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Q → Quality factor.

Quadrature coil: analogous to quadrature detection in that the in phase and 90° out of phase components are used to improve the efficiency of the RF pulse and the S/N ratio.

Quadrature detector: a phase sensitive detector or demodulator that detects the com­po­nents of the signal in phase with a reference oscillator and 90° out of phase with the reference oscillator.

Quality factor (Q): applies to any electrical circuit component; most often the coil Q is limiting. Inversely related to the fraction of the energy in an oscillating system lost in one oscillation cycle. Q is inversely related to the range of frequency over which the sys­tem will exhibit resonance. It affects the signal-to-noise ratio, because the detected sig­nal in­crea­ses proportionally to Q while the noise is proportional to the square root of Q. The Q of a coil will depend on whether it is unloaded (no patient) or loaded (patient).

Quench, quenching: loss of superconductivity of the current carrying coil that may oc­cur unexpectedly in a superconducting magnet. As the magnet becomes resistive, heat will be released that can result in rapid evaporation of liquid helium in the cryostat.

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