Liquid-state carbon-13 hyperpolarization generated in an MRI system for fast imaging.

Hyperpolarized (HP) tracers dramatically increase the sensitivity of magnetic resonance imaging (MRI) to monitor metabolism non-invasively and in vivo. Their production, however, requires an extra polarizing device (polarizer) whose complexity, operation and cost can exceed that of an MRI system itself. Furthermore, the lifetime of HP tracers is short and some of the enhancement is lost during transfer to the application site. Here, we present the production of HP tracers in water without an external polarizer: by Synthesis Amid the Magnet Bore, A Dramatically Enhanced Nuclear Alignment (SAMBADENA) is achieved within seconds, corresponding to a hyperpolarization of ∼20%. As transfer of the tracer is no longer required, SAMBADENA may permit a higher polarization at the time of detection at a fraction of the cost and complexity of external polarizers. This development is particularly promising in light of the recently extended portfolio of biomedically relevant para-hydrogen-tracers and may lead to new diagnostic applications.

Do twisted laser beams evoke nuclear hyperpolarization?

The hyperpolarization of nuclear spins promises great advances in chemical analysis and medical diagnosis by substantially increasing the sensitivity of nuclear magnetic resonance (NMR). Current methods to produce a hyperpolarized sample, however, are arduous, time-consuming or costly and require elaborate equipment. Recently, a much simpler approach was introduced that holds the potential, if harnessed appropriately, to revolutionize the production of hyperpolarized spins. It was reported that high levels of hyperpolarization in nuclear spins can be created by irradiation with a laser beam carrying orbital angular momentum (twisted light). Aside from these initial reports however, no further experimental verification has been presented. In addition, this effect has so far evaded a critical theoretical examination. In this contribution, we present the first independent attempt to reproduce the effect. We exposed a sample of immersion oil or a fluorocarbon liquid that was placed within a low-field NMR spectrometer to Laguerre-Gaussian and Bessel laser beams at a wavelength of 514.5nm and various topological charges. We acquired (1)H and (19)F NMR free induction decay data, either during or alternating with the irradiation that was parallel to B0. We observed an irregular increase in NMR signal in experiments where the sample was exposed to beams with higher values of the topological charge. However, at no time did the effect reach statistical significance of 95%. Given the measured sensitivity of our setup, we estimate that a possible effect did not exceed a hyperpolarization (at 5mT) of 0.14-6%, depending on the assumed hyperpolarized volume. It should be noted though, that there were some differences between our setup and the previous implementation of the experiment, which may have inhibited the full incidence of this effect. To approach a theoretical description of this effect, we considered the interaction of an electron with a plane wave, which is known to be able to induce electronic (e.g. in rubidium) and subsequent nuclear hyperpolarization. Compared to the plane wave, the additional transitions caused by a twisted wave are of the order of 10(-3) less. This suggests that the twist of the laser is unlikely to be responsible for the hyperpolarization of nuclear spins, unless a new mechanism of momentum transfer is identified.

Microstructural effects of a neuro-modulating drug evaluated by diffusion tensor imaging.

In a longitudinal mouse study we evaluated whether diffusion tensor imaging (DTI) can monitor microstructural changes after administration of the neuromodulating drug EPO and whether erythropoietin (EPO) has an effect on cognitive performance. Twelve mice (2 groups with 6 mice each) were scanned in a 7T Bruker Biospin animal scanner with a highly resolved DTI sequence before and 16 days after intraperitoneal injections of EPO or saline. All mice underwent behavioral testing (Morris water maze) and histologic evaluation of hippocampal and corpus callosum cell proliferation and oligodendrogenesis. Whole brain DTI analysis showed significant Trace, RD and AD decrease within the dentate gyrus, subiculum, primary motor, somatosensory, and supplementary somatosensory areas and FA increase in the hippocampus, corpus callosum, and fimbria fornix in EPO treated mice only. ROI-based DTI analysis showed significant Trace and RD decrease and FA increase only in the corpus callosum of EPO treated mice, whereas in the dentate gyrus significant Trace, RD, and AD decrease occurred in both, EPO- and control-group. Behavioral tests showed that EPO treated mice performed better and learned faster than controls. Histologically, the number of BrdU-positive nuclei and optical density of DCX-labeled juvenile neurons significantly increased within the dentate gyrus, corpus callosum and fimbria fornix and the number of NG2-positive oligodendrocyte progenitors in corpus callosum and fimbria fornix, respectively. In conclusion we were able to monitor microstructural changes with DTI and showed EPO treatment-related alterations correlating with enhanced dentate gyrus and corpus callosum cell proliferation and better learning capabilities.

