The DICOM standard allows to save the image data using different data encoding. It is called the Transfer Syntax. The basic is no encoding, meaning the file contains the raw pixel values. But it can also be JPEG, JPEG Lossless, JPEG 2000…
For example, the data you see above this text uses JPEG 2000 encoding, below is a JPEG Lossless.
Check this page on the DWV wiki to view the current transfer syntax support: Dicom-Support#transfer-syntax.