Reducing the overhead associated with accessing archive media (such as retrieval time and number of retrieves) can be achieved by storing studies in cache for a longer period of time, either by increasing the size of the cache, which can be costly, or by allowing more studies to be stored in the cache. IMPAX uses lossy JPEG compression to allow more studies to be stored in cache by:
Reducing the average study size
Removing redundant copies in the cache after lossy compression is performed
Reduced file size also means it takes less time for the IMPAX Client to access images. Image access can be an issue with large images regardless of network bandwidth; for example, digitized film, CR, and mammography images can be 15 MB or more.
To save storage space on the archive, you can configure IMPAX to archive lossy compressed studies. However, when a study is lossy compressed, the lossless version is permanently removed from the IMPAX cluster.
Lossy JPEG compression modes
While the most conservative storage method is lossless compression, the IMPAX cluster can also work in one of these two modes:
Cache only
Lossy compression is enabled in the cache after the lossless copy is archived (Enable Lossy Archive checkbox is cleared in Source Manager - Compressor Scheduler Configuration)
After the conditions for archiving are satisfied (so that the lossless copy of the study is fully archived) and the conditions for compression are satisfied, the study is lossy JPEG compressed in the cache and the lossless copy is removed. The value of the lossy_compressed_status in the dosr_study table is set to C (Cache).
Lossy archiving
Lossy compression is enabled in the cache and the lossy copy is archived (Enable Lossy Archive checkbox is selected)
After the conditions for compression are satisfied, the study is lossy JPEG compressed in the cache and the lossless copy is removed. After the conditions for archiving are satisfied, the lossy compressed copy of the study is archived and the value of the lossy_compressed_status in the dosr_study table is set to T (True).
 | CAUTION! We do not recommend lossy archiving, particularly not for mammography studies. |
Note the following:
When Enable Lossy Archive is selected, studies must meet the conditions for compression before they can be archived. If they are diagnostically read, they must reach the age defined in Read Study Age in Compressor Scheduler. If they are not diagnostically read, they must reach the age defined in Study Age. Therefore, enabling lossy compression in archives may delay the archiving of studies.
If compression in the archive is enabled after compression in the cache, lossless JPEG studies may already exist in the archive. To ensure that the system is consistent, if these lossless studies are retrieved from the archive, they are lossy compressed and the value of lossy_compressed_status in the dosr_study table for the study is set to C (Cache).
Each component in the IMPAX cluster can distinguish between a lossy and lossless study based on its lossy_compressed_status. If the status is C (Cache), you can retrieve a lossless copy from the archive. If it is T (True), only a lossy compressed copy exists in the system.