  Absorbed power of nanoparticles during magnetic hyperthermia can be well
determined from changes in the quality factor ($Q$ factor) of a resonator, in
which the radiofrequency (RF) absorbent is placed. We present an order of
magnitude improvement in the $Q$ factor measurement accuracy over conventional
methods by studying the switch-on and off transient signals of the resonators.
A nuclear magnetic resonance (NMR) console is ideally suited to acquire the
transient signals and it also allows to employ the so-called pulse
phase-cycling to remove transient artifacts. The improved determination of the
absorbed power is demonstrated on various resonators in the 1-30 MHz range
including standard solenoids and also a birdcage resonator. This leads to the
possibility to detect minute amounts of ferrite nanoparticles which are
embedded in the body and also the amount of the absorbed power. We demonstrate
this capability on a phantom study, where the exact location of an embedded
ferrite is clearly detected.
