Interferometric scattering (iSCAT) microscopy has demonstrated
unparalleled performance among label-free optical methods for
detecting and imaging isolated nanoparticles and molecules. However,
when imaging complex structures such as biological cells, the
superposition of the scattering fields from different locations of the
sample leads to a speckle-like background, posing a significant
challenge in deciphering fine features. Here, we show that by
controlling the spatial coherence of the illumination, one can
eliminate the spurious speckle without sacrificing sensitivity. We
demonstrate this approach by positioning a rotating diffuser coupled
with an adjustable lens and an iris in the illumination path. We
report on imaging at a high frame rate of 25 kHz and across a
large field of view of 100µm×100µm, while maintaining
diffraction-limited resolution. We showcase the advantages of these
features by three-dimensional (3D) tracking over 1000 vesicles in a
single COS-7 cell and by imaging the dynamics of the endoplasmic
reticulum (ER) network. Our approach opens the door to the combination
of label-free imaging, sensitive detection, and 3D high-speed tracking
using wide-field iSCAT microscopy.