Technology aids clinical use of 16-slice scanners

November 30, 2003

Since multidetector CT scanners began rolling off the production line five years ago, CT anigography has become a routine procedure, in many cases the procedure of choice, for screening patients for arterial disease. The reason is the ease with which it can be applied. Patients can undergo CTA without being admitted to the hospital and without invasive catheter placement. An added benefit is the extraordinarily low cost of a scan compared with the gold standard x-ray angiography.

 

Since multidetector CT scanners began rolling off the production line five years ago, CT anigography has become a routine procedure, in many cases the procedure of choice, for screening patients for arterial disease. The reason is the ease with which it can be applied. Patients can undergo CTA without being admitted to the hospital and without invasive catheter placement. An added benefit is the extraordinarily low cost of a scan compared with the gold standard x-ray angiography.

In healthcare, the promise of lower costs thanks to higher technology brings welcome relief. And CTA has distinguished itself among its competitors. It provides more details about blood vessels than diagnostic ultrasound. Compared with MR angiography, CTA is more available, more flexible, and safer. Patients with pacemakers and other metal implants need not avoid it. But one overriding advantage is responsible for CT's rising popularity in angiography, according to Dr. John Fountain, codirector of neuroradiology at Emory University.

"CTA is just so fast," he said.

CTA is commonly used to identify aortic dissection, carotid stenosis, aneurysm, and vascular disease of such organs as the kidney. MDCT has made these applications possible, while new user-oriented software is continuing to make the technology easier to use.

But the role of CTA is not a given in all settings. At institutions where competing modalities are readily available, the strengths and interests of practitioners may overshadow those of the technologies. At George Washington University, for example, MRA has gained on CTA over the last year. Medical staff there see MR taking on more of the advanced applications once ascribed to CTA, including vascular studies. The opposite is true at the cardiovascular MR and CT facilities of the Cleveland Clinic Heart Center, however, where angiography falls in the CT camp and MR is used to evaluate motion and heart function. Exceptions at these facilities are driven by patient needs. Patients with pacemakers who visit the Cleveland Clinic Heart Center go to CT. Those with a dye allergy go to MR.

Some physicians are hesitant about CT, because of its vulnerability to artifacts due to beam hardening, dental amalgam, or heavy arterial calcification. And broader isues may give referring physicians and radiologists pause. Patients and referring physicians are becoming increasingly sensitive to x-ray exposure, and MR and Doppler ultrasound have the upper hand on this score.

CTA does deliver, however, in ways that competitors cannot. In addition to its speed, automated procedures and algorithms that optimize images make results easy to reproduce. And patients who fear tight spaces prefer CTA. Then there are the exclusionary clinical reasons.

"Every year 200,000 self-expanding (metal) stents are implanted," said Dr. Kieran Murphy, an associate professor of radiology and neurological surgery at Johns Hopkins Medical Institutions. "MR can't see inside a stent. End of story."

COMPETITION OR COLLABORATION?

It's easy to view matchups among modalities as competitive, but the relationship among vascular technologies may be more collaborative. At Emory University Hospital, carotid bifurcation suggested by clinical presentation, ultrasound, or MRA is verified with CTA images that define the stenosis in terms of its length and position. CTA establishes the context-the surrounding vessels and soft tissue-as well as the degree of calcification in the plaque. This has led to more accurate placement of smaller surgical incisions, according to Emory staff.

CTA will play an increasingly important role in screening patients suspected of ischemic or hemorrhagic stroke, according to Dr. Randall T. Higashida, a professor of radiology and neurological surgery at the University of California, San Francisco. He and his colleagues already depend on CTA for this purpose.

"It is an excellent tool for looking at carotid atherosclerotic disease, as well as at intracranial blood vessels to find sources of stroke," he said.

At Emory, stroke patients are screened for lesions in the anterior and posterior circulation to rapidly identify these who might best respond to intravenous or intra-arterial therapy. Areas at risk for infarction can also be identified, as the vertebral and carotid circulation down to the aortic arch can be examined for the source of the ischemia.

