1. A method of registering two imaging systems, the method comprising: imaging a first plane of an object that contains a plurality of spaced-apart calibration sites, each calibration site comprising a first compound detectable by a first imaging modality and a second compound detectable by a second imaging modality; imaging a second plane of the object that contains a plurality of spaced-apart calibration sites, each calibration site comprising a first compound detectable by a first imaging modality and a second compound detectable by a second imaging modality; obtaining a volumetric representation of the object using a first imaging system; obtaining a volumetric representation of the object using a second imaging system; and registering image data from the first and the second imaging systems in at least one dimension.