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Label-free measurement of subcellular morphology can be used to track dynamically cellular function under various
conditions and has important applications in cellular monitoring and in vitro cell assays. We show that optical filtering
of scattered light by two-dimensional Gabor filters allows for direct and highly sensitive measurement of sample
structure. The Gabor filters, which are defined by their spatial frequency, orientation and Gaussian envelope, can be used
to track locally and in situ the characteristic size and orientation of structures within the sample. Our method consists of
sequentially implementing a set of Gabor filters via a spatial light modulator placed in a conjugate Fourier plane during
optical imaging and identifying the filters that yield maximum signal. Using this setup, we show that Gabor filtering of
light forward-scattered by spheres yields an optical response which varies linearly with diameter between 100nm and
2000nm. The optical filtering sensitivity to changes in diameter is on the order of 20nm and can be achieved at low
image resolution. We use numerical simulations to demonstrate that this linear response can be predicted from scatter
theory and does not vary significantly with changes in refractive index ratio. By applying this Fourier filtering method
in samples consisting of diatoms and cells, we generate false-color images of the object that encode at each pixel the size
of the local structures within the object. The resolution of these encoded size maps in on the order of 0.36μm. The pixel
histograms of these encoded images directly provide 20nm resolved “size spectra”, depicting the size distribution of
structures within the analyzed object. We use these size spectra to differentiate the morphology of apoptosis-competent
and bax/bak null apoptosis-resistant cells during cell death. We also utilize the sensitivity of the Gabor filters to object
orientation to track changes in organelle morphology, and detect mitochondrial fission in cells undergoing apoptosis.
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