Minimal invasive surgery guided by NMR techniques is receiving increased attention for different medical applications. The surgery can be realized in standard and preferably open magnets. NMR images, which are displayed online on a screen, are used to follow the surgery and the path of the endoscope. The NMR endoscope itself is a diagnostic instrument, by which specific local information about tissues, body liquids, implants, etc. can be obtained. The endoscopic probe consists of a flexible shaft with a micro coil at the tip. The coil is part of a radio-frequency resonant circuit which is used as a surface coil. By relaxation and spectroscopy experiments one can get information during the surgical operation or for intra-vascular analysis. Similar to NMR imaging in a medical tomograph the endoscope can be employed to obtain images close to the micro coil by the application of appropriate pulse squences. High sensitivity and good spatial resolution due to an excellent filling factor are advantages of such micro coils in comparison with standard surface and whole body-coils. For visualization of the position, the endoscope is equipped with a switchable marker. For use in medical offices independent from a tomograph the endoscope will be provided with permanent magnets. With its own static magnetic field the probe becomes a transportable NMR instrument for medical diagnostics. A prototype of the NMR endoscope has been constructed and tested. Exploration of clinical application is in progress.
A new class of NMR rf volume coils has been developed that permits improved B1 homogeneity, tuning range, tuning stability, and sensitivity compared to birdcages under a wide range of practical conditions, especially for microscopy and wrap-around flexible applications. They are denoted litz coils, as their flux transparency and current distribution is obtained from woven foil patterns with insulated crossovers. Contrary to design criteria in phased arrays, the parallel routes in litz coils require high coupling coefficients to achieve an optimal current distribution that is independent to first order of tuning, balancing, and matching adjustments and is compatible with multiple capacitive segmentation. The motivation for exploring novel current topologies arose primarily from work in vertical bore magnets (7 T to 14 T) where tight space constraints make it extremely difficult to obtain high B1 homogeneity for asymmetric loads in birdcages, as the high-order rf homogeneity errors can only be addressed by a complex tuning procedure and the parasitics cannot be precisely controlled. Numerical modeling and experimental comparisons with balanced-high-pass birdcages are presented for a variety of birdcage and litz coils with frequency-diameter products from 5 to 30 MHz-m for frequencies at least to 600 MHz. Both linear and quadrature versions are presented.
Modern magnetig resonance imaging techniques require more and more powerful gradient systems. In order to optimize the whole gradient system performances, the gradient specifications must be balanced in respect to a cost function. The specifications of a gradient system usually include : 1. the maximum gradient strength GM [mT/m], 2. the slew rate SM [T/m/s], or the gradient switching time Ts from 0 to GM, 3. the gradient linearity over the region of interest, 4. the free access to the region of interest. The dimension of the gradient system is the first parameter to consider : the electric power P required to operate a gradient is P~GM2 D5 / Ts, where D represents the characteristic dimension of the coil [1]. For a given dimension and linearity, one must then consider a compromise between the maximum gradient strength GM and the slew rate SM considering a cost function. The simple cost function Cf = GM SM is often adequate because it is proportional to the instantaneous power required to drive the gradient. Cf is also directly linked to the manufacturing cost of the system because of increasing technological constraints following the increase of the current IM and applied voltage VM (power supply unit, gradient amplifier, and gradient coils). On the other hand, considering the image quality for a given voxel size, the time Tm allowed for coding, refocusing, or spoiling the magnetization must be minimized [2]. Then a global optimization of {Cf, Tm} must be realized. Illustrations of this optimization are given for various imaging situations showing that the balance between the maximum gradient strength and the slew rate must be carefully chosen. [1] Saint-Jalmes H., et al., Magn Reson. Med., 2, 245 (1985) [2] Reeder S.B., et al., Magn Reson. Med., 32, 612 (1994)
"Florida-Bitter" resistive magnets at the National High Magnetic Field Laboratory in Tallahassee, FL provide the opportunity to carry out low resolution NMR measurements in magnetic fields up to 33 T at cryogenic temperatures. The associated challenges include an available probe space of only 17 mm diameter, a magnetic field variation of 10-100 ppm over mm-sized samples, and a low electrical breakdown field when operating in a low helium vapor pressure. In this paper, we describe a pulsed NMR probe design that operates up to 1.3 GHz within these constraints and which uses a single variable component for tuning and matching the NMR coil. It also includes a goniometer for rotating the sample and coil about a horizontal axis in the vertical magnetic field. Several examples of GHz-range proton NMR measurements taken with these probes will be displayed. This work was supported by NSF Grant DMR-9705369.
Solid-state NMR studies of proteins and other biopolymers benefit from high magnetic fields because of increased sensitivity and resolution, especially for 1H chemical shifts. The 550 and 700 MHz high-field spectrometers at the Resource for Solid-State NMR of Proteins at the University of Pennsylvania are capable of running the full range of multiple-pulse, multiple resonance, and sample spinning experiments that constitute high-resolution solid-state NMR spectroscopy. Multiple resonance probes tuned for 1H frequencies over the range 360-700 MHz will be described in detail. All phases of design, construction, and testing of digital, analog, and radio frequency components are performed in the electronic shop of the Resource. Information on how to utilize the Resource will be available.
