We use nano particles in mice with 4.6 up to 46 μg/ 100/150 μl to have a significant signal intensity decrease in the liver and spleen. But this is for diagnostic imaging, not knoewing what you mean with therapeutic..hope that helps

This is indeed a good question and the answer is highly dependant on the type of nanoparticle you are going to use, the type of MR sequence, the magnetic field to which you will performed your experiment and on the type of tissue you will target.

For exemple, at 7T, in a rat brain trauma model the injection of about 150µg of iron oxyde (particle size around 20nm) induces a signal reduction of 30-40% on a T2*w image (Acquisitions were performed with the following parameters: TR=110 ms, 6 TEs, TE1 = 3ms, interecho time = 3.14 ms.).