MR Microimaging Research Articles

Solid-state 31P and 1H chemical MR micro-imaging of hard tissues and biomaterials with magic angle spinning at very high magnetic field

In this work, we show that it is possible to overcome the limitations of solid-state MRI for rigid tissues due to large line broadening and short dephasing times by combining Magic Angle Spinning (MAS) with rotating pulsed field gradients. This allows recording ex vivo31P 3D and 2D slice-selected images of rigid tissues and related biomaterials at very high magnetic field, with greatly improved signal to noise ratio and spatial resolution when compared to static conditions. Cross-polarization is employed to enhance contrast and to further depict spatially localized chemical variations in reduced experimental time. In these materials, very high magnetic field and moderate MAS spinning rate directly provide high spectral resolution and enable the use of frequency selective excitation schemes for chemically selective imaging. These new possibilities are exemplified with experiments probing selectively the 3D spatial distribution of apatitic hydroxyl protons inside a mouse tooth with attached jaw bone with a nominal isotropic resolution nearing 100 µm.

Metabolic Study of Breast MCF-7 Tumor Spheroids after Gamma Irradiation by (1)H NMR Spectroscopy and Microimaging.

Multicellular tumor spheroids are an important model system to investigate the response of tumor cells to radio- and chemotherapy. They share more properties with the original tumor than cells cultured as 2D monolayers do, which helps distinguish the intrinsic properties of monolayer cells from those induced during cell aggregation in 3D spheroids. The paper investigates some metabolic aspects of small tumor spheroids of breast cancer and their originating MCF-7 cells, grown as monolayer, by means of high–resolution (HR) 1H NMR spectroscopy and MR microimaging before and after gamma irradiation. The spectra of spheroids were characterized by higher intensity of mobile lipids, mostly neutral lipids, and glutamine (Gln) signals with respect to their monolayer cells counterpart, mainly owing to the lower oxygen supply in spheroids. Morphological changes of small spheroids after gamma-ray irradiation, such as loss of their regular shape, were observed by MR microimaging. Lipid signal intensity increased after irradiation, as evidenced in both MR localized spectra of the single spheroid and in HR NMR spectra of spheroid suspensions. Furthermore, the intense Gln signal from spectra of irradiated spheroids remained unchanged, while the low Gln signal observed in monolayer cells increased after irradiation. Similar results were observed in cells grown in hypoxic conditions. The different behavior of Gln in 2D monolayers and in 3D spheroids supports the hypothesis that a lower oxygen supply induces both an upregulation of Gln synthetase and a downregulation of glutaminases with the consequent increase in Gln content, as already observed under hypoxic conditions. The data herein indicate that 1H NMR spectroscopy can be a useful tool for monitoring cell response to different constraints. The use of spheroid suspensions seems to be a feasible alternative to localized spectroscopy since similar effects were found after radiation treatment.

Enhanced characterization of the zebrafish brain as revealed by super-resolution track-density imaging

In this study, we explored the use of super-resolution track-density imaging (TDI) for neuroanatomical characterization of the adult zebrafish brain. We compared the quality of image contrast and resolution obtained with T2* magnetic resonance imaging (MRI), diffusion tensor-based imaging (DTI), TDI, and histology. The anatomical structures visualized in 5 μm TDI maps corresponded with histology. Moreover, the super-resolution property and the local-directional information provided by directionally encoded color TDI facilitated delineation of a larger number of brain regions, commissures and small white matter tracks when compared to conventional MRI and DTI. In total, we were able to visualize 17 structures that were previously unidentifiable using MR microimaging, such as the four layers of the optic tectum. This study demonstrates the use of TDI for characterization of the adult zebrafish brain as a pivotal tool for future phenotypic examination of transgenic models of neurological diseases.

