Challenges for molecular magnetic resonance imaging

Time domain para hydrogen induced polarization

Para hydrogen induced polarization (PHIP) is a powerful hyperpolarization technique, which increases the NMR sensitivity by several orders of magnitude. However the hyperpolarized signal is created as an anti-phase signal, which necessitates high magnetic field homogeneity and spectral resolution in the conventional PHIP schemes. This hampers the application of PHIP enhancement in many fields, as for example in food science, materials science or MRI, where low B(0)-fields or low B(0)-homogeneity do decrease spectral resolution, leading to potential extinction if in-phase and anti-phase hyperpolarization signals cannot be resolved. Herein, we demonstrate that the echo sequence (45°-τ-180°-τ) enables the acquisition of low resolution PHIP enhanced liquid state NMR signals of phenylpropiolic acid derivatives and phenylacetylene at a low cost low-resolution 0.54 T spectrometer. As low field TD-spectrometers are commonly used in industry or biomedicine for the relaxometry of oil-water mixtures, food, nano-particles, or other systems, we compare two variants of para-hydrogen induced polarization with data-evaluation in the time domain (TD-PHIP). In both TD-ALTADENA and the TD-PASADENA strong spin echoes could be detected under conditions when usually no anti-phase signals can be measured due to the lack of resolution. The results suggest that the time-domain detection of PHIP-enhanced signals opens up new application areas for low-field PHIP-hyperpolarization, such as non-invasive compound detection or new contrast agents and biomarkers in low-field Magnetic Resonance Imaging (MRI). Finally, solid-state NMR calculations are presented, which show that the solid echo (90y-τ-90x-τ) version of the TD-ALTADENA experiment is able to convert up to 10% of the PHIP signal into visible magnetization.

DNA surface modified gadolinium phosphate nanoparticles as MRI contrast agents

Oligonucleotide modified gadolinium phosphate nanoparticles have been prepared and their magnetic resonance relaxivity properties measured. Nanoparticles of GdPO4·H2O were synthesized in a water/oil microemulsion using IGEPAL CO-520 as surfactant, resulting in 50 to 100 nm particles that are highly dispersible and stable in water. Using surface modification chemistry previously established for zirconium phosphonate surfaces, the particles are directly modified with 5'-phosphate terminated oligonucleotides, and the specific interaction of the divalent phosphate with Gd(3+) sites at the surface is demonstrated. The ability of the modified nanoparticles to act as MRI contrast agents was determined by performing MR relaxivity measurements at 14.1 T. Solutions of nanopure water, Feridex, and Omniscan (FDA approved contrast agents) in 0.25% agarose were used for comparison and control purposes. MRI data confirm that GdPO4·H2O nanoparticles have relaxivities (r1, r2) comparable to those of commercially available contrast agents. In addition, the data suggest that biofunctionalization of the surface of the nanoparticles does not prevent their function as MRI contrast agents.

1H chemical shift magnetic resonance imaging probes with high sensitivity for multiplex imaging

Proton-based chemical shift imaging probes were encapsulated inside nano-carriers to increase the sensivitity of the reporters. Co-encapsulation with a relaxation agent results in improved sensitivity and suppresses background signals. Simultaneous imaging of different chemical shift reporters allows multiplexed detection.

NaDyF4 Nanoparticles as T2 Contrast Agents for Ultrahigh Field Magnetic Resonance Imaging

A major limitation of the commonly used clinical MRI contrast agents (CAs) suitable at lower magnetic field strengths (<3.0 T) is their inefficiency at higher fields (>7 T), where next-generation MRI scanners are going. We present dysprosium nanoparticles (β-NaDyF4 NPs) as T2 CAs suitable at ultrahigh fields (9.4 T). These NPs effectively enhance T2 contrast at 9.4 T, which is 10-fold higher than the clinically used T2 CA (Resovist). Evaluation of the relaxivities at 3 and 9.4 T show that the T2 contrast enhances with an increase in NP size and field strength. Specifically, the transverse relaxivity (r2) values at 9.4 T were ∼64 times higher per NP (20.3 nm) and ∼6 times higher per Dy(3+) ion compared to that at 3 T, which is attributed to the Curie spin relaxation mechanism. These results and confirming phantom MR images demonstrate their effectiveness as T2 CAs in ultrahigh field MRIs.

