Alan R Rath

Optimization of modulation functions to improve insensitivity of adiabatic pulses to variations in B1 magnitude

Abstract Amplitude- and frequency-modulated pulses or the equivalent amplitude- and phase-modulated pulses based on adiabatic principles achieve their transformations of magnetization vectors over a large range of variation in B1 magnitude. The B1-insensitivity of the pulses depends on the particular modulation functions employed. In this paper, we examine the factors that govern the B1 tolerance of these pulses and illustrate that previously used amplitude/frequency modulation functions sin/cos, sech/tanh, or constant/tan are far less than optimum in achieving maximal B1 insensitivity on resonance. We describe a new method by which optimized modulation functions can be constructed to impart insensitivity to B1 inhomogeneities over a predetermined B1 range. This is accomplished by dictating that the pulses based on the new modulation functions fulfill the adiabatic condition over the predetermined B1 range and for the complete duration of the pulse. The new functions also provide the additional advantage of operating at lower pulse power. Experimental and theoretical data are presented to illustrate the superiority of the new modulation functions.

Amplitude- and frequency/phase-modulated refocusing pulses that induce plane rotations even in the presence of inhomogeneous B1 fields

Amplitude- and frequency/phase-modulated 180† plane rotation pulses that can achieve both inversion and refocusing transformations in the presence of large B1 variations are presented. Such pulses are required especially in in vivo applications where some of the most commonly used coils generate highly inhomogeneous RF fields. The principles involved in constructing such pulses are discussed in detail together with five different types of problems encountered that affect performance of these pulses off resonance. The refocusing transformation is achieved by inverting the effective field halfway through the pulse. The different pulses discussed tolerate very large and similar range of variations in B1 magnitude on resonance, but behave differently off resonance. Two of the pulses achieve 180† plane rotations with constant phase over a useful off resonance range and in the presence of large B1 variations. Therefore, these pulses are expected to be useful in high-resolution as well as in vivo NMR applications.

Design and performance of a double-tuned bird-cage coil

Abstract The “high-pass” and “low-pass” designations commonly given to bird-cage coils derive in each case from the relationship of the individual sections of the bird cage to simple filters. By extension of the concept of constructing a bird-cage coil from elementary filter sections, it is possible to create coils which support “bird-cage mode” resonances at more than one frequency. Designs based on both simple bandpass and band-stop filters, for example, provide means to realize bird-cage coils which may be tuned simultaneously to two frequencies. Construction and performance of a band-stop bird cage operating at both 200 and 81 MHz are described.

Opposed coil magnet calculations for large sample and unilateral nuclear‐magnetic resonance

A magnet configuration has been devised, consisting of two nested coaxial coils with the current in the inner coil circulating in an opposite sense from that in the outer coil. The magnetic field generated by such an opposed pair exhibits a small region of homogeneity outside of the magnet itself, creating the possibility of constructing a unilateral nuclear‐magnetic‐resonance device capable of examining an object from only one side. Calculations on one special case, the inside‐out Helmholtz configuration, indicate a volume of 10 cm3 with homogeneity of ±1% for a coil of outer radius 10 cm, and 0.1 cm3 with 100‐ppm homogeneity. A second special case, consisting of two sets of opposed solenoids arranged in a quasi‐Helmholtz manner, offers homogeneity comparable to a conventional Helmholtz pair of similar diameter but with an increase in separation of as much as a factor of 2.

A spatially selective opposed-loop surface coil

Abstract We describe a variation of the conventional surface coil for use in large-sample NMR. It consists of two concentric and coplanar circular loops of wire with different diameters and possibly different numbers of turns, and with currents flowing in opposite directions. The ratio of currents may be chosen so that the field near the center of the coil is zero, resulting in a region at some distance from the coil in which the radiofrequency field intensity is uniform. A parallel winding scheme is described which allows operation at high frequencies. A 5 cm overall diameter opposed-loop coil was built to operate at 80 MHz and exhibited an axial maximum in sensitivity between 1.5 and 2 cm from the plane of the coil.