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Query: UMLS:C0276640 (TEM)
 20,729 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

R(F) performance in high-field MRI applications is improved by filling the resonator with material of relative dielectric constant approximating that of human soft tissue. We demonstrate this by filling a toroidal cavity resonator operating in TEM(00) (cyclotron) mode with titanium dioxide (TiO(2)) in powdered rutile form, and acquiring phantom, human lower leg and human breast images of good quality at 4.1 T. Images made with this resonator had unusually high SNR, while the level of R(f) power required to produce a 90 degrees flip angle pulse was about a quartes as high for the filled resonator as for the same resonator before filling. Phantom images obtained with the filled resonator had an SNR of nearly 300 at a resolution of 256 x 256 voxels, nearly three times that of images of the same phantom obtained using a standard volume R(f) coil in frequent use at this laboratory. Breast images made at 256 x 256 voxels resolution had an SNR of 174, also unusually high for a volume coil. High-resolution (512 x 512 voxels) were also obtained, with SNR = 60. Preliminary phantom and in vivo human images are presented in this article. Acquiring the phantom and leg images required significantly less R(f) power than did comparable imaging using a conventional coil. In addition, the field lines were focused as they penetrated into the sample, and this resulted in a more homogeneous B(1)-field. We believe that these improvements occurred because the dielectric presence minimizes the large dielectric mismatch between air and sample.
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PMID:A TiO(2) dielectric filled toroidal radio frequency cavity resonator for high-field NMR. 1135 83

B(1) Field inhomogeneity and the relative effects of dielectric resonances are analyzed within the context of ultra high field MRI. This is accomplished by calculating the electromagnetic fields inside spherical phantoms and within a human head model in the presence and absence of an RF coil. These calculations are then compared to gradient echo and RARE images, respectively. For the spherical phantoms, plane incident wave analyses are initially presented followed by full wave finite difference time domain (FDTD) calculations. The FDTD methods are then utilized to examine the electromagnetic interactions between the TEM resonator and an anatomically detailed human head model. The results at 340 MHz reveal that dielectric resonances are most strongly excited in objects similar in size to the human head when the conducting medium has a high dielectric constant and a low conductivity. It is concluded that in clinical UFHMRI, the most important determinants of B(1) field homogeneity consist of 1) the RF coil design, 2) the interaction between the RF coil, the excitation source and the sample, and finally 3) the geometry and electrical properties of the sample.
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PMID:Dielectric resonances and B(1) field inhomogeneity in UHFMRI: computational analysis and experimental findings. 1135 60

We present an analytic solution for the B1 field produced in a gapped toroidal cavity resonator designed as a probe for high field MRI. This resonator supports standing TEM waves, so its electric and magnetic fields are identical to those produced by a stationary planar current source with the same (constant) cross-section multiplied by a complex exponential propagation factor. An explicit expression for the field may therefore be found by solving Laplace's equation for the static potential, which is accomplished with a two-dimensional logarithmic conformal transformation algorithm. The equipotential curves are also the contours of the field strength B, and the B (vector) field at any point is directed along the contour passing through that point. With this information, we construct the solution by computing the angle made by the equipotential curve with the horizontal axis at each point, using this angle to analyze the B field into its x and y components, and adding the contributions from the current sources to obtain the magnitude and direction of B at each point in the region of interest. Some proposed extensions of this algorithm are also discussed.
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PMID:Computing the B1 field of the toroidal MRI coil. 1586 92

