An apparatus for determining a fractional flow reserve value of the coronary artery system of a living being, the apparatus being a computed tomography system and comprising: - a representation providing unit for providing a representation of the coronary artery system, - a projection data providing unit being an acquisition unit for providing projection data by acquiring the projection data, wherein the acquisition unit comprises a radiation source and a detector, wherein the detector has several rows of detection elements used for generating the projection data, wherein the radiation source is adapted such that not all rows of detection elements are irradiated, but only one row is irradiated by a single fan beam or some rows are irradiated by several fan beams, wherein a fan beam irradiates only a single row of detection elements and wherein, if some rows are irradiated by several fan beams, in between the irradiated rows of detection elements non-irradiated rows of detection elements are present, and wherein the acquisition unit is adapted such that the projection data are generated by the detector over time based on radiation, which is generated by the radiation source and which has traversed the aorta of the living being, while a contrast agent has flown through the aorta, - a boundary condition determination unit for determining a boundary condition which is specific for the living being based on the provided projection data generated by the one or several fan beams, and - a fractional flow reserve value determination unit for determining the fractional flow reserve value by using a fractional flow reserve value determination algorithm that is adapted to determine the fractional flow reserve value based on the determined boundary condition and the provided representation of the coronary artery system. - a representation providing unit for providing a representation of the coronary artery system, - a projection data providing unit being an acquisition unit for providing projection data by acquiring the projection data, wherein the acquisition unit comprises a radiation source and a detector, wherein the detector has several rows of detection elements used for generating the projection data, wherein the radiation source is adapted such that not all rows of detection elements are irradiated, but only one row is irradiated by a single fan beam or some rows are irradiated by several fan beams, wherein a fan beam irradiates only a single row of detection elements and wherein, if some rows are irradiated by several fan beams, in between the irradiated rows of detection elements non-irradiated rows of detection elements are present, and wherein the acquisition unit is adapted such that the projection data are generated by the detector over time based on radiation, which is generated by the radiation source and which has traversed the aorta of the living being, while a contrast agent has flown through the aorta, - a boundary condition determination unit for determining a boundary condition which is specific for the living being based on the provided projection data generated by the one or several fan beams, and - a fractional flow reserve value determination unit for determining the fractional flow reserve value by using a fractional flow reserve value determination algorithm that is adapted to determine the fractional flow reserve value based on the determined boundary condition and the provided representation of the coronary artery system.