Spiral CT begins take off as technology matures

Although somewhat slow out of the starting gate, spiral CT haspicked up speed commercially this year. The technology was a hottopic at the European Congress of Radiology conference in Viennalast month and is sure to be featured prominently at the RadiologicalSociety of North America meeting next month in Chicago.

Few could have foreseen this development when CT's very existenceappeared compromised by the clinical emergence of MRI a decadeago. The question now is not how CT will fare against MRI, butwhat applications it might reclaim from MRI, and what inroadsit will make against angiography and standard x-ray.

Vendors acknowledge that this is a pivotal period for CT, althoughthey are not all in agreement as to its expanded clinical potential.At a minimum, spiral technology will improve the economics ofconventional CT at a marginal incremental cost to users.

"What was a very stable technology has experienced anorder of magnitude improvement in performance," said ReubenLevinson, CT product manager for Elscint.

The basic concept of using a CT scanner with a moving patientbed and a continuously rotating gantry to gather a volume of imagedata is common to all the manufacturers. However, marketing terminologyrelated to continuous-rotation CT varies among the vendors:

• Siemens, Elscint and Picker use spiral;

• Toshiba and GE prefer helical; and

• Philips opts for volumetric.

SCAN uses spiral CT as a generic term for the technology sinceit is used most frequently in clinical papers and offers fewersyllables to trip over.

SPIRAL CT HAS BECOME A PRACTICAL clinical tool, greatly advancedfrom the early days of dynamic CT scanning, or even four yearsago when Siemens introduced the first commercial continuous-rotationspiral CT system. All major CT vendors now offer spiral scanners.

Some manufacturers have extended the technology from the premiumto the mid-range of their product lines. Both Elscint (SCAN 10/6/93)and Siemens showed mid-priced spiral systems for the first timeat the ECR meeting.

It remains unclear, however, how rapidly--if at all--spiralscanning will penetrate the low end of the CT market.

Several technological developments in spiral CT have been combinedto offer equivalent or even superior image quality relative tostandard, single-slice axial scanners, while opening up new avenuesin the presentation of volumetric CT data, such as CT angiography.

In the past, users who ran their CT scanners fast to overcomemotion artifact problems knew they had to accept poorer resolution.

"When dynamic scanning first came about, the scans wereusually data-starved," said John Barni, director of CT marketingand planning for Picker International. "Although there wasa benefit to scanning the patient quickly, the outcome was imagesinferior to those acquired in a routine axial way."

Since the onset of spiral CT in 1989, users have benefitedfrom a race among vendors to provide greater volume and higherresolution images. High heat-capacity x-ray tubes pumped up spiralCT acquisition speeds, providing greater single-breath-hold volumecapabilities. At the same time, compact high-frequency generatorsrunning with low-voltage slip rings polished spiral CT's imagequality.

Larger heat capacity x-ray tubes--as high as 5 million heatunits--allow scanners to operate continuously for longer periodsof time. Longer running periods, in turn, enable greater volumesof continuous image data acquisition.

Improvements in x-ray generator and slip-ring technology allowedvendors to place compact high-frequency generators directly onthe CT gantries. These generators boosted the mAs (milliampere-second)level of spiral CT systems. The higher the mAs level, the bettera scanner's image quality, according to Levinson.

Vendors have battled over the past four years to bring themAs level of spiral scanning from below 100 to 300, where standardnonspiral scanners operate, he said. This mAs performance hasbeen met and exceeded by some spiral CT manufacturers.

Application of slip rings and continuous gantry rotation openedup new possibilities for fast CT. Slice acquisition speed--andtherefore the number of slices possible during single exams--rosedramatically. Ten years ago, users were happy to obtain a three-secondscan, said Diane Cerny, CT product marketing manager for Siemens.Now, most spiral systems operate with one-second slice acquisitionspeed or faster.

CT gained the capability to acquire continuous volumes of datafor the first time with spiral scanning. This x-ray-based modalitybegan to take on some of the functionality of MRI.