Drug-loaded and superparamagnetic iron oxide nanoparticle surface-embedded amphiphilic block copolymer micelles for integrated chemotherapeutic drug delivery and MR imaging

We report on the fabrication of organic/inorganic hybrid micelles of amphiphilic block copolymers physically encapsulated with hydrophobic drugs within micellar cores and stably embedded with superparamagnetic iron oxide (SPIO) nanoparticles within hydrophilic coronas, which possess integrated functions of chemotherapeutic drug delivery and magnetic resonance (MR) imaging contrast enhancement. Poly(ε-caprolactone)-b-poly(glycerol monomethacrylate), PCL-b-PGMA, and PCL-b-P(OEGMA-co-FA) amphiphilic block copolymers were synthesized at first by combining ring-opening polymerization (ROP), atom transfer radical polymerization (ATRP), and post- modification techniques, where OEGMA and FA are oligo(ethylene glycol) monomethyl ether methacrylate and folic acid-bearing moieties, respectively. A model hydrophobic anticancer drug, paclitaxel (PTX), and 4 nm SPIO nanoparticles were then loaded into micellar cores and hydrophilic coronas, respectively, of mixed micelles fabricated from PCL-b-PGMA and PCL-b-P(OEGMA-co-FA) diblock copolymers by taking advantage of the hydrophobicity of micellar cores and strong affinity between 1,2-diol moieties in PGMA and Fe atoms at the surface of SPIO nanoparticles. The controlled and sustained release of PTX from hybrid micelles was achieved, exhibiting a cumulative release of ~61% encapsulated drugs (loading content, 8.5 w/w%) over ~130 h. Compared to that of surfactant-stabilized single SPIO nanoparticles (r(2) = 28.3 s(-1) mM(-1) Fe), the clustering of SPIO nanoparticles within micellar coronas led to considerably enhanced T(2) relaxivity (r(2) = 121.1 s(-1) mM(-1) Fe), suggesting that hybrid micelles can serve as a T(2)-weighted MR imaging contrast enhancer with improved performance. Moreover, preliminary experiments of in vivo MR imaging were also conducted. These results indicate that amphiphilic block copolymer micelles surface embedded with SPIO nanoparticles at the hydrophilic corona can act as a new generation of nanoplatform integrating targeted drug delivery, controlled release, and disease diagnostic functions.

Exploring the mechanisms of metal-based pharmacological agents via an integrated approach

The peculiar chemical properties of metal-based drugs impart innovative pharmacological profiles to this class of therapeutic and diagnostic agents, most likely in relation to novel molecular mechanisms still poorly understood. However, inorganic drugs have been scarcely considered for medicinal applications with respect to classical organic compounds due to the prejudice of the relevant toxic effects evidenced in certain cases. Thus, the development of improved metallodrugs requires clearer understanding of their physiological processing and molecular basis of actions. Among the various issues in the area of medicinal inorganic chemistry, the possibility of target elucidation is essential for the identification of new therapeutic applications for metal compounds or as molecular biological tools. Here we present the results of our recent research in the field, which in our opinion constitute the basis of a systematic and interdisciplinary approach to address some of the critical issues in the study of the molecular mechanisms of metallodrugs' action via the implementation of high-resolution biophysical techniques coupled with more pharmacological methods.

Development and in vivo imaging of a PET/MRI nanoprobe with enhanced NIR fluorescence by dye encapsulation

AIM

To monitor cells in vivo or to detect the sentinel lymph node, we developed a PET/MRI silica nanoprobe with an enhanced near-infrared fluorescence signal.

METHODS

We developed enhanced near-infrared fluorescent (NIRF) magnetic silica nanoparticles, MNP-SiO2(NIR797), that encapsulate NIRF dye in the silica. We applied this probe to visualizing cells in the deep tissue of mice using NIRF imaging. After labeling with a radioisotope, (68)Ga, on the surface of MNP-SiO2(NIR797), we injected it into the forepaw of mice to visualize the sentinel lymph node.

RESULTS

This encapsulated nanoprobe showed enhancement of fluorescent intensity and stability compared with the nanoprobe, which had the same dyes on the surface of the silica nanoparticles. We also obtained multimodal in vivo imaging of (68)Ga-{MNP-SiO2(NIR797)} applied to sentinel lymph node detection of mice using PET/MRI/NIRF images.

CONCLUSION

This multimodal nanoprobe with enhanced fluorescence may provide a useful tool for imaging diagnostics and cell tracking.

