If you're trying to figure out how to represent imaging data in SDTM, it may be helpful to think about the similarities between an image and a specimen.
A sample taken from a subject for testing at a lab is a surrogate for the subject. Results of tests on the specimen tell us something about the subject at the time the specimen was taken.
An image, such as an x-ray, CT scan, or MRI scan, is a surrogate for the subject. Observations made on the image tell us something about the subject at the time the image was taken.
Specimens are routinely given identifiers, which are often represented in a --REFID variable in SDTM-based datasets.
Images are also routinely given identifiers, which can be represented in a --REFID variable.
The process in which a specimen is collected need not be represented in SDTM-based datasets, though it can be represented in the Biospecimen Events (BE) domain, if necessary. The record for a result from a specimen will include the --SPEC variable, and the date of specimen collection in the --DTC variable, and usually this is the only information of interest about the specimen collection.
The procedure during which images are taken need not be represented in an SDTM-based dataset, though it can be represented in the Procedures (PR) domain, if necessary. The record for a result from the image will include the imaging modality in the --METHOD variable, and the date the image was made in the --DTC variable, and often this is the only information of interest about the imaging procedure.