Optical Molecular Imaging

The terms functional imaging and molecular imaging describe a new direction in imaging technology. As opposed to structural or morphological imaging, functional imaging refers to the capability of monitoring physiological processes primarily based on such indicators as blood flow, cellular metabolism, and neuronal activity. Molecular imaging is a subfield of functional imaging that refers to imaging specifically targeted processes and pathways in cells and tissue at a molecular level.

Although a number of imaging modalities can provide physiologic, metabolic, and molecular information from small animal models for health-related research, optical imaging provides functional in vivo molecular imaging at a comparatively lower cost with high sensitivity and minimal toxicity.

In vivo optical imaging technologies rely on the analysis of photon propagation through a medium. Typically, photons generated by an external light source are directed at the medium, where they are subject to various phenomena, such as scattering, absorption, radiative or non-radiative relaxation, that are identified using a photon detector. In order to enhance the image contrast or the selectivity of the images with respect to the biological processes under investigation, a contrast agent, or probe, may be added to the medium prior to imaging. In some optical imaging modalities, such probes are fluorescent. In vivo imaging of deeper tissue usually prompts the use of the near-infrared wavelengths between 650 and 900 nm where tissue absorption is low.

The advantages of using fluorescence probes to visualize and quantify cells and molecular processes in vivo include:

  • Labeling is flexible. Antibodies, proteins, genes, and small molecules can be easily conjugated with readily available fluorescence dyes.
  • Targeted probes stay within the animal for an extended time.
  • Near infrared fluorescence can be detected from deep within the tissue.