Immuno-magnetoliposomes targeting activated platelets as a potentially human-compatible MRI contrast agent for targeting atherothrombosis.

To detect unstable atherosclerotic plaques early and noninvasively would be of great clinical interest. Activated platelets are an interesting molecular target for detecting early lesions or unstable plaques. We therefore developed an MRI contrast agent consisting of magnetoliposomes (ML) linked to an antibody (anti-LIBS) specifically targeting the ligand-induced binding site of the activated GPIIb/IIIa receptor of platelets. ML were prepared by dual centrifugation (DC). ML pegylation up to a total PEG content of 7.5 mol% positively influenced the stability and amount of entrapped SPIOs, and also reduced SPIO-membrane interactions, while higher PEG contents destabilized PEG-ML. Stable anti-LIBS-ML with high amounts of entrapped SPIOs (∼86%, ∼0.22 mol Fe/mol liposomal lipid) and high MRI sensitivity (relaxivity r2 = 422 s(-1) mM(-1) and r2(∗) = 452 s(-1) mM(-1)) were obtained by coupling anti-LIBS to ML in a two-step post-insertion technique. We confirmed specific binding to the GPIIb/IIIa receptor's activated conformation on activated human platelets and cell lines expressing activated GPIIb/IIIa receptor ex vivo. The immuno-ML obtained in this study constitute an important step towards developing a potentially human-compatible MRI contrast agent for the timely detection of plaque rupture by targeting activated platelets.

Phased-array of microcoils allows MR microscopy of ex vivo human skin samples at 9.4 T.

PURPOSE: The aim of this study was to demonstrate the feasibility of a custom-made phased-array microcoil within a 400 MHz animal system for the morphological characterization of human skin tissue in correlation with histopathology. MATERIALS AND METHODS: A dedicated 7-channel microcoil-based MR detector arranged in a phased-array geometry was developed to combine the advantages of both a large field of view and a high signal-to-noise ratio. Standard gradient echo sequences were adapted for the characterization of skin morphology ex vivo. RESULTS: In this study, the feasibility of using this type of microdetector, combined with specially manufactured sample holders, to achieve high-resolution MR images of fresh and formalin-fixed, normal and hidradenitis suppurativa diseased skin was successfully demonstrated. The setup presented in this work allows reliable acquisitions of high-resolution images with in-plane resolution up to 25 × 25 µm², and 100 µm in the orthogonal direction, thereby allowing the differentiation of typical layers of the skin, sebaceous glands and hair follicle. CONCLUSION: This study demonstrates that MR microscopy on skin biopsies can be applied at low cost on a standard animal MR imaging system. The successful imaging of different skin structures ex vivo is a prerequisite for non-invasive, in vivo application of skin MR microscopy for accurate complementary disease diagnosis in dermatology.

Cell type-dependent pathogenic functions of overexpressed human cathepsin B in murine breast cancer progression.

