SPECT images are acquired, typically also a CT or MR image for anatomical reference. The radioactivity concentration is measured from the PET or SPECT images for the various organs of interest. This may include measuring the volume of these organs e.g. from the CT image (rather than weighing the organs as in the dissection procedure) or assessing the radioactivity concentration in a representative part of the organ. Normalizing the tissue radioactivity concentrations to the injected dose gives values in units of percent of the injected dose per milliliter of organ or biological tissue. A benefit of imaging is that the animals can be anaesthetized for imaging for several or all the required time points, that is few animals are required for this procedure and all of them are kept alive. This is considered a non-invasive procedure. In addition, the procedure is in essence the same as for medical diagnostic imaging in the clinic with two main differences: (1) novel compounds under development may be injected into animals subject to scrutiny and approval of the detailed experimental plan while clinicians can only inject radiolabelled compounds that had been tested rigorously and approved for use in humans; (2) animals usually need to be anaesthetized for the duration of the scan (on the order of minutes) while humans are awake and simply need to stay still during the scan. Non-invasive biodistribution imaging in gene therapy 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. References Applied genetics Medical genetics Clinical research Molecular biology