Gadolinium-induced fibrosis testing by protein targeting assay and superparamagnetic iron oxide nanoparticle-based magnetic resonance microscopy of skin and kidneys

Abstract Gadolinium-based magnetic resonance imaging (MRI) contrast agents cause undefined fibrosis in kidneys and skin damage. Magnetic resonance microimaging of rat skin and kidney was used first time to identify the physical factors modulating the gadolinium Omniscan®-induced fibrosis by protein targeting. A 500-MHz MR imaging was done to visualize fibrosis in gadolinium-treated animals. Cationic superparamagnetic iron oxide magnetoferritin (SPIOM) was injected in rat to target basement membrane (in rat kidney and different skin structures including epidermis glycolipids and dermis proteins. After MR imaging, excised rat skin and kidneys tissues were imaged by ex vivo 900 MHz MR microimaging to confirm renal fibrosis and skin epidermis thickening. The proton density-weighted images visualized micro details of skin structures and nephron territories while T2-weighted images showed better contrast of tissue structures in both skin and kidney. The gadolinium further enhanced the image contrast and targeted the proteins in renal basement membrane and viable proteins in epidermis. SPIOM enhanced the tissue contrast due to dephasing effect caused by SPIOM on structural changes in nephron and epidermis. Conclusion: Tissue membrane protein and chelate ligand group binding with gadolinium biophysical interaction at molecular level may develop fibrosis. SPIOM injection improved the dephased image contrast of different structures in both skin and nephrons. The epidermis thickening and nephrofibrosis changes may be associated with nephrogenic systemic fibrosis or fibrosing dermatopathy.

21 Tesla Rat Heart Magnetic Resonance Microimaging by Paramagnetic Anti-Troponin Bound Polyethylene Based Iron-Oxide Nanoparticles and Image Processing

Abstract21T MR microimaging with the first use of troponin nanoparticles enhances the visualization of cardiac muscle fibers and offers technical advancement for the future. Diffusion weighting offers fiber tracking and functional analysis. Image processing offers heart probabilistic atlas and maps.http://www.ismrm.org/10/program/ELECTRONICPOSTERS.htm

Microcoil-based MRI: feasibility study and cell culture applications using a conventional animal system

The aim of this study was to demonstrate the feasibility of MR microimaging on a conventional 9.4 T horizontal animal MRI system using commercial available microcoils in combination with only minor modifications to the system, thereby opening this field to a larger community. Commercially available RF microcoils designed for high-resolution NMR spectrometers were used in combination with a custom-made probehead. For this purpose, changes within the transmit chain and modifications to the adjustment routines and image acquisition sequences were made, all without requiring expensive hardware. To investigate the extent to which routine operation and high-resolution imaging is possible, the quality of phantom images was analysed. Surface and solenoidal microcoils were characterized with regard to their sensitive volume and signal-to-noise ratio. In addition, the feasibility of using planar microcoils to achieve high-resolution images of living glioma cells labelled with MnCl(2) was investigated. The setup presented in this work allows routine acquisition of high-quality images with high SNR and isotropic resolutions up to 10μm within an acceptable measurement time. This study demonstrates that MR microscopy can be applied at low cost on animal MR imaging systems, which are in widespread use. The successful imaging of living glioma cells indicates that the technique promises to be a useful tool in biomedical research.

MR microimaging detects subfield-specific neuronal loss in the hippocampus

MR microimaging detects subfield-specific neuronal loss in the hippocampus

Abstract LB-382: MR microscopy detects architectural changes in the pancreas of mutant Kras mouse models of pancreatic cancer

