Multidetector CT: Handling Image Volume

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hiit041002The name of the game is speed. Facilities upgrading to the latest generation of 16-, 32-, 40-, 64-slice and dual-source CT scanners have quickly learned that boosting clinical capacity in CT goes hand-in-hand with escalating network depth and data storage to smooth access to the torrents of data these scanner create. Fine-tuning workflow also is key to manipulating and managing the larger volumes of images generated — and more quickly — the higher the slice count climbs. Adaptive facilities are sending automatically generated multi-planar reformatted images from the scanner directly to PACS — which allows radiologists to get 3D data faster and avoid a trip to the workstation. Expectation is high for quick results throughout the healthcare enterprise, especially from emergency departments dealing with trauma cases.

Avoiding multidetector shock

You don't need to have a 64-slice CT system to see a huge jump in data. That happens as soon as you leap into the multislice arena.

"The reality is that once you've gone from a single- or dual-slice system to 16 slices, you have already gone over your biggest hurdle," says Pedro J. Diaz, PhD, vice chair of the Department of Radiology at Case Western Reserve University/MetroHealth Medical Center in Cleveland, Ohio, a Level 1 trauma center. MetroHealth's department performs nearly 50,000 CT studies yearly, many of which serve the hospital's emergency department. They currently have four Philips Brilliance CT systems, three of which are 16 slice scanners and one 64 slice. Hence, Diaz has a pretty firm understanding of how to deal with load after load of CT images.

In his view, the initial move to multidetector scanners is challenging largely because the older single-slice systems were limited in how small a slice they could acquire. While typical acquisitions with single/dual systems were performed with slice thicknesses between 2 mm and 7 mm, the move to 8- or 16-slice scanners brought acquisition slice thicknesses fell to 0.625 mm or 0.5 mm.

"For a given anatomic protocol, the accompanying increase in the number of slices generated could be dramatic," says Diaz. "However, minimum/typical slice thickness has remained fairly constant as we've moved from 16- to 32- to 64- and higher-slice CT scanners. As such, while advanced applications on 64 and larger scanners will add to the storage burden, if you've solved the basic storage problem with the 16-slice scanner, you've pretty much solved it for larger-slice scanners."

The biggest change is that you're just scanning more quickly. Additionally, when you go from 64 to 128, you add more applications, but the slices remain essentially the same. And generally speaking, the new applications largely pose a workflow obstacle rather than a storage one.

"In more and more instances, we generate thin-slice isotropic data that we will want to store indefinitely, and we also do more post-processing, which requires easy access to this data as well as additional storage," says Diaz.

Now that you've got all of these data and images — including more post-processed images such as 3D — where do you put them, and for how long? The decision at MetroHealth comes down to what radiologists need to see in PACS to review axial images. For a number of the imaging protocols, they will use the thicker slices that are kept in the PACS for five years. The thin slices — used for 3D rendering post-processing — are kept in a temporary processing store for three months.

Ideally, everything — including thin-slice isotropic data — would end up in PACS, says Diaz. Storing that much data isn't the challenge it used to be, as sophisticated storage area network (SAN) and network attached storage (NAS) costs have dropped in price. MetroHealth worked closely with its PACS vendor, GE Healthcare, to deploy mirrored EMC Centera NAS devices, which can be upgraded in capacity easily, for long-term storage. For faster, short-term storage, about a year's worth of data are cached on a RAID (redundant array of independent disks) subsystem.

Additionally, while hammering through tons of CT data, workflow is another weapon to hurry things along. "I'm a big advocate of making it easy for everyone to access thin-slice data and post-processed images from PACS. On our CT scanners, a trained technologist is able to do most of the required post-processing as part of an acquisition protocol," says Diaz. "However, there are instances where a radiologist may want to do some post-processing at the