  Medical research suggests that the anterior-posterior (AP)-diameter of the
inferior vena cava (IVC) and its associated temporal variation as imaged by
bedside ultrasound is useful in guiding fluid resuscitation of the
critically-ill patient. Unfortunately, indistinct edges and gaps in vessel
walls are frequently present which impede accurate estimation of the IVC
AP-diameter for both human operators and segmentation algorithms. The majority
of research involving use of the IVC to guide fluid resuscitation involves
manual measurement of the maximum and minimum AP-diameter as it varies over
time. This effort proposes using a time-varying circle fitted inside the
typically ellipsoid IVC as an efficient, consistent and novel approach to
tracking and approximating the AP-diameter even in the context of poor image
quality. In this active-circle algorithm, a novel evolution functional is
proposed and shown to be a useful tool for ultrasound image processing. The
proposed algorithm is compared with an expert manual measurement, and
state-of-the-art relevant algorithms. It is shown that the algorithm
outperforms other techniques and performs very close to manual measurement.