The cysteine protease cathepsin B (CTSB) is frequently overexpressed in human breast cancer and correlated with a poor prognosis. Genetic deficiency or pharmacological inhibition of CTSB attenuates tumor growth, invasion and metastasis in mouse models of human cancers. CTSB is expressed in both cancer cells and cells of the tumor stroma, in particular in tumor-associated macrophages (TAM). In order to evaluate the impact of tumor- or stromal cell-derived CTSB on Polyoma Middle T (PyMT)-induced breast cancer progression, we used in vivo and in vitro approaches to induce human CTSB overexpression in PyMT cancer cells or stromal cells alone or in combination. Orthotopic transplantation experiments revealed that CTSB overexpression in cancer cells rather than in the stroma affects PyMT tumor progression. In 3D cultures, primary PyMT tumor cells showed higher extracellular matrix proteolysis and enhanced collective cell invasion when CTSB was overexpressed and proteolytically active. Coculture of PyMT cells with bone marrow-derived macrophages induced a TAM-like macrophage phenotype in vitro, and the presence of such M2-polarized macrophages in 3D cultures enhanced sprouting of tumor spheroids. We employed a doxycycline (DOX)-inducible CTSB expression system to selectively overexpress human CTSB either in cancer cells or in macrophages in 3D cocultures. Tumor spheroid invasiveness was only enhanced when CTSB was overexpressed in cancer cells, whereas CTSB expression in macrophages alone did not further promote invasiveness of tumor spheroids. We conclude that CTSB overexpression in the PyMT mouse model promotes tumor progression not by a stromal effect, but by a direct, cancer cell-inherent mode of action: CTSB overexpression renders the PyMT cancers more invasive by increasing proteolytic extracellular matrix protein degradation fostering collective cell invasion into adjacent tissue.

Enzymatic single-chain antibody tagging: a universal approach to targeted molecular imaging and cell homing in cardiovascular disease.

RATIONALE: Antibody-targeted delivery of imaging agents can enhance the sensitivity and accuracy of current imaging techniques. Similarly, homing of effector cells to disease sites increases the efficacy of regenerative cell therapy while reducing the number of cells required. Currently, targeting can be achieved via chemical conjugation to specific antibodies, which typically results in the loss of antibody functionality and in severe cell damage. An ideal conjugation technique should ensure retention of antigen-binding activity and functionality of the targeted biological component. OBJECTIVE: To develop a biochemically robust, highly reproducible, and site-specific coupling method using the Staphylococcus aureus sortase A enzyme for the conjugation of a single-chain antibody (scFv) to nanoparticles and cells for molecular imaging and cell homing in cardiovascular diseases. This scFv specifically binds to activated platelets, which play a pivotal role in thrombosis, atherosclerosis, and inflammation. METHODS AND RESULTS: The conjugation procedure involves chemical and enzyme-mediated coupling steps. The scFv was successfully conjugated to iron oxide particles (contrast agents for magnetic resonance imaging) and to model cells. Conjugation efficiency ranged between 50% and 70%, and bioactivity of the scFv after coupling was preserved. The targeting of scFv-coupled cells and nanoparticles to activated platelets was strong and specific as demonstrated in in vitro static adhesion assays, in a flow chamber system, in mouse intravital microscopy, and in in vivo magnetic resonance imaging of mouse carotid arteries. CONCLUSIONS: This unique biotechnological approach provides a versatile and broadly applicable tool for procuring targeted regenerative cell therapy and targeted molecular imaging in cardiovascular and inflammatory diseases and beyond.

Resorption rate assessment of adipose tissue-engineered constructs by intravital magnetic resonance imaging.

