Lossless compression (DICOM)
Lossless compression reduces the size of DICOM Pixel Data while allowing the exact original pixel values to be reconstructed, unlike lossy compression, which permanently discards image information to achieve a smaller file.
What Lossless compression (DICOM) means in practice
DICOM carries both kinds as separate transfer syntaxes, and which one was used is recorded in the object itself rather than left to inference. The distinction matters for archiving because a lossless-compressed image is bit-for-bit recoverable, whereas a lossy-compressed one cannot be returned to its original state once written.
Source
This definition follows NEMA PS3.5 §10 — Compression of Pixel Data, published by NEMA as part of the DICOM standard.
Related terms
Transfer Syntax
A transfer syntax is the set of encoding rules — byte ordering, explicit or implicit value representation, and any compression — that a DICOM object is written in, and it is negotiated between two devices before they exchange data.
Pixel Data
Pixel Data is the DICOM data element, tag (7FE0,0010), that holds the actual image content of an instance, encoded according to the file's transfer syntax.