Magnetic Resonance Imaging Probes Research Articles

A dual magnetic resonance imaging/fluorescent contrast agent for Cathepsin‐D detection

Currently there are no approved biomarkers for the pre-symptomatic diagnosis of Alzheimer's disease (AD). Cathepsin-D (Cat-D) is a lysosomal protease that is present at elevated levels in amyloid plaques and neurons in patients with AD and is also elevated in some cancers. We have developed a magnetic resonance imaging (MRI)/fluorescent contrast agent to detect Cat-D enzymatic activity. The purpose of this study was to investigate the cellular and tissue uptake of this MRI/fluorescent contrast agent. The agent consists of an MRI probe [DOTA-caged metal ion (Gd³⁺ or Tm³⁺)] and a fluorescent probe coupled to a cell-penetrating-peptide sequence by a Cat-D recognition site. The relaxivity of Gd³⁺-DOTA-CAT(cleaved) was measured in 10% heat-treated bovine serum albumin (BSA) phantoms to assess contrast efficacy at magnetic fields ranging from 0.24 mT to 9.4 T. In vitro, Tm³⁺-DOTA-CAT was added to neuronal SN56 cells over-expressing Cat-D and live-cell confocal microscropy was performed at 30 min. Tm³⁺-DOTA-CAT was also intravenously injected into APP/PS1-dE9 Alzheimer's disease mice (n = 9) and controls (n = 8). Cortical and hippocampal uptake was quantified at 30, 60 and 120 min post-injection using confocal microscopy. The liver and kidneys were also evaluated for contrast agent uptake. The relaxivity of Gd³⁺-DOTA-CAT(cleaved) was 3.3 (mM s)⁻¹ in 10% BSA at 9.4 T. In vitro, cells over-expressing Cat-D preferentially took up the contrast agent in a concentration-dependent manner. In vivo, the contrast agent effectively crossed the blood-brain barrier and exhibited a distinct time course of uptake and retention in APP/PS1-dE9 transgenic mice compared with age-matched controls. At clinical and high magnetic field strengths, Gd³⁺-DOTA-CAT produced greater T₁ relaxivity than Gd³⁺-DTPA. Tm³⁺-DOTA-CAT was taken up in a dose-dependent manner in cells over-expressing Cathepsin-D and was shown to transit the blood-brain barrier in vivo. This strategy may be useful for the in vivo detection of enzyme activity and for the diagnosis of Alzheimer's disease.

Amphiphilic starlike dextran wrapped superparamagnetic iron oxide nanoparticle clsuters as effective magnetic resonance imaging probes

Starlike polymers have been widely used in various fields, such as tissue engineering, imaging, gene and drug delivery because of their unique structures and properties. Dextran has long been used as a temporary plasma substitute because of its excellent biocompatibility. In this study, starlike polysaccharide with multiple dextran arms was designed and developed by attaching dextran to a β-cyclodextrin core through click chemistry. Next, starlike dextran was modified with aliphatic chains and these amphiphilic polymers can self-assemble into nanoscale micelles in water, and their critical micelle concentration values (3.7 × 10−8 M) are much lower comparing to its linear analogs (1.7 × 10−7 M), resulting in more stable nanostructures in aqueous environment. These micelles can encapsulate multiple superparamagnetic iron oxide nanoparticles and forming clustering particle nanostructures in water, and the resulting nanocomposites have a high T2 relaxivity of 436.8 Fe mm−1 s−1 under a 1.5T clinical magnetic resonance imaging (MRI) scanner. Further, dual functional probes were developed by loading both superparamagnetic iron oxide nanoparticles and small molecule anticancer drug doxorubicin into polymeric micelles. Multidrug-resistant breast cancer cells MCF-7/Adr treated with these probes can be characterized under MRI.

