24 May 2010
A highly sensitive EMCCD camera helps to redraw the boundaries of super-resolution 3D imaging.
With the help of a highly sensitive Andor iXon+ EMCCD camera, US researchers have developed a super-resolution, 3D imaging technique that can resolve single fluorescent molecules with greater than 10 times more precision than conventional optical microscopy. By being able to locate molecules to within 12–20 nm in all three axes, the team hopes to be able to observe interactions between nanometre-scale intracellular structures previously too small to see (Nano Letters 10 211).
This major advance has been achieved by combining super-resolution imaging by sparse photoactivation of single-molecule labels (PALM, STORM, F-PALM) with a double-helix point spread function (DH-PSF) to provide accurate z-position information.
Rafael Piestun from the University of Colorado, US, and his students developed a PSF with two rotating lobes where the angle of rotation depends on the axial position of the emitting molecule. This means that the PSF appears as a double helix along the z-axis of the microscope. W E Moerner at Stanford University, US, and his team realized that the DH-PSF could be used for super-resolution imaging with single molecules.
With the DH-PSF, a single emitting fluorescent molecule emits a pattern corresponding to a standard PSF, but the image this creates is convolved with the DH-PSF using Fourier optics and a reflective mask outside the microscope. At the detector, the image from a single molecule appears as two spots, rather than one. The orientation of the pair can be used to decode the z-location of a molecule, which combined with the 2D localization data, enables the 3D position to be accurately defined.
Furthermore, the DH-PSF approach has been shown to extend the depth of field to around 2 µm in the specimen, approximately twice that which has been achieved in other 3D super-resolution techniques.
Commenting on the role played by the Andor iXon+ EMCCD camera in this breakthrough, Moerner said: "As the localization precision of our super-resolution technique improves at a rate of one over the square root of the number of photons detected, it was essential to use a camera that allowed us to detect every possible photon from each single molecule. Put simply, the more photons we detect, the greater the x, y, and z precision. However, the speed of imaging is also important. Since we need to acquire multiple images for each reconstruction, it is always best to record the images as fast as possible."
The DH-PSF's usefulness was recently validated in a 3D localization experiment involving imaging of a single molecule of the new fluorogen, DCDHF-V-PF4 azide. This photoactivatable molecule was chosen as it emits a large number of photons before it bleaches, and is easily excited.
By operating the EMCCD camera at a constant EM gain setting of x250, to eliminate the read noise detection limit, it was possible to acquire many images of the single photoactivated molecule. From these images, the x-y-z position of the fluorophore could be determined with 12–20 nm precision, depending on the dimension of interest.
Moerner and his team have called this new technique single molecule Double Helix Photoactivated Localization Microscopy (DH-PALM), and are confident that it will provide far more useful information than is the case for other approaches to extracting 3D positional information.
"We expect that the DH-PSF optics will become a regular attachment on advanced microscopes, either for super-resolution 3D imaging of structures, or for 3D super-resolution tracking of individually labelled bio-molecules in cells or other environments," he said.