Theory 20/Nanoscale/MRI contrast
Contrast Is Size Against Relaxation
The claim
The relaxivity of a nanoparticle contrast agent is set by the ratio of particle size to the water diffusion length during relaxation, so larger particles do not always contrast better.
01Mechanism
Relaxation depends on how many water molecules visit the particle's field; above the diffusion length, far water does not return in time.
02Prediction
Relaxivity should peak at an intermediate ratio.
03Refutation
Relaxivity increasing with size would remove the diffusion limit.