Multifunction Multislice

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 Winkler, MD, adjunct professor of health sciences at the University of Nevada in Las Vegas and founding member of Steinberg Diagnostic Medical Imaging, describes his experience with their six CT scanners in a variety of slice configurations. They have four 4-slice (Aquilion 4), one 16-slice (Aquilion 16) and one 64-slice (Aquilion 64), all made by Toshiba America Medical Systems.

"Although it is only a 4-slice system, it can do 0.5 mm slices, so it allows us to do fancy ENT sinus or temporal bone examinations with isotropic voxel studies for musculoskeletal studies of small joints, such as the wrist or ankle," says Winkler, who describes this as a very robust general imager. He considers that for all but the most esoteric applications, it would suffice for the majority of CT imaging studies.

Winkler maintains that one advantage of the 16-slice scanner lies in its capacity to acquire four times as many slices per unit time as well as reconstruct images faster because of its more powerful computer. Therefore, he considers that if a clinician were in a high throughput setting such as a trauma center where speed is of the essence, a 16-slice machine might prove more appropriate.

Anthony Minotti, MD, chairman of radiology at MetroHealth Medical System in Cleveland, describes their extremely busy radiology department that accomplishes 250,000 exams per year. The facility utilizes a 16-slice Brilliance 16 multidetector system from Philips Medical Systems, Brilliance 40 40-slice system and a dual-slice scanner. The healthcare system, associated with Case Western Reserve, is the only county hospital in Cleveland and features a Level 1 Trauma Center.

Multiple improvements on their CT scanners have brought on significant improvements in patient care, Minotti says, including the dramatic increase in the speed of gantry rotation, and significant rise in the numbers of detectors that produce a greater volume of images in a study. He considers that not only have previous applications for CT improved, but these advances have "opened the door" for numerous new imaging studies, and brought CT into the mainstream of clinical practice.

The Brilliance 40 is the primary workhorse of the department, seeing use in "bread and butter" studies as well as more sophisticated imaging scans.

"Along with the scanner itself, the Brilliance Workspace [workstation] is very user-friendly and it post-processes very quickly," says Minotti. The speed of image reconstruction facilitates rapid diagnosis. "That's where the strength of some of these systems lies, in the tools to manipulate the data quickly. If we have more and more data, and don't have a way to manage it efficiently, then it just slows us down."

Stacie Kuzmiak, RT(R), CT, is a CT technologist at the Cleveland Clinic Foundation. The facility has progressed from four slice to 16 slices (Siemens Medical Solutions SOMATOM Sensation 16) and in August added 64-slice capabilities with the SOMATOM Sensation 64.

With the 16-slice machine, she notes a significant decrease in the amount of time for patient breathholds during acquisition of a CT scan, which is of paramount importance in some critically ill patients.

"For simple imaging with routine studies, the four-slice will meet most needs," says Kuzmiak. "But if you get into non-invasive angiography studies, the 16 or 64 would be required."

Because the 64-slice scanner is three times faster than the 16 slice, they've experienced added benefits for cardiac and vascular studies for peripheral disease as well as perfusion studies.

Nathan Peled, MD, head of the department of radiology at Carmel Medical Center in Haifa, Israel, describes their use of the Brilliance 40 scanner last year when they did 500 cases in collaboration with cardiologists. This 500-bed hospital serves as the cardiovascular medical center for the largest HMO in Israel, and they do 2,500 cardiac catheterizations per year, 500 carotid studies, renal and peripheral studies as well as supporting surgical and interventional activity with CT angiography.

"We're doing anything the cardiologists need: following bypass patients, failed arteriograms, patients with difficult chest pain, anything in their daily practice," Peled explains. They now do a CT on any patient with a suspected stent re-stenosis. Those patients no longer require another invasive cardiac catheterization within weeks or months of the intervention.

Peled has been using the Brilliance 64 since its installation in November. To date, it has completed 350 patient studies, including 70 cardiac CTAs and 150 CTAs of other body parts including carotids, and runoff studies. He anticipates that within five years, 40 to 50 percent of their [CT] workload will be cardiac CT.

