money tracing

since 1990, the Treasury has placed a nylon strip inside the paper of most of the printed money ($10's and above, I believe). This strip has letters on it, spelling out "USA TWENTY" (for the $20 bill) such that it can only be seen if light is passed through the note. This was officially meant to prevent the high quality scanners and color printers from being able to forge money, since the scanner would not be able to reproduce the reflectively-invisible letters in the strip. Wonder if they could include such a technique in money tracking. Has anyone ever checked the green ink on money for magnetic resonance?