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November 19 , 2007

256 Slices — Imaging Whole Organs in One Pass
By Beth W. Orenstein
Radiology Today
Vol. 8 No. 23 P. 26

Earlier this year, researchers at Johns Hopkins Medicine in Baltimore spent three months testing Toshiba’s Aquilion 256-slice CT scanner. The company plans to take the results from Hopkins and two test sites in Japan and finalize the scanner’s reconstruction algorithms.

Doug Ryan, senior director of Toshiba’s CT business unit, expects the 2-metric-ton device to be available for general clinical use by next summer. Ryan notes that the scanner’s development has been a priority for Japan-based Toshiba for the past 10 years. Toshiba hopes to earn approval from the FDA for general clinical use while the beta test results are being incorporated into the final product.

The Hopkins investigators, who used the 256-slice CT for studies of the heart and brain (and their surrounding arteries) and imaging joint function, were impressed with its capabilities and expect it could change the way some diagnostic procedures are done in the future.

Five-inch Coverage
“It really is a magnificent feat of engineering,” says interventional neuroradiologist Kieran J. Murphy, MD, an associate professor of radiology at Hopkins who led the radiology testing with the scanner, the first of its kind in North America.

The 256-CT consists of a patient table surrounded by a central gantry that looks like a massive, doughnut-shaped metal ring. Because of its larger number of detectors, it can cover an area of 12.8 centimeters, or 5 inches, a slice thick enough to capture most individual organs, including the brain, heart, joints, and most of the lungs and liver, in one rotation. By comparison, a 64-slice CT can cover approximately 3.2 centimeters per image and requires several rotations to fully image an organ.

The neuroradiologists at Hopkins took advantage of the 256-CT’s power to image the whole head in one swoop. Currently, the scan takes 1 second for a complete head CT, Murphy says, “but it will be 0.5 seconds in the future.”

Being able to gather the data on the brain at once with such speed is a tremendous advantage, Murphy says. When there are several scans, the slices have to be stacked or put together like a jigsaw puzzle, but the wide-area detector system eliminates that problem. “Previously, postprocessing of the images was a technologically complex procedure. Now, we can do it with the flick of a switch,” says Murphy.

Brain Perfusion Studies
Murphy believes that when it is commercially available, the 256-CT will be used for whole brain perfusion studies. Neuroradiologists should be able to use this technique to find brain areas where blood flow has slowed, and thus are vulnerable to strokes, before a stroke happens, he says.

With the 256-CT, a whole brain perfusion study with both an arteriogram and venogram takes roughly 10 minutes of a patient’s time, Murphy notes.

Richard George, Jr, MD, an assistant professor of medicine at Johns Hopkins School of Medicine and Heart Institute, says the 256-CT scanner will provide the same opportunity for the heart. With it, doctors will be able to detect early signs of restricted blood flow in the heart attributable to symptoms such as chest pain or shortness of breath.

The current practice is for patients with symptoms to undergo stress testing induced by exercise or chemicals. Patients walk on a treadmill and heart activity is monitored by an electrocardiogram (EKG). Or patients are injected with a radioactive tracer, usually thallium or technetium, and images of the resting and stressed heart are taken with a gamma camera. If the images show possible blockages, the patient may undergo a catheterization, an invasive procedure where a catheter is inserted into an artery in the arm or leg and advanced into the heart chambers or coronary arteries. The catheters also are used to inject contrast into the coronary arteries so images can be taken of blood flow.

Reducing Catheterizations
Only roughly one half of patients who undergo a cardiac catheterization require further treatment such as balloon angioplasty, stenting, or bypass surgery. George does not expect that CT angiograms done on a wide-area detector will ever replace catheterizations. However, they could be used to better triage patients to the cath lab. “If, after looking at the images from the CT, we can say definitively, ‘Hey, there is arteriosclerosis present in the vessel, and it is causing a decrease in blood flow,’ those are the patients who should be triaged to the cardiac cath lab because they are going to most likely benefit from an invasive strategy,” he says.

Stress testing has a fairly high false-positive rate. As a result, many patients are sent to the catheterization lab unnecessarily. “If, with the wide-area detector, we can develop a test that can eliminate the unnecessary referrals to the catheterization lab, we can not only save the healthcare system a lot of money, but it also would be safer for the patients,” he explains. While serious complications from catheterizations are rare, there is a small risk of negative outcomes such as heart attack, stroke, or even death, from the invasive procedure, George says.

