Decompress the data, and move any overlays which are encoded with the pixel data (a
method deprecated in DICOM many years ago) to stand-along overlay data in 60xx,3000.
DicomDataSet overlayDs = new DicomDataSet("Overlay_Dataset.dcm");
var overlayData = overlayDs[Keyword.OverlayData].Value; // null
overlayDs.SimplifyOverlays();
overlayData = overlayDs[Keyword.OverlayData]?.Value; // pixel data is encoded