This paper presents a computer vision framework for detecting and tracking
diffraction images of linear structures in differential interference contra
st (DIC) microscopy, The tracker can resolve image displacements of 1/10 of
a pixel despite the weak and orientation-dependent contrast in DIG. as wel
l as the variable blur in such image data caused by vertical specimen movem
ent. In our high numerical aperture, high magnification microscope set-up,
this resolution corresponds to 5 nm in object space. In video DIC similar r
esolution has been reported hitherto only for rotationally symmetric target
s such as bead images. The tracker was developed for measuring deflections
of clamped microtubules with a freely moving second end. By analysing the t
hermal fluctuations of such microtubules it was possible to derive their el
asticity.
The paper describes a filtering scheme for the detection and localization o
f DIC diffraction line images which represent loci of microtubules. For tra
cking the movements of the extracted lines we adopted the sum of squared (b
rightness) differences algorithm from computer vision. The analysis of the
fluctuation measurements demonstrates the high sensitivity of this tracking
technique in quantifying positional and orientational changes. We derived
that the theoretical limit in tracking displacements of such diffraction li
ne images is 1.25 nm, four times below the experimentally verified sensitiv
ity. This indicates that the proposed tracker is still suboptimal. Neverthe
less, the tracking precision was sufficient to reveal subtle deviations in
the distribution of microtubule deflection from free diffusion. They were i
nduced by pivotal points and multiple positions of relaxation. Also, the re
sults suggest that there were defects in the polymer structure which caused
very small but significant bends in the microtubule axis.