"For anyone looking for a new CT scanner, it's important to remember they can do 99 percent of their studies on an average 16-slice scanner, but the 64-slice scanners are best for cardiac CTAs and CTAs in general," says Peled. "If I'm looking to do cardiac CTs, I'll take a 40- or 64-slice scanner."

64-Channels

Although diagnostic angiography can be accomplished on a 16-slice scanner, W. Dennis Foley, MD, professor of radiology at Medical College of Wisconsin and director of the section of digital imaging and attending physician at Froedert Hospital in Milwaukee, asserts that these are performed more effectively with higher spatial and temporal resolution and without artifact with a 64-channel scanner. And that comes from experience. His department used the prototype of the LightSpeed VCT64, which was installed in July. It was recently upgraded.

In their evaluation work and beyond, they've focused on two application categories: the coronary arteries and non-coronary vascular cases that include studies of the carotid cerebral territory, abdominal aorta, the aorta/iliac system including abdominal visceral branches, and extremity arterial circulation.

"The resolution is not changed from the 16 [slice scanner], but the speed is increased," explains Foley. "That means you can do these studies faster, and get less pulsation effect ... and that means you can study syncopations because the exam times are shorter and you're using less contrast." Besides cardiac applications, the scanner performs gated studies that also include pulmonary arteries for embolism evaluation and scans for suspected aortic dissection. The utility in performing a differential diagnosis in the patient who presents with acute chest pain is of significant benefit.

While Foley is quite enthusiastic about the capabilities of the 64-channel scanner, he adds the caveat that prior to purchase radiologists need to: assess their PACS infrastructure that will be required to manage the huge data sets - a peripheral vascular study can yield up to 2,000 slices, each at approximately a megabyte in size; training for technologists in running the scanner and performing the 3D reconstructions of the image datasets; and all of the other components of providing access to the scanner 24/7. He also stresses the importance of robust workstation and software components to render 3D reconstructions quickly.

Kuzmiak agrees that infrastructure aspects are important, and that education of technologists is vital to excellence. She relates that with Siemens and their syngo platform, technologists who crosstrain on different machines or across modalities are able to understand the software on the latest scanners.

"If you have a technologist who has been working on the 4 slice, it looks identical to the 16 and the 64," says Kuzmiak. The important issue is learning about the speed of each system, so that they can accurately time a contrast bolus. She explains that some of the software that is included with the Sensation 64 offers automated imaging protocols that regulate the technique for each patient to optimize the image quality with the lowest possible dose.

Tanenbaum from Edison Imaging relates that as they have progressed from single channel to four channel (LightSpeed Qx/i) to 8 (LightSpeed Ultra), 16 (LS Pro) and now to 64 (LightSpeed VCT), these systems have provided progressively increased acquisition speed which positively impacts patient comfort, contrast requirements, and image resolution.

"The 64 channel CTs are producing astonishing quality coronary artery angiography images, which are sharper, with less artifact and much more consistently excellent," Tanenbaum says. "With a 360-degree rotation in 0.35 seconds and a four times boost in longitudinal coverage with respect to a 16-channel system, the LS [LightSpeed] VCT 64-channel system does have unique benefits in cardiac. For the first time, coronary CTA is almost automatic."

In addition to cardiac applications, he believes 64-slice systems will eventually be used for routine, day-to-day medical imaging throughout the body. "This is a mainstream imager than can bring its benefits to imaging the whole body."

Winkler in Las Vegas echoes the notion that the primary benefit to 64-slice systems comes with cardiac CT in that during a five to six second breathhold, the entire study can be completed, and be reproducible on a consistent basis. Besides that application, they are beginning to explore the advantages of using this equipment for other imaging studies.

"We're now doing all of our brain scans with isotropic pixels at 0.5 mm resolution, so I no longer read my brains in the axial plane. I can now read them in the sagittal or coronal plane, just like I'd read an MRI," says Winkler.

Besides the obvious benefits of being able to have an alternative to MR scanning, Winkler says they use CT to capture arterial studies in the brain.

Stephen Green, MD, associate director of the catheterization laboratory at North Shore University Hospital in Manhasset, N.Y., describes a unique collaboration between the cardiologists and radiologists as they install their new LightSpeed VCT 64 to perform coronary CT angiography studies. They've hired a radiologist to read all non-cardiac portions of imaging studies (both CT and MR) in tandem with the cardiologists.

