Joel B. Miller

NMR images of solids

Abstract A way of obtaining two-dimensional NMR images of solids using multiple-pulse line narrowing is demonstrated. The method is adaptable to any fast recovery solid-state spectrometer having 2D FT NMR software and provisions for applying magnetic gradients. The use of molecular mobility as an NMR image contrast mechanism is demonstrated.

Time-suspension multiple-pulse sequences : applications to solid-state imaging

In this Communication, we introduce a 48-pulse homonuclear dipolar decoupling cycle which also refocuses chemical shifts and other resonance offsets. When applied by itself to a system of spins then it appears that the spin system does not evolve in the absence of irreversible effects. By carefully combining this multiple-pulse sequence with a time-dependent magnetic field gradient, the gradient-induced evolution can be preserved without destroying the line-narrowing benefits of the cycle. With this method we hope to obtain liquid-like images of solid samples. One approach to solid-state imaging, then, is to combine a multiple-pulse cycle which averages time-independent linear and bilinear Z, Hamiltonians with a temporally and spatially dependent linear Z, Hamiltonian in such a manner that this spatially dependent Hamiltonian does not interfere (I ) with the multiple-pulse averaging of the time-independent Hamiltonians. This approach allows high-resolution solid-state images to be obtained with modest magnetic field gradient strengths thereby avoiding the sensitivity cost associated with large detection bandwidths (2,3). In addition, the resulting images have nearly uniform spatial resolution unlike solid-state images that are acquired with multiple-pulse sequences in a time-independent magnetic field gradient. These “time-suspension” cycles average both linear and bilinear Z, Hamiltonians by toggling Z, and Z,Z: operators through a variety of states the temporal average of which is zero. We use time suspension here to connote setting to zero the time evolution of a propagator rather than the equivalent picture in which the spin-dependent part of the Hamiltonian is set to zero: this phrase is chosen in analogy to “time-reversal” sequences which change the sign of the spin Hamiltonian. Such discussions are only useful when one remembers that irreversible effects are excluded, and spin-lattice relaxation and molecular motions place a limit on the interval over which manipulations of the Hamiltonian may be equated to manipulations of time. Naturally, timesuspension only refers to the sampling point for the cycle, and spin evolution during the cycle is much more complicated. Time-suspension sequences are well known in solid-state imaging: an eight-pulse version was the basis of an early imaging scheme suggested by Mansfield and Grannel (4); Weitekamp et al. have used this approach for radio frequency gradient solid-state imaging (5, 6), and we have employed this approach for solid-state imaging with

NQR detection using a meanderline surface coil

14N pure NQR detection in a thin-layer sample using a meanderline, or zig-zag, surface coil is reported. The RF magnetic field of the meanderline drops off approximately as exp−πhb), where b is the conductor spacing and h is the distance from the plane of the coil, and therefore is ideally suited for NQR detection in thin, planar samples. A meanderline with 11 conducting strips of 4 cm spacing was used to observe the 14N ν-resonance line in sodium nitrite at sample distances up to 1.6 cm, where the RF field has dropped to less than 1e. The meanderline results are compared with those obtained using a solenoid of comparable dimensions. It is also shown that the sensitivity of NQR detection using surface coils is greatly enhanced by using fast-pulsing techniques such as the strong off resonance comb, which can produce an order-of-magnitude or better improvement in the signal-to-noise ratio over more conventional (T1-limited) techniques.

Three-frequency nuclear quadrupole resonance of spin-1 nuclei

Abstract We introduce a new nuclear quadrupole resonance (NQR) method for the detection of spin-1 nuclei, where the transition excited and directly detected is not irradiated at all. It is demonstrated, theoretically and experimentally, that the irradiation of a powder sample containing spin-1 nuclei by two of the three characteristic NQR frequencies can result in free induction decay (FID) and echo signals at the third NQR frequency. We present the optimal conditions for such three-frequency NQR experiments and compare theory with experiment using 14 N ( I =1) in a powder sample of sodium nitrite.

Generation of short, intense gradient pulses

Abstract A method of generating intense, short pulses of magnetic field gradient is described, for application in solid-state, multiple-pulse imaging. The gradient coil is resonated with an energy storage capacitor, producing a half-sinusoidal current waveform. Clean turnoff occurs automatically at a zero-crossing, so RF pulses may be used immediately before and/or after gradient pulses.

Pulsed field gradient NMR imaging of solids

Abstract The use of multiple pulse line narrowing for NMR imaging of solids is hampered because large magnetic field gradients can place some spins sufficiently far from resonance so that the local line narrowing efficiency is reduced. We propose that this interference can be modeled as a phase error of the rf pulses and can be reduced by applying the gradient only during selected windows in the pulse cycle. We demonstrate this interference with the use of a digital phase and compare one-dimensional images obtained with the standard static gradient technique and this new pulse gradient method.

Narcotics and explosives detection by 14 N pure nuclear quadrupole resonance

Pure nuclear quadrupole resonance (NQR) of 14N nuclei is quite promising as a method for detecting explosives such as RDX and contraband narcotics such as cocaine and heroin in quantities of interest. Pure NQR is conducted without an external applied magnetic field, so potential concerns about damage to magnetically encoded data or exposure of personnel to large magnetic fields are not relevant. Because NQR frequencies of different compounds are quite distinct, we do not encounter false alarms from the NQR signals of other benign materials. We have constructed a proof-of-concept NQR explosives detector which interrogates a volume of 300 liters (10 ft3). With minimal modification to the existing explosives detector, we can detect operationally relevant quantities of (free base) cocaine within the 300-liter inspection volume in 6 seconds. We are presently extending this approach to the detection of heroin base and also examining 14N and 35,37Cl pure NQR for detection of the hydrochloride forms of both materials. An adaptation of this NQR approach may be suitable for scanning personnel for externally carried contraband and explosives. We first outline the basics of the NQR approach, highlighting strengths and weaknesses, and then present representative results for RDX and cocaine detection. We also present a partial compendium of relevant NQR parameters measured for some materials of interest.

NMR Imaging of Solids with a Surface Coil

Nuclear magnetic resonance (NMR) imaging [1] usually exploits the dependence of the resonance frequency (generally in the rf band) on the magnetic field strength to map spatial position onto a frequency spectrum. The magnetic field strength is given a known spatial dependence and the spins are uniformly excited with an rf field. The uniform rf field is generally produced inside a cylindrical rf coil.

A broadband nuclear magnetic resonance spectrometer: Digital phase shifting and flexible pulse programmer

A simple, low‐cost pulse nuclear magnetic resonance (NMR) spectrometer is described. There are two key features of the instrument: (1) all radio frequency phase shifting is done digitally with a new‐generation frequency synthesizer and (2) the pulse programmer is based on a high‐speed 32‐bit parallel interface card in a desktop computer. The spectrometer is well suited for single‐frequency channel experiments such as nuclear quadrupole resonance and deuterium quadrupole echo NMR experiments. Because the rf phase shifting is done digitally, the spectrometer operates easily over a wide frequency range, here 10–300 MHz. The pulse programmer and other software functions are written in LabView, a graphical programming language.

14N magnetic resonance for materials detection in the field.

Nitrogen is prevalent in many materials, both naturally occurring and man-made. In particular, it is found in many explosives and other contraband materials. One technique for the detection of such materials in the field is the use of the magnetic resonance signal from the nearly 100% abundant, spin-1, 14N nuclei. Some of the difficulties with such measurements in the field include spurious signals from acoustic resonances, radio-frequency interference, and generally low signal-to-noise ratios. A summary of recent work by the authors to help mitigate these difficulties is presented.