In the current study, amine surface modified iron-oxide nanoparticles of 6 nm diameter without polymer coating were fabricated in an aqueous solution by organic acid modification as an adherent following chemical coprecipitation. Structure and the superparamagnetic property of magnetite nanoparticles were characterized by selected area electron diffraction (SAED) and superconducting quantum interference measurement device (SQUID). X-ray photoelectron spectrometer (XPS) and zeta potential measurements revealed cationic surface mostly decorated with terminal -NH(3)(+). This feature enables them to function as a magnetic carrier for nucleotides via electrostatic interaction. In addition, Fe(3)O(4)/trypsin conjugates with well-preserved functional activity was demonstrated. The nanoparticles displayed excellent in vitro biocompatibility. The NMR and the in vitro MRI measurements showed significantly reduced water proton relaxation times of both T(1) and T(2). Significantly reduced T(2) and T(2)*-weighted signal intensity were observed in a 1.5 T clinical MR imager. In vivo imaging contrast effect showed a fast and prolonged inverse contrast effect in the liver that lasted for more than 1 week. In addition, it was found that the spherical Fe(3)O(4) assembled as rod-like configuration through an aging process in aqueous solution at room temperature. Interestingly, TEM observation of the liver tissue revealed the rod-like shape but not the spherical-type nanoparticles being taken up by the Kupffer cells 120 h after tail vein infusion. Combining these results, we have demonstrated the potential applications of the newly synthesized magnetite nanoparticles in a broad spectrum of biomedical applications.
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PMID:Aqueous dispersions of magnetite nanoparticles with NH3+ surfaces for magnetic manipulations of biomolecules and MRI contrast agents. 1596 22

The literature concerning nuclear cardiology and cardiac MRI has been particularly rich in the fields of diagnosis, prognosis and therapeutic evaluation of coronary artery disease and cardiac failure. Almost 18 million 'conventional' myocardial scintigraphies (SPECT-single photon emission tomography, or TEM: tomography by monophotonic emission in French) are routinely performed worldwide each year. Nuclear cardiology represents the 3rd scientific domain of application for scintigraphy, after oncology and neurology. The advent of new conventional gamma cameras and PET (positron emission tomography) combined with CT will allow considerable improvement in the quality of investigation in obese or tri-truncal patients and women. We will limit ourselves to original clinical studies, based on scintigraphical techniques or magnetic resonance imaging, applied to the classic cardiological themes: myocardial infarction and ischaemia, cardiomyopathy and cardiac failure. We will also consider the new directions in nuclear cardiology regarding a new tracer and some innovative technology: rubidium-82 and TEP-CT.
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PMID:[The best of nuclear cardiology and MRI in 2005]. 1647 61

SPION with appropriate surface chemistry have been widely used experimentally for numerous in vivo applications. In this study, SPION stabilized by alginate (SPION-alginate) were prepared by a modified coprecipitation method. The structure, size, morphology, magnetic property and relaxivity of the SPION-alginate were characterized systematically by means of XRD, TEM, ESEM, AFM, DLS, SQUID magnetometer and MRI, respectively, and the interaction between alginate and iron oxide (Fe(3)O(4)) was characterized by FT-IR and AFM. The results revealed that typical iron oxide nanoparticles were Fe(3)O(4) with a core diameter of 5-10 nm and SPION-alginate had a hydrodynamic diameter of 193.8-483.2 nm. From the magnetization curve, the Ms of a suspension of SPION-alginate was 40 emu/g, corresponding to 73% of that of solid SPION-alginate. This high Ms may be due to the binding of Fe(3)O(4) nanoparticles to alginate macromolecule strands as visually confirmed by AFM. SPION-alginate of several hundred nanometers was stable in size for 12 months at 4 degrees C. Moreover, T1 relaxivity and T2 relaxivity of SPION-alginate in saline (1.5 T, 20 degrees C) were 7.86+/-0.20 s(-1) mM(-1) and 281.2+/-26.4 s(-1) mM(-1), respectively.
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PMID:Preparation and characterization of superparamagnetic iron oxide nanoparticles stabilized by alginate. 1707 54

In high-field MRI severe problems with respect to B(1) (+) uniformity and specific absorption rate (SAR) deposition pose a great challenge to whole-body imaging. In this study the potential of a phased array transmit coil is investigated to simultaneously reduce B(1) (+) nonuniformity and SAR deposition. This was tested by performing electromagnetic simulations of a phased array TEM coil operating at 128 MHz loaded with two different homogeneous elliptical phantoms and four dielectric patient models. It was shown that the wave interference of a circularly polarized RF field with an ellipse and a pelvis produces largely identical B(1) (+) and electric field patterns. Especially for obese patients, this results in large B(1) (+) nonuniformity and global areas with elevated SAR deposition. It is demonstrated that a phased array transmit coil can reduce these phenomena. The technique was especially successful in suppressing SAR hotspots with a decrease up to 50%. The application of optimized settings for an ellipse to the patient models leads to comparable results as obtained with the patient-specific optimizations. This suggests that generic phase/amplitude port settings are possible, requiring no preinformation about patient-specific RF fields. Such a scheme would, due to its simultaneous B(1) (+) homogenization and extra SAR margin, have many benefits for whole-body imaging at 3 T.
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PMID:Simultaneous B1 + homogenization and specific absorption rate hotspot suppression using a magnetic resonance phased array transmit coil. 1732 85