Abstract Pancreatic cancer (PC) is a devastating disease due to the lack of early detection resulting in late diagnosis and poor prognosis. Thus, there is a need for a technology that can visualize morphological and cellular changes in the pancreas when the cancer cells are at a pre-invasive stage. Nearly 90% of human PC have a mutation in Kras and several mouse models that express mutant Kras develop precancerous lesions resembling those in humans. Using MR microimaging, we examined pancreata from EL-Kras (ELK) and Pdx1-Cre/LSL-Kras (PCK) mice which target mutant Kras to the pancreas via unique approaches. EL targeting employs human mutant Kras where nearly all acinar cells express this oncogene, while Pdx1 targeting uses a mutation in an endogenous mouse Kras allele. The source (human vs. mouse), targeting (EL vs. Pdx1) and delivery (artificial vs. endogenous) of mutant Kras contribute to altered phenotypes including differences in normal appearing parenchyma, frequency of acinar-ductal metaplasia, precancerous lesion formation, and progression to advanced disease. We examined pancreas from both mutant Kras mouse models using MR microimaging and compared their MR signature to that of control mice. Fixed pancreata were imaged on a 14.1T microimager. T2-weighted, high spatial resolution images were acquired with fat suppression using a fast spin-echo 3D imaging protocol and isotropic pixel size of ∼70μm. MR data was reconstructed to provide volume images of the pancreatic components and their spatial relationship to each other. T2-weighted MR microimages had high spatial resolution and excellent endogenous contrast that enabled the visualization of acinar cells, islets and stroma in the normal pancreas. T2-weighted images of ELK pancreas showed poor contrast and delineation between acinar cells, islets and stroma indicating changes in their T2-relaxation times possibly as a result of changes at the cellular level. Contrast between pancreatic components in the PCK pancreas was similar to control. Volume images of the pancreas from both mutant Kras mice showed unique differences from control in stromal architecture. While the normal mouse pancreas had a dense, concentrated network of stroma, the ELK mouse pancreas showed only a sparse network with reduced signal intensity, and the PCK mouse pancreas had a more extensive network of stroma. These differences in stromal architecture of mutant Kras pancreas detected by MR microscopy most likely represent transgene-induced alterations in the composition of extracellular matrix (ECM) and might have implications for the development of pancreatic pre-cancerous lesions and cancer. Ours is the first report of MR microimaging of ECM in the intact mouse pancreas of relevant mouse models of PC. Deciphering the differences in ECM among these two models is the focus of ongoing work. Our results suggest that MR volume imaging at high spatial resolution may have potential in detecting morphological and cellular changes associated with pre-invasive and invasive stages of PC. (Supported by Jay Wertheimer Pancreatic Cancer Research Fund and NIH S10 RR13880) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-382.

MR Microimaging of amyloid plaques in Alzheimer’s disease transgenic mice

Alzheimer's disease (AD) is the most prevalent neurological condition affecting industrialized nations and will rapidly become a healthcare crisis as the population ages. Currently, the post-mortem histological observation of amyloid plaques and neurofibrillary tangles is the only definitive diagnosis available for AD. A pre-mortem biological or physiological marker specific for AD used in conjunction with current neurological and memory testing could add a great deal of confidence to the diagnosis of AD and potentially allow therapeutic intervention much earlier in the disease process. Our group has developed MRI techniques to detect individual amyloid plaques in AD transgenic mouse brain in vivo. We are also developing contrast-enhancing agents to increase the specificity of detection of amyloid plaques. Such in vivo imaging of amyloid plaques will also allow the evaluation of anti-amyloid therapies being developed by the pharmaceutical industry in pre-clinical trials of AD transgenic mice. This short review briefly discusses our progress in these areas.

Diffusion tensor imaging of articular cartilage as a measure of tissue microstructure

To use diffusion tensor MR micro-imaging to observe differences in magnitude and anisotropy of water diffusion between 'healthy' cartilage and cartilage enzymatically degraded to simulate arthritic damage. Diffusion tensor images (156 x 156 microm in-plane resolution, 2mm slice thickness) of bovine cartilage were obtained at either 4.7 or 7.0 T using pulsed field gradient spin echo sequences. The parameters determined were: maximum and mean diffusivity, direction of the maximum diffusion eigenvector with respect to the normal to the articular surface and fractional anisotropy (FA) of diffusion. Both maximum and mean diffusion eigenvalues were found to decrease, respectively, from approximately 1.95 x 10(-9) and 1.80 x 10(-9) m2 s(-1) at the articular surface to approximately 1.08 x 10(-9) and 0.79 x 10(-9) m2 s(-1) in the deep zone. A systematic change was observed in the direction of the eigenvector corresponding to maximum diffusivity, reflecting the expected change in orientation of the collagen macrofibrillar bundles. Degradation with trypsin to remove proteoglycans resulted in a 10-15% increase in apparent diffusion coefficient of water in the cartilage, with no apparent change in FA. These methods have the potential to be used to probe local changes in tissue microstructure and the hydrodynamic status of cartilage during development of osteoarthritis.

