Gadolinium for CT

Is iodine contrast visible on MRI?
Is gadolinium contrast visible on CT?  

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.

Gadolinium, like iodine, is a heavy metal capable of attenuating x-rays. The atomic number of gadolinium (Z=64) is higher than that of iodine (Z=53). The k-edge of gadolinium is also more closely matched to the peak of the CT spectrum, meaning gadolinium absorbs a greater fraction of the x-ray beam than does iodine. However, the lower concentration and total number of gadolinium atoms administered in current formulations means that gadolinium contrast has a much lower visibility on CT than iodine contrast.

From the early 1990's to the mid-2000's, gadolinium contrast was occasionally used in CT and angiography in patients with severe allergies to iodinated contrast. An example from our institution is shown below. With the recognition that high doses of gadolinium might precipitate nephrogenic systemic fibrosis, this technique was abandoned but still has some historical interest. Even today, a trace amount of contrast enhancement in the bladder or renal collecting systems may be noted in patients receiving a CT scan shortly after a gadolinium-enhanced MR study.

High-dose Gd seen on CT. On left, noncontrast head image. On right CT obtained after intravenous administration of 60 cc of gadopentetate dimeglumine (Magnevist) shows weak enhancement of choroid plexus and vessels.

Advanced Discussion (show/hide)»

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.

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References
     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.

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Related Questions
     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? 

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