Verrier and Fournier present a method where this tracking can be improved; after an initial rough tracking step, small changes in the object’s position between images can be used to create a super-resolved image of it. The resulting image has both a higher resolution and better signal-to-noise ratio than the individual frames. Fitting the model to this higher-quality image allows them to estimate some parameters (for example axial position and size) more accurately. Once those parameters are established, they can be used to calculate a better template image for the particle, which can then be used to estimate for the particle’s position more accurately in each video frame. This leads to an improvement in the lateral tracking accuracy as well. Absorbing particles were simulated for this paper, the diffraction patterns of which can be parameterised by their radius and axial position. The lateral motion of the particles was used to create enhanced images to find radius and axial position, which in turn led to improved template images for lateral tracking.
The authors suggest their technique should be generalised to include axial position in the super-resolution algorithm as a parameter that can change from frame to frame. In most microscopic situations, this would be optimal: Brownian motion means a micro-object’s position changes quite rapidly in three dimensions, but generally other parameters (like size or refractive index) do not change on short timescales. Using a super-resolution technique to create a high-quality image that allows these constant parameters to be estimated more accurately then means that the particle’s motion in 3D can be tracked better, by comparing each image with the best possible template image.
As the method presented already copes with multiple objects, which may move independently, it should be suitable for a wide range of situations where particles must be tracked as accurately as possible. This should further improve the accuracy with which in-line holography can localise particles, in applications from micro-particle imaging velocimetry to optical tweezers.
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