3D Structured Illumination Microscopy (SIM) is a well established method of extendinglateral and axial resolution in fluorescence microscopy, up to two times
that of the diffractive limit of resolution of wide-field microscopy. The operating
principle of SIM is (spatial) frequency mixing between the illumination light and
the specimen structure. The structured illumination pattern is generated inside
the sample using three monochromatic polarized beams diffracted by a grating
which interfere to form an extruded modulation field. The sinusoidal grating is
shifted laterally to laterally translate the modulation field through the specimen,
axial sample translation passes the specimen through the axial modulation component
and scrambles axial modulation within the captured image plane.
Commercial 3D SIM microscopes are available from several vendors but are
expensive and typically only available in academic imaging facilities. Challenges in
application of 3D SIM to study living biological specimens include the limited 3D
acquisition time required to record each 3D SIM image volume. Multi-focus SIM
(MF-SIM) has been previously demonstrated in a proof of concept experiment
on a fixed specimen in a single color. My thesis is part of a research project to
provide a live specimen compatible MF-SIM system.
My work has included leveraging modern electronic and opto-mechanical devices,
alternative illumination and data reconstruction strategies to reduce the
complexity of the illumination path, by removing any unnecessary moving or
modulated devices, and utilizing a modern phase-based Liquid Crystal on Silicon
(LCoS) Spatial Light Modulator (SLM) utilizing calibrated sinusoidal phase grating
patterns to avoid spurious orders generated from aliased pixel lattice patterns
caused by binary gratings.
This approach frees us of some mechanical components and allows us to extend
the functionality of our SIM system to easily accept two or more optimized SIM
patterns, which can be interleaved for multi-color acquisitions. This allows us
to run samples with dual flourophores and study the dynamics and interactions
of fine structures in samples such as the synaptonemal complex in the model
organism C. elegans.
Combining 3D SIM with Multifocus Microscopy (MFM) techniques which utilize
specialized custom diffractive optics. This approach gives us the ability to
simultaneously image 7, 9, or more planes while running a 3D SIM pattern. This
greatly speeds up our SIM temporal resolution at the expense of the total intensity
being split between the focal planes. Due to the fundamental difference
between translating the sample axially through a modulation field, and collecting
multiple focal planes sitting in a modulation field, a novel approach is required to
recover the encoded axial data and perform a true 3D reconstruction. To evaluate
MF-SIM more closely I have been creating tools to simulate 3D SIM modulation
fields, synthetic data sets and synthetic MF-SIM data.