I have been working with dicom files that are about 4 MB each but I recently received some which are 280 KB each. I am not sure whether this is because they are from different CT scanners or if the new dicoms were compressed before being given to me. Is there a way to find out and if they are compressed is there a way to uncompressed them to the original size?
This is in continuation to the other answer from @kritzel_sw.
If you see any of the following UIDs in (0002,0010) Transfer Syntax UID element:
1.2.840.10008.1.2 Implicit VR Endian: Default Transfer Syntax for DICOM
1.2.840.10008.1.2.1 Explicit VR Little Endian
1.2.840.10008.1.2.2 Explicit VR Big Endian
then the Pixel Data (7FE0,0010) Pixel Data is uncompressed. You will generally observe bigger file size here.
Not a part of your question, but objects other than image (PDF may be in case of Structured Report) can be encapsulated with following Transfer Syntax:
1.2.840.10008.1.2.1.99 Deflated Explicit VR Little Endian
Other well known values for Transfer Syntax mean that the Pixel Data is compressed.
Note that there are also private Transfer Syntax values possible for data set. Implementation of those values is generally private to the respective manufacturer.