Hyperpolarized 129Xe NMR signature of living biological cells

We show that the differentiation between internal and external compartments of various biological cells in suspension can be made via simple NMR spectra of hyperpolarized (129) Xe. The spectral separation between the signals of (129) Xe in these two compartments is already known for red blood cells, because of the strong interaction of the noble gas with hemoglobin. The observation of two separate peaks in the 200-ppm region can be seen with both eukaryotic and prokaryotic cells, some of which are not known to contain paramagnetic proteins in large quantities. Using different experiments in which the cells are lysed, swell or are blocked in G2 phase, we demonstrate that the low-field-shifted peak observed corresponds to xenon in the aqueous pool inside the cells and not in the membranes. The presence of this additional peak is a clear indication of cell integrity, and its integration allows the quantification of the total cell volume. The relaxation time of intracellular xenon is sufficiently long to open up promising perspectives for cell characterization. The exchange time between the inner and outer cell compartments (on the order of 30 ms) renders possible the targeting of intracellular receptors, whereas the observation of chemical shift variations represents a method of revealing the presence of toxic species in the cells.

Lanthanide-DTPA grafted silica nanoparticles as bimodal-imaging contrast agents

The design and synthesis of a combined MRI-optical probe for bio-imaging are reported. The materials studied join the properties of lanthanide (Ln(3+)) complexes and nanoparticles (NPs), offering an excellent solution for bimodal imaging. The hybrid SiO(2)@APS/DTPA:Gd:Ln (Ln = Eu(3+) or Tb(3+)) (APS: 3-aminopropyltriethoxysilane, DTPA: diethylenetriamine pentaacetic acid) system increases the payload of the active magnetic centre (Gd(3+)) and introduces a Ln(3+) long-life excited state (Eu(3+): 0.35 ± 0.02 ms, Tb(3+): 1.87 ± 0.02 ms), with resistance to photobleaching and sharp emission bands. The Eu(3+) ions reside in a single low-symmetry site. Although the photoluminescence emission is not influenced by the simultaneous presence of Gd(3+) and Eu(3+), a moderate r(1) increase and a larger enhancement of r(2) are observed, particularly at high fields, due to susceptibility effects on r(2). The presence of Tb(3+) instead of Eu(3+) further raises r(1) but decreases r(2). These values are constant over a wide (5-13) pH range, indicating the paramagnetic NPs stability and absence of leaching. The uptake of NPs by living cells is fast and results in an intensity increase in the T(1)-weighted MRI images. The optical properties of the NPs in cellular pellets are also studied, confirming their potential as bimodal imaging agents.

Perspectives and opportunities for nanomedicine in the management of atherosclerosis

The use of nanotechnology for medical purposes--nanomedicine--has grown exponentially over the past few decades. This is exemplified by the US Food and Drug Administration's approval of several nanotherapies for various conditions, as well as the funding of nanomedical programmes worldwide. Although originally the domain of anticancer therapy, recent advances have illustrated the considerable potential of nanomedicine in the diagnosis and treatment of atherosclerosis. This Review elaborates on nanoparticle-targeting concepts in atherosclerotic disease, provides an overview of the use of nanomedicine in atherosclerosis, and discusses potential future applications and clinical benefits.

Mn2+ complexes of 1-oxa-4,7-diazacyclononane based ligands with acetic, phosphonic and phosphinic acid pendant arms: stability and relaxation studies

