Nanoparticles in magnetic resonance imaging: from simple to dual contrast agents

Abstract

Magnetic resonance imaging (MRI) has become one of the most widely used and powerful tools for noninvasive clinical diagnosis owing to its high degree of soft tissue contrast, spatial resolution, and depth of penetration. MRI signal intensity is related to the relaxation times ( T 1 , spin–lattice relaxation and T 2 , spin–spin relaxation) of in vivo water protons. To increase contrast, various inorganic nanoparticles and complexes (the so-called contrast agents) are administered prior to the scanning. Shortening T 1 and T 2 increases the corresponding relaxation rates, 1/ T 1 and 1/ T 2 , producing hyperintense and hypointense signals respectively in shorter times. Moreover, the signal-to-noise ratio can be improved with the acquisition of a large number of measurements. The contrast agents used are generally based on either iron oxide nanoparticles or ferrites, providing negative contrast in T 2 -weighted images; or complexes of lanthanide metals (mostly containing gadolinium ions), providing positive contrast in T 1 -weighted images. Recently, lanthanide complexes have been immobilized in nanostructured materials in order to develop a new class of contrast agents with functions including blood-pool and organ (or tumor) targeting. Meanwhile, to overcome the limitations of individual imaging modalities, multimodal imaging techniques have been developed. An important challenge is to design all-in-one contrast agents that can be detected by multimodal techniques. Magnetoliposomes are efficient multimodal contrast agents. They can simultaneously bear both kinds of contrast and can, furthermore, incorporate targeting ligands and chains of polyethylene glycol to enhance the accumulation of nanoparticles at the site of interest and the bioavailability, respectively. Here, we review the most important characteristics of the nanoparticles or complexes used as MRI contrast agents.

Bioimaging predictors of rilpivirine biodistribution and antiretroviral activities

“…MRI pre-scans were taken before injection of EuCF-RPV (G4) or EuCF-PCL plus NRPV (G5) to establish a baseline signal and representative pre-scans are shown in Fig.5B. MRI was chosen as the first modality because of its superior spatial resolution, which provides the most accurate information about the localization of these particles in soft tissues[43]. Post-injection MR images were acquired 2 and 5 days after particle intravenous administration (Fig.5B).…”

Synthesis, characterization, applications, and challenges of iron oxide nanoparticles

“…There is a need to formulate standard procedures to control and compare different NPs in terms of cytotoxic effects and uptake efficiency. The effects of iron oxide NPs should be carefully evaluated on cultured cells prior to assessing their clinical potential in cell transplantation research 27,48,99,113…”

Nanoparticles for imaging and treatment of metastatic breast cancer

“…NP-based contrast agents for MRI have advantages over conventional contrast agents due to improved sensitivity and prolonged blood circulation time. [35] Qualitative and quantitative information on NP distribution could be obtained from MRI generated transverse ( R 2 or 1/ T 2 ) or longitudinal relaxation rates ( R 1 or 1/ T 1 ) maps with the assumption that the changes in relaxation rates are dominated by the changes in NP concentration. [36, 37] In one study, the distribution of Gd 3+ conjugated polymeric NPs in brains showed enhanced T 1 signal.…”