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What’s New 3/03/08 . Development
of the PISEMA probe 1/28/08 . Recent
results from Doty XC MAS probes 1/28/08 . Preprint
available – Probe
Design and Construction. Abstract: A brief review of some interesting aspects of the history of NMR probes is presented, followed by an overview of the basics of NMR probe design and optimization for liquids, solids, and imaging applications. Simple analytical methods are presented for signal-to-noise optimization of single, double-, and triple resonance circuits based on Q-values, magnetic filling factor, and circuit efficiency. Effects of non-ideal components and some data on construction materials are presented. Thermal engineering challenges, with reference to some recent solids probes, are also introduced. 1/28/08 . Preprint
available – Solid
State NMR Probe Design. Abstract: An overview is presented of the special requirements for components found in NMR probes for solid samples, including high-power ceramic capacitors, variable capacitors, coils, construction materials, and spinner assemblies. Key data, including magnetic susceptibility and dielectric properties, are tabulated for the various ceramics, plastics, composites, and alloys often preferred in probe construction. Overviews are presented of the primary types of solids probes, including wideline, magic angle spinning, single crystal, and switched angle spinning. Some of the special design challenges are discussed in more detail, including spinner design, triple resonance high-power rf circuits, and operation at extreme temperatures. 2/16/07 . Preprint
available – RF
Coil Technology for Small-Animal MRI. Abstract:
A review of the theory, technology, and practice of RF coils
for small-animal MRI is presented that includes a brief overview
of MR S/N analysis and discussions of the various coils commonly
used in small-animal MR – surface coils, linear volume
coils, birdcages, and their derivatives. The scope is limited
to mid-range coils – coils where the product (fd) of
the frequency f and the coil diameter d is in the range of
2-30 MHz-m. Common applications include mouse brain and body
coils from 125 MHz to 750 MHz, rat body coils up to 500 MHz,
and small surface coils at all fields. In this regime, all
the sources of loss (coil, capacitor, sample, shield, and
transmission lines) are important. All such losses may be
accurately captured in some modern full-wave 3D EM software,
and new simulation results are presented for a selection
of surface coils using Microwave Studio (MWS) 2006 by Computer
Simulation Technology (CST), showing the dramatic importance
of the “lift-off effect”. Standard linear circuit
simulators have been shown to be useful in optimization of
complex coil tuning and matching circuits. There appears
to be considerable potential for trading S/N for speed using
phased arrays, especially for a larger field of view. Circuit
simulators are shown to be useful for optimal mismatching
of ultra-low-noise preamps based on the E-PHEMT (Enhancement
mode Pseudomorphic High Electron Mobility Transistor) for
optimal coil decoupling in phased arrays. Cryogenically cooled
rf coils are shown to offer considerable opportunity for
future gains in S/N in smaller samples. 9/8/06 . 55%
gain in S/N in OptiMASTM probe
with DI-4 Spinner. Problems with the previous VT gas flow control and thermocouple placement led to poor sample temperature accuracy when the Cold Zone in the first OptiMAS probe was cooled for S/N enhancement. The latest spinner revisions have completely solved these problems, and thermometry is now much better on the new 4mm “Drop-in” spinner, the DI-4, than on any of our previous MAS spinners. When the cold zone was at 85 K with the sample near RT, the difference between the temperature indicated by the sample thermocouple and the actual sample temperature, as measured by the chemical shift in PbNO3, was only ~3oC prior to calibration. The experiments were done in a triple-resonance fully equipped OptiMASTM probe at 7 T. 6/26/06 . Preprint
available – Reducing
Decoupler heating. 4/20/06 . OptiMAS Update. 2/24/06 . OptiMAS Update. We are expecting to have preliminary NMR data on the next-generation OptiMAS probe by the ENC in April 23. Our next-generation NMR MAS probe for fields up to 950 MHz will combine: (1) The highest S/N (we’ve been 30-80% beyond the competition*, now we’re taking it up another 50%); (2) Automatic sample exchange; (3) Order-of-magnitude lower decoupler heating than the solenoid (or half that of the scroll coil); (4) Highest resolution in a solids probe; (5) Highest spinner stability (axial vibrations under 2 microns rms); (6) Quad-resonance, multi-nuclear, H/X/Y/lock; (7) High-performance Magic Angle Gradient coil (250 G/cm at 500 MHz); (8) Extended VT range; (9) Outstanding tuning stability and robustness; (10) Lowest phase transients.