A new class of macrocyclic ligands based on 1-oxa-4,7-diazacyclononane was synthesized and their Mn(2+) complexes were investigated with respect to stability and relaxation properties. Each ligand has two pendant arms involving carboxylic (H(2)L(1)--1-oxa-4,7-diazacyclononane-4,7-diacetic acid), phosphonic (H(4)L(2)--1-oxa-4,7-diazacyclononane-4,7-bis(methylenephosphonic acid)), phosphinic (H(2)L(3)--1-oxa-4,7-diazacyclononane-4,7-bis(methylenephosphinic acid)) or phenylphosphinic (H(2)L(4)--1-oxa-4,7-diazacyclononane-4,7-bis[methylene(phenyl)phosphinic acid]) acid moieties. H(2)L(3) and H(2)L(4) were synthesized for the first time. The crystal structure of the Mn(2+) complex with H(2)L(4) confirmed a coordination number of 6 for Mn(2+). The protonation constants of all ligands and the stability constants of their complexes with Mn(2+) and some biologically or biomedically relevant metal ions were determined by potentiometry. The protonation sequence of H(2)L(3) was followed by (1)H and (31)P NMR titration and the second protonation step was attributed to the second macrocyclic nitrogen atom. The potentiometric data revealed a relatively low thermodynamic stability of the Mn(2+) complexes with all ligands investigated. For H(2)L(3) and H(2)L(4), full Mn(2+) complexation cannot be achieved even with 100% ligand excess. The transmetallation of MnL(1) and MnL(2) with Zn(2+) was too fast to be followed at pH 6. Variable temperature (1)H NMRD and (17)O NMR measurements have been performed on MnL(1) and MnL(2) to provide information on water exchange and rotational dynamics. The (17)O chemical shifts indicate hydration equilibrium between mono- and bishydrated species for MnL(1), while MnL(2) is monohydrated. The water exchange is considerably faster on MnL(1) (k(ex)(298) = 1.2 × 10(9) s(-1)) than on MnL(2) (k(ex)(298) = 1.2 × 10(7) s(-1)). Small endogenous anions (phosphate, carbonate, citrate) do not replace the coordinated water in either of the complexes, but they induce their slow decomposition. All Mn(2+) complexes are stable toward air-oxidation.

MRI evaluation of the antitumor activity of paramagnetic liposomes loaded with prednisolone phosphate

The design of long circulating liposomes co-loaded with the glucocorticoid prednisolone phosphate (PLP) and the amphiphilic paramagnetic contrast agent Gd-DOTAMA(C(18))(2) allowed the MRI-guided in vivo visualization of the delivery and biodistribution of PLP, as well as the monitoring of drug efficacy. The performance of this theranostic probe was investigated in a mouse model bearing a melanoma B16 syngeneic tumor. The release kinetics of the drug were evaluated in vitro where it displayed a peculiar behavior characterized by a fast process (completed in few hours) involving only a small portion (<5%) of the drug. Interestingly, the incorporation of the amphiphilic imaging reporter in the liposomal bilayer slightly increased the amount of the fast-release portion (<10%), thus suggesting that it could be attributed to a drug fraction embedded in the liposomal bilayer. In fact, the release of a hydrophilic imaging probe encapsulated in the inner core of the same long circulating liposomes formulated for carrying the drug, displayed different, single-step, kinetics. The in vivo monitoring of the antitumor activity of the nanomedicine revealed that the incorporation of the MRI probe into the liposome bilayer did not significantly affect the drug efficacy. The in vivo experiments also indicated a relevant and fast liposome uptake from macrophage-rich organs like spleen and liver, which reduced the tumor accumulation of the liposomes. The accumulation of the amphipatic MRI label caused the occurrence of a long-term residual T(1) contrast still detectable 1week after injection.

Layer-by-layer capsules for magnetic resonance imaging and drug delivery

Layer-by-layer (LbL) self-assembled polyelectrolyte capsules have demonstrated their unique advantages and capability in drug delivery applications. These ordered micro/nano-structures are also promising candidates as imaging contrast agents for diagnostic and theranostic applications. Magnetic resonance imaging (MRI), one of the most powerful clinical imaging modalities, is moving forward to the molecular imaging field and requires the availability of advanced imaging probes. In this review, we are focusing on the design of MRI visible LbL capsules, which incorporate either paramagnetic metal-ligand complexes or superparamagnetic iron oxide (SPIO) nanoparticles. The design criteria cover the topics of probe sensitivity, biosafety, long-circulation property, targeting ligand decoration, and drug loading strategies. Examples of MRI visible LbL capsules with paramagnetic or superparamagnetic moieties were given and discussed. This carrier platform can also be chosen for other imaging modalities.

