Iodine-based radiographic contrast agents contain 127I, the only stable isotope in nature. 127I has a spin of 5/2, so it can undergo NMR if placed in a magnetic field. Its gyromagnetic ratio is approximately 8.52 MHz/T, meaning it resonates at about 1/5 the rate of an 1H nucleus. 127I also has a powerful quadrupolar moment, so its signal decays very quickly. In brief, an MR signal from iodine contrast could be recorded by a special laboratory NMR spectrometer, but would would not be detectable in a conventional MRI system.
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Notwithstanding the above discussion, the iodine-based contrast iopamidol has been investigated for potential use as an MR contrast agent. The iopamidol molecule is highly water-soluble with hydrophilic protons in three amide groups. These protons can undergo chemical exchange and decrease the signal intensity in free water through the so-called chemical exchange saturation transfer (CEST) mechanism. The CEST technique obtains images before and after application of a train of ~1000 Hz off-resonance saturation pulses, similar to those used in magnetization transfer (MT) experiments. By subtracting these images, subtle differences in image contrast may be observed. CEST contrast agents cause a decrease in T2 values, resulting in darkening of the areas of accumulation on T2-weighted images.
The potential use of iopamidol as a CEST MR contrast agent has nothing to do with the iodine moiety, just the amide protons. However, the presence of iodine raises the possibility that iopamidol might function as a dual-use contrast agent for both MR and CT.
Aime S, Calabi L, Biondi L, et al. Iopamidol: exploring the potential use of a well-established contrast agent for MRI. Magn Reson Med 2005; 53:830-834. (see Advanced Discussion).
Bloem JL, Wondergem J. Gd-DTPA as a contrast agent in CT. Radiology 1989; 171:578-579.
Quinn AD, O'Hare NJ, Wallis FJ, Wilson GF. Gd-DTPA: an alternative contrast medium for CT. J Comput Assist Tomogr 1994;18:634-636.
Why are most MR contrast agents based on the element gadolinium?
Is gadolinium contrast nephrotoxic? Can it be given safely to patients with mild renal insufficiency?