The quality factor (Q) applies to any resonant circuit component; most often the quality factor of the
coil determines the overall Q of the circuit.
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.
In a parallel tuned circuit (such as used in a MR
coil), the quality factor is defined as:
Q =
vL/R
where L is the
coil inductance, R is the circuit resistance, and
v is the
angular frequency.
Increasing quality factor results in improving the
signal to noise ratio SNR by a factor √Q and also produces a sharper
frequency response (decreased band width). The Q of a
coil will depend on the circumstances under which it is measured, e.g. whether it is 'unloaded' (no patient) or 'loaded' (patient).