MR Microimaging of Benign and Malignant Nodes in the Neck

We evaluated the diagnostic criteria of high-resolution MRI in differentiating benign and malignant cervical nodes that were palpable and superficial in the neck. We performed MR microimaging on 24 histologically proven metastatic nodes, 14 histologically proven lymphomas, and 35 histologically or clinically proven benign nodes in the necks of 26 patients. The lymph nodes were imaged with T1-weighted spin-echo, fat-suppressed T2-weighted turbo spin-echo, and spin-echo diffusion-weighted echo-planar sequences using a 47-mm microscopy coil. MR microimaging provided high-resolution images of the nodes. Hilar fat was lost in 92%, 79%, and 46% of the metastatic nodes, lymphomas, and benign nodes, respectively. Smooth nodal margins were lost in 58%, 23%, and 9% of metastatic nodes, lymphomas, and benign nodes, respectively. Heterogeneous nodal parenchyma on T1- or fat-suppressed T2-weighted images, or both, was observed in 88%, 29%, and 23% of metastatic nodes, lymphomas, and benign nodes, respectively. The apparent diffusion coefficients were significantly different among these three node groups (p < 0.001), with metastatic nodes being the highest, followed by benign nodes. Logistic regression analyses showed that heterogeneous nodal parenchyma and apparent diffusion coefficient levels were significant in discriminating metastatic nodes, and apparent diffusion coefficient levels in discriminating lymphomas. Combined use of these MR microscopic criteria on nodal architecture and apparent diffusion coefficients yielded 90% accuracy (86% sensitivity, 94% specificity) and 93% accuracy (85% sensitivity, 95% specificity) for discriminating metastatic nodes and lymphomas, respectively. The nodal architecture and apparent diffusion coefficient levels on MR microimaging may provide useful information in diagnosing benign and malignant nodes in the neck.

Functional MR Microimaging of Pancreatic β-Cell Activation

The increasing incidence of diabetes and the need to further understand its cellular basis has resulted in the development of new diagnostic and therapeutic techniques. Nonetheless, the quest to noninvasively ascertain beta-cell mass and function has not been achieved. Manganese (Mn)-enhanced MRI is presented here as a tool to image beta-cell functionality in cell culture and isolated islets. Similar to calcium, extracellular Mn was taken up by glucose-activated beta-cells resulting in 200% increase in MRI contrast enhancement, versus nonactivated cells. Similarly, glucose-activated islets showed an increase in MRI contrast up to 45%. Although glucose-stimulated Ca influx was depressed in the presence of 100 microM Mn, no significant effect was seen at lower Mn concentrations. Moreover, islets exposed to Mn showed normal glucose sensitivity and insulin secretion. These results demonstrate a link between image contrast enhancement and beta-cell activation in vitro, and provide the basis for future noninvasive in vivo imaging of islet functionality and beta-cell mass.

Diffusion-Weighted MR Microimaging of the Lacrimal Glands in Patients with Sjögren's Syndrome

The purpose of this study was to detect quantitative diffusion-weighted abnormalities in the lacrimal glands of patients with Sjogren's syndrome. Diffusion-weighted MRI was performed on 31 healthy volunteers and 11 Sjogren's syndrome patients with impaired lacrimal function. The volunteers and patients underwent MRI with a single-shot spin-echo echo-planar technique using a 47-mm microscopy coil. The apparent diffusion coefficient (ADC) of the lacrimal and parotid glands was obtained with b factors of 500 and 1,000 sec/mm(2). T1-weighted and fat-suppressed T2-weighted MR microscopic images were also obtained to evaluate the gland morphology and signals. MR microscopy provided high-resolution images of the lacrimal glands that enabled ADC measurements. The ADCs of the normal lacrimal glands showed no significant sex- or age-related changes. The ADCs for the lacrimal glands were significantly higher than those of the parotid glands in the same subjects (mean +/- SD, 891 +/- 103 vs 703 +/- 84 x 10(-6) mm(2)/sec, respectively; p < 0.0001, Mann-Whitney U test). We found that ADCs of the lacrimal glands in Sjogren's syndrome patients were significantly lower than those from the normal glands of age-matched healthy volunteers (736 +/- 34 vs 923 +/- 84 x 10(-6) mm(2)/sec; p < 0.0001, Mann-Whitney U test). These findings suggest that the measurement of ADCs may be a useful tool to assess abnormalities of the lacrimal glands in patients with Sjogren's syndrome.

