This property may not be used for DicomLabels of type Bitmap. The length is the length of the entire circumference of the label. Where pixel dimensions for the whole image are present within the image data, these are used to scale the length appropriately, but at present multiple region calibration areas (e.g. in U/S images) are not currently supported. The units used (which for un calibrated data may simply be pixels) is returned by ROIDistanceUnits.