Dear Aranaz i think you can use chitosan nanoparticles (CNP) and after CNP accumulation you can do PET imaging.

It depends on context and funds. Radio labelling or isotopic labelling require  a complex instrumentation and an isotope facility. Fluorescent la belling is possible but dyes can be released whereas chitosan is precipitated. Fot CNP RG you may follow http://jairjp.com/SEPTEMBER%202012/12%20CHITRA.pdf

Thanks for the link,  my concern about fluorescent labeling is that the size of the probe is big compared to oligosaccharide size. Can you recommend me any paper about radio or isotopic labeling?. To see the protocol.  Keeping in mind that it is very likely that I cannot do it in my lab and I have to search for a colleague in Spain with the facilities.

The simplest way is selecting among different sources of chitosan the one that differs clearly from your meam medium 13c or 2H or 18O composition. This would require the use of an IRMS facility. Isotope ratio analysers are quite easy to find as service. The hard way is synthetic 13C labelling and this may require an investement but Isolife or any other firm can evaluate an offer. Also Dr Tiwari geve labelling suggestions for PET and you could ask him about a protocol