Water uptake and oil distribution during imbibition of seeds of western white pine (Pinus monticola Dougl. ex D. Don) monitored in vivo using magnetic resonance imaging

Dry or fully imbibed seeds of western white pine (Pinus monticola Dougl. ex D. Don) were studied using high-resolution magnetic resonance imaging (MRI). Analyses of the dry seed revealed many of the gross anatomical features of seed structure. Furthermore, the non-invasive nature of MRI allowed for a study of the dynamics of water and oil distribution during in situ imbibition of a single seed with time-lapse chemical shift selective MRI. During soaking of the dry seed, water penetrated through the seed coat and megagametophyte. The cotyledons of the embryo (located in the chalazal end of the seed) were the first to show hydration followed by the hypocotyl and later the radicle. After penetrating the seed coat, water in the micropylar end of the seed likely also contributed to further hydration of the embryo; however, the micropyle itself did not appear to be a site for water entry into the seed. A model that describes the kinetics of the earlier stages of imbibition is proposed. Non-viable pine seeds captured with MRI displayed atypical imbibition kinetics and were distinguished by their rapid and uncontrolled water uptake. The potential of MR microimaging for detailed studies of water uptake and distribution during the soaking, moist chilling ("stratification"), and germination of conifer seeds is discussed.

Intermolecular double-quantum coherence MR microimaging of pig tail with unique image contrast.

Image contrast in intermolecular double-quantum coherence (iDQC) imaging of a pig tail was investigated on a 7.05-T microimaging scanner. In addition to TR (repetition time) and TE (echo time), the time interval tau between radio frequency pulses during iDQC evolution and the areas under the iDQC-encode gradients in the iDQC imaging sequence were also used to manipulate image contrast. When suitable imaging parameters were selected, images with unique contrast, such as those with certain regions of the sample highlighted, were obtained without using contrast agents. The effects of iDQC-encode gradient on image contrast were studied quantitatively, and the unique contrast imposed by the related diffusion weighting was also shown. Experimental results demonstrated that the iDQC images have contrast fundamentally different from the conventional single-quantum coherence images.

Analysis of the diffusion weighted MR microscopy data of excised spinal cord of a rat on the basis of the model of restricted diffusion

Anisotropic diffusion in the excised rat spinal cord saturated with 0.9% saline was investigated using MR microimaging with b-values up to 8000 s/mm 2 for different diffusion times. Non-exponential transversal diffusion decay found in white matter (WM) and gray matter (GM) was fitted with 2 components (the “fast” and the “slow”). Significantly smaller non-exponential dependence was found for the longitudinal diffusion in the WM. Obtained results corresponding to restricted diffusion in the range from ∼2 to ∼7 μm were correlated with axon diameter distribution in the WM obtained from transmission electron micrographs. It was concluded that observed diffusion anisotropy in the spinal cord might be entirely explained by presence of the slow transversal component, arising from the restricted diffusion. The strict analytical description of the diffusion decay in nervous tissue requires taking into account continuous distribution of the space-scale of the restricting barriers. The simplified two-component analysis may be applicable for visualization of the nervous tissue in clinical practice.

Receptor-mediated endocytosis of iron-oxide particles provides efficient labeling of dendritic cells for in vivo MR imaging.