*The
low-end of this S/N advantage range refers to low-loss samples. For
example, our XC-MAS gets over 200:1 on natural-abundance 13C
glycine in a triple-resonance 750 MHz narrow-bore (or standard
bore) probe, while competitors apparently get about 120-150
in
such probes, depending on various probe features. We understand
a competitor's 900 MHz 2.5 mm probe gets 50:1 on this same
test. Click here for a recent technical progress poster.
2/5/06 . Dramatically Improved Lineshape in Diffusion Probes. We’ve not worried too much about the resolution and lineshape in our probes for self-diffusion measurements in liquids, as it’s usually not important in conventional diffusion experiments. However, with the growing interest in diffusion-ordered spectroscopy (DOSY) and other HR techniques needing very high performance gradients, we realized we needed to better address the resolution and lineshape issues in diffusion probes. We’ve re-designed the gradient coils with precise magnetic compensation and developed the needed precision magnetic compensation technology for the rf coils in liquids probes. We anticipate NMR data (from both 5 mm and 10 mm diffusion probes) demonstrating resolution and lineshape in our diffusion probes (with 2000 G/cm gradients) that are competitive with state-of-the-art liquids probes.
1/20/06 . Improved Surface Coil Optimization. Small surface coils at moderate fields (especially, for frequency-diameter products less than 5 MHz-m, such as a 25 mm coil at 200 MHz or less) were found to benefit much more than was previously expected from an increase in the conductor cross section. The S/N benefit is greater than is indicated by either of the simple Q metrics commonly used (Q0/QL, or simply QL), as the larger conductor cross-section also improves magnetic filling factor from the reduction in the field intensity very near the conductor surface. Going with a significantly heavier conductor makes it necessary to utilize zero-susceptibility conductors to minimize magnetic susceptibility artifacts.
Another important surface coil matter is decoupling from the transmit field. We have switched from a simple diode clamp to a numerically optimized passive detuning network for more effective surface coil isolation from the transmitter. We are considering beginning to develop pre-amp decoupling to allow surface coils to be positioned arbitrarily for array reception.
1/10/06 . Ultra-stable MAS Spinner for Auto Sample Exchange. A novel inflow Bernoulli axial bearing for MAS has been developed that provides exceptional axial stability as well as compatibility with automatic sample exchange. Typical axial oscillations are under 2 microns rms (an order of magnitude improvement) over a wide range of spinning conditions. The 3 mm rotor was spun to 30 kHz MAS, and the 4 mm rotor was spun to 23 kHz, as well as 0.5 Hz spectral resolution on 1H with liquids-like line shape. The spinner designs are compatible with extended VT ranges, automatic sample exchange, ultra-low-E cross-coils, a magic-angle-gradient coil, and a ceramic dewar suitable for CryoMAS in which the sample is above RT and the coils are at 25 K. The improved axial stability is beneficial in improving spectral resolution and is essential in most gradient methods. Click here for a more detailed abstract.
12/20/05 . CryoMAS Update. Steady progress continues in the development of a quad-resonance ( 1 H/ 13C/ 15N/lock) 3 mm CryoMAS probe (for wide-bores only) with full-range VT capability, automatic sample exchange, and high-performance magic-angle gradients. We are initially expecting a factor-of-five gain in S/N (compared to available 3.2 mm MAS probes with auto sample exchange), and ultimately we anticipate a gain of a factor of eight. The potential S/N gain in MAS appears to be greater than for the narrow-bore liquids cryoprobes mainly because the extra space available in the widebore allows for more effective minimization of the losses in all the circuit components.
