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 pi/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.

Radiofrequency heating effects around resonant lengths of wire in MRI.

Several recent reports agree that the potentially dangerous heating around extended wires or coaxial cables inside the bodycoil of a magnetic resonance imager is related to resonant effects. No quantitative description of this idea has been given so far. We analyse a simplified situation, where a straight metallic wire is completely surrounded by a large volume of homogeneous dielectric with a small conductivity. If it has the correct length, the wire acts as a receiving-and-retransmission antenna, changing the axial symmetry of the incoming electric field into a radially outgoing electric field near the wire ends. The latter field points into the conducting surroundings, causing dissipation. Some simple experiments on geometries related to this theoretical model provide support to the main conclusions. These suggest that under actual imaging conditions resonant effects might be avoided by choosing a wire length of about 2 m. However, more experimental work remains to be done to validate this suggestion.