Engineering of adipose tissue by implantation of preadipocytes within biodegradable materials has already been extensively reported. However, a method that allows to accurately determine the resorption rate of adipose tissue constructs has not been described to date. The purpose of this study was to determine whether the non-invasive and non-destructive technique of magnetic resonance imaging (MRI) could be used to assess the resorption rate of adipose tissue substitutes after injection of human preadipocytes within fibrin into athymic nude mice. Different concentrations of undifferentiated preadipocytes were injected within fibrin into athymic nude mice. Two days, 3 months and 6 months post-implantation, the mice were anaesthetised and an MRI was performed using a 9.4 Tesla device in order to determine both volume and resorption rate of the implants. Subsequently, the specimens were explanted and qualitative analysis of adipose tissue formation was performed by histological examination. After implantation, a progressive resorption of all constructs was macroscopically observed. Implants could be easily visualised and delimited from the surrounding tissues by MRI. Magnetic resonance analysis demonstrated a resorption rate of the implants of 99-100% at 6 months, which was also confirmed by histological analysis. In the remaining implants, formation of human adipose tissue could be immunohistologically confirmed. Here, we show that MRI provides an efficient and non-invasive method for the assessment of implant resorption in adipose tissue engineering.

Characterization of a 3D MEMS fabricated micro-solenoid at 9.4 T.

We present for the first time a complete characterization of a micro-solenoid for high resolution MR imaging of mass- and volume-limited samples based on three-dimensional B(0), B(1) per unit current (B(1)(unit)) and SNR maps. The micro-solenoids are fabricated using a fully micro-electromechanical systems (MEMS) compatible process in conjunction with an automatic wire-bonder. We present 15 µm isotropic resolution 3D B(0) maps performed using the phase difference method. The resulting B(0) variation in the range of [-0.07 ppm to -0.157 ppm] around the coil center, compares favorably with the 0.5 ppm limit accepted for MR microscopy. 3D B(1)(unit) maps of 40 µm isotropic voxel size were acquired according to the extended multi flip angle (ExMFA) method. The results demonstrate that the characterized microcoil provides a high and uniform sensitivity distribution around its center (B(1)(unit) = 3.4 mT/A ± 3.86%) which is in agreement with the corresponding 1D theoretical data computed along the coil axis. The 3D SNR maps reveal a rather uniform signal distribution around the coil center with a mean value of 53.69 ± 19%, in good agreement with the analytical 1D data along coil axis in the axial slice. Finally, we prove the microcoil capabilities for MR microscopy by imaging Eremosphaera viridis cells with 18 µm isotropic resolution.

Imaging monocytes with iron oxide nanoparticles targeted towards the monocyte integrin MAC-1 (CD11b/CD18) does not result in improved atherosclerotic plaque detection by in vivo MRI.

Imaging of macrophages with superparamagnetic iron oxide particles (SPIO) has been performed to improve detection of atherosclerotic plaque inflammation in human and mouse studies by molecular magnetic resonance imaging (MRI). Since affinity of the monocyte/macrophage integrin MAC-1 (CD11b/CD18) is upregulated in inflammation, we generated a contrast agent targeting CD11b (CD11b-SPIOs) for improved macrophage detection in plaques. CD11b-SPIOs and non-targeted SPIOs (control-SPIOs) were incubated in vitro with human monocytes/macrophages. As quantified by SPIO-induced MRI signal extinction, intracellular iron-content was significantly higher in monoytes/macrophages incubated with CD11b-SPIO than with control-SPIO in vitro (p < 0.05), suggesting an improved uptake of CD11b-SPIOs into monocytes. Therefore, the aortic arch (AA) and vessel branches of ApoE(-/-)-knockout mice on a Western-type diet were imaged before and 48 h after contrast agent injection of either CD11b-SPIOs or control-SPIOs, using a 9.4 T animal MRI system. The SPIO-induced change in the MRI signal was quantified, as well as the macrophage-content by anti-CD68 immunhistochemistry and the iron-content by Prussian-blue staining. However, SPIO-induced signal extinction in in vivo-MRI was similar in CD11b-SPIO and control-SPIO-injected animals, with a non-significant trend towards an improved uptake of CD11b-SPIOs in the subclavian artery and subsections of the AA. These data correlated well with the results obtained by histology. Although in vitro MRI-data indicated an increased uptake of targeted CD11b-SPIOs in monocytes/macrophages, in vivo mouse data do not allow improved atherosclerotic plaque detection compared WITH non-targeted SPIOs. Therefore, CD11b-targeted MRI contrast labelling of monocytes/macrophages does not seem to be a successful strategy in stable atherosclerotic plaques such as found in the ApoE(-/-)-knockout-model. However, the impressive correlation between MRI and histology data encourages further development of inflammation- and plaque-specific contrast agents for vulnerable plaque imaging.