Another advantage of the 256-CT vs. the 64-slice scanner is that the more powerful scanner requires less contrast. Because the scanner has a greater number of detectors, “We can cut the contrast dose by about 40% to 50% compared to the 64-slice,” George says. Since the scanner requires less contrast, the researchers at Hopkins were able to safely perform two scans—CT angiography (CTA) and CT perfusion imaging—on each patient.

George says the 256-CT has great potential for noninvasive CTA. “Right now, one of the Achilles’ heels of CT angiography is that it detects arteriosclerosis noninvasively very well, but it doesn’t detect ischemia very well. Currently, the positive predictive value for CT angiography to predict ischemia is in the 30% range in multiple studies,” he explains. “So what we’re trying to do is build upon the CT angiogram and add measurement of blood flow and perfusion to a CT study. The 256-CT allows us to do that because we have a lower radiation dose and lower contrast dose. We’re able to do the two scans safely on a patient.”

One Heartbeat
George is also excited about the possibility of using the wide-area detector when treating patients with arrhythmia or irregular heartbeats. The 256-CT can acquire a full image of the heart in the time it takes for just one beat while the 64-slice CT takes as many as seven or eight heartbeats. “If you are taking pictures of different parts of the heart over seven or eight heartbeats and those heartbeats are different distances apart, when you try to paste the images back together, they’re going to be misaligned,” George says. “You don’t have that problem with the 256-CT because it takes the pictures over one heartbeat. This is a very big advancement because right now, we really can’t scan these patients at all. They are basically stuck with having traditional stress testing studies or stress testing with radionuclide imaging or with echo imaging because they are not eligible for CT angiograms.”

Many times, patients with atrial fibrillation, one of the most common heart arrhythmias, can be treated with an ablation procedure. During the procedure, a catheter is guided into the heart to the offending area, which is then ablated with radiofrequency energy. Before having the ablation, patients undergo a cardiac CT. “Right now, we tolerate the degraded image quality because of the patient’s abnormal rhythm,” George says. “With the 256-CT, we will be able to get a much more accurate image before the patients undergo these procedures.”

The Hopkins cardiology research team presented its findings at the American Heart Association’s annual Scientific Sessions in Orlando, Fla., earlier this month. At the time of this interview, George could not talk about the results but says, “They were very good compared to [SPECT] perfusion imaging.” The neuroradiologists will also present their findings at RSNA 2007 in Chicago.

Hopkins has purchased an Aquilion beta 256 from Toshiba, and the device is expected to have a price tag of more than $1 million. However, Ryan says Toshiba cannot provide exact pricing until after it receives FDA 510K clearance.

Researchers at Hopkins also used the scanner to study wrist and ankle motion.

Ryan says interest in the scanner seems to come from both high-end academic institutions and mainstream hospitals. “Where we see the 256-CT fitting in is in a hospital that has two or three CT systems and is looking for that additional high-end, advanced imaging system,” he explains.

The system does not require any major site specifications other than a slightly larger area than a 64-slice CT would take up, Ryan says. Nine technicians also were on loan to Hopkins during the three-month test run.

Murphy says the 256-CT is capable of providing physicians with neuroimaging of a quality they never had before. “It’s absolutely a new paradigm,” he says. However, none of its capabilities will make any difference unless radiologists improve how they interpret CTA “and put the time into them they require,” he adds.

If busy radiologists do not take the time and effort to interpret these images carefully, he says, CTAs will continue to disappoint, and patients will have to undergo unnecessary cerebral catheter angiograms, which are much more risky
.
Murphy also believes that if the CTAs are interpreted by angiographers, “we will begin to see how excellent this technique is.

“I am a catheter angiographer. It’s what I do every day,” says Murphy, whose division at Hopkins performs roughly 4,500 exams per year. “This tool has the potential to remove at least 25% of that workflow because it is so effective, but it will require a behavioral change on the part of the radiologist,” he adds.

— Beth W. Orenstein is a freelance medical writer and regular contributor to Radiology Today. She writes from her home in Northampton, Pa.