Kurt Luescher

Designs for an asymmetric gradient set and a compact superconducting magnet for neural magnetic resonance imaging

Imaging of the head and neck is the most commonly performed clinical magnetic resonance imaging (MRI) examination [R. G. Evans and J. R. G. Evans, AJR 157, 603 (1991)]. This is usually undertaken in a generalist MRI instrument containing superconducting magnet system capable of imaging all organs. These generalist instruments are large, typically having a bore of 0.9–1.0 m and a length of 1.7–2.5 m and therefore are expensive to site, somewhat claustrophobic to the patient, and offer little access by attending physicians. In this article, we present the design of a compact, superconducting MRI magnet for head and neck imaging that is less than 0.8 m in length and discuss in detail the design of an asymmetric gradient coil set, tailored to the magnet profile. In particular, the introduction of a radio-frequency FM modulation scheme in concert with a gradient sequence allows the epoch of the linear region of the gradient set to be much closer to the end of the gradient structure than was previously possible...

The design of biplanar, shielded, minimum energy, or minimum power pulsed B0 coils

By extending the formalism previously developed for the design of unshielded, biplanar gradient coils, shielded biplanarB0 coils optimized for homogeneity and either minimum energy or minimum power may be designed. We present results from an integrated approach to shielded biplanar coil design, the results of which are also applicable to gradient coils, enabling the design of shielded coils with a concomitant decrease in total inductance of the coil. Length constraints are also included in the integrated minimization procedure. Results from a preliminary design indicate that high-homogeneity, low-impedance, well-shielded coils result from this design approach.

Currents and fields in shielded RF resonators for NMR/MRI

A new method for calculating the current densities in shielded RF probes for NMR imaging and spectroscopy is presented. From these current densities, accurate field distributions can be calculated and the effects of shielding the probe deduced. Without accurate calculations of the current density in the shielded coil conductors, the performance of such coils cannot be reliably predicted. Shielded coils are shown not only to drastically reduce external flux leakage but also to improve the homogeneity of the coil structure. Preliminary experimental results for circular cross section RF resonators confirm the accuracy of the calculated fields. The method of current density calculation is applicable to coils of various cross sectional shapes. As an example, the current density and fields generated by a shielded ellipsoidal RF coil are calculated.

Incorporating phase retardation effects into radiofrequency resonator models: application to magnetic resonance microscopy

A method is presented for including path propagation effects into models of radiofrequency resonators for use in magnetic resonance imaging. The method is based on the use of Helmholtz retarded potentials and extends our previous work on current density models of resonators based on novel inverse finite Hilbert transform solutions to the requisite integral equations. Radiofrequency phase retardation effects are most pronounced at high field strengths (frequencies) as are static field perturbations due to the magnetic materials in the resonators themselves. Both of these effects are investigated and a novel resonator structure presented for use in magnetic resonance microscopy.

Asymmetric MRI systems: shim and RF coil designs

We have recently introduced the concept of asymmetric clinical MRI systems. The potential advantages of these systems include a reduced perception of claustrophobia by patients and better physician access to the patient. For asymmetric magnet systems to be useful as a clinical system, asymmetric shims and RF coils must be implemented, and in this work we describe new design methodologies for both.