Computer-implemented method for determining the blood volume flow (I BI) through a portion (90 1, 90 2, 90 3,...) of a blood vessel in an operating region using a fluorophore, in which a plurality of images (80 1, 80 2, 80 3, 80 4,,...) 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 (90 2, 90 2, 90 3,...) of the blood vessel at different recording times (t1, t2, t3, t4,...), in which, by processing the provided images (80 1, 80 2, 80 3, 80 4,,...), a diameter and a length of the portion of the blood vessel and a time interval for a propagation of the fluorophore through the portion (90 2, 90 2, 90 3,...) of the blood vessel are determined, which time interval describes a characteristic transit time (τ) for the fluorophore in the portion (90 2, 90 2, 90 3,...) of the blood vessel, in which a blood vessel model ( M B Q), which describes the portion (90 2, 90 2, 90 3,...) of the blood vessel as a flow channel having a length, having a wall with a wall thickness (d), and having a free cross section Q, on at least one of the provided images (80 1, 80 2, 80 3, 80 4,,...) is adapted by means of image processing; in which a fluid flow model M F Q for the adapted blood vessel model M B Q is provided, which fluid flow model describes a local flow velocity at different positions over the free cross section Q of the flow channel in the adapted blood vessel model M B Q; characterized in that a fluorescent light model M L Q is provided, which describes a spatial probability density for the intensity of the diffusely reflected light at different positions over the free cross section Q of the flow channel in the adapted blood vessel model M B Q, which light is emitted by a fluid, which is mixed with fluorophore and flows through the free cross section Q of the flow channel in the adapted blood vessel model M B Q, when said fluid is irradiated with fluorescence excitation light; and the blood volume flow (I BI) is determined as a fluid flow guided through the flow channel in the adapted blood vessel model M B Q, which fluid flow is calculated from the length and the diameter of the portion (90 2, 90 2, 90 3,...) of the blood vessel and from the characteristic transit time (τ) for the fluorophore in the portion (90 2, 90 2, 90 3,...) of the blood vessel, using the fluid flow model M F Q provided and the fluorescent light model M L Q provided. in which a plurality of images (80 1, 80 2, 80 3, 80 4,,...) 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 (90 2, 90 2, 90 3,...) of the blood vessel at different recording times (t1, t2, t3, t4,...), in which, by processing the provided images (80 1, 80 2, 80 3, 80 4,,...), a diameter and a length of the portion of the blood vessel and a time interval for a propagation of the fluorophore through the portion (90 2, 90 2, 90 3,...) of the blood vessel are determined, which time interval describes a characteristic transit time (τ) for the fluorophore in the portion (90 2, 90 2, 90 3,...) of the blood vessel, in which a blood vessel model ( M B Q), which describes the portion (90 2, 90 2, 90 3,...) of the blood vessel as a flow channel having a length, having a wall with a wall thickness (d), and having a free cross section Q, on at least one of the provided images (80 1, 80 2, 80 3, 80 4,,...) is adapted by means of image processing; in which a fluid flow model M F Q for the adapted blood vessel model M B Q is provided, which fluid flow model describes a local flow velocity at different positions over the free cross section Q of the flow channel in the adapted blood vessel model M B Q; characterized in that a fluorescent light model M L Q is provided, which describes a spatial probability density for the intensity of the diffusely reflected light at different positions over the free cross section Q of the flow channel in the adapted blood vessel model M B Q, which light is emitted by a fluid, which is mixed with fluorophore and flows through the free cross section Q of the flow channel in the adapted blood vessel model M B Q, when said fluid is irradiated with fluorescence excitation light; and the blood volume flow (I BI) is determined as a fluid flow guided through the flow channel in the adapted blood vessel model M B Q, which fluid flow is calculated from the length and the diameter of the portion (90 2, 90 2, 90 3,...) of the blood vessel and from the characteristic transit time (τ) for the fluorophore in the portion (90 2, 90 2, 90 3,...) of the blood vessel, using the fluid flow model M F Q provided and the fluorescent light model M L Q provided.