OPTICAL

It is possible to image particular molecules, cells or organisms using optical imaging techniques. Bioluminescence imaging provides a relatively inexpensive means of imaging genetically-engineered luciferase-expressing cells. When luciferase-labelled cells or genes are exposed to substrate (e.g., luciferin, injected just prior to imaging), photons are emitted that can be detected and quantified by a charge-coupled device (CCD) camera. Relatively small numbers of cells (i.e., < 1,000) can be detected with this technique because background noise levels are inherently low. The availability of genetically-altered luciferases and natural luciferase variants, characterized by differing emission spectra, may allow simultaneous tracking of more than one molecular target.

Using fluorescently labeled antibodies or other molecules, or inducing expression of fluorescence proteins, fluorescence imaging can also be used for cellular and molecular imaging. Unlike bioluminescence imaging, the animal is typically illuminated with an external light source. The resulting autofluorescence reduces the signal-to-noise ratio, but background filtration can be used to improve sensitivity. Diverse fluorescent dyes are now commercially available, allowing imaging of several fluorophores simultaneously in a single animal. In addition, dual-function reporter genes, e.g, a luciferase-GFP fusion reporter, have been utilized to couple in vivo imaging with ex vivo assays. Recently, triple-fusion genes have been developed to link optical and PET imaging.

In vivo optical imaging is limited by the absorption and scatter of light as it passes through living tissue. The development of optical probes that transmit in the near infra-red spectrum has improved resolution, but optical imaging will likely remain a lower resolution modality in comparison to PET, SPECT, CT and MRI. Nevertheless, it has widespread and important application in many fields of biological and biomedical research, including tumor growth and response to therapy, efficacy of gene transfer/gene expression, bacterial infection and immune cell trafficking, beta-amyloid plaque development, and drug efficacy.

Our IVIS 100 Imaging System (Xenogen Corp.), consisting basically of a light-tight chamber and a highly sensitive cooled CCD camera, is equipped for inhalation (isoflurane) anesthesia and uses Windows-based data acquisition and analysis software. As many as 5 mice can be imaged simultaneously, and imaging times are typically 1-20 sec (fluorescence) and 1-5 min (bioluminescent). Although the data from this system are 2-dimensional and have lower resolution than the other imaging modalities, optical imaging has the advantage of high throughput screening afforded by multi-animal scanning and short image acquisition times.           

For additional information:

Contag CH, Bachmann MH. Advances in in vivo bioluminescence imaging of gene expression. Annu Rev Biomed Eng; 4:235-60, 2002.

Ntziachristos V, Bremer C, Weissleder R. Fluorescence imaging with near-infrared light: new technological advances that enable in vivo molecular imaging. Eur Radiol; 13:195-208, 2003.

Bremer C, Ntziachristos V, Weissleder R. Optical-based molecular imaging: contrast agents and potential medical applications. Eur Radiol; 13:231-43, 2003.

Cherry SR. In vivo molecular and genomic imaging: new challenges for imaging physics. Phys Med Biol; 49:R13-R48, 2004.

Contag CH. In vivo pathology: Seeing with molecular specificity and cellular resolution in the living body. Annu Rev Pathol Mech Dis; 2:277-305, 2007.