Spiral CT's increased functionality was limited, however, bythe speed at which the rotating tube could process across thebody within the time a patient can hold a single breath. Breathingintroduces motion and misregistration artifacts between sectionsof the spiral-acquired volume.

"Many applications we are seeing now were not possiblewith step scanners," Cerny said. "You have to go intoa continuous volume acquisition mode and then try to get longervolumes out of the same period of time."

Faster spiral scanning speeds improved CT's ability to tracka single bolus injection of a contrast medium. This, in turn,allowed for imaging during the period of peak contrast enhancementand decreased the required use of contrast media. High-speed spiralscanning also improves imaging of hard-to-scan patients in trauma,pediatric and geriatric cases.

Lower contrast use per procedure is one of the strongest economicarguments to be made for spiral CT. Initial results show thata high-volume site might save as much as $1 million a year throughreduced contrast administration, one vendor said.

More work is required on peripheral technology to optimizecontrast use with spiral CT. Pressure rates for contrast injectorsand injection timing sequences are being adjusted to match theincreased speed of acquisition, according to Irene Jemczyk, CTproduct manager for Philips.

When spiral CT was first introduced, the technology was slowto be adapted because speed and, therefore, single-breath-holdvolume were limited, Jemczyk said. This negated some of the advantagesin contrast reduction and reduced procedure time.

"You couldn't use (spiral CT) for a complete study,"she said. "First you had to do a regular axial scan, isolatean area and then go back and rescan. This took a significant amountof time and more than likely a second contrast injection."

Proliferation of spiral technology into lower priced CT systemsusually entails some compromise in the heat capacity of the CTtube and power of the x-ray generator. The result is decreasedvolume and resolution capabilities in the mid-tier spiral scanners.

Low-end spiral CT users interested in vascular imaging wouldbe faced with a smaller volume scanning capability, reducing theirprospects of lower contrast use, Jemczyk said.

It is not clear, however, whether low-end CT users would beinterested in the capabilities provided by spiral CT, she said.

"Customers who would be purchasing (a lower end scanner)are taking a general look," Jemczyk said. "They mightwant more of a general-information scanner rather than the specificdata (spiral) CT can provide."

SPIRAL CT WILL BEST REALIZE its potential if it can be appliedto general imaging purposes in a manner transparent to the users,said Brian Duchinsky, CT product manager for GE.

One reason spiral scanning did not catch on rapidly at firstis that the systems were designed for specialized volumetric routinesthat did not relate to the routine clinical imaging needs of users,he said.

"We almost took a big step backwards," he said. "Insteadof being able to do multi-organ studies, (spiral was moving) tosingle one-shot studies that tied the machine up, as in the earlydays of dynamic scanning."

Improvements in spiral systems technology, however, providefor simpler user operation, sequential scanning at multiple slicethicknesses and rapid postprocessing of data, he said. This willallow physicians to apply the benefits of spiral in their routineclinical work.

"What you are trying to study in the human body hasn'tchanged," Duchinsky said. "You scan the body using thesame techniques. Contrast utilization may be different, but thatis more a matter of timing and how much contrast you use. Basically,you are going after the same medical problems with a slightlydifferent spin on how the data are acquired."

While the technical specifications of certain spiral CT components,such as the tube and generator, can make a difference in systemprice, there has not been a major rise in CT scanner prices dueto the emergence of spiral scanning. CT prices have stayed withina range of 10% to 15% over the past decade, noted Janet Lippincott,CT sales support manager for GE.

Any additional cost of spiral upgrades has been counteractedby a general downward price pressure in the CT market. Furthermore,the slip ring, having evolved first in nonmedical applications,is already a mature and reasonably priced component technology.

Cost-containment pressures make it unlikely that CT manufacturerswill introduce new, more expensive technology than is availabletoday unless it can provide clearly unique applications, saidGE's Duchinsky.

The greater speed of today's CT equipment actually providesusers with more imaging bang for the buck. It is possible fora top-of-the-line spiral scanner to replace two older installedunits, Barni said.

"We have some sites doing six to eight patients an hour,"he said. "Many replacement customers are buying spiral knowingthat it improves efficiency and economics."

SPIRAL SCANNERS ARE EXERTING a strong impact on the structureof CT's annual sales and installed base. About 1000 of the 7000CT scanners installed worldwide are spiral or spiral-capable,Levinson said. Some slip-ring systems are introduced to the userbase with the promise of a spiral capability once applicationsprogramming has been developed.

Figures from the National Electrical Manufacturers Associationindicate that spiral CT has struck pay dirt in the U.S., accordingto Cerny. More than 60% of the CT units sold this year in thismarket were spiral or spiral-ready. More than 10% of the U.S.installed base may already have been switched over to spiral,she said.

It is not as clear, however, whether the growth in spiral CTinstallations has translated into a greater usage of CT relativeto other imaging techniques. Data acquired by the American HospitalAssociation indicate a basically flat level of CT utilizationand procedures, Barni said.

Most vendors do not see spiral CT and CTA blowing MRI/MRA orconventional angiography out of the water. Chances are that allthree modalities will maintain competitive strengths in particularapplications and jockey for clinical acceptance.

MRI will continue to provide superior brain and CNS imaging,while spiral CT exploits resolution and speed advantages in bodyimaging. Angiography should still provide the highest spatialresolution, but it may be susceptible to competition from CTAbecause of the latter's lower cost, greater speed and noninterventionalnature.

"CTA looks at large vessels," said Bryan Westerman,CT clinical sciences manager for Toshiba. "It will neverget down to the very small vessels you can see on conventionalangiography."

There are applications, however, in which CTA provides sufficientresolution to function alone, not merely as a less interventionalscreening tool for angiography, he said.

"(Spiral CT) can stand by itself in certain areas,"Westerman said. "The base of the brain, carotids and theaorta--both in the thoracic and abdominal regions--are susceptibleto CT because they are our largest vessels."

If spiral scanning doesn't propel CT beyond other imaging modalities,it does help level the clinical playing field. CTA and volumetricreformatting are genuine advances in the versatility of CT imaging.With its large, well-established user base and inherent cost advantages,spiral CT will be a power to reckon with.

MRI's spatial resolution when imaging large body parts is lessthan half that of CT, Levinson said. Longer scanning times withMRI make single-breath-hold imaging harder. Techniques to increaseMRI speed can sacrifice resolution. Both imaging modalities mustprovide special procedures to overcome problems in angiography.While MRA requires fat suppression techniques, CTA needs to getrid of the bone, he said.

CTA can provide both tissue and blood vessel information froma single study, while MRI requires a second study using differentpulse sequences. MRA also has a problem with turbulence artifactsbecause it images blood movement rather than the presence of contrastmedia, Levinson said.

Post-acquisition image processing techniques are common toboth MRI and CT, but image processing combined with the uniquefeatures of CT volume data can provide another competitive advantagefor spiral CT, he said. For instance, a connectivity computeralgorithm has been developed that distinguishes veins from arteriesin CTA and indicates when an arterial venous malformation is connectedto a particular vessel, Levinson said. This technique improvesthe already high capability of spiral CT for surgical planning.

Volumetric image processing of spiral CT data also improvesthe economy of CT imaging by reducing the need for retakes. Alldata required are present within the volume of data taken in oneacquisition. Z-axis slices can be produced through postacquisitionprocessing of spiral data without the need to scan again.

While 3-D and multiplanar reconstruction offer increased diagnosticpower, the rapid speed of spiral CT acquisition, along with establishedsiting advantages, has provided inroads for the modality in emergencyrooms and other standard x-ray settings.

CT exams used to take 30 minutes to an hour. Spiral scanning,however, cuts down equipment use time for a single exam to betweenfive and 10 minutes, Barni said. Times are quicker in emergencysituations.

"This is really a breakthrough for CT imaging technology,"he said. "You can put trauma patients on (a spiral system)and be done with a case in 20 seconds."