For a water-soluble polyethylene glycol (PEG) magnetic resonance imaging (MRI) contrast agent, it has been demonstrated that the contrast efficiency was increased with increased branched structure of the contrast agent. However, the cause of enhanced contrast efficiency by the branched structure has not been clarified. Hence, we investigate the cause of the contrast agent enhancement by changing the Gd introduction ratio of the eight-arm PEG from 1.97 to 4.07; furthermore, the terminal mobility of the contrast agents with different structures was evaluated using proton nuclear magnetic resonance (1H-NMR) spectroscopy. It was shown that the relaxivity and contrast luminance of the synthesized branched PEG-Gd contrast agents are larger than those of linear PEG-Gd and commercially available contrast agents. Additionally, the change in the Gd introduction ratio did not affect the contrast efficiency. The terminal mobility results measured by NMR show that the linewidth at half height became broader with an increased number of branches, implying that the mobility of branched PEG-Gd is slower than that of linear PEG-Gd. Interestingly, the linewidth at half height of different structures did not change in an organic solvent; this phenomenon appeared specifically in water. It is suggested that the stable branched structure enabled the improvement in the relaxivity and contrast luminance.
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