Catheter systems and methods for determining blood flow rates based on light reflection measurements. The catheter may include a lumen extending between a proximal end of the catheter and a distal end of the catheter. The catheter may include fluid infusion openings at the distal end region of the catheter that are configured to permit the indicator fluid to exit the catheter from the lumen. The catheter system may include an optical fiber having one or more sensors thereon for sensing light reflected by blood particles in a body vessel lumen. A blood flow rate may be determined based on the sensed light reflected by blood particles in the body vessel lumen.