Optical Imaging (OI)
Molecular imaging by means of light detection
Optical imaging (OI) is a sensitive tool for molecular imaging, based on the detection of light emitted from vital cells or tissues. The two most often used optical imaging approaches rely on fluorescence or bioluminescence as a source of light. The emitted light is detected by sensitive, intensified and cooled charge-coupled device (CCD) cameras.
Advantages of OI compared to other imaging modalities:
Bioluminescence imaging (BLI) requires genetic engineering of the cells or tissues to image with a reporter gene that encodes one of a number of light-generating enzymes (luciferases).
Intensified detectors use photocathodes to convert captured photons to electrons that are amplified, converted back to photons using a phosphorscreen, and finally detected on the CCD. Dependent on the photocathode used, the light detection is limited to specific regions of the light spectrum (bialkali photocathodes are best at detecting light in the blue-green range but perform less well above 600nm; gallium arsenide photocathodes extend the spectrum into the red range). CCD cameras can be cooled down to -120°C to reduce the thermal noise, greatly improving the signal-to- noise ratio while preserving the spectral sensitivity of the CCD.
Luciferases are enzymes that can generate visible light through the oxidation of an enzyme-specific substrate in the presence of oxygen (and sometimes other co-factors like ATP). Luciferases are present in certain bacteria, marine crustaceans, fish and insects. Because mammalian tissues do not naturally emit bioluminescence, the advantage of in vivo BLI is the inherently low background signal.
The most commonly used luciferases are the ones from the North American firefly (Photinus pyralis) and the sea pansy (Renilla reniformis). The firefly luciferase enzyme converts the substrate D-luciferin to oxyluciferin, resulting in green light emission at 562nm. The renilla luciferase enzyme uses the substrate coelenterazine to produce blue light at 482nm.
An industrial cooperation with Berthold allows us to study certain applications by use of the Berthold Nightowl Camera. Furthermore, in collaboration with the Centre for Molecular Medicine Cologne (CMMC) we have access to the IVIS 200 system (Xenogen).
Bioluminescence imaging of luciferase expressing cells. Cells with regulated expression of luciferase show luciferase activity only in the presence of the inducer doxycycline (Dox). (top – Dox, bottom + Dox; Xenogen, IVIS 200)
Bioluminescence image of a growing glioma. The growth profile of a luciferase expressing tumour is displayed by means of bioluminescence after injection of the substrate D-luciferin.