic sampling of this polarization over the spatial region of the tissue being examined forms the basis for construction of the image. MRI contrast agents may be administered by injection into the blood stream or orally, depending on the subject of interest. Oral administration is well suited to G.I. tract scans, while intravascular administration proves more useful for most other scans. A variety of agents of both types enhances scans routinely. MRI contrast agents can be classified in many ways, including by their: Gadolinium(III) containing MRI contrast agents (often termed simply 'gado' or 'gad') are the most commonly used for enhancement of vessels in MR angiography or for brain tumor enhancement associated with the degradation of the blood–brain barrier. For large vessels such as the aorta and its branches, the gadolinium(III) dose can be as low as 0.1 mmol per kg body mass. Higher concentrations are often used for finer vasculature. Gd(III) chelates do not pass the intact blood–brain barrier because they are hydrophilic. Thus, these are useful in enhancing lesions and tumors where blood-brain barrier is compromised and the Gd(III) leaks out. In the rest of the body, the Gd(III) initially remains in the circulation but then distributes into the interstitial space or is eliminated by the kidneys. Gadolinium(III) contrast agents can be categorized into: As of July 2018, nine different types of gadolinium-containing contrast agents are available in different territories. In European countries, Gd chelated contrast agents approved by the European Medicines Agency (EMA) include: In the United States, Gd chelated contrast agents approved by the U.S. Food and Drug Administration (FDA) include: