Biomedical Engineering Reference
In-Depth Information
useful in collecting the membrane potential information from many spatially distributed points, either
from a single neuron or a population of neurons.
As an alternative solution to faster scanning is the recently developed SLM microscopy [52], in which
the laser beam is multiplexed in space by a spatial light modulator, so that every part of the sample of
interest is illuminated simultaneously. Then, one can use a camera as a detector to collect photons that
emerge from every area of interest. Thus, with SLM, one does not scan the sample any more, and this
solves the problem of small pixel dwell time, introduced by faster scanning methods. “Scanless” SLM
microscopy works well with femtosecond lasers and is therefore poised to be very helpful to advance fast
SHG measurements.
Finally, similar to most imaging techniques, the signal-to-noise ratio needs to be improved for SHG
imaging. This is especially crucial in voltage measurement since better signal-to-noise ratio is directly
translated into better ability to detect smaller voltage fluctuations with higher confidence. Photon coun-
ter has been implemented to increase signal-to-noise ratio in the SHG recording of membrane potential
in neurons, which is generally photon-limited [53]. Use of photon-counting mode has two advantages:
(1) it reduces background signals arising from thermal noises, and (2) it allows accumulation of SHG
photons in a given recording time. Increase in signal and reduction in noise would lead to better sig-
nal-to-noise ratio and therefore better recordings. Just as photon counting is shown to improve SHG
imaging, we can expect that application of other techniques used in different imaging researches may
improve the SHG imaging as well.
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