By reflecting the different stages of gene action, molecular imaging strategies have equivalent biotechnological techniques that assess molecular genetic events in tissue samples. Broadly speaking, indirect molecular imaging corresponds to genomics or proteomics, direct imaging to proteomics and surrogate imaging to physiomics or metabonomics. It should be noted that, unlike molecular imaging, some biotechnology approaches such as DNA microarrays assess many hundreds or thousands of molecular genetic events simultaneously. However, molecular imaging has some distinct advantages over tissue-sampling methods. Firstly, the need for invasive biopsy is avoided. In some cases, obtaining tissue for genetic analysis can be difficult and it is possible that tissue perturbation resulting from the biopsy process can alter gene expression. Secondly, spatial heterogeneity of gene expression, well recognised within tumours, may result in biological features being missed on biopsy due to sampling error, whereas imaging can assess the whole tumour. Such heterogeneity has been shown to produce discrepancies in the apparent degree of expression of pgp-mediated multidrug resistance as assessed in vitro by immunohistochemistry and in vivo by MIBI imaging. Differences in gene expression at different disease sites in the same patient and changing expression over time are also well-recognised phenomena that will be more readily appreciated through molecular imaging, whereas multiple biopsies are likely to be undesirable or impracticable. Furthermore, molecular imaging can assess molecular and genetic processes in the target tissue and other organs simultaneously.
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