With the growing technical developments in CT, the traditional practice of categorizing systems by the number of slices they can produce has become inadequate. Slices are axial images (two-dimensional cross sections) reconstructed following data acquisition, and in today’s scanners, the number of slices obtained during a study depends largely on the study protocol chosen by the radiologist, and much less so on the technical limitations of the hardware. In addition, there are significant differences between the designs used by different manufacturers (especially in the more advanced systems), making apples-to-apples comparisons difficult.
NEW INNOVATIONS CALL FOR NEW CATEGORIES
CT systems are now divided into four categories: 16-channel, 20- to 40-channel, 64- to 80-channel, and premium. (ECRI Institute expects that the premium category will need further revision as CT technology continues to advance.) In addition to the number of channels/rows/slices, this categorization takes into account criteria such as coverage, advanced applications, and acquisition capabilities.
“ROW” AND “CHANNEL” ARE BETTER THAN “SLICE”
The number of rows and channels is a better way to describe CT hardware than the number of slices. More important, these terms better characterize the strengths and limitations of the technology. Here’s what these terms mean:
ROWS, CHANNELS, AND SLICES IN DIFFERENT SYSTEMS
In most sub-64-channel systems, the number of channels and the maximum number of slices are the same, but the number of detector rows is greater. This allows users to choose slice thickness.
For example, a 16-channel CT system has more than 16 detector rows. There are 16 rows in the center of the detector that are narrower and are used to acquire thinner slices; during such acquisitions, the detectors along the outer edges of the array are not used. If, on the other hand, thicker slices are selected, the entire width of the detector is used, but the thin rows in the center of the detector are merged to match the thickness of the slices obtained from the wider ones. This means that when thinner slices are obtained, a narrower area of the detector array is used, and consequently a shorter section of the patient is imaged with each rotation. If greater anatomical coverage is required, thicker slices may have to be used. In either case, 16 channels will be used to transmit the information.
In systems that fall within the 64- to 80-channel category, on the other hand, the number of detector rows, and the number of channels are identical. On these systems, there are 64 to 80 detector rows, all of them the same width. This means that, regardless of how many rows are being used, the narrowest slice information is always being obtained, even if thicker slices are reconstructed from that information. So unlike on 16-channel systems, the narrowest slices can always be acquired without reducing the coverage per rotation, thus reducing the scan time.
Some CT systems use advanced sampling techniques to generate twice as many slices than there are detector rows or channels. But doubling the slice count may not result in thinner slices and improved resolution. Instead, the slices overlap, making the images less susceptible to certain artifacts, and making any 3-D reconstructions smoother (i.e., the “stair-step” artifact — in which smooth surfaces lose their continuity — will be reduced).
The top CT tier consists of more highly developed models that we call premium systems. Most CT manufacturers offer premium systems. These models have more functionality — for example, wider-coverage detectors, multiple detectors, or dual-energy acquisition options. In many cases, the additional acquisition options afforded by the advanced technology are not yet widely used in clinical settings. Also, each manufacturer has very different ideas about the future of the technology, so there are many differences between the systems and their capabilities.
This article is an excerpt from a Health Devices article posted on ECRI Institute’s Health Devices System, Health Devices Gold, and SELECTplus membership websites on March 19, 2014. The full article includes more guidance on CT system classifications and detailed classification tables. The article also includes detailed diagrams explaining rows, channels, and slices. More detailed cost information for these systems can be provided by ECRI Institute’s SELECTplus Capital Equipment Pricing Tool. To purchase this article or to learn more about membership programs, visit www.ecri.org, contact email@example.com, or call (610) 825-6000, ext. 5891.
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