Prepolarized MRI is an ultra-low-cost MRI technique that uses distinct magnets for polarization and readout. Polarization is achieved using a strong (near 0.5T) but non-uniform magnet and the signal is acquired in a weak (above 40 mT) but homogeneous ``readout field.'' Here we introduce an efficient algorithm for designing homogeneous electromagnets with arbitrary geometrical constraints. We have found that minimum power electromagnet design is equivalent to minimizing the sum of |Ik|ak subject to known homogeneity constraints. Here Ik and ak are the current and radius of the kth coil among a dense set of feasible candidate coils. This can be cast as a linear programming (LP) problem and solved very efficiently to yield a global minimum solution. It also automatically picks the minimum number of non-zero-current coils. Using the above algorithm, we designed a readout magnet that is ideal for head and neck imaging. The challenge is to design a homogeneous electromagnet with 40 cm clear bore and under 10 kW power at 4 MHz (93 mT). To image the neck, we wish to push the FOV towards the edge of the coil. We designed an asymmetric magnet that has one large coil (60 cm diameter) on one side for shoulder access and 6 small coils with 40 cm diameter. The 10 ppm FOV region is a cylinder with 24 cm diameter and 15 cm length and it is located only 5 cm away from the edge of the 40 cm solenoid. This unusual FOV yields a shorter magnet and the asymmetrical design saves power. The magnet comsumes 9.1 kW of power at 4 MHz and it weighs 282 kg. This design greatly improves access to the FOV and its geometry allows easy integration with the polarizing magnet. As a comparison, a classical Garrett 4 coil design with 20 cm bore will require 5.76 kW of power at 4 MHz. But it has a 11 cm diameter FOV and it is 13 cm away from the edge, so we can not image the neck. This new design approach has several advantages: it allows for arbitrary coil location constraints; the algorithm avoids nonlinear optimization; it automatically chooses the minimal number of non-zero currents; and it guarantees the minimum power design.
We have redesigned our real time RF pulse and acquisition NMR spectrometer monitor first described in [1]. The changes include new logarythmic detectors with dynamic range covering all available spectrometer power levels and analog recording channels capable of constinously monitoring parameters like lock level, VT temperature, etc. The new detectors, instead of simple RF diode, use frequency conversion stage and Analog Devices 8-stage logarythmic detector AD606. Dynamic range in such a arrangement by far exceeds the one of diode detector and allows observation of pulses down to 0.04mW ( tpwr=0). Control functions and analog signal monitoring are performed by microcontroller PIC16C74 which has buit in an 8-bit A/D converter and can communicate with the host computer via RS323 port. Detailed schematics are presented along with example results involving complex pulse sequences. *Research sponsored by the National Cancer Institute, DHHS, under contract with ABL. 1. R.A. Byrd, F.S. DiGennaro, "A Real Time RF Pulse Sequence and Acquisition Monitor for NMR Spectrometers", JMR A 112, 250-254, 1995
A microwave frequency synthesizer driven 40 GHz DNP/ESR spectrometer is described, in which a quick in-situ ESR measurement is conveniently used to set up the optimum DNP condition. The spectrometer can be readily switched from the ESR to the DNP condition with a single switch. A large volume sample is also used to enhance the overall NMR sensitivity. Preliminary results are obtained on carbazole and purine doped with free-radicals. A 1H DNP enhancement factor of 36 is obtained on carbazole doped with BDPA free-radicals and a 1H enhancement factor as large as 70 is obtained on purine doped with a mixture of BDPA and DPPH free-radicals. With such a large 1H DNP enhancement, it is possible to obtain the 15N CSA powder patterns at 15N natural abundance levels in only tens of minutes using the 15N DNP-CP experiment. Results on direct 15N DNP, low temperature DNP, rotating frame DNP experiments as well as the exploration of liquid DNP experiments will also be reported.
A DNP - NMR probe was designed and constructed to operate at a field strength of 1.4 Tesla, corresponding to a 1H frequency of 60 MHz, a 13C frequency of 15 MHz, a 15N frequency of 6 MHz, and an electron frequency of 40 GHz. The efficiency of the microwave magnetic field at the sample and the configuration of the NMR sample coil for obtaining an optimal NMR signal enhancement were studied and tested. The sample temperature can be controlled from room temperature to approximately -165 degrees C. By using a double tuned circuit design, 1H and 13C pulse lengths are typically 2.5 and 4.0 microseconds, respectively. The features of the design and experimental results will be presented.
Improved NMR sensitivity can be achieved by increasing the signal strength (e.g. by using instruments with higher magnetic fields) or by reducing the noise in the detection system. A recently developed probe constructed with receiver coils containing high temperature superconducting (HTS) materials is one method of accomplishing the latter.1 In this poster, we show some preliminary results obtained from an HTS probe constructed for a 600 MHz spectrometer. The probe contains dual HTS coils tuned for the 1H and 19F resonance frequencies. Sensitivity gains of 300-400% are observed compared to room temperature triple resonance probes originally received with our spectrometer. Examples of applications in metabolite research and polymer science are used to show the sensitivity gain and its applications. 1 H. Hill et al., Bull. Magn.Resonance, 17, 98 (1995).
