Modality upgrades threaten to overwhelm PACS storage

The new capabilities that result from upgrades to imaging modalities may have a greater effect on imaging volume than does growth in the number of patients who undergo imaging. Even as hospitals planning for a PACS carefully project their storage needs, improved imaging technology may lead to significant increases in storage requirements. New strategies are emerging, however, to tackle this data deluge.

The University Medical Center in Groningen, the Netherlands, was unprepared for the vast quantity of data sets generated by new modality upgrades. As radiologists fully exploit the new technology, the facility's PACS archiving capabilities are being stretched beyond the hospital's expectations and their PACS vendor's promises.

Researchers at Groningen began tracking the impact of modality upgrades on their facility's archive demands in 2000. By the end of 2004, the imaging data stored in the archive had increased from 1.2 to 5.4 TB. Because the growth seems to be tracking linearly, the medical center anticipates that it will need more than 10 TB of storage in 2008, said Peter M.A. van Ooijen, Ph.D, who spoke at the 2005 Computer Assisted Radiology and Surgery meeting in June.

The number of patient exams entered into the radiology information system increased from 140,000 to 155,000 during the same four-year period, van Ooijen said.

CT has been the leader in generating additional imaging volume. The hospital installed a spiral CT scanner in 2000 and subsequently added four-slice, 16-slice, and electron-beam systems. Last November, it installed a 64-slice scanner. CT image volume in 2000 was slightly less than 20,000 per month; in 2004 it averaged nearly 553,000 per month.

The growth was not anticipated, and the hospital's PACS vendor had downplayed the prospect, van Ooijen said. It seems that when radiologists discover what they can do with the new scanners, they begin to order larger studies. One emergency room study, for instance, totaled 10,000 images.

New CT scanners enable radiologists to use thin collimation in acquiring images, which they then reconstruct using thicker sections for storage in the PACS. This strategy helps them cope with CT-driven imaging overload but places an increased burden on the hospital's PACS. Researchers in Korea have turned to a miniPACS to provide cost-effective additional storage.

Radiologists at Seoul National University Hospital offloaded over 72% of their CT data and 32.8% of total data for all modalities from their main PACS to a miniPACS, said Dr. Kyoung Ho Lee, a clinical instructor of diagnostic imaging at the hospital. Only thick-section reconstructed data sets, which are the images radiologists often prefer to review, were pushed to the main PACS.

In February 2004, 867 CT exams, which had been compressed and represented 26.8 GB of data, were stored in the main PACS. Of these, volumetric data from 604 exams were stored in the miniPACS, accounting for 78.1 GB of data.

Over a five-month period, 1.48 TB of data were stored in the main PACS, with 278 GB of the data coming from CT exams. Volumetric data sets, which made up 72.6% of those CT exams, were successfully stored in the miniPACS, Lee said at the Society for Computer Applications in Radiology meeting in June.

At the end of the five months, all data on the main PACS were kept online, while 245 GB of data in the miniPACS were retained online and 650 GB were placed in nearline storage. The online data in the miniPACS will eventually be moved onto digital linear tapes, which are less expensive then maintaining online storage on the main PACS, Lee said.

CT is not the only source of growth in volume at many institutions, however. At Groningen, MR volume increased from 66,000 images per month in 2000 to 104,000 in 2004. In the same period, x-ray angiography increased from 1200 per month to 3700; ultrasound from 10,000 per month to 20,145; and mammography from 22 per month (priors that were digitized) to 627 (images that were captured digitally beginning in 2003).

Only computed radiography volume remained relatively constant at 10,338 per month in 2000 and 12,651 in 2004.

New postprocessing systems compound the data volume problem, although they were not evaluated as a separate factor, van Ooijen said. Although some facilities place limits on which images are stored, all images generated at Groningen are entered into the PACS. Legal constraints and concerns about future legislation related to the preservation of medical data have prevented the hospital from developing a system to purge old data. Even when patients die, removing old data can be so time-consuming and difficult that it is not done.

The continuing evolution of storage technology and the need to migrate from one system to another complicate the situation further, he said.