Magnetic resonance imaging contrast agent targeted toward activated platelets allows in vivo detection of thrombosis and monitoring of thrombolysis.

BACKGROUND: Platelets are the key to thrombus formation and play a role in the development of atherosclerosis. Noninvasive imaging of activated platelets would be of great clinical interest. Here, we evaluate the ability of a magnetic resonance imaging (MRI) contrast agent consisting of microparticles of iron oxide (MPIOs) and a single-chain antibody targeting ligand-induced binding sites (LIBS) on activated glycoprotein IIb/IIIa to image carotid artery thrombi and atherosclerotic plaques. METHODS AND RESULTS: Anti-LIBS antibody or control antibody was conjugated to 1-microm MPIOs (LIBS MPIO/control MPIO). Nonocclusive mural thrombi were induced in mice with 6% ferric chloride. MRI (at 9.4 T) was performed once before and repeatedly in 12-minute-long sequences after LIBS MPIO/control MPIO injection. After 36 minutes, a significant signal void, corresponding to MPIO accumulation, was observed with LIBS MPIOs but not control MPIOs (P<0.05). After thrombolysis, in LIBS MPIO-injected mice, the signal void subsided, indicating successful thrombolysis. On histology, the MPIO content of the thrombus, as well as thrombus size, correlated significantly with LIBS MPIO-induced signal void (both P<0.01). After ex vivo incubation of symptomatic human carotid plaques, MRI and histology confirmed binding to areas of platelet adhesion/aggregation for LIBS MPIOs but not for control MPIOs. CONCLUSIONS: LIBS MPIOs allow in vivo MRI of activated platelets with excellent contrast properties and monitoring of thrombolytic therapy. Furthermore, activated platelets were detected on the surface of symptomatic human carotid plaques by ex vivo MRI. This approach represents a novel noninvasive technique allowing the detection and quantification of platelet-containing thrombi.

Superparamagnetic iron oxide binding and uptake as imaged by magnetic resonance is mediated by the integrin receptor Mac-1 (CD11b/CD18): implications on imaging of atherosclerotic plaques.

INTRODUCTION: Superparamagnetic iron oxide nanoparticles (SPIONs) have been successfully used for magnetic resonance imaging (MRI) of atherosclerotic plaques. Endocytosis into monocytes/macrophages has been proposed as the mechanism for SPION uptake, but a specific receptor has not been identified yet. A potential candidate is the versatile integrin Mac-1 (CD11b/CD18, alphaMbeta2), which is involved in leukocyte adhesion, complement activation and phagocytosis. METHODS AND RESULTS: Intracellular SPION-accumulation was confirmed in cultured human monocytes using immunohistochemistry and iron staining. Recombinant cells expressing Mac-1 in different activation states as well as human monocytes with or without PMA stimulation were incubated either with an unspecific IgG or a CD11b-blocking antibody. Thereafter, cells were incubated with FITC-labeled amino-covered SPIONs or ferumoxtran-10 SPIONs and signal intensity was quantified by flow cytometry. Depending on the activation status of Mac-1, a significant increase in SPION binding/uptake was observed, independent on surface coating. Furthermore, SPION binding/uptake was significantly reduced after CD11b blockade. Results were confirmed in recombinant cells incubated with amino-PVA SPIONs and ferumoxtran-10, using T2(*)-weighted 3T MRI. CONCLUSION: The integrin Mac-1 is directly involved in SPION binding/uptake. Thus, monocytes abundantly expressing Mac-1 and especially activated monocytes expressing activated Mac-1 may be useful vehicles for high resolution MRI labeling of atherosclerotic plaques.