GREEK SYMBOLS COMMONLY USED
IN MR IMAGING
and their transcriptions in the Glossary


(alpha)
flip angle


(gamma)
gyromagnetic ratio


(delta)
chemical shift


(mu)
vector of the magnetic moment,
and


(mu)
permeability


(nu)
frequency


(pi)
ratio of the circumference of the circle to its diameter


(rho)
proton density


(tau)
time delay (TE/2)


(tau c)
correlation time


(chi)
susceptibility


(omega)
Larmor frequency
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Q

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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