EMCCD camera redraws the boundaries of superresolution 3-D imaging

Charles T. Troy, charlie.troy@photonics.com

Researchers in the US have developed a superresolution 3-D imaging technique that resolves single fluorescent molecules with >10 times the precision of conventional optical microscopy. Using an iXon+ electron multiplying CCD (EMCCD) camera from Andor Technology plc, they can locate molecules to within 12 to 20 nm in all three axes and hope to be able to observe interactions between nanometer-scale intracellular structures previously too small to see.

Combining the concepts of superresolution imaging by sparse photoactivation of single-molecule labels, three-dimensional stochastic optical reconstruction microscopy (STORM) and fluorescence photo-activation localization microscopy (F-PALM) together with a double-helix point spread function (DH-PSF) to provide accurate Z-position information resulted in the 3-D superresolution imaging.

Associate Professor Rafael Piestun of the University of Colorado at Boulder and his students developed a point spread function 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, lending it the distinctive name of “Double Helix PSF.”

Gaining dimension

Professor William E. Moerner at Stanford University in California and his team realized that the DH-PSF could be used for superresolution 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 2-D localization data, enables the 3-D position to be accurately defined.

Furthermore, the DH-PSF approach has been shown to extend the depth of field to ~2 μm in the specimen, approximately twice that achieved with other 3-D superresolution techniques.

Commenting on the role played by the iXon+ camera in this breakthrough, Moerner said, “As the localization precision of our superresolution 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 detected, 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.”

Validation

The DH-PSF’s usefulness was validated recently in a 3-D localization experiment involving imaging of a single molecule of the new fluorogen, DCDHF-V-PF4 azide. This photoactivatable molecule was chosen because it is easily excited and emits a large number of photons before photobleaching takes effect. By operating the EMCCD camera at a constant electron multiplying 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- to 20-nm precision, depending upon 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 3-D positional information.

“We expect that the DH-PSF optics will become a regular attachment on advanced microscopes, either for superresolution 3-D imaging of structures or for 3-D superresolution tracking of individually labeled biomolecules in cells or other environments,” he said.