1. A method for patient-dependent optimization and reduction of a contrast medium amount before an imaging, x-ray-based measurement for creating image data of a patient, the method comprising: provisioning patient-specific data, at least containing information about a patient cross-section relevant for the imaging measurement; establishing at least one of CT values and CNR values of a contrast medium to be expected for at least one region of interest of the patient for different candidate settings of at least one of x-ray source arrangement and an image reconstruction device for reconstruction of monoenergetic image data from multi-energy measurements relative to a CT value or CNR value for the region of interest for a reference setting of the at least one of x-ray source arrangement and image reconstruction device as a function of the patient-specific data; determining a relevant candidate contrast medium amount for different candidate settings which would lead to a CT value or CNR value of the contrast medium which corresponds to the CT value or CNR value of the contrast medium at the reference setting of the at least one of x-ray source arrangement and image reconstruction device; and displaying an optimum relative candidate contrast medium amount and the associated candidate setting.