FKBP5 and emotional neglect interact to predict individual differences in amygdala reactivity

Individual variation in physiological responsiveness to stress mediates risk for mental illness and is influenced by both experiential and genetic factors. Common polymorphisms in the human gene for FK506 binding protein 5 (FKBP5), which is involved in transcriptional regulation of the hypothalamic-pituitary-adrenal (HPA) axis, have been shown to interact with childhood abuse and trauma to predict stress-related psychopathology. In the current study, we examined if such gene-environment interaction effects may be related to variability in the threat-related reactivity of the amygdala, which plays a critical role in mediating physiological and behavioral adaptations to stress including modulation of the HPA axis. To this end, 139 healthy Caucasian youth completed a blood oxygen level-dependent functional magnetic resonance imaging probe of amygdala reactivity and self-report assessments of emotional neglect (EN) and other forms of maltreatment. These individuals were genotyped for 6 FKBP5 polymorphisms (rs7748266, rs1360780, rs9296158, rs3800373, rs9470080 and rs9394309) previously associated with psychopathology and/or HPA axis function. Interactions between each SNP and EN emerged such that risk alleles predicted relatively increased dorsal amygdala reactivity in the context of higher EN, even after correcting for multiple testing. Two different haplotype analyses confirmed this relationship as haplotypes with risk alleles also exhibited increased amygdala reactivity in the context of higher EN. Our results suggest that increased threat-related amygdala reactivity may represent a mechanism linking psychopathology to interactions between common genetic variants affecting HPA axis function and childhood trauma.

Recent Progress of Molecular Imaging Probes Based on Gadofullerenes

Abstract Paramagnetic gadofullerenes and their derivatives exhibit more advantages than commercial Magnevist™ (Gd-DTPA) therefore have become competitive candidates for MR imaging probes. One of the most important properties is the stable fullerene cages are believed to hinder both chemical attacks on the inner clusters and the escape of the toxic lanthanide ions, which should effectively suppress the toxicity of naked Gd 3+ ions. Some functionalized gadofullerides are also available for further conjugation to fabricate multifunctional probes. This review presented the latest findings of MRI imaging probes based on various gadofullerenes, including Gd@C 60 , Gd@C 82 and Gd 3 N@C 80 and discussed the derivative strategy of gadofullerenes determining their relaxivity properties, biodistributions as well as their potential applications as nanoplatform for multifunctional probes.

Hollow manganese phosphate nanoparticles as smart multifunctional probes for cancer cell targeted magnetic resonance imaging and drug delivery

Multifunctional probes for simultaneous magnetic resonance imaging (MRI) and drug delivery have attracted considerable interest due to their promising potential applications in the early-stage diagnosis and therapy of the diseases. In this study, hollow manganese phosphate nanoparticles (HMP NPs) with an average diameter of 18 nm were synthesized and aminated through silanization, which enabled the covalent conjugation of biocompatible poly(ethylene glycol) (PEG) on their surfaces. The anti-tumor drug doxorubicin (DOX) could be loaded into the hollow cavities. Under physiological conditions (pH 7.4), the NPs showed low MRI T 1 contrast (r 1 = 1.19 L·mmol−1·s−1), whereas high T 1 enhancement (r 1 = 5.22 L·mmol−1·s−1) was achieved after dissolving them in endosome/lysosome mimetic conditions (pH 5.4). This is due to the fact that the NPs were easily eroded, which resulted in the release of Mn2+ at low pH. To use this interesting phenomenon for targeted DOX drug delivery, we conjugated the tumor-targeting ligand folic acid (FA) on HMP NPs and investigated their drug delivery capacity and cytotoxicity to cell lines expressing different amount of folate receptor (FR). KB cells showed more significant cellular uptake than HeLa cells and A549 cells, as confirmed by confocal laser scanning microscopy (CLSM), flow cytometry and cellular T 1-weighted MRI. Furthermore, the drug-loaded HMP NPs exhibited greater cytotoxicity to KB cells. Our results suggest that functionalized HMP NPs can act as an effective multifunctional probe for selective diagnosis with MRI, as well as giving efficient targeted drug delivery. Open image in new window

A d-f heteronuclear complex for dual-mode phosphorescence and magnetic resonance imaging

A d-f heteronuclear complex (Ir–Gd) by coupling an iridium(III) complex to a macrocycle-based gadolinium complex has been developed as as a dual-mode bioimaging probe for luminescence imaging and magnetic resonance imaging (MRI). The cyclometalated iridium complex showed an intense visible metal-to-ligand charge transfer (MLCT) absorption and strong yellow phosphorescent luminescence. It has specificity of staining in murine mitochondria confirmed by confocal laser scanning microscopy. The macrocycle-based gadolinium complex provides the longitudinal relaxivity of 3.36 mm−1s−1 on a 3 T system, which has been applied to MRI for liver in mice. Furthermore, the biodistribution of Ir–Gd has been investigated using inductively coupled plasma–atomic emission spectroscopy (ICP-AES) analyses. Such platform should be extended to further applications in the identification and diagnosis of liver diseases.

