Figure 1.

A) The FFC experiment: The nuclear spin polarization is built up during the pre‐polarization phase, at B_p. Relaxation occurs during the evolution period (τ) at B_r, then the NMR signal is detected at B_d. The sequence is repeated, staggering τ each time. For B_r>7 MHz, the cycle starts in the absence of any polarization field. B) Photographs of the FFC‐NMR relaxometer showing the introduced modifications for the in vivo acquisition: a) the FFC magnet; b) the mouse holding system; c) the transmitter/receiver coil around the mouse's leg.