Macromolecular Crowding May Significantly Affect the Performance of an MRI Contrast Agent: A ¹H NMR Spectroscopy, Microimaging, and Fast‐Field‐Cycling NMR Relaxometry Study

Abstract

Contrast enhancement agents are often employed in magnetic resonance imaging (MRI) for clinical diagnosis and biomedical research. However, the current theory on MRI contrast generation does not consider the ubiquitous presence of macromolecular crowders in biological systems, which poses the risk of inaccurate data interpretation and misdiagnosis. To address this issue, herein the macromolecular crowding effects on MRI contrast agent are investigated with the 1H relaxation rate of water in aqueous solutions of Dotarem with different concentrations of macromolecules. Two representative macromolecular crowder systems are used: polyethylene glycol (with no specific secondary structure) and bovine serum albumin (with compact secondary and tertiary structures). The water 1H relaxation rates in various solutions are measured in a fixed magnetic field and in variable magnetic fields. The results show significant crowding effects for both crowders. The relaxation rate is proportional to the concentration of the MRI contrast agent but shows conspicuous superlinearity with respect to the concentration of the crowder. The size of polyethylene glycol does not affect the relaxivity of water in Dotarem solutions. The above effects are verified with T 1‐ and T 2‐weighted NMR microimages. These results highlight the importance of the effect of macromolecular crowding on the MRI contrast agent and are valuable for understanding the mechanism of MRI contrast agents and designing new‐generation MRI contrast agents.

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“…Some representative results of the longitudinal relaxation times are shown in the right four columns of Table . The estimated relaxivities from above data, which increase with the concentration of the crowder molecules (PEG), are r 1 = 4.0–80 s –1 mM –1 , significantly increasing the maximum longitudinal relaxivity in the absence of crowder molecules . This implies that the contrast agent has higher relaxivities in real cellular environments where extra macromolecules abound.…”

“…The transverse and longitudinal relaxivities measured from both NMR relaxation and micro-MRI images are found to increase with the concentration of PBMP17 (r 1 = 4.0−40 s −1 mM −1 , r 2 = 1.2−10.0 s −1 mM −1 , depending on the concentration of the contrast agent and the medium 39,40 ). The detailed relaxivity data are summarized in Tables 1 and 2 (we list both linewidth and T 2 because the contribution from magnetic inhomogeneity to linewidth may depend on the concentration of the contrast agent).…”

“…The estimated relaxivities from above data, which increase with the concentration of the crowder molecules (PEG), are r 1 = 4.0−80 s −1 mM −1 , significantly increasing the maximum longitudinal relaxivity in the absence of crowder molecules. 40 This implies that the contrast agent has higher relaxivities in real cellular environments where extra macromolecules abound. The crowder-enhanced relaxivity is also confirmed in the following image experiments in real biological environments.…”