A design methodology for short, whole-body, shielded gradient coils for MRI

A series of designs is presented for restricted length, whole-body, shielded gradient coils. By using the real space optimization technique, simulated annealing (SA), it is possible to produce viable gradient sets with a length-to-diameter ratio (LDR) of just 1.0. Radially remote return paths for the transverse coils aid in producing such short coils. While the linear regions of such coils cannot be as large as longer coils, they produce homogeneous linear regions suitable for use in whole-body imaging. The coil sets are well shielded even at such small LDRs

A selective excitation/B0 gradient technique for high-resolution 1H NMR studies of metabolites via zero-quantum coherence and polarization transfer.

A new method for selective observation of scalar coupled metabolites by either zero-quantum coherence transfer or polarization transfer with concurrent water suppression in a single acquisition was developed. Gaussian shaped RF pulses were used to selectively generate multiple-quantum and zero-quantum coherence in the metabolite of interest, single quantum (including water) and double quantum coherences were then dephased under the influence of a B0 field gradient and the surviving zero-quantum coherence was converted to observable metabolite signal. The duration of the gradient application and the frequency and angle of the final selective read pulse determined whether a polarization transfer or a coherence transfer signal was observed. Water suppression factors of around 8000 were achieved which allowed operation of the receiver at high gain levels resulting in greatly improved signal to noise in the metabolite spectra. The CH3 and CH resonances of lactate in a mouse brain homogenate were selectively edited and the method was also applied to selective editing of ethanol

An analysis and optimization of elliptical RF probes used in magnetic resonance imaging

In magnetic resonance imaging of the entire body, it is often desirable to use an elliptical RF probe, rather than a circular one. As an ellipse more closely conforms to the anatomical cross section of the human thorax and head, better filling factors and therefore improved signal-to-noise ratios may be achieved by the use of elliptical RF coils. The probe is usually of bird-cage type, but the rungs are of finite width due to the high-frequency signals involved. This paper presents a method for computing the magnetic fields produced inside elliptical probes, and the current distributions on the rungs. A slotted shield is assumed to surround the probe and its influence on field homogeneity is studied. In particular, the currents in a 16-runged unshielded elliptical coil of practical interest were determined optimally in one case, using simulated annealing to optimize the homogeneity of the magnetic field within the probe. The effects of a segmented shield of both elliptical and circular cross section on this coil are discussed, and the results are confirmed by experiment

Design of Shielded Quadrupolar Gradient Coils for Magnetic Resonance Microscopy by Simulated Annealing

A novel design for shielded, magnetic pulsed field gradients with transverse geometry is presented. The gradients are intended for use in NMR microscopy and localized spectroscopy. The integrated coil design is based on simulated annealing (SA) optimization and results in well sheilded coil sets of high linearity and sensitivity. The design approach is a flexible one and is amenable to the design of gradients in a variety of configurations

Quadrature detection in F1 induced by pulsed field gradients

The improved understanding and control of induced eddy current effects has led to the increased application of pulsed field gradients in high-resolution 2D NMR spectroscopy. Of particular interest is the ability to select specific coherences in a single acquisition with concomitant "quadrature detection" in the F1 dimension. A product-operator analysis of this effect for an AX spin system which describes the selection of N-type or P-type signals using appropriate gradient episodes in the COSY and 2D double-quantum experiments is presented

A Novel Approach to the Calculation of RF Magnetic and Electric Fields for NMR Coils of Arbitrary Geometry

This paper presents a novel approach for determining the magnetic and electric fields generated by NMR RF coils of arbitrary shape, Maxwell′s equations are solved using Helmholtz retarded potentials, The model incorporates the effects of radiofrequencies and allows the calculation of both electric and magnetic fields at any position, Discussion of the numerical evaluation of the resulting formulas is presented, A comparison of the numerical results with an exact solution is made for the case of a singular circular loop of wire, Theoretical results for a birdcage coil are also given. This approach provides fast, accurate results and will easily lend itself to coil optimization