Engineered nanoparticles for biomolecular imaging

In recent years, the production of nanoparticles (NPs) and exploration of their unusual properties have attracted the attention of physicists, chemists, biologists and engineers. Interest in NPs arises from the fact that the mechanical, chemical, electrical, optical, magnetic, electro-optical and magneto-optical properties of these particles are different from their bulk properties and depend on the particle size. There are numerous areas where nanoparticulate systems are of scientific and technological interest, particularly in biomedicine where the emergence of NPs with specific properties (e.g. magnetic and fluorescence) for contrast agents can lead to advancing the understanding of biological processes at the biomolecular level. This review will cover a full description of the physics of various imaging methods, including MRI, optical techniques, X-rays and CT. In addition, the effect of NPs on the improvement of the mentioned non-invasive imaging methods will be discussed together with their advantages and disadvantages. A detailed discussion will also be provided on the recent advances in imaging agents, such as fluorescent dye-doped silica NPs, quantum dots, gold- and engineered polymeric-NPs, superparamagnetic iron oxide NPs (SPIONs), and multimodal NPs (i.e. nanomaterials that are active in both MRI and optical methods), which are employed to overcome many of the limitations of conventional contrast agents (e.g. gadolinium).

Receptor-targeted iron oxide nanoparticles for molecular MR imaging of inflamed atherosclerotic plaques

In a number of literature reports iron oxide nanoparticles have been investigated for use in imaging atherosclerotic plaques and found to accumulate in plaques via uptake by macrophages, which are critical in the process of atheroma initiation, propagation, and rupture. However, the uptake of these agents is non-specific; thus the labeling efficiency for plaques in vivo is not ideal. We have developed targeted agents to improve the efficiency for labeling macrophage-laden plaques. These probes are based on iron oxide nanoparticles coated with dextran sulfate, a ligand of macrophage scavenger receptor type A (SR-A). We have sulfated dextran-coated iron oxide nanoparticles (DIO) with sulfur trioxide, thereby targeting our nanoparticle imaging agents to SR-A. The sulfated DIO (SDIO) remained mono-dispersed and had an average hydrodynamic diameter of 62 nm, an r(1) relaxivity of 18.1 mM(-1) s(-1), and an r(2) relaxivity of 95.8 mM(-1) s(-1) (37 °C, 1.4 T). Cell studies confirmed that these nanoparticles were nontoxic and specifically targeted to macrophages. In vivo MRI after intravenous injection of the contrast agent into an atherosclerotic mouse injury model showed substantial signal loss on the injured carotid at 4 and 24 h post-injection of SDIO. No discernable signal decrease was seen at the control carotid and only mild signal loss was observed for the injured carotid post-injection of non-sulfated DIO, indicating preferential uptake of the SDIO particles at the site of atherosclerotic plaque. These results indicate that SDIO can facilitate MRI detection and diagnosis of vulnerable plaques in atherosclerosis.

Magnetite Nanoparticles for Medical MR Imaging

Nanotechnology has given scientists new tools for the development of advanced materials for the detection and diagnosis of disease. Iron oxide nanoparticles (SPIONs) in particular have been extensively investigated as novel magnetic resonance imaging (MRI) contrast agents due to a combination of favorable superparamagnetic properties, biodegradability, and surface properties of easy modification for improved in vivo kinetics and multifunctionality. This review discusses the basics of MR imaging, the origin of SPION's unique magnetic properties, recent developments in MRI acquisition methods for detection of SPIONs, synthesis and post-synthesis processes that improve SPION's imaging characteristics, and an outlook on the translational potential of SPIONs.

The fluorous effect in biomolecular applications

From being a niche area only a few decades ago, fluorous chemistry has gained momentum and is, nowadays, a fervent area of research. It has brought forth, in fact, numerous applicative innovations that stretch among different fields: from catalysis to separation science, from supramolecular to materials and analytical chemistry. Recently, the unique features of perfluorinated compounds have reached the attention of the biochemists' audience. This tutorial review introduces the basic concepts of fluorous chemistry and illustrates its main biomolecular applications. Special attention has been given to fluorous microarrays and their combination with Mass-Spectroscopy (MS) techniques, to protein properties modification by the introduction of local fluorous domains, and to the most recent applications of (19)F-Magnetic Resonance Imaging ((19)F-MRI).

18F-Labeled magnetic-upconversion nanophosphors via rare-Earth cation-assisted ligand assembly

A novel method of rare-earth cation-assisted ligand assembly has been developed to provide upconversion nanophosphors with T(1)-enhanced magnetic resonance (MR), radioactivity, and targeted recognition properties, making these nanoparticles potential candidates for multimodal bioimaging. The process of modifying the surface of the nanophosphors has been confirmed by transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, proton nuclear magnetic resonance, Fourier-transform infrared spectroscopy, energy-dispersive X-ray analysis, and so on. The versatility of this surface modification approach for incorporating functional molecules and fabricating fluorine-18-labeled magnetic-upconversion nanophosphors as multimodal bioprobes has been demonstrated by targeted cell imaging, in vivo upconversion luminescence, MR imaging, and positron emission tomography imaging of whole-body small animals.

