Peter Bernard Roemer

Optimization of two-dimensional spatially selective NMR pulses by simulated annealing

Abstract Two-dimensional spatial localization can be achieved with a single NMR inverting or refocusing pulse, by applying the RF pulse in the presence of a net magnetic field gradient which reorients through two dimensions during the course of the pulse. Simple rotating (“ϱ”) pulses of this type, however, have the disadvantage of producing ringing outside the central selected volume of the 2D sensitivity profile. In order to eliminate this ringing, a simulated annealing technique has been used to simultaneously optimize the RF and gradient waveforms, producing pulses which leave the NMR signal localized to a well-defined cylinder. These pulses have been implemented on a whole-body NMR system to produce restricted field-of-view images of agarose disk phantoms and of the head. Applications of optimized ϱ pulses include localization in 31 P spectroscopy and elimination of aliasing artifacts in NMR imaging.

Susceptibility, lineshape, and shimming in high-resolution NMR

Abstract The field inhomogeneity caused by an object of known susceptibility and shape is calculated. The field perturbation is derived using the method of equivalent magnetic charges. Lineshapes are calculated by summing Lorentzians over the sample volume and are verified experimentally by observing changes caused by introducing material into the vicinity of a previously shimmed NMR sample. Such lineshapes can be used for the determination of susceptibility of materials.

High temperature superconducting resonator for use in nuclear magnetic resonance microscopy

We describe surface coil resonators made with thin‐film Y1Ba2Cu3O7 (YBCO) for use in high‐resolution nuclear magnetic resonance imaging. At the operating temperature of the coil, 10 K, we have measured Q values (quality factor) in excess of 50 000 at 300 MHz in magnetic fields as high as 9 T. Noise temperature measurements of this superconducting probe indicate that the dominant noise source is Johnson noise. We describe a coupling circuit comprised of the YBCO coil that is inductively coupled to a room‐temperature copper resonator. This circuit preserves the high signal‐to‐noise ratio that accrues to the YBCO resonator.

Off‐axis spatial localization with frequency modulated nuclear magnetic resonance rotating ( ρ) pulses

Nuclear magnetic resonance (NMR) rotating ( ρ) pulses produce two‐dimensional spatially selective inversion of nuclear magnetization with a single NMR excitation. A technique employing frequency modulation of the excitation waveform is described here for offsetting the sensitive volume defined by these pulses to any preselected location within the sample. The technique is demonstrated in a whole‐body 1 H NMR imaging system, and can be used to provide simple, automated relocation or interleaving of selectively excited volumes in NMR imaging and spectroscopy.

A high-frequency 4-switch GTO speed-up inverter for the generation of fast-changing MRI gradient fields

Magnetic resonance imaging (MRI) systems require stronger gradient coil currents with faster ramp times for high-speed image taking and in order to obtain a smaller field of view or a magnification effect. A new simplified four-switch circuit topology is described which is based not on the parallel but on the series connection of a high-voltage inverter and a low-voltage linear amplifier. It is implemented with gate turn-off thyristors (GTOs). This nonresonant circuit combines the advantages of linear amplifiers with those of high-efficiency switchmode power conversion techniques for the generation of up to megawatt pulses. An order of magnitude increase in available gradient power is achieved with excellent coil current waveforms.<<ETX>>

A GTO speed-up inverter for fast-scan magnetic resonance imaging

Magnetic resonance imaging (MRI) machines require unique high-power electronic systems for the generation of pulsed B gradient fields, especially for echo planar MRI, where images are taken in 25-30 ms instead of minutes. The speedup inverter described has been implemented with gate turn-off thyristors (GTOs). A novel nonresonant circuit combines the advantages of high-efficiency switch-mode power conversion techniques for the generation of megawatt pulses. An order-of-magnitude increase in available gradient power has been achieved with excellent fidelity and stability.<<ETX>>