Description: The objective is to construct an NMR spectrometer based on permanent magnets that, using RF pulses applied through arrays of coils operates without electrically controlled magnetic field shims. In Phase II, the system should be actively and dynamically stabilized with respect to the fluctuation of external parameters such as temperature and meet additional performance metrics. In Phase III, the concept should be extended to MRI experiments incorporating pulsed magnetic field gradients.
When used for chemical identification or anatomical imaging, magnetic resonance experiments are conducted in strong magnetic fields that are homogeneous within parts per million everywhere within the volume under study. Superconducting MRI magnets rely upon a complex set of auxiliary electromagnets, called shim coils that are adjusted empirically to compensate for inhomogeneities in the static magnetic field. Portable NMR and MRI systems based on permanent magnet assemblies are generally designed to be intrinsically homogeneous. They employ adjustable mechanical shims but still rely upon electrically controlled auxiliary coils; both compromise their portability in applications ranging from composite materials inspection, the monitoring of curing in industrial settings, to petro-physical and petrochemical studies of extracted core samples. Both also make it impossible for permanent magnet-based NMR systems to be easily produced in volume, since each must be shimmed by hand.
The implementation of fully functional mobile NMR systems therefore requires an alternate means of counteracting these inhomogeneities. Signal acquisition over a very small region can lead to homogeneous spectra [1,2]; however, many applications require signal acquisition over volumes that are much larger than the length scale of the magnetic field inhomogeneities. An alternative method to accomplish something equivalent to shimming involves the preparation of the quantum state of the spins under study (i.e. their initial phases), such that inhomogeneity is intrinsically compensated by subsequent evolution of the spin system under a pulse sequence. In general, this requires addressing, in a time-dependent manner, the initial phase of spins in each voxel with spatial resolution comparable to the length over which the magnetic field varies. Such methods typically reduce the linewidth of single-sided systems from ~100-500 kHz to <1 kHz[3,4,5]. Such methods have been demonstrated only in laboratory settings using high amplitude pulsed magnetic field gradient systems and RF coils that enclose the sample, and with known or imposed static magnetic field inhomogeneities. The principal technical and scientific challenge to be addressed in Phase I is the generalization of this technique to a planar, single-sided geometry of magnet and surface coils in which the inhomogenities arise from the magnet and its interaction with the sample under study and are not known a priori. This SBIR opportunity thus involves the use of the shim pulse technique to produce single-sided, permanent-magnet based systems of increasing complexity and capability for a wide spectrum of analytical applications.
Phase I: Design a single-sided NMR system incorporating an array of electrically controlled radiofrequency coils (a parallel phased array) for spectroscopy with shim pulses. The system should have an average equivalent magnetic field of >0.35T in an active fluid volume of 1 mL and produce linewidths of better than 50 ppm. The goal at the end of phase I is a working, bench-scale prototype with no elements that preclude its miniaturization to a handheld portable unit. The sensitivity and molecular specificity of the unit must be estimated in the solution phase using mixtures of the fluorinated compounds hexafluorobenzene and perfluorohexane.
Phase II: Based on the Phase I design, construct and demonstrate a spectrometer with an average equivalent magnetic field of >0.5T in an active volume of 1 mL, yielding linewidths of better than 5 ppm. By dynamically adjusting the shim pulses, the system should be compensated against variations in external parameters such as the temperature. Phase II also requires the development of an efficient algorithm to measure and counteract the field inhomogeneity. Further, the spectrometer should implement a graphical software interface for the automated generation and optimization of shim pulse coefficients.
Phase III: The goal of Phase III is the commercial production of a completely portable self-contained automated NMR spectrometer unit capable of solving a variety of spectroscopic analytical problems in a single-sided inspection package.