Much effort in coil design has focused in cylindrical and elliptical geometry devoted to medical and biomedical applications. However, today's advances of NMR and NMRI applications in industrial processes, such in the food industry [McCarthy et al.], requires new and specific designs. We present several optimized designs of RF coils with rectangular and square sections (width to high ratio from 0.5 to 1) having transverse field. Numerical optimizations have been carried out solving the full set of Maxwell's equations in the time varying (AC) case with a 2D finite element analysis software (QuickField, from Tera Analysis). This approach takes into account the current distribution in the conductors due to Eddy currents and gives a more realistic idea of the magnetic field created by a RF coil. All simulations have been performed at 26 MHz, but they can be easily transposed to other fields. We verified that the Eddy current effects become saturated at 1 MHz giving similar results for higher frequencies. The results show that the optimized coils have high homogeneity: depending on the design, the +/- 5% field homogeneity contour covers from 65% up to 90% of the dimensions of the coil. Besides the high spatial homogeneity, several advantages of these type of coils can be pointed out. This geometry has an improved filling factor for samples having square section such as packaged food, rectangular pipes and multi-screw extruders. The coils are easy to built, avoiding tedious calculations and practical difficulties often encountered in bird cage designs. The rectangular coils can be mounted in "U" shape supports allowing side loading. Moreover, this type of coil has a low inductance and is thus suitable for high field applications. A prototype of a rectangular coil has been built to work at 26 MHz in our 0.6 T imaging system and shows excellent homogeneity as expected from the simulation results. M.J. McCarthy and K.L. McCarthy, "Applications of Magnetic Resonance Imaging to Food Research", Magnetic Resonance Imaging, Vol. 14, Nos. 7/8, pp.799-902, 1996.
There has been recent renewed interest in NMR as an on-line analysis technique for process monitoring and control. Magnetic resonance imaging has also captured the interest of process engineers intrigued by the possibility of seeing real-time images of materials undergoing various manufacturing operations (mixing, flow, extrusion, etc.). We have been studying how NMR and MRI might be used to monitor composition, degree of mixing, and flow behavior of materials encountered in solid explosives and propellant manufacturing processes. The materials are challenging because they are often very viscous, highly loaded with solids, and sometimes contain additives which are paramagnetic (e.g. iron oxide or carbon black). Relaxation times and images are analyzed to indicate some of the possibilities for determining composition and homogeneity variables of interest to process engineers. Guidelines for interpretation of such data and applicable ranges of application of the methods are discussed. The data is used to infer design criteria for low-field NMR or MRI instrumentation that might be incorporated directly in-line for extrusion process monitoring.
Cryogenic probes are moving rapidly from prototype demonstrations1,2 to user-friendly products with significant sensitivity advantages over conventional probes. We present results obtained with a variety of cryogenic high-resolution NMR probes featuring either superconductive or cooled normal-metal RF coils. In this paper, the system aspects of such probes are discussed, followed by a presentation of the results obtained with various types of probes. The probes range from single-nucleus types to heteronuclear versions including pulsed gradients and cryogenic preamplifiers. The data will cover a number of aspects including: (1) basic probe characteristics like the S/N ratio, lineshape, B1 homogeneity, pulse widths, salt tolerance, and solvent suppression; and (2) applications (including homo- and heteronuclear 2D experiments) that demonstrate the power of these probes in making practical measurements with very small sample quantities. 1W.-H. Wong et al., "Superconductive carbon-observe probes," 1997 ENC. 2D. Marek et al., "Ultra sensitive high resolution NMR probes," 1997 ENC.
The utility of microcoil NMR for nanoliter-volume, mass-limited samples arises from its enhanced mass sensitivity (S/N per unit mass) compared to traditional NMR probes. Typically, the solenoidal microtransceiver coils are fabricated by wrapping copper wire directly around a capillary. However, the differences in magnetic susceptibility of the coil and its surroundings disrupt the magnetic field homogeneity within the sample volume. Diminished spectral resolution becomes more pronounced as the distance between the coil and the sample decreases to the micrometer size regime. We are exploring several approaches to minimize field nonuniformities to improve resolution for a variety of coil and capillary dimensions and configurations. One approach surrounds the coil and capillary region with a perfluorinated fluid which approximates the volume magnetic susceptibility of the coil components in a manner that improves line shape and decreases line width without spinning. With this strategy, sub-Hz line widths are maintained as the fill factor is varied from 4% to 34%. In addition, configuration of the microcoil and capillary at the magic angle with respect to the external magnetic field results in line shape improvement and simplified shimming. We also describe a probe with an air-driven, spinning capillary of 700 m o.d. within a solenoidal coil of 1.3 mm i.d. Compared to similar non-spinning configurations, resolution and signal-to-noise ratio are significantly improved for liquid samples. These advances allow a variety of probe design options and indicate a strong future for high-resolution microcoil NMR, especially in mass-limited situations such as analyses of samples of biological origin and pharmaceutical interest.