Image storage strategy changes with the times

To meet about 90% of all of its radiologists' image-viewing needs, one academic facility found it need only provide about three months of short-term image storage.

Radiologists at Northwestern Memorial Hospital in Chicago perform about 270,000 radiological procedures per year. The hospital installed a PACS in March 1999, which currently handles about 780,000 studies and 44.5 million images.

"We looked at our PACS database and asked: What are people looking at?" said Dr. David S. Channin, chief of imaging informatics at Northwestern.

In June 2001, Channin and colleagues analyzed over 24,000 viewing episodes drawn from their PACS short-term storage subsystem and found that 90% of the episodes involved studies less than 65 days old.

By 2002, as their PACS database aged, 90% of radiologists' viewing needs were met by allowing for 90 days' worth of studies, using lossless image compression in storage, Channin said at the RSNA meeting Tuesday.

"PACS is not like a modality. A PACS continuously evolves," he said.

Channin suggested historical curiosity as one of the reasons the amount of short-term storage had to be increased. As the PACS database matured, more radiologists knew about, and were subsequently interested in, the older studies they could still access.

In addition, as the number of exams performed per year increased, so did the average data size of the exam, adding to the amount of information that had to be stored.

The facility now uses a 2.5-terabyte redundant array of inexpensive disks (RAID) for short-term storage and pushes older studies to three 2.5-terabyte optical disk jukeboxes. Studies that are even older are sent to an application service provider, and the facility hopes to eventually send all its older studies to an ASP.

"Basically, our entire storage system fits into one rack," Channin said.

Despite Moore's law, which states that the number of transistors (and hence the processing power) per chip will double every few years, Northwestern is not worried about its PACS' ability to meet future image viewing needs, he said.

"Thank goodness, Moore's law is advancing faster than clinical use," Channin said.