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