CTA and MRA often give comparable results, UCSF radiologists agree, but CTA is usually the preferred method. CT scanners are more available, Higashida said, and the technology is faster and safer for seriously ill patients. Speed is a critical consideration in emergencies, particularly in children. Murphy described a nine-year-old child who fell at the beach and presented with symptoms consistent with acute vascular trauma. CTA defined the problem quickly and accurately.

"Conventional angiography in a kid is traumatic," Murphy said. "It requires anesthesia. It scares the hell out of the parents. To do something noninvasively in that setting is perfect."

Increasingly, x-ray angiography is being relegated to therapeutic situations. Murphy, who performs numerous neurointerventions under x-ray guidance, foresees a day when physicians will specialize entirely in the diagnostic or therapeutic use of imaging technologies.

"We are moving toward an imaging therapist-somebody who treats based on imaging and for whom the diagnostic part of imaging is separate," he said. "There are a tremendous number of similarities in postprocessing and interpretation of images, whether the cardiologist or the radiologist is doing them."

Speaking volumes in favor of CTA are providers who do not invest in the highest technology yet reap the advantages of CTA. Radiology Associates, which runs two imaging centers in New Port Richey and Bayonet Point, FL, sports the tag line "an image you can trust." Staff radiologist Dr. Denis Stewart reads an increasing number of CTAs from the practice's updated quadslice Volume Zoom, predecessor to Siemens' Sensation and Emotion MDCT CT families.

"The number of CTAs we do has been climbing, and part of the reason is that we are able to market CTA so effectively to doctors," he said. "It's very easy to use, results look good, and turnaround is quick."

GETTING THE BUGS OUT

Being a step or two behind the curve can have advantages. Radiologists at Emory University hit more than their share of bumps on the road to advanced applications. Artifacts cropped up unexpectedly after the installation of a 16-slice scanner, and Fountain and his staff had to work out the problems themselves. About a year after the system was delivered, the company came back with improved protocols, but by then the work was done.

"We spent a lot of valuable time screwing around with this, trying to find out what really works," Fountain said.

Experiences at Emory may be more the norm than the exception. Vendors have taken the approach of letting physicians define how and when to use new CT systems. Early adopters are consequently left to fill in the blanks themselves. Enough experience has been gained, however, that vendors can predict future clinical needs, taking them into account when designing the next generation.

Murphy hopes this generation will be fast enough to prevent venous overlap and provide enough coverage to show dynamic changes. Fountain hopes the equipment will produce fewer artifacts, such as those caused by beam hardening. He would also like to visualize dynamic flow, as can be done with x-ray angiography.

"We do a lot of perfusion, and we'd like to go beyond that to see flow going through the vessels," he said.

Making good on that wish list will likely take some time. While vendors talk about machines capable of 32 or 64 slices-or even whole volumes through the use of flat panels-the commercial arrival of this technology is still years away. Vendors have been coming up with tools for capitalizing on 16-slice technology, however, and more will be on display at the RSNA meeting.

These will be surrogates for the next generation of CT scanner, as 16-slice scanners continue to change the practice of medicine, notably in cardiology. A paper published earlier this year in Circulation (107[5]:664-666) found that a 16-slice scanner could detect coronary artery stenoses with high accuracy and a low rate of unevaluable arteries.

Cardiology represents the most challenging area of CT clinical application, Murphy said. Technologies developed for cardiology, therefore, should be able handle just about anything else.

"When you are capable of operating at those high levels of performance, everything else is easy," he said.

CTA and MRA often give comparable results, UCSF radiologists agree, but CTA is usually the preferred method. CT scanners are more available, Higashida said, and the technology is more time-efficient and safer for seriously ill patients.

Higashida and colleagues at UCSF first order a CT scan to determine whether the stroke is due to either hemorrhage or ischemia. If it is, a bolus infusion of contrast is administered to narrow the cause to emboli from the carotids, or occlusions or stenoses in the intracranial blood vessels.

"This approach is appealing, because it can be done very rapidly," he said.