DICOM supports image compression by defining transfer syntaxes which are used for different lossless (bit preserving) and lossy compression schemes. With lossless compression, all original pixel data is recovered and displayed after the image is uncompressed. It means that there is no loss in pixel data of the image and the uncompressed image is bit-for-bit exact replicas of the original image. Unlike lossless compression, Lossy compression does not retain the original pixel data and eliminates some unnecessary and redundant pixel data to reduce the size of the image, so there might be some loss in pixel data after the image is uncompressed.
You are notified by a visual cue "LOSSY" that is emphasized on the bottom-right corner of the image when you view a lossy compressed image.