Development and application of a new method to shim first harmonic in compact cyclotron

In order to adequately eliminate 1st harmonic in the magnetic field of the compact cyclotron, CIAE has developed a 1st harmonic shimming method that can be applied to the four-sector compact cyclotron. The amplitude of the 1st harmonic can be reduced by adjusting the azimuthal width of the shimming bars at both sides of the sector in this method. Based on the beam dynamics requirement on the amplitude of 1st harmonic, the principle of 1st harmonic shimming method is illustrated through numerical analysis, and meanwhile, the shimming has been implemented on the magnet of CYCIAE-CRM by processing the azimuthal width using NC milling machining. The shimming result shows that it satisfies the requirement of beam dynamics. The shimming effect using this method is also analyzed and expounded in detail. The extension of this method is conducted so that it can be used to shim β_θ at the median plane induced by the installation error of the upper and lower sector poles.     

Volume parcellation for improved dynamic shimming

Introduction The need for a homogeneous magnetic field in magnetic resonance imaging is well established, especially at high static magnetic field strengths where susceptibility-induced image distortions and signal losses become excessively large. Dynamic shim updating, where the optimal set of shim currents is applied for each slice during a multi-slice acquisition, has been shown to improve magnetic field homogeneity to a greater extent than conventional global shimming. Methods Here, in an initial feasibility study, we show via simulation that improved efficacy of shimming can be achieved by using the novel parcellated dynamic shimming method. Results The results of these simulations indicate that parcellated dynamic shimming based on just linear shim terms can perform approximately as well as slice-based dynamic shimming with up to third-order shim terms. Conclusions This work shows that the effective magnetic field inhomogeneity can be further reduced if shimming and image data acquisition are sequentially performed over a series of compact, cuboidal sub-volumes rather than planes. Further work is needed to develop an imaging approach that can be used for the optimal implementation of parcellated dynamic shimming.     

Spurious signals in DQF spectroscopy: two-shot stimulated echoes

The most widely used technique for double-quantum filtered (DQF) single-voxel spectroscopy (SVS) is based on a symmetric PRESS sequence with two additional spatially unselective /2 pulses, one of which is usually frequency selective. The actual filtering, rejecting signals from all uncoupled resonances, can be done by suitable phase cycling of the rf pulses in successive shots, but in practice gradient filtering is always used. Under usual conditions the sequence repetition time is comparable to the spin-lattice relaxation time, and a stimulated echo is formed by five out of the ten rf pulses in two consecutive shots. This echo is not filtered out by the gradients, and additional phase cycling is needed to eliminate it. Its spatial origin is the full transverse slice selected by the last pulse of the PRESS sequence. The SVS shimming procedure may create an important field variation in this slice (outside the volume of interest VOI). Water singlet signals therefore appear in a band of frequencies other than 4.7 ppm, and remain unaffected by water suppression pulses. In practice phase-alternation schemes can reduce these spurious signals by several orders of magnitude, but even then they may mask the weak metabolite signals of interest. We describe a strategy to minimize these spurious signals and propose a 16-step phase cycling scheme that attenuates the stimulated echo in every two-step subcycle.