In gene therapy, gene delivery vectors, such as viruses, can be imaged according either to their particle biodistribution or their transduction pattern. The former means labeling the viruses with a contrast agent, being visible in some imaging modality, such as MRI or SPECT/PET and latter means visualising the marker gene of gene delivery vector to be visible by the means of immunohistochemical methods, optical imaging or even by PCR. Non-invasive imaging has gained popularity as the imaging equipment has become available for research use from clinics.
For example, avidin-displaying baculoviruses could be imaged in rat brain by coating them with biotinylated iron particles, rendering them visible in MR imaging. The biodistribution of the iron-virus particles was seen to concentrate on the choroid plexus cells of lateral ventricles.