We are expecting to have very preliminary NMR test data by the time of the ENC, April 23. Click here for a recent technical progress poster.
12/5/05 . SAS progress. Switched Angle Spinning (SAS) techniques are expected to provide useful dynamic control over bicelle alignment in protein structure determinations. Two SAS probes suitable for such experiments in high-field magnets are into testing – one for wide bore and one for narrow bore magnets. Each includes the following capabilities: very high efficiency multi-nuclear triple-resonance, field gradient coil, 25 ms sample flipping, and high resolution. A novel 4 mm spinner compatible with automatic sample exchange demonstrated 10 kHz spinning while flipping in 25 ms. The 600 MHz 7 mm 1H/X/Y SAS PFG probe achieved 55 kHz 1H decoupling with 120 W. The latest WB version achieved angle reproducibility of 0.02 degree and lead lifetime of 300,000 flips. Click here for a more detailed abstract.
11/29/05 . New Silicon-nitride Rotor Material Identified. Kyocera has developed a very promising, new grade of Si3N4 , SN240, that appears to have better strength and toughness than any previous grade. We are optimistic that this new material, along with improved processing, will allow us to offer thick-walled rotors rated to higher spinning speeds with greatly reduced chances of failure. We are expecting to have preliminary test results on thick-walled rotors from this new material in early summer, 2006.
11/3/05 . Improved MAS Turbine Caps. Glass-fiber-reinforced torlon grade 5030 will now be used on new MAS turbine caps (except for low proton Kel-F caps) for greatly improved VT performance and all-round better reliability and performance. This new material stands out from the rest with respect to isotropic thermal expansion (only 16E-6/ 0C), tensile strength at 200oC (120 MPa), and heat distortion temperature (282oC). It also has rather low moisture absorption, high wear resistance, and high impact strength. Click here for more detailed information.
6/25/05. High-performance Head Gradient Coil passes preliminary tests. Preliminary tests were satisfactorily completed on a novel prototype head gradient coil. The coil in all essential respects performed as expected from the detailed simulations. The coil permits a substantial reduction in acoustic noise, a substantial increase in continuous gradient capability, and substantially reduced nerve stimulation. The initial tests were performed at 1.5 T, and the coil is being sent to MGH for further testing at 3 T. Work is beginning on the next generation, which will correct several deficiencies limiting current handling in the first prototype. The next generation is expected to permit 70 mT/m continuous gradients in fields at least up to 8 T. Click here for more detailed information.
4/10/05. Double-balanced, Double-tuned 125 MHz Head Coil Preliminary tests were carried out on a novel, double-balanced, double-tuned birdcage which permits more fully minimized sample losses on the high frequency channel than is obtained in prior designs. Click here for more detailed information.
11/16/04. 500 MHz, 11 cm, hybrid birdcage. An 8-section 500 MHz shielded hybrid birdcage of 11 cm diameter was completed and tested on baboon brain. Performance agreed well with expectations from full-wave simulations using CST MWS 5.1, which showed that the capacitor ratios must be such that the coil is much closer to high-pass than low-pass conditions. The coil tuned well over a wide range of samples, with excellent channel isolation. Click here for more detailed information.
7/28/04. Highest MAS S/N and Order of Magnitude Lower Decoupler Heating. Simulation results and preliminary test results at 750 MHz, as reported in April, 2004, at the ENC in Asilomar, showed rf decoupler heating to be lower in the XC MAS probe by at least an order of magnitude compared to conventional solenoid probes. Now, 13C test results on natural glycine show S/N to exceed 200:1 in the H/X/Y XC4 750 MHz probe. This apparently exceeds that of competitors’ probes for use in narrow-bore magnets by 30-80%, depending on the various features.
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