11 Digital Micro PIV (μPIV) and Velocity Profiles In Vitro and In Vivo Aristotle G. Koutsiaris Bioinformatics Lab, School of Health Sciences, Technological Educational Institute (TEI) of Larissa, Larissa, Greece 1. Introduction The term Particle Image Velocimetry (PIV), in the technical world, is used to describe a powerful, automated flow visualization method which quantifies the instantaneous flow velocity field in 2 dimensions. PIV gives valuable information on how velocity field changes at a specific measurement plane, at regular time intervals, selected by the operator. Correlation techniques were used extensively for the measurement of fluid velocity. In temporal correlation techniques, velocity is inversely proportional to the transit time for flow tracers to cross a fixed distance. In spatial correlation techniques, the velocity is directly proportional to the displacement of flow tracers in a fixed time interval. In this sense, many video techniques, high speed cinematography and PIV can be classified as spatial correlation techniques. The term “Digital” refers to the fully digital implementation of the method, namely the use of a digital camera connected directly to a digital electronic computer in order to acquire flow images and then to estimate the correlation function. This may seem a common place today, but a lot of early implementations used to have special electrooptical and electromechanical hardware for the estimation of the correlation function (optical correlation) or film cameras with laser scanners (optical – digital correlation). Later, fully electronic PIV implementations appeared using analog together with digital hardware: analog CCD cameras connected to video recorders produced video tape images which were digitized by computers (Gardel et al., 2005). These implementations were limited by their analog components. The term “Micro” refers to the length scale of the microfluidic environment ranging between 1Ǎm and 1000 Ǎm. The word “environment” is a general term for every possible microstructure inside which the fluid under study flows. This chapter is a medium size review on Digital micro PIV (μPIV) and its applications on the velocity profiles in vitro and in vivo. Parts of sections 6 and 7 come from a mini review paper (Koutsiaris, 2010b). www.intechopen.com