New generation of multi-channel CT scanners brings diagnostic power and speed
April 1, 2005
The second 64-slice computed tomography scanner ever produced by Philips Medical Imaging, and the first to reach the United States, has been installed and is now in clinical use at the University of Chicago Hospitals.
The scanner, which has four times as many detectors as a typical multi-detector CT scanner, combines unrivaled image quality with remarkable speed. It can produce detailed pictures of any organ in a few seconds and provide sharp, clear, three-dimensional images, including 3-D views of the blood vessels, in an instant.
"The technology is stunning," said Michael Vannier, M.D., professor of radiology at the University of Chicago. "We can perform much more detailed analysis of very complex anatomy with a speed and confidence that we couldn't aspire to a year ago. It has changed what we can look for and how we look at it when we find it."
A complete chest scan used to include about 100 to 200 slices and take about 30 minutes, Vannier said. "Now we typically collect ten times as many slices in less than half the time."
It has also changed the way radiologists look at CT scans. Prior to the introduction of multi-detector CT scanners, doctors examined individual slices. "Now we look at reconstructed three-dimensional views," Vannier said, "and the doctors who send us patients insist on reconstructions. They won't tolerate a collection of slices anymore."
"This new generation of scanners is going to revolutionize the way we detect and diagnose heart disease," said Dianna Bardo, M.D., assistant professor of radiology at the University of Chicago. "We have already become reliant upon to these images in working up children with congenital heart disease, and multi-detector CT scans of the heart are increasingly being used instead of angiograms to detect clogged arteries in older patients."
"We're still learning," she adds, "what this technology can do."
Last spring, the University of Chicago Hospitals entered a partnership with Philips to serve as a clinical testing and development site for Philips' newest equipment. The arrangement brings Philips' most advanced imaging technologies to the University of Chicago. In exchange, the University's renowned experts in computer detection and diagnosis help Philips test and improve their image-processing software.
Last summer, Philips installed four new 16-slice scanners at the Hospitals as well as one of their first 40-slice CT scanners, at the time one of eight such devices in the world. In March 2005 they brought in the 64-slice scanner.
Other companies, including General Electric, Siemens and Toshiba are competing to produce and distribute similar multi-detector CT scanners, which combine the highest possible image resolution with breathtaking speed.
Many standard scanners have only two, four, six or ten detectors. One year ago a top-of-the-line scanner might have 16.
Each detector picks up an x-ray beam as it spins around the body and then computes the densities of the tissues that beam has passed through to produce a thin image of that narrow slice of the body.
The more detectors a scanner has, the closer they can be packed together, which improves resolution, and the faster they can gather slices, which increases speed. The computer then collects the images, stacks up the slices like a loaf of bread with each slice thinner than a penny, and presents a three-dimensional picture of the inside of the body. Software allows the radiologist to adjust the images to highlight specific tissues.
The multi-detector scanners can be packed less than a millimeter apart and take less than half a second to circle the body. So a 16-slice scanner can cover 8 to 12 millimeters in one pass or about an inch a second. A 40-slice scanner collects images covering 20 to 32 millimeters in a single pass and a tightly packed 64-slice device can cover about 40 millimeters at a pass, which takes 0.4 seconds.
At that rate, a 64-slice scanner can gather a high-resolution image of a heart, brain or a pair of lungs in about five seconds. A scan of the whole body, (in search of a blood clot, for example, that has become a source of emboli) takes about 30 seconds.
The technology has been particularly exciting for studying the beating heart, providing the first clear non-invasive images of the heart and its major vessels. The scans can be timed to use only images gathered between contractions, so that the heart and its vessels can be seen without the blurring caused by motion.
In fact, these scanners have already provoked "turf" battles between radiologists and the cardiologists who perform diagnostic angiograms -- until now the gold standard for assessing the coronary arteries. The best multi-detector CT scans can rival angiography for detail and are quicker, more convenient, less expensive and safer than an angiogram, as well as exposing the patient to less radiation.
"We've already switched from catheter-based to CT-based imaging in the brain," said Bardo. "The heart may come next."
"We are still sorting this issue out," cautions David Faxon, M.D., chief of cardiology at the University of Chicago and past president of the American Heart Association. "CT images have improved dramatically in the last few years," he said, "but there are still certain patients for whom angiography is more informative, especially for the small and distal vessels. I am sure, however, that the detail and resolution of these images will improve with time."
"What is clear right now," Faxon added, "is that when they combine tools and experience, radiologists and cardiologists can now look at the precise anatomy of the heart and its vessels in ways we only dreamed of not all that long ago."
The scanners are beginning to have an impact on cancer diagnosis and treatment as well. Nearly 60 percent of CT scans at the University of Chicago Hospitals are done for cancer. The speed and precision of these new scanners not only improves the image quality, but also "lets us look at dynamic processes," Vannier explained. "Instead of just monitoring changes in tumor size, we can watch the perfusion of a contrast agent as it moves toward, around and through a tumor," he said. "This can provide an early view of how a patient is responding to therapy. It helps us predict, rather than simply describe responses to treatment."
Other promising indications for multi-slice scanners include evaluation of plaque within the carotid arteries (5 to 8 seconds), searching for pulmonary emboli (5 seconds, less than an easy breath hold), coronary artery imaging (10 seconds, including distal segments and multiple arterial branches).
The scans have their own limitations. Although the scanner table is built to support up to 450 pounds, it can be difficult to accommodate patients who are morbidly obese. Each scanner costs between $1.5 million to $2 million.
One remaining challenge is image storage. The Philips "Brilliance Workstation" collects and processes the images within seconds but the enormous files can overwhelm the computers that store them.
"We never used to gather so much information so quickly," points out Vannier. "We currently perform about 4,000 CT scans a month, each including 1,000 to 2,000 discreet slices, and those numbers are increasing. We have to rethink how we can compress and store so much data, yet retain easy access to these images when we need them."
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