"The best people to read CT angios on the cardiac side are the interventional cardiologists, the people who look at angios every day," says Green. He believes this technology will not replace angiography, but enhance the information available in treating these patients. "We view this as a screening device to actually bring more people to the intervention table, where we want to treat disease."

Multidetector CT/PET hybrids

Hybrid scanners that combine CT and PET (positron emission tomography) technologies, use multidetector CTs for the anatomic portion of the scan.

Robert D. Burke, MD, president of Midtown Imaging LLC in West Palm Beach, Fla., describes their anticipated use for their two biograph PET/CT scanners from Siemens that include a 16-slice CT component. They plan to do central nervous system applications, as well as use data to overlay a coronary artery lesion with a perfusion component.

"We chose Siemens PET/CT because they use an LSO crystal which has the highest efficiency, and enables reconstruction 'on the fly' which means that after the scan is completed, the images are available to review," says Burke. This has a very practical benefit because the clinician can make an immediate decision about whether or not additional scans are required before the patient gets off the table.

Kevin Berger, MD, director of PET and CT and assistant professor of radiology at Michigan State University in East Lansing, describes their use of CT for standard whole body imaging studies including virtual colonoscopy, standard oncology applications and diagnosis of renal stones. They are using a GE Discovery PET/CT for cardiac studies.

"We are the GE beta development site for HeartFusion, a software program that allows us to merge a scan of a perfusion study from PET with a coronary angiogram," says Berger. "So if a patient has a stenosis in the coronary artery, we can see the area of the heart with the perfusion deficit." And the reverse is true as well. If a perfusion deficit is detected on the PET scan, they can trace to see which blood vessel empties into that area. The result is that they can non-invasively work-up patients with suspected coronary artery disease. As the technology continues in development, in areas such as Alzheimer's disease, Burke anticipates new applications will follow.

Conclusion

"In today's practice of medicine, imaging plays a vital role in patient care," says Minotti. "Staying on the cutting edge of technology is the best way to take care of patients, and the evolution of CT has advanced imaging in healthcare."

Most clinicians believe that they have just barely scratched the surface in learning about additional capabilities of multi-detector CT scanners, and that the future holds great promise as new applications are advanced.

But clinicians and IT professionals must keep a watchful eye on the display, storage and retrieval of the volume data sets created. Providing an efficient workflow is key to realizing the true benefits of higher slice count CT.

INTO THE CRYSTAL BALL: WHAT COMES AFTER 64?

"When you consider that the heart is 12 to 13 cm long, not until we reach 256-slice detectors do we get to the point where we can do the entire heart in a single rotation. I think we'll make the leap from 64 all the way to 256 to allow whole organ perfusion studies. Plus, you get a lot of benefit when you can cover the entire brain in one rotation because you can start to do imaging between arterial and venous phases."

• The challenges? Computing speed and cost. Toshiba has two 256-multidetector scanners in operation, powered by supercomputers - but that adds significantly to the cost. That will come down, he says.
  Douglas Ryan, director of CT business unit, Toshiba America Medical Systems
• "If you look at where CT is now with 64 slice, there is no problem with speed, and even resolution is good. But CT needs improvement in temporal resolution, so moving forward, this is our focus: improving temporal and spatial resolution."
  Dynamic scans that show perfusion in many organs and dynamic interventional studies will come to be as well.
  Murat Gungor, marketing manager for high-end CT, Siemens Medical Solutions
• "Some of the breakthrough work we're doing these days with perfusion applies in essentially all of the organs. We've done stroke assessment, and CTA plus perfusion is useful to determine function in organs such as the heart, liver or pancreas. We will continue working on software improvements to facilitate ease of use, to enable physicians to find information in the optimal format for their diagnosis."
  James (Jim) Green, senior vice president and general manager of business unit CT, Philips Medical Systems
• "CT is leading detector technology, for example with the V-Res detector on LightSpeed VCT covering 40mm in a single rotation and a heart in five beats. There have been tremendous advances in CT software capabilities as well, especially in areas like cardiac, vascular, neurology and oncology. CT technology is progressing rapidly with new detector technology in both CT and PET."
  Domenic Smith, CT product manager, GE Healthcare