NMR spiral surface microcoils: Applications

Cell Viability and Noninvasive In Vivo MRI Tracking of 3D Cell Encapsulating Self-Assembled Microcontainers

Several molecular therapies require the implantation of cells that secrete biotherapeutic molecules and imaging the location and microenvironment of the cellular implant to ascertain its function. We demonstrate noninvasive in vivo magnetic resonance imaging (MRI) of self-assembled microcontainers that are capable of cell encapsulation. Negative contrast was obtained to discern the microcontainer with MRI; positive contrast was obtained in the complete absence of background signal. MRI on a clinical scanner highlights the translational nature of this research. The microcontainers were loaded with cells that were dispersed in an extracellular matrix, and implanted both subcutaneously and in human tumor xenografts in SCID mice. MRI was performed on the implants, and microcontainers retrieved postimplantation showed cell viability both within and proximal to the implant. The microcontainers are characterized by their small size, three dimensionality, controlled porosity, ease of parallel fabrication, chemical and mechanical stability, and noninvasive traceability in vivo.

Cell encapsulation and oxygenation in nanoporous microcontainers

With strides in stem cell biology, cell engineering and molecular therapy, the transplantation of cells to produce therapeutic molecules endogenously is an attractive and achievable alternative to the use of exogenous drugs. The encapsulation of such cell transplants in semi-permeable, nanoporous constructs is often required to protect them from immune attack and to prevent their proliferation in the host. However, effective graft immunoisolation has been mostly elusive owing to the absence of a high-throughput method to create precisely controlled, high-aspect-ratio nanopores. To address the clinical need for effective cell encapsulation and immunoisolation, we devised a biocompatible cell-encapsulating microcontainer and a method to create highly anisotropic nanopores in the microcontainer's surface. To evaluate the efficacy of these nanopores in oxygenating the encapsulated cells, we engineered 9L rat glioma cells to bioluminesce under hypoxic conditions. The methods described above should aid in evaluating the long term survival and efficacy of cellular grafts.

Radiofrequency coils for magnetic resonance microscopy

Given the several orders of magnitude fewer spins per voxel for MR microscopy than for conventional MRI, efficient coil design is important to obtain sufficient signal-to-noise within reasonable data acquisition times. As MR microscopy is typically performed using very high magnetic fields, coil design must also incorporate the effects of increased component losses and skin-depth-dependent resistance, as well as radiation losses and phase effects for coils when conductor dimensions constitute a substantial fraction of the electromagnetic wavelength. For samples much less than 1 mm in size, wire solenoids or microfabricated planar coils are used. For samples with diameters of several millimeters, saddle, birdcage, Alderman-Grant or millipede coils become the preferred choice. Recent advances in multiple-coil probes and phased arrays have been used to reduce data acquisition time and/or increase sample throughput, and small superconducting coils have shown significant improvements in signal-to-noise over equivalently sized room-temperature coils.

NMR planar micro coils for micro spectroscopy: design and characterisation

The goal of this study is to determine the concentration sensitivity and the limit of detection of a SNMR receiver planar micro coil with ellipsoidal geometry 1000x500 microm, fabricated using an electroplating technique and used as SNMR receiver coil at 200 MHz. The maximum signal intensity on the NMR images and simulation of RF field distribution allows defining an active volume of 0.8 microL. The localised spectroscopy based on a PRESS sequence shows that the concentration sensitivity is closed to S(C)=2.33 M(-1) and the limit of detection LOD=0.8 M. This micro-system offers the possibility of new investigation techniques based on implantable micro coils used for in vivo study of local cerebral metabolites occupying a small volume (microL to nL order).

Nuclear magnetic resonance of mass-limited samples using small RF coils

Figure Schematic diagram of a typical arrangement used for hyphenating chemical microseparations (e.g. capillary HPLC, CE, or CEC) with microcoil NMR detection.

Functional MR microimaging of pancreatic beta-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.

Sample patterning on NMR surface microcoils

Aligned microcontact printing for patterning the sample in areas of homogeneous RF-field on the highly sensitive surface of planar NMR microprobes is presented. We experimentally demonstrate that sample patterning allows drastic improvement of the spin excitation uniformity. The NMR microprobes are designed for cell analysis and characterized using lipid vesicles as cell substitutes. Lipid vesicles are advantageous as composition and concentration of the confined solution are precisely controlled and because of their similarity to living cells. Using aligned microcontact printing, a monolayer of lipid vesicles is immobilized on the surface of the planar NMR microprobe in a patterned way. 1H NMR spectra and CPMG spin echoes of sucrose solution confined within the lipid vesicles are successfully recorded. Nutation curves of the sample structured in different patterns demonstrate the impact of patterning on the spin excitation uniformity. The total detection volumes are between 1 and 2 nL and derived with help of a theoretic model based on 3D finite element simulation. This model predicts the signal-to-noise ratio and the progression of the nutation curves.

Towards nuclear magnetic resonance μ-spectroscopy and μ-imaging

The first successful experiments demonstrating Nuclear Magnetic Resonance (NMR) were a spin-off from the development of electromagnetic technology and its introduction into civilian life in the late forties. It was soon discovered that NMR spectra held chemically relevant information making it useful as an analytical tool. By introducing a new way of detection, moving away from continuous wave spectroscopy, Fourier Transform NMR helped to overcome sensitivity problems and subsequently opened the way for multi-dimensional spectroscopy. As a result NMR has developed into one of the most powerful analysis techniques with widespread applications. Still sensitivity is a limiting factor in the applicability of NMR. Therefore we witness a renaissance of technique development in magnetic resonance striving to improve its receptiveness. This tutorial review introduces the efforts currently made in miniaturizing inductive detection by designing optimal radio-frequency microcoils. A second approach is to introduce a new way of detecting magnetic resonance signals by means of very sensitive micromechanical force detectors. This shows that the detection limits in terms of absolute sensitivity or imaging resolution are still open to significant improvements.