Reversible pH-responsive MRI contrast with paramagnetic polymer micelles

Paramagnetically-doped polymer micelles, containing an ionizable poly(acrylic acid) (PAA) block, support high-contrast MR imaging at clinically relevant field strengths in a manner that is strongly pH responsive. A reversible switch in polymer strand charge specifically has a direct impact on local rigidity, and rotational correlation time characteristics, of the integrated Gd-chelate, driving a ∼50% amplitude switch in positive contrast.

Stimulus-Responsive Nanoparticle Magnetic Resonance Imaging Contrast Agents: Design Considerations and Applications

Magnetic resonance imaging (MRI) has been widely used for disease diagnosis because it can noninvasively obtain anatomical details of various diseases through accurate contrast between soft tissues. Over one-third of MRI examinations are performed with the assistance of contrast agents. Traditional contrast agents typically display an unchanging signal, thus exhibiting relatively low sensitivity and poor specificity. Currently, advances in stimulus-responsive contrast agents which can alter the relaxation signal in response to a specific change in their surrounding environment provide new opportunities to overcome such limitation. The signal changes based on stimulus also reflects the physiological and pathological conditions of the site of interests. In this review, how to design stimulus-responsive nanoparticle MRI contrast agents from the perspective of theory and surface design is comprehensively discussed. Key structural features including size, clusters, shell features, and surface properties are used for tuning the T₁ and T₂ relaxation properties. The reversible or non-reversible signal changes highlight the contrast agents have undergone structural changes based on certain stimulus, as an indication for disease diagnosis or therapeutic efficacy.

Mesoporous Silica Nanoparticles in Bioimaging

A biomedical contrast agent serves to enhance the visualisation of a specific (potentially targeted) physiological region. In recent years, mesoporous silica nanoparticles (MSNs) have developed as a flexible imaging platform of tuneable size/morphology, abundant surface chemistry, biocompatibility and otherwise useful physiochemical properties. This review discusses MSN structural types and synthetic strategies, as well as methods for surface functionalisation. Recent applications in biomedical imaging are then discussed, with a specific emphasis on magnetic resonance and optical modes together with utility in multimodal imaging.

Iron Oxide Nanoparticles: An Alternative for Positive Contrast in Magnetic Resonance Imaging

Iron oxide nanoparticles have been extensively utilised as negative (T2) contrast agents in magnetic resonance imaging. In the past few years, researchers have also exploited their application as positive (T1) contrast agents to overcome the limitation of traditional Gd3+ contrast agents. To provide T1 contrast, these particles must present certain physicochemical properties with control over the size, morphology and surface of the particles. In this review, we summarise the reported T1 iron oxide nanoparticles and critically revise their properties, synthetic protocols and application, not only in MRI but also in multimodal imaging. In addition, we briefly summarise the most important nanoparticulate Gd and Mn agents to evaluate whether T1 iron oxide nanoparticles can reach Gd/Mn contrast capabilities.

Magnetic Nanoparticles Supporting Bio-responsive T1/T2 Magnetic Resonance Imaging

: The use of nanoparticulate systems as contrast agents for magnetic resonance imaging (MRI) is well-established and known to facilitate an enhanced image sensitivity within scans of a particular pathological region of interest. Such a capability can enable both a non-invasive diagnosis and the monitoring of disease progression/response to treatment. In this review, magnetic nanoparticles that exhibit a bio-responsive MR relaxivity are discussed, with pH-, enzyme-, biomolecular-, and protein-responsive systems considered. The ability of a contrast agent to respond to a biological stimulus provides not only enriched diagnostic capabilities over corresponding non-responsive analogues, but also an improved longitudinal monitoring of specific physiological conditions.