Techniques are described for the design of shielded gradient coils for superconducting MRI systems. These design methods are suited for constructing the most efficient gradient coil that meets a specified homogeneity requirement. Tradeoffs in coil design of efficiency with coil size and gap size are discussed. Residual eddy currents from coils constructed with a finite number of wires are calculated and give guidelines for the construction of efficient, whole-body gradient coils.
We consider a quadrature nuclear magnetic resonance (NMR) probe with imperfect isolation between the two channels arising from a parasitic mutual impedance, and show that the theoretically achievable signal-to-noise ratio is reduced by a factor of (1+η2)−1/2, relative to that of a probe with perfect isolation, where −20 log η is the isolation in dB; provided that all relevant receiver components are matched to a common impedance. The theoretical result is verified by experimental determination of the response of actual NMR imaging probes when driven with coherent signal, and the response of model circuits simulating actual probes when driven with sources of excess noise; the simulated circuits are needed to reduce extraneous interference in the noise measurements. The theory is also confirmed by computer simulations of noise response in coupled resonator circuits.
Lineshapes of spectra obtained through chemical-shift imaging are often distorted due to the delay in sampling necessary for application of phase-encoding gradients. We have developed an automated fitting procedure which simultaneously performs signal quantification, phase correction, and baseline deconvolution of such spectra. The fit is based on the maximum likelihood method and can be implemented in either the time or the frequency domain.
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