Dendritic cells (DCs) function as antigen presenting cells in vivo and play a fundamental role in numerous diseases. New methods are described for high-efficiency intracellular labeling of DCs with superparamagnetic iron-oxide (SPIO) utilizing a receptor-mediated endocytosis (RME) mechanism. Bone marrow-derived DCs or a fetal skin-derived DC line were incubated with SPIO conjugated to anti-CD11c monoclonal antibody (mAb) under conditions favoring RME. These cells exhibited approximately a 50-fold increase in uptake relative to DCs incubated with SPIO without the mAb. Flow cytometry studies assaying cell surface markers showed a down-modulation of CD11c, but no other changes in phenotype. Immunological function of the DCs was unmodified by the labeling, as determined by cytokine secretion assays. The RME mechanism was confirmed using electron microscopy, endocytosis inhibition assays, and incubation experiments with SPIO conjugated to mAbs against accessory molecules that are not expressed on DCs. Labeled DCs were injected into murine quadriceps and monitored in vivo for several days using MR microimaging at 11.7 T. DCs were observed to remain within the muscle for >24 hr. The use of RME is an efficient way to label immune cells for in vivo MRI and can be applied to a wide variety of cell types.

Three-Dimensional Diffusion Tensor Magnetic Resonance Microimaging of Adult Mouse Brain and Hippocampus

The use of diffusion tensor information as an additional contrast in MR microimaging was investigated in ex vivo mouse brain and isolated hippocampus. Diffusion Tensor Imaging (DTI) provided unique contrast to identify many internal structures of the gray matter such as hippocampus, thalamus, and cortex. In hippocampus, stratum granulosum and stratum pyramidale could be identified using the isotropic water diffusion constant. Stratum moleculare and stratum radiatum were identified from their characteristic fiber architecture revealed by color-coded DTI. Identification of these structures allowed reconstruction of their 3-D volume. Thus, high-resolution DTI has excellent potential as a tool for 3-D characterization of murine brains.

Ex vivo MR microimaging of neuronal damage after kainate‐induced status epilepticus in rat: Correlation with quantitative histology

The present study was designed to investigate whether T2-weighted signal changes obtained by microimaging of paraformaldehyde-fixed brain correlate with the histologically quantified damage in a model of status epilepticus (SE) induced by kainic acid in the rat. Animals were killed at several time points up to 8 weeks after a single intraperitoneal kainate (KA) injection (9 mg/kg). Perfusion-fixed brains were embedded in gelatin for MR microimaging at 9.4T. After the MRI analysis, the gelatin was removed and the brains were cryoprotected and processed for quantitative histology. Severity of neuronal damage and gliosis were assessed from thionin-stained serial sections. Correlative analysis of microimaging and histology data was done in the hippocampus, amygdala, parietal rhinal cortex (PaRH), piriform cortex (Pir), and entorhinal cortex. The relative signal intensities in T2-weighted images correlate with the severity of neuronal damage in the matched histological sections (correlation coefficients of 0.752–0.826). Our data show that MR microimaging ex vivo detects the degree of neuronal damage and its anatomical distribution after KA-induced SE, thus providing a useful tool for detecting the dynamics of progressive neuronal damage after prolonged seizures. Magn Reson Med 46:946–954, 2001. © 2001 Wiley-Liss, Inc.

In vivo magnetic resonance micro-imaging of the human toe at 3 tesla

The feasibility of in vivo high-resolution magnetic resonance micro-imaging of fine anatomic structures of human toes was tested. Five healthy subjects were investigated on an experimental 3 Tesla whole body scanner, using standard 3D gradient echo sequences. A radio-frequency surface coil was used for signal detection. Feet, toes and surface coil were comfortably fixed using a home built device for positioning and reduction of motion artifacts. The spatial resolution of 117 × 313 × 375 μm 3 allowed detailed visualization of anatomic structures like skin layers, vessels and nerves. In addition, oval structures with diameters ranging from 500 to 1000 μm were observed in all subjects, which could represent the sensory nerve endings of Vater-Pacinian bodies. Thus, high resolution MR micro-imaging at 3 Tesla may provide improved morphologic information in distal extremities of humans in vivo.