Zn2+ Responsive Bimodal Magnetic Resonance Imaging and Fluorescent Imaging Probe Based on a Gadolinium(III) Complex

A Zn(2+)-responsive bimodal magnetic resonance imaging (MRI) and luminescence imaging probe GdL was synthesized. The relaxivity and luminescence properties were examined. In the presence of 0.5 equiv of Zn(2+), the longitudinal relaxivity is increased from 3.8 mM(-1) s(-1) to 5.9 mM(-1) s(-1) at 23 MHz and 25 °C with 55% enhancement, whereas the fluorescence exhibits a 7-fold increase. The Zn(2+) responsive imaging probe shows favorable selectivity and tolerance over a variety of biologically relevant anions and metal ions in physiological pH range for both relaxivity and luminescence. In vitro phantom images and confocal fluorescence images in living cells show that the bimodal Zn(2+) probe can effectively enhance T(1)-weighted imaging contrast and luminescence imaging effect through Zn(2+) coordination with excellent cellmembrane permeability and biocompatibility. Spectral and electrospray ionization mass spectrometry (ESI-MS) studies indicate that two different Zn(2+)-bound species, (GdL)(2)Zn and GdLZn, are formed when 0.5 and 1 equiv of Zn(2+) are bound to GdL complex, respectively. Crystal structural determination and dysprosium-induced (17)O NMR shift (DIS) experiment demonstrate that the increased molecular weight and the improved molecular rigidity upon complexation of Zn(2+) with GdL is the primary factor for relaxivity enhancement. Significant enhancement of the luminescence is due to a heavy atom effect and much increased molecular rigidity upon Zn(2+) binding to 8-sulfonamidoquinoline chromophore.

Bioengineered Probes for Molecular Magnetic Resonance Imaging in the Nervous System

The development of molecular imaging probes has changed the nature of neurobiological research. Some of the most notable successes have involved the use of biological engineering techniques for the creation of fluorescent protein derivatives for optical imaging, but recent work has also led to a number of bioengineered probes for magnetic resonance imaging (MRI), the preeminent technique for noninvasive investigation of brain structure and function. Molecular MRI agents are beginning to be applied for experiments in the nervous system, where they have the potential to bridge from molecular to systems or organismic levels of analysis. Compared with canonical synthetic small molecule agents, biomolecular or semibiosynthetic MRI contrast agents offer special advantages due to their amenability to molecular engineering approaches, their properties in some cases as catalysts, and their specificity in targeting and ligand binding. Here, we discuss an expanding list of instances where biological engineering techniques have aided in the design of MRI contrast agents and reporter systems, examining both advantages and limitations of these types of probes for studies in the central nervous system.

ChemInform Abstract: The Solution Structure and Dynamics of MRI Probes Based on Lanthanide(III) DOTA as Investigated by DFT and NMR Spectroscopy

AbstractReview: ca. 100 refs.

