Basic MPI Principle. Basic MPI principle . The drive field H
(t) generates a particle response M(t) that induces a voltage in the receive coils. The time-dependent voltage is measured and constitutes the raw signal s(t) ∝ dM(t)/dt. Due to the non-linear magnetization curve, the signal spectrum S
contains higher harmonics of the drive frequency ω
0, which are used for particle detection and imaging. For reference, the derivative of the magnetization curve ∂M/∂H is also shown (blue curve).