Molecular Imaging

Molecular Medical Imaging (MMI) permits visualising, in qualitative and quantitative manner, the function of biological processes in the human body. It uses synthetic molecules that, upon being administered to the patientís body, will respond to the local environment and produce imaging contrast. The imaging is achieved non-invasively, without the need for biopsy or surgery. Currently, MMI uses mainly nuclear medicine modalities, such as PET and SPECT. However, in recent years, increased interest is observed in the application of contrast-enhanced ultrasound, photoacoustic and optical imaging. These new modalities are enabled by rapid advances in instrumentation, including endoscopy methods.

The outstanding properties of light make it a valuable tool for imaging: light can be delivered with very high spatiotemporal precision and control over intensity and wavelength; furthermore, photons do not contaminate the patientís body and have low (wavelength-dependent) cytotoxicity. The wide-spread recognition of the possibilities offered by photocontrolled processes is supported by the fact that the year 2015 has been proclaimed by the United Nations (General Assembly 68th Session) as the Year of Light and Light-based Technologies.

Figure 1. Existing (a-c) and potential (d) ways of using light in therapy and imaging: a) In photopharmacology and photodynamic therapy, light is used to locally and temporally activate a drug, avoiding systemic side effects and development of resistance; b) In optical imaging, a fluorescent compound is administered that has the ability to bind to a specific, disease-related target. Excitation with a light of certain energy results in absorption and subsequent emission of a photon of lower energy, which is then detected; c) In photoacoustic imaging, the absorption of a photon by a chromophore leads to temperature rise, thermo-elastic expansion and production of a sound wave which is subsequently detected; d) Alternatively, the light emitted by a fluorophore upon external excitation could be used to locally activate contrast agents used in other imaging modalities.