The novel Gd(III) complexes of heptadentate ligands NE3TA and NE3TA-Bn were prepared, and their relaxivities were measured and favorably compared to the commercially available MRI contrast enhancement agent Gd(DOTA). NE3TA was conjugated with cholic acid (CA) to produce CA-NE3TA. TEM images of Gd(CA-NE3TA) indicate that the complex self-assembles forming nano-sized micelles and displays an over threefold increased relaxivity compared to Gd(DOTA). The new cholic acid-conjugated nanoparticle MR contrast enhancement agent, Gd(CA-NE3TA) possesses great promise for use in targeted MRI.
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PMID:A novel cholic acid-based contrast enhancement agent for targeted MRI. 1833 94

Three hyperbranched fluoropolymers were synthesized and their micelles were constructed as potential (19)F MRI agents. A hyperbranched star-like core was first synthesized via atom transfer radical self-condensing vinyl (co)polymerization (ATR-SCVCP) of 4-chloromethyl styrene (CMS), lauryl acrylate (LA), and 1,1,1-tris(4'-(2''-bromoisobutyryloxy)phenyl)ethane (TBBPE). The polymerization gave a small core with M n of 5.5 kDa with PDI of 1.6, which served as a macroinitiator. Trifluoroethyl methacrylate (TFEMA) and tert-butyl acrylate (tBA) in different ratios were then "grafted" from the core to give three polymers with M(n) of about 120 kDa and PDI values of about 1.6-1.8. After acidolysis of the tert-butyl ester groups, amphiphilic, hyperbranched star-like polymers with M(n) of about 100 kDa were obtained. These structures were subjected to micelle formation in aqueous solution to give micelles having TEM-measured diameters ranging from 3-8 nm and DLS-measured hydrodynamic diameters from 20-30 nm. These micelles gave a narrow, single resonance by (19)F NMR spectroscopy, with a half-width of approximately 130 Hz. The T1/T2 parameters were about 500 and 50 ms, respectively, and were not significantly affected by the composition and sizes of the micelles. (19)F MRI phantom images of these fluorinated micelles were acquired, which demonstrated that these fluorinated micelles maybe useful as novel (19)F MRI agents for a variety of biomedical studies.
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PMID:Amphiphilic hyperbranched fluoropolymers as nanoscopic 19F magnetic resonance imaging agent assemblies. 1879 85

In the field of nanomedicine there is a great demand for technologies that allow the creation of self-assembled structures of which the size and morphology can be accurately controlled. In the current study, we report a nanoparticle platform that is composed of a paramagnetic lipid and a fluorescently labeled lipopeptide. By judiciously controlling the ratio of the aforementioned amphiphilic molecules, a variety of well-defined nanosized supramolecular structures with different sizes and morphologies could be created. The hydrodynamic radii of the different structures were determined by dynamic light scattering. Cryo-TEM revealed the aggregate morphology to vary from small micellar structures to plate-like and even full grown ribbons of which the aspect ratios varied from a diameter of 5-8 nm to structures with a width of up to 25 nm and infinite length. Interestingly, nuclear magnetic resonance dispersion profiling revealed excellent properties for MRI and also showed that the relaxivity of the structures was tunable and morphology dependent. Finally, macrophage cells were treated with two selected nanoparticles and were shown to be avidly taken up. In conclusion we demonstrate a methodology to create structures that (1) are paramagnetic to enable their detection with MRI, (2) exhibit fluorescent properties, (3) can be tuned to defined sizes and shapes, and (4) are efficiently taken up by macrophage cells in vitro.
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PMID:Well-defined, multifunctional nanostructures of a paramagnetic lipid and a lipopeptide for macrophage imaging. 1910 54


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