MRl of Prostate Cancer Antigen Expression for Diagnosis and lmmunotherapy

BackgroundTumor antigen (TA)–targeted monoclonal antibody (mAb) immunotherapy can be effective for the treatment of a broad range of cancer etiologies; however, these approaches have demonstrated variable clinical efficacy for the treatment of patients with prostate cancer (PCa). An obstacle currently impeding translational progress has been the inability to quantify the mAb dose that reaches the tumor site and binds to the targeted TAs. The coupling of mAb to nanoparticle-based magnetic resonance imaging (MRI) probes should permit in vivo measurement of patient-specific biodistributions; these measurements could facilitate future development of novel dosimetry paradigms wherein mAb dose is titrated to optimize outcomes for individual patients.MethodsThe prostate stem cell antigen (PSCA) is broadly expressed on the surface of prostate cancer (PCa) cells. Anti-human PSCA monoclonal antibodies (mAb 7F5) were bound to Au/Fe3O4 (GoldMag) nanoparticles (mAb 7F5@GoldMag) to serve as PSCA-specific theragnostic MRI probe permitting visualization of mAb biodistribution in vivo. First, the antibody immobilization efficiency of the GoldMag particles and the efficacy for PSCA-specific binding was assessed. Next, PC-3 (prostate cancer with PSCA over-expression) and SMMC-7721 (hepatoma cells without PSCA expression) tumor-bearing mice were injected with mAb 7F5@GoldMag for MRI. MRI probe biodistributions were assessed at increasing time intervals post-infusion; therapy response was evaluated with serial tumor volume measurements.ResultsTargeted binding of the mAb 7F5@GoldMag probes to PC-3 cells was verified using optical images and MRI; selective binding was not observed for SMMC-7721 tumors. The immunotherapeutic efficacy of the mAb 7F5@GoldMag in PC-3 tumor-bearing mice was verified with significant inhibition of tumor growth compared to untreated control animals.ConclusionOur promising results suggest the feasibility of using mAb 7F5@GoldMag probes as a novel paradigm for the detection and immunotherapeutic treatment of PCa. We optimistically anticipate that the approaches have the potential to be translated into the clinical settings.

Structure-Guided Directed Evolution of Highly Selective P450-Based Magnetic Resonance Imaging Sensors for Dopamine and Serotonin

New tools that allow dynamic visualization of molecular neural events are important for studying the basis of brain activity and disease. Sensors that permit ligand-sensitive magnetic resonance imaging (MRI) are useful reagents due to the noninvasive nature and good temporal and spatial resolution of MR methods. Paramagnetic metalloproteins can be effective MRI sensors due to the selectivity imparted by the protein active site and the ability to tune protein properties using techniques such as directed evolution. Here, we show that structure-guided directed evolution of the active site of the cytochrome P450-BM3 heme domain produces highly selective MRI probes with submicromolar affinities for small molecules. We report a new, high-affinity dopamine sensor as well as the first MRI reporter for serotonin, with which we demonstrate quantification of neurotransmitter release in vitro. We also present a detailed structural analysis of evolved cytochrome P450-BM3 heme domain lineages to systematically dissect the molecular basis of neurotransmitter binding affinity, selectivity, and enhanced MRI contrast activity in these engineered proteins.

MRI Tracking of FePro Labeled Fresh and Cryopreserved Long Term In Vitro Expanded Human Cord Blood AC133+ Endothelial Progenitor Cells in Rat Glioma

BackgroundEndothelial progenitors cells (EPCs) are important for the development of cell therapies for various diseases. However, the major obstacles in developing such therapies are low quantities of EPCs that can be generated from the patient and the lack of adequate non-invasive imaging approach for in vivo monitoring of transplanted cells. The objective of this project was to determine the ability of cord blood (CB) AC133+ EPCs to differentiate, in vitro and in vivo, toward mature endothelial cells (ECs) after long term in vitro expansion and cryopreservation and to use magnetic resonance imaging (MRI) to assess the in vivo migratory potential of ex vivo expanded and cryopreserved CB AC133+ EPCs in an orthotopic glioma rat model.Materials, Methods and ResultsThe primary CB AC133+ EPC culture contained mainly EPCs and long term in vitro conditions facilitated the maintenance of these cells in a state of commitment toward endothelial lineage. At days 15–20 and 25–30 of the primary culture, the cells were labeled with FePro and cryopreserved for a few weeks. Cryopreserved cells were thawed and in vitro differentiated or IV administered to glioma bearing rats. Different groups of rats also received long-term cultured, magnetically labeled fresh EPCs and both groups of animals underwent MRI 7 days after IV administration of EPCs. Fluorescent microscopy showed that in vitro differentiation of EPCs was not affected by FePro labeling and cryopreservation. MRI analysis demonstrated that in vivo accumulation of previously cryopreserved transplanted cells resulted in significantly higher R2 and R2* values indicating a higher rate of migration and incorporation into tumor neovascularization of previously cryopreserved CB AC133+ EPCs to glioma sites, compared to non-cryopreserved cells.ConclusionMagnetically labeled CB EPCs can be in vitro expanded and cryopreserved for future use as MRI probes for monitoring the migration and incorporation to the sites of neovascularization.

