of interest, the objects to be tracked have widely differing and even time-varying
appearances. As a consequence, full automation usually can be achieved only by
developing very dedicated algorithms that are tuned for the specific application.
This explains why existing commercial tracking software tools, which are devel-
oped for broad, general applicability, often fail to yield satisfactory results for
specific tracking tasks.
In this section we discuss published approaches to automated object tracking
in time-lapse microscopy images. A distinction is made between cell tracking and
particle tracking. Two different strategies exist for both problems. The first
consists of the identification of the objects of interest in the entire image
sequence, separately for each frame, followed by temporal association, which
tries to relate identified objects either globally over the entire sequence or from
frame to frame. In the second strategy, objects of interest are identified only in
a first frame and are subsequently followed in time by image matching or by
model evolution. In either case, the algorithms usually include a detection or
segmentation stage and a temporal association stage. Both are essential to
performing motion analysis of individual objects. Alternative approaches
based on optic flow have also been studied [17, 19, 40, 41], but these are limited
to computing collective cell motion and intracellular particle flows, unless
additional detection algorithms are applied.
15.4.1 Cell Tracking
Cell motility and migration are of fundamental importance to many biological
processes [4, 10, 42, 43]. In embryonic development, for example, cells migrate
and differentiate into specific cell types to form different organs. Failures in this
process may result in severe congenital defects and diseases. In adult organisms
as well, cell movement plays a crucial role. In wound healing, for example,
several interrelated cell migration processes are essential in regenerating dam-
aged tissue. The immune system consists of many different proteins and cells
interacting in a dynamic network to identify and destroy infectious agents.
Many disease processes, most notably cancer metastasis, depend heavily on
the ability of cells to migrate through tissue and reach the bloodstream. Because
of its importance for basic cell biology and its medical implications, cell migra-
tion is a very active field of research. Automated methods for segmenting cells
and following them over time (Table 15.1) are becoming essential in quantifying
cell movement and interaction under normal and perturbed conditions.
15.4.1.1 Cell Segmentation
The simplest approach for separating cells from the background is intensity
thresholding (see Chapter 9). This involves a single threshold parameter that can
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15.4 Image Analysis

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