The suitability of paramagnetic complexes as contrast agents depends not only on their relaxivity but also on their stability and inertness towards transmetallation processes by endogenous ions. In this work, we describe a convenient method to study the stability of paramagnetic Gd complexes through the evolution of the paramagnetic longitudinal relaxation rate of water protons at 37 °C.

Paramagnetic magnetic resonance imaging (MRI) contrast agents are the compounds most used in clinical studies. Among these, the derivatives of Gd‐DTPA (gadoliniumdiethylenetriaminepentaacetic acid, Magnevist®, Bayer HealthCare) have been extensively studied. The first part of this work consists of a comparison of various derivatives of Gd‐DTPA [mono‐, bis‐ or pentaamides; C‐functionalised (C2, C4, C5, N6) or diester derivatives] according to their synthesis and to the most important parameters that regulate their efficacy. The synthesis of bis(amide)s consists of only one step and is thus easier than the synthesis of the C4 derivatives, which needs several steps and requires a tedious purification. These last compounds are, however, more stable against transmetallation with another ion such as zinc [more than 50 % of the gadolinium complex remains after 4300 min while this percentage falls under 40 % for most of the bis(amide) compounds] and generally have higher relaxivities. Moreover, the residence time of the water molecule in the first coordination sphere of the gadolinium complex (τM) is shorter for the C4 derivatives and does not limit the relaxivity. In the second part of this work, an overview of different kinds of recently developed paramagnetic contrast agents is presented: new systems to improve the relaxivity, like fullerenol compounds, apoferritin or zeolite complexes, lipophilic complexes (in structures like micelles or liposomes), macromolecular products as well as small complexes interacting with human serum albumin (HSA).

Knowledge of the longitudinal and transverse relaxation times (T(1) and T(2)) of water protons in an aqueous solution of an MRI contrast agent is essential for its characterization. These parameters can be measured at low field on low resolution spectrometers or at high field on high resolution spectrometers. The reliability and the accuracy of T(1) and T(2) measurements rely on several experimental settings and on the equation used to fit the data. Examples of the importance of careful adjustment of the most important parameters are illustrated through several measurements performed on a low-resolution, low-magnetic field instrument. In addition, some specificities of T(1) and T(2) measurements on high-resolution, high-magnetic field spectrometers are pointed out.

The questionable mechanism initially proposed to explain how 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) allows us to ring-open β-lactones, such as β-butyrolactone (BL), is reinvestigated here.

Lanthanides, holmium(iii), dysprosium(iii), and terbium(iii), were coordinated to an amphiphilic DOTA bis-coumarin derivative and then further assembled with an amphiphilic europium(iii) DTPA bis-coumarin derivative into mono-disperse micelles. The self-assembled micelles were characterized and assessed for their potential as bimodal contrast agents for high field magnetic resonance and optical imaging applications. All micelles showed a high transverse relaxation (r) of 46, 34, and 30 s mM at 500 MHz and 37 °C for Dy(iii), Ho(iii) and Tb(iii), respectively, which is a result of the high magnetic moment of these lanthanides and the long rotational correlation time of the micelles. The quantum yield in aqueous solution ranged from 1.8% for Tb/Eu to 1.4% for Dy/Eu and 1.0% for the Ho/Eu micelles. Multi-photon excited emission spectroscopy has shown that due to the two-photon absorption of the coumarin chromophore the characteristic Eu(iii) emission could be observed upon excitation at 800 nm, demonstrating the usefulness of the system for in vivo fluorescence imaging applications. To the best of our knowledge, this is the first example reporting the potential of a holmium(iii) chelate as a negative MRI contrast agent.