Providing an item of conversion information relating to an image dataset

An X-ray apparatus comprising:

• a recording device comprising:
• an X-ray detector;
• and
• an X-ray source;
• and
• a controller configured to provide an item of conversion information describing an allocation rule of at least one physical property value of a material in a voxel relating to an image value of the voxel in a three-dimensional (3D) image data set recorded with the X-ray apparatus, the provision of the item of conversion information comprising:
• determination of a calibration dataset that is used for determining the allocation rule, the determination of the calibration dataset comprising scanning a phantom in the X-ray apparatus, the phantom comprising at least one calibration material, wherein the 3D image data set is recorded with a receiving spectrum geared to an X-ray detector of the X-ray apparatus, the receiving spectrum being described by at least one spectral parameter;
• and
• determination of the allocation rule dependent upon the at least one spectral parameter, the determination of the allocation rule comprising use of calibration data derived from the determined calibration dataset describing different receiving spectra.