In vivo Imaging and CCD cameras
Choosing an In Vivo Optical Imaging system entails the evaluation of scientific grade CCD cameras.
Important parameters to consider are cooling,intensification, quantum efficiency, spectrum, dynamic range, frame rate, resolution, etc.
Cooling to absolute - 90oC will allow for maximal sensitivity by maximal suppression of dark current. Essential for snesitivity of detection of bioluminescent signals.
The quantum efficiency (QE) graph of a CCD is a measure of the detector's capability to produce an electronic charge in response to the detected signal as a function of wavelength. High QE over a broad spectrum (450-850nm) allows for maximal sensitivity of all reporters in the visible spectrum.
Signal to noise ratio: a comparison of CCD's. The CCD approximates "ideal" >1000 ph/s. Remember that the faint autoluminescent signal of an animal is ~ 5000 ph/s, setting the biological threshold. Even though the camera threshold is usually <100 ph/s.
Gain aging effect: the gain components in an ICCD and an EMCCD i.e. the MCP and the EM register suffer from gain aging effects. For a given applied voltage to the gain components you would expect the gain to remain constant. However the gain components suffer from parasitic effects which lower their gain as a function of the total charge extracted from them. Therefore if you measure the gain over a period of time at a fixed voltage and temperature the gain will fall off and the fall off will be more pronounced the more extracted charge that passes though the gain component. The gain is not lost however and a certain gain can be recovered by simply increasing the voltage in the gain component to compensate for the aging effect. Ultimately the gain components can be damaged by excessive voltage so they do have an ultimate lifetime. To minimize the effects of gain aging it is desirable not to have the gain turned up when the device is not recording signal and to use the gain sparingly i.e. set the gain to give you sufficient sensitivity and not to use the gain at the maximum level needlessly.
Intensified CCD’s also suffer from an additional aging effect due to the poisoning of the photocathode. The photocathode gets damaged by contaminates in the image intensifier tube and this damage is accelerated when the tube is gated on. The damage is not recoverable so again it is good practice to keep the gain turned down when it is not required and minimise the light falling on the photocathode.
To learn more:
CCD, EMCCD and iCCD Camera comparisons
Digital camera fundamentals PDF
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