Patent ID: 8509875

Claim:
An MR process to locate in the examination volume of an MR device a medical instrument with a microcoil attached thereto which is part of a resonant circuit that is non-switchably tuned to the resonant frequency of the MR device and having no external controls, with the following process steps: generating a plurality of temporally successive RF pulses within the examination volume, a temporal spacing between successive RF pulses being less than a longitudinal relaxation time of core magnetization such that the core magnetization is driven toward a steady state in which the generation and relaxation of the core magnetization are balanced, the microcoil causing the core magnetization in tissue adjacent the microcoil to approach the steady state in tissue differently than in tissue displaced from the microcoil; before the core magnetization reaches the steady state, applying a magnetic field gradient pulse in a first direction after an earlier RF pulse to generate a first gradient echo signal frequency encoded in the first direction, wherein the generating of the plurality of RF pulses and the generating of the first and second gradient echoes are performed without altering the resonance frequency of the microcoil resonant circuit; when the core magnetization has been driven closer to the steady state, re-applying the magnetic field gradient pulse in the first direction after a subsequent RF pulse to generate a second gradient echo signal frequency encoded in the first direction; subtracting the first and second gradient echo signals to generate a difference signal; determining a position of the medical instrument in the first direction by analysis of the difference signal.