Iridium N-heterocyclic carbene complexes as efficient catalysts for magnetization transfer from para-hydrogen

While the characterization of materials by NMR is hugely important in the physical and biological sciences, it also plays a vital role in medical imaging. This success is all the more impressive because of the inherently low sensitivity of the method. We establish here that [Ir(H)₂(IMes)(py)₃]Cl undergoes both pyridine (py) loss as well as the reductive elimination of H₂. These reversible processes bring para-H₂ and py into contact in a magnetically coupled environment, delivering an 8100-fold increase in ¹H NMR signal strength relative to non-hyperpolarized py at 3 T. An apparatus that facilitates signal averaging has been built to demonstrate that the efficiency of this process is controlled by the strength of the magnetic field experienced by the complex during the magnetization transfer step. Thermodynamic and kinetic data combined with DFT calculations reveal the involvement of [Ir(H)₂(η²-H₂)(IMes)(py)₂]⁺, an unlikely yet key intermediate in the reaction. Deuterium labeling yields an additional 60% improvement in signal, an observation that offers insight into strategies for optimizing this approach.

Lanthanide dota-like complexes containing a picolinate pendant: structural entry for the design of Ln(III)-based luminescent probes

In this contribution we present two ligands based on a do3a platform containing a picolinate group attached to the fourth nitrogen atom of the cyclen unit, which are designed for stable lanthanide complexation in aqueous solutions. Potentiometric measurements reveal that the thermodynamic stability of the complexes is very high (log K = 21.2-23.5), being comparable to that of the dota analogues. Luminescence lifetime measurements performed on solutions of the Eu(III) and Tb(III) complexes indicate that the complexes are nine coordinate with no inner-sphere water molecules. A combination of density functional theory (DFT) calculations and NMR measurements shows that for the complexes of the heaviest lanthanides there is a major isomer in solution consisting of the enantiomeric pair Λ(δδδδ) and Δ(λλλλ), which provides square antiprismatic coordination (SAP) around the metal ion. Analysis of the Yb(III)-induced paramagnetic shifts unambiguously confirms that these complexes have SAP coordination in aqueous solution. For the light lanthanide ions however both the SAP and twisted-square antiprismatic (TSAP) isomers are present in solution. Inversion of the cyclen ring appears to be the rate-determining step for the Λ(δδδδ) ↔ Δ(λλλλ) enantiomerization process observed in the Lu(III) complexes. The energy barriers obtained from NMR measurements for this dynamic process are in excellent agreement with those predicted by DFT calculations. The energy barriers calculated for the arm-rotation process are considerably lower than those obtained for the ring-inversion path. Kinetic studies show that replacement of an acetate arm of dota by a picolinate pendant results in a 3-fold increase in the formation rate of the corresponding Eu(III) complexes and a significant increase of the rates of acid-catalyzed dissociation of the complexes. However, these rates are 1-2 orders of magnitude lower than those of do3a analogues, which shows that the complexes reported herein are remarkably inert with respect to metal ion dissociation.

Chemical exchange saturation transfer (CEST): what is in a name and what isn't?

Chemical exchange saturation transfer (CEST) imaging is a relatively new magnetic resonance imaging contrast approach in which exogenous or endogenous compounds containing either exchangeable protons or exchangeable molecules are selectively saturated and after transfer of this saturation, detected indirectly through the water signal with enhanced sensitivity. The focus of this review is on basic magnetic resonance principles underlying CEST and similarities to and differences with conventional magnetization transfer contrast. In CEST magnetic resonance imaging, transfer of magnetization is studied in mobile compounds instead of semisolids. Similar to magnetization transfer contrast, CEST has contributions of both chemical exchange and dipolar cross-relaxation, but the latter can often be neglected if exchange is fast. Contrary to magnetization transfer contrast, CEST imaging requires sufficiently slow exchange on the magnetic resonance time scale to allow selective irradiation of the protons of interest. As a consequence, magnetic labeling is not limited to radio-frequency saturation but can be expanded with slower frequency-selective approaches such as inversion, gradient dephasing and frequency labeling. The basic theory, design criteria, and experimental issues for exchange transfer imaging are discussed. A new classification for CEST agents based on exchange type is proposed. The potential of this young field is discussed, especially with respect to in vivo application and translation to humans.

