The invention relates to a blood flow determination apparatus (5) comprising a speckle density determination unit (7) for determining a speckle density value being indicative of a level of a speckle density in an ultrasound image of an inner lumen of a blood vessel, which has been generated at an imaging location being behind an introduction location at which a fluid has been introduced into the blood vessel (20) with a known volumetric fluid flow rate. A blood flow rate determination unit (8) determines a blood flow value being indicative of a volumetric blood flow rate based on the determined speckle density value and the volumetric fluid flow rate. This allows determining the intravascular blood flow with an improved accuracy.