Multifunctional nanocomposite based on graphene oxide for in vitro hepatocarcinoma diagnosis and treatment

Because of its unique chemical and physical properties, graphene oxide (GO) has attracted a large number of researchers to explore its biomedical applications in the past few years. Here, we synthesized a novel multifunctional nanocomposite based on GO and systemically investigated its applications for in vitro hepatocarcinoma diagnosis and treatment. This multifunctional nanocomposite named GO-PEG-FA/Gd/DOX was obtained as the following procedures: gadolinium-diethylenetriamine-pentaacetic acid-poly(diallyl dimethylammonium) chloride (Gd-DTPA-PDDA) as magnetic resonance imaging (MRI) probe was applied to modify GO by simple physical sorption with a loading efficiency of Gd(3+) up to 0.314 mg mg(-1). In order to improve its tumor targeting imaging and treatment efficiency, the obtained intermediate product was further modified with folic acid (FA). Finally, the nanocomposite was allowed to load anticancer drug doxorubicin hydrochloride via π-π stacking and hydrophobic interaction with the loading capacity reaching 1.38 mg mg(-1). MRI test revealed that GO-PEG-FA/Gd/DOX exhibit superior tumor targeting imaging efficiency over free Gd(3+). The in vitro release of DOX from the nanocomposite under tumor relevant condition (pH 5.5) was fast at the initial 10 h and then become relatively slow afterward. Moreover, we experimentally demonstrated that the multifunctional nanocomposite exhibited obviously cytotoxic effect upon cancer cells. Above results are promising for the next in vivo experiment and make it possible to be a potential candidate for malignancy early detection and specific treatment.

Europium-doped gadolinium sulfide nanoparticles as a dual-mode imaging agent for T1-weighted MR and photoluminescence imaging

We present a facile synthesis of europium-doped gadolinium sulfide (GdS:Eu3+) opto-magnetic nanoparticles (NPs) via sonochemistry. Their photoluminescence and strong paramagnetic properties enable these NPs to be utilized as an in vitro cell imaging and in vivo T1-weighted MR imaging probe. The GdS:Eu3+ NPs have a prominent longitudinal (r1) relaxivity value, which is a critical parameter for T1-weighted MR imaging. Here, we showed not only their strong positive contrast effect to blood vessels and organs of mice, but also blood half-life and biodistribution including clearance from organs, in order to assess the GdS:Eu3+ NPs as a competent nanocrystal-based T1 contrast agent. We further showed confocal images of breast cancer cells containing GdS:Eu3+ NPs to evaluate as a photoluminescence probe. Dual-mode imaging capability obtained from the GdS:Eu3+ NPs will allow target-oriented cellular imaging as well as the resulting disease-specific MR imaging.

Nitroxide Radicals@US‐Tubes: New Spin Labels for Biomedical Applications

AbstractThe encapsulation of nitroxide radicals within ultrashort (ca. 50 nm) single‐walled carbon nanotubes (US‐tubes) is achieved. Tempo‐ and Iodo‐Tempo@US‐tubes are characterized by thermogravimetric analysis (TGA), X‐ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electron paramagnetic resonance (EPR) spectra display characteristic signals due to the detection of the spin probes within the US‐tubes. Longitudinal proton relaxivities (r1) of both nitroxide@US‐tubes samples are 7 to 13 times greater than the free nitroxide radicals in solution, giving relaxivities comparable to the clinical contrast agent (CA) Magnevist. In addition, transverse proton relaxivities (r2) show unprecedented proton relaxation enhancement in comparison to any other reported nitroxide radical‐based system or the clinically approved T2 CA, Resovist, under the same conditions. T2‐weighted magnetic resonance imaging (MRI) phantom images show that the encapsulation of nitroxide radicals within the US‐tubes produces good contrast enhancement due to their high r2 relaxivities. The nitroxide radicals@US‐tube agents are a new promising class of spin probes for MRI and electronic paramagnetic resonance imaging (EPRI) labeling, tracking, and diagnosis.

