The invention relates to a computer-implemented method (10) for determining the blood volume flow (IBI) through a portion (90i, i=1, 2, 3, . . . ) of a blood vessel (88) in an operating region (36) using a fluorophore. A plurality of images (801, 802, 803, 804, . . . ) are provided, which are based on fluorescent light in the form of light having wavelengths lying within a fluorescence spectrum of the fluorophore, and which show the portion (90i) of the blood vessel (88) at different recording times (t1, t2, t3, t4, . . . ). By processing at least one of the provided images (801, 802, 803, 804, . . . ), a diameter (D) and a length (L) of the portion (90i) of the blood vessel (88) and also a time interval for a propagation of the fluorophore through the portion (90i) of the blood vessel (88) are determined, which time interval describes a characteristic transit time (τ) for the fluorophore in the portion (90i) of the blood vessel (88), in which a blood vessel model (MBQ) for the portion (90i) of the blood vessel (88) is specified, which blood vessel model describes the portion (90i) of the blood vessel (88) as a flow channel (94) having a length (L), having a wall (95) with a wall thickness (d), and having a free cross section Q. A fluid flow model MFQ for the blood vessel model (MBQ) is assumed, which fluid flow model describes a local flow velocity (122) at different positions over the free cross section Q of the flow channel (94) in the blood vessel model (MBQ), and a fluorescent light model MLQ is assumed, which describes a spatial probability density for the intensity of the remitted light at different positions over the free cross section Q of the flow channel (94) in the blood vessel model (MBQ), which light is emitted by a fluid, which is mixed with fluorophore and flows through the free cross section Q of the flow channel (94) in the blood vessel model (MBQ), when said fluid is irradiated with fluorescence excitation light. The blood volume flow (IBI) is determined as a fluid flow guided through the flow channel (94) in the blood vessel model (MBQ), which fluid flow is calculated from the length (L) and the diameter (D) of the portion (90i) of the blood vessel (88) and from the characteristic transit time (τ) for the fluorophore in the portion (90i) of the blood vessel (88), using the fluid flow model MFQ and the fluorescent light model MLQ.