Wolfgang U. Roffmann

Respiratory Triggered Imaging with An Optical Displacement Sensor

Motion of abdominal organs with respiration is a major problem in NMR spectroscopy and imaging thereof. Triggering each phase-encoding step with respiration or gating a number of phase-encoding steps is one approach to the problem. The design of a sensor for small animal experiments has not been as simple. An optical device, implemented with polymer optical fibres is described, along with associated hardware and electronics which can act as a trigger for small animal NMR experiments. A brief description of a similar device for human application is also given. 2DFT spin-echo and B0 susceptibility images, both triggered and untriggered, are presented to validate the technique.

Direct visualisation of B1 inhomogeneity by flip angle dependency

Inhomogeneities in the spatial distribution of the excitatory Radio Frequency (RF) field, are still a dominant source of artifacts and loss of signal to noise ratio in MR imaging experiments. A number of strategies have been proposed to quantify this distribution. However, in this technical note we present a relatively simple MR imaging procedure which can be used to visualise RF inhomogeneities directly either by means of the magnitude or the phase of an image. To visualise the RF field distribution in both the inner and outer volumes of the coil, we have performed experiments in which the entire coil is submerged in a non-conducting fluid. To the best of our knowledge this strategy has not been used previously in order to evaluate coil performance. Finally, we demonstrate that the method is sensitive enough to reveal the effects of the sample properties on the effective RF wavelength of the transmitted field.

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...

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.