Ultra-fast method to synthesize mesoporous magnetite nanoclusters as highly sensitive magnetic resonance probe

An ultra-fast method to synthesize mesoporous magnetite (Fe3O4) nanoclusters is reported. These mesoporous magnetite can be used as a highly sensitive magnetic resonance imaging (MRI) probe. The nanoclusters were synthesized by reducing iron (III) acetylacetonate with hydrazine in ethylene glycol under microwave irradiation and only 5min was needed in the synthesis. The diameter of the nanoclusters could be controlled effectively between 75nm and 115nm by increasing the amount of iron (III) acetylacetonate. Brunauer–Emmett–Teller (BET) results reveal a mesoporous structure and a large surface area of 72.3m2g−1. Cytotoxicity test performed in HepG2 cell line indicated that the as-prepared nanoclusters were non-cytotoxic. The nanoclusters exhibited an enhanced T2 relaxivity value of 417.4±29.9s−1mM−1. In vitro and in vivo MRI confirmed the high sensitivity of the magnetite nanoclusters as MRI probe. The biodistribution of the nanoclusters in rat liver and spleen after intravenous injection was also investigated.

Giant Vesicles Containing Superparamagnetic Iron Oxide as Biodegradable Cell-Tracking MRI Probes

A major breakthrough in in vivo cellular imaging has been the clinical/preclinical use of magnetic resonance imaging (MRI) with contrast agent. Superparamagnetic iron oxide (SPIO) is a promising candidate for the development of smart MRI probes for cell-tracking. In the present study, we describe biodegradable probes made of giant vesicles (GVs; closed lipid membranes with diameters >1 μm) that encapsulate SPIO for use as an MRI contrast agent. These SPIO-containing GVs (SPIO-GVs) exhibited excellent contrast enhancement in the single cell of medaka fish (Oryzias latipes) embryos immediately after their microinjection, and this enhancement disappeared when the GV membranes were destroyed. Our results demonstrate that SPIO-GVs are useful MRI probes for single cell-tracking that have minimum cytotoxicity and will greatly improve clinical/preclinical in vivo cellular imaging techniques.

Lanthanide Loaded Zeolites, Clays, and Mesoporous Silica Materials as MRI Probes New Bisaqua Picolinate‐Based Gadolinium Complexes as MRI Contrast Agents with Substantial High‐Field Relaxivities Supramolecular Adducts of Negatively Charged Lanthanide(III) DOTP Chelates and Cyclodextrins Functionalized with Ammonium Groups: Mass Spectrometry and Nuclear Magnetic Resonance Studies (Eur. J. Inorg. Chem. 12/2012)

AbstractThe front cover picture shows gives an impression on the breadth of coverage of this Cluster Issue on Metal‐Based Magnetic Resonance Imaging Probes. Superimposed on an angiographic MRI image are the zeolite cages for Gd3+ and the mode of action of this probe from the review by J. A. Peters and K. Djanashvili (top) and a depiction of the principle of magnetoluminescent contrast agents for dual MRI and luminescence imaging (V. C. Pierre et al., bottom right). Representative of papers on different approaches to optimize the efficacy of contrast agents are the PARACEST properties of the large Tm‐DOTP–cyclodextrin complex of C. F. G. C. Geraldes et al. (bottom left) and the high relaxivity associated with two bis(aqua) complexes of M. Mazzanti et al. (center left). We thank the authors for making their graphics available for use on the cover and Dr. Giorgio Gatti for the time‐consuming creation of this attractive picture. magnified image

Polysuccinimide graft copolymer nano aggregates encapsulating magnetites for imaging probe

Polysuccinimide (PSI) grafted with methylpoly(ethylene glycol) (MPEG) was synthesized and employed to encapsulate pre-synthesized magnetite using a co-precipitation method. This encapsulation was stabilized by the chemical bonding between PSI and magnetite via the introduction of a bridging compound, 3-aminopropylethoxysi- lane (APS). The morphology and properties of the magnetite-encapsulated polymer particles were widely character- ized to investigate their applications as potential magnetic resonance (MR) imaging probes. The Fourier transform infrared and Fourier transform nuclear magnetic resonance spectroscopy confirmed the successful synthesis of the MPEG-g-PSI- and APS-coated magnetites in addition to X-ray diffraction of their crystalline structure. The size and distribution of the pure and polymer-coated magnetite aggregates were examined using transmission electron microscopy and light scattering analysis. The magnetite-encapsulated aggregates prepared in this study showed excellent superparamagnetic and biocompatible properties that enabled their successful use as MR imaging probes.

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.