Abstract
Wide-field temporal focused (WF-TeFo) two-photon microscopy allows for the simultaneous imaging of a large planar area, with a potential order of magnitude enhancement in the speed of volumetric imaging. To date, low repetition rate laser sources with over half a millijoule per pulse have been required in order to provide the high peak power densities for effective two-photon excitation over the large area. However, this configuration suffers from reduced signal intensity due to the low repetition rate, saturation effects due to increased excitation fluences, as well as faster photobleaching of the fluorescence probe. In contrast, with the recent advent of high repetition rate, high pulse energy laser systems could potentially provide the advantages of high repetition rate systems that are seen in traditional two-photon microscopes, while minimizing the negatives of high fluences in WF-TeFo setups to date. Here, we use a 100 microjoule/high repetition rate (50–100 kHz) laser system to investigate the performance of a WF-TeFo two-photon microscope. While using micro-beads as a sample, we demonstrate a proportionate increase in signal intensity with repetition rate, at no added cost in photobleaching. By decreasing pulse intensity, via a corresponding increase in repetition rate to maintain fluorescence signal intensity, we find that the photobleaching rate is reduced by ~98.4%. We then image live C. elegans at a high repetition rate for 25 min. as a proof-of-principle. Lastly, we identify the steady state temperature increase as the limiting process in further increasing the repetition rate, and we estimate that repetition rate in the range between 0.5 and 5 MHz is ideal for live imaging with a simple theoretical model. With new generation low-cost fiber laser systems offering high pulse energy/high repetition rates in what is essentially a turn-key solution, we anticipate increased adoption of this microscopy technique by the neuroscience community.
Highlights
In recent years there is ever-increasing interest in the neuroscience community for obtaining access to a more holistic view of neural activity, which has motivated the development of newer bio-optical technologies
The acquisition rate is fundamentally limited by the number of photons that can be captured by a detector during the exposure time
This is especially critical in multiphoton microscopy, where excitation/emission processes are limited by power-law dependency on excitation intensity [16]
Summary
In recent years there is ever-increasing interest in the neuroscience community for obtaining access to a more holistic view of neural activity, which has motivated the development of newer bio-optical technologies. Two-photon laser scanning microscopy is one of the tools of choice often employed for this purpose While it offers excellent resolution and is minimally invasive, it is constrained by the speed at which scanning can be realized [1]. An extension of this technique using adaptive optics to generate multi-focal excitation is one way to increase the scan speed [2] Another way is to employ the temporal focusing phenomenon, which offers a substantial increase in the area that can be accessed per single exposure cycle combined with useful axial sectioning characteristics [3]. Wide-field temporal focusing microscopy (WF-TeFo) is a far-field multiphoton imaging technique, which allows for excitation over a large area of the sample, which eliminates temporal drift in the lateral plane It could potentially enable a dramatic increase in acquisition rate for volumetric fluorescence imaging. Information from all pixels can be gathered in parallel, since the whole image plane need not be scanned sequentially
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