Multifunction Multislice

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Differentiating CT scanners by slice count - 4, 8, 16, 32, 40 and 64 - and clinical need - cardiac, musculoskeletal, pulmonary, angiography or liver - is essential to picking the right scanner for the long term. Here are some pointers to choose the right scanner based on acquisition speed, contrast requirements, image resolution and patient comfort.

Industry and clinicians have been hard at work in recent years driving advancements in multislice CT, creating more sophisticated volumetric imaging studies designed to capitalize on advances in CT scanners, workstations and software components.

Many experts anticipate a pause in the "slice wars" that have waged over the past few years to enable clinicians to fully explore and refine advanced applications while manufacturers work to further enhance hardware and software capabilities.

As the industry has moved from single-slice scanners through four- and eight-slice and now to 16, 32, 40 and 64, terminology is beginning to change. Rather than referring to "slice" capabilities, most industry leaders and some clinicians have begun to refer to the latest generation as multidetector or multi-channel volumetric CT scanners. Although that is certainly the coming lingo, many clinicians still speak in terms of "slices."

"I usually refer to systems that are 16 channel and beyond as volumetric multichannel CT," says Lawrence Tanenbaum, MD, FACR, section chief MRI, CT and Neuro-radiology, Edison (N.J.) Radiology Group. Once one considers volumetric renderings, the data are not presented as individual slices. Multiple simultaneous channels of data feed into one reconstruction engine that produces images in any plane. "So we're not getting multiple slices when we scan, we're acquiring a volume with multiple channels of data."

Thomas D. Hedrick, MD, medical director of radiology for The Methodist Health Care System in Houston describes their busy practice. They have two GE LightSpeed4 slice scanners, one LightSpeed Ultra 8 slice, and three LightSpeed16 systems. In addition, they're in the process of installing their first 64-channel scanner, the LightSpeed VCT 64, and have two more ordered that are scheduled for installation later this year.

Hedrick describes the different categories of examinations that are at the core of the slice issue. The first type of exam is one where the clinician must scan a large section of the patient's body in a short period of time. For example, a contrast-enhanced CT of the chest, abdomen and pelvis, where the goal is to obtain the scan while the contrast material remains in arterial circulation. He says that a four-slice scanner can do an adequate job with a long breathhold by the patient, but that it becomes an easier task to accomplish for both the technologist and the patient with 16-slice scanners or above.

The next cluster of exams consists of those that involve scanning a moderate section of territory, but with repeated passes. An example would include multiphase studies of the liver, pancreas or kidney, where the exam includes a view before contrast, during arterial contrast, during venous phase contrast and perhaps a subsequent view. With only 15 to 20 seconds between phases, the challenge is to turn the machine quickly enough to capture the different segments as they occur. This domain is the strength of a 16-slice system, Hedrick says.

The final groups of studies are probably more easily accomplished on a 64-slice scanner. The first example includes a study covering an extremely long interval with very thin slices, such as a CT aortogram and runoff study involving images from the arch of the aorta down to the ankles. It would be very difficult to accomplish this task with a 16-slice scanner before the bolus of contrast became diluted.

The final example involves coronary CT angiography. Methodist Health Care currently performs these exams on a 16-slice machine, although they are challenged by timing, breathholds and arrhythmias. This is the domain of 64-slice scanners, and Hedrick will know that better when the facility gets its new system.

"Every one of these scanners has had a significant jump in price as we've gone forward, and I think especially for mid-sized institutions or outpatient imaging centers, you must carefully weigh if it's worth the extra money for your additional capabilities," says Hedrick. "You can do a great majority of your 'bread and butter' applications on a 4-slice scanner."

The numbers game: comparing slices

Mark Louis