Fig. 4.

Tracer distributions in hydrogel for a needle inserted at (a) 1.8 mm/s and (b) 0.2 mm/s. Both figures show tracer infusions with a flow rate of 0.5 μ/min. Z is the reported backflow distance and $e$ is the perpendicular tracer penetration distance in hydrogels (0.7176 mm needle diameter).