Rubber is soft like skin, muscle and tendon. So why not use magnetic resonance imaging (MRI) to visualize cross-link density distributions and layer structures of rubber products with magnetic resonance, when MRI works so well for diagnosing illness in soft biological tissue?
The problem comes when tires are to be studied. They are too large to fit into a whole-body magnet of a clinical MRI scanner, and they contain steel belts, which are vigorously attracted by the magnet. The solution to this problem came with the invention of the NMR-Mouse.
This is a compact MRI device with a permanent magnet of particular shape where the object is placed not inside but next to the magnet. The measured signal is collected in the stray field of the magnet so that the object can be arbitrarily large.
Moreover, the magnetic attraction between NMR-Mouse and the steel belt is not overwhelming so that information contained in one MRI pixel readily can be collected locally and non-destructively with the NMR-Mouse from tires with steel belts and from other rubber products. The pixel has the shape of a less than 0.1 mm thick nickel coin and can be shifted through the object to acquire depth profiles to study the layer structure at any location. The contrast can be adjusted in such a way that it scales with crosslink density or with proton density, which can be converted into gravimetric density.