Magnetoliposomes as multimodal contrast agents for molecular imaging and cancer nanotheragnostics

In the emerging field of molecular and cellular imaging, flexible strategies to synthesize multimodal contrast agents with targeting ligands are required. Liposomes have the ability to combine with a large variety of nanomaterials, including superparamagnetic iron oxide nanoparticles, to form magnetoliposomes (MLs). MLs can be used as highly efficient MRI contrast agents. Owing to their high flexibility, MLs can be associated with other imaging modality probes to be used as multimodal contrast agents. By using a thermosensitive lipid bilayer in the ML structure, these biocompatible systems offer many possibilities for targeting and delivering therapeutic agents for ‘theragnostics’, a coincident therapy and diagnosis strategy. This article deals with the fast-growing field of MLs as biomedical diagnostic tools. Different kinds of MLs, their preparation methods, as well as their surface modification with different imaging probes, are discussed. ML applications as multimodal contrast agents and in theragnostics are reviewed. Some important issues for the biomedical uses of magnetic liposomes, such as toxicity, are summarized.

Development of intravascular contrast agents for MRI using gadolinium chelates

Two MRI contrast agents (CAs) composed of Gd-DO3A conjugated to amino acid building blocks derived from glutamic acid (CA1) and lysine (CA2) have been synthesized by using novel alkyne and propionate linkers, and subsequently characterized. In vitro cell viability assays showed insignificant cytotoxicity of both CAs at low concentrations up to 0.2 mM. The longitudinal relaxivities (r(1) ) of CA1 and CA2 measured at 9.4 T are 6.4 and 5.4 mM(-1)  s(-1) in H(2) O at 25 °C, respectively. Both r(1) values are higher than those of CAs in clinical use: Gd-DTPA (Magnevist, Bayer Schering, Germany) and Gd-DOTA (Dotarem, Guerbet, France). In vivo imaging in Wistar rats demonstrated considerable signal enhancement (∼50 %) in the brain artery by CA2, but lower signal enhancement (∼30 %) by CA1. In contrast to Dotarem, which showed a similar signal enhancement as CA2, the enhancement by CA2 remained high (∼30 %), even at 52 min post-injection. This demonstrates that CA2 has a much longer blood half-life (68.1 min), which could be advantageous for angiography and tissue targeting.

A hydrogen peroxide-responsive hyperpolarized 13C MRI contrast agent

We report a new reaction-based approach for the detection of hydrogen peroxide (H(2)O(2)) using hyperpolarized (13)C magnetic resonance imaging ((13)C MRI) and the H(2)O(2)-mediated oxidation of α-ketoacids to carboxylic acids. (13)C-Benzoylformic acid reacts selectively with H(2)O(2) over other reactive oxygen species to generate (13)C-benzoic acid and can be hyperpolarized using dynamic nuclear polarization, providing a method for dual-frequency detection of H(2)O(2). Phantom images collected using frequency-specific imaging sequences demonstrate the efficacy of this responsive contrast agent to monitor H(2)O(2) at pre-clinical field strengths. The combination of reaction-based detection chemistry and hyperpolarized (13)C MRI provides a potentially powerful new methodology for non-invasive multi-analyte imaging in living systems.

Magnetic nanocomposites with mesoporous structures: synthesis and applications

Magnetic nanocomposites with well-defined mesoporous structures, shapes, and tailored properties are of immense scientific and technological interest. This review article is devoted to the progress in the synthesis and applications of magnetic mesoporous materials. The first part briefly reviews various general methods developed for producing magnetic nanoparticles (NPs). The second presents and categorizes the synthesis of magnetic nanocomposites with mesoporous structures. These nanocomposites are broadly categorized into four types: monodisperse magnetic nanocrystals embedded in mesoporous nanospheres, microspheres encapsulating magnetic cores into perpendicularly aligned mesoporous shells, ordered mesoporous materials loaded with magnetic NPs inside the porous channels or cages, and rattle-type magnetic nanocomposites. The third section reviews the potential applications of the magnetic nanocomposites with mesoporous structures in the areas of heath care, catalysis, and environmental separation. The final section offers a summary and future perspectives on the state-of-the art in this area.