The system was characterized and validated in several liquid and solid phantom experiments. First, a liquid phantom system with a submerged flow tube was developed to evaluate the impact of tube depth, flow speed, and background liquid optical properties on BFI. To simulate the DCS signals relevant to breast tissue, flow rates were selected to yield rBF (tumor/normal) values (from 1.90 to 2.70) at a tube located 15 mm below the surface based on previously measured flow changes of 32 patients.20 Increasing the depth of the flow tube not only decreases the measured flow rate but also reduces flow rate sensitivity and dynamic range. We found that with 28 mm s-d separation, a linear flow signal was accessible up to 15 mm deep. However, while flow changes were accessible for a 20-mm deep flow tube, its linear dynamic range was greatly reduced. values derived from linear regression analysis of BFI at depths of 0, 5, 10, 15, and 20 mm were 0.98, 0.98, 0.98, 0.86, and 0.76, respectively.