What Is a DICOM Segmentation (SEG) Object?

What Is a DICOM Segmentation (SEG) Object

A DICOM Segmentation, usually written SEG, is a DICOM object that stores which voxels belong to what: one or more labeled segments — an organ, a lesion, a target volume — each referencing the frames of the source series they were derived from. The segmentation travels as its own file, so the original images are never modified.

Every segment carries coded metadata: an anatomical category and type, and often the algorithm that produced it, which is how a viewer can tell an automatically generated segment from a manually drawn one.

SEG objects come in two flavors. A binary segmentation marks each voxel as in or out. A fractional one stores a value per voxel, expressing either the fraction of the voxel occupied by the structure or a probability — which is what many AI models produce, and what lets a viewer show a soft, thresholded overlay rather than a hard edge.

Because a segmentation references its source frames, it is displayed over the series it belongs to. Its geometry is defined independently of the source resolution, so a viewer may have to resample it to draw the overlay.

In Weasis, see DICOM SEG viewer.