Opportunities abound for cardiovascular CT
Faster scanners, thinner slices, and better resolution reshape CT's future
By: Catherine Carrington
Every time CT makes another technological leap, imagers are presented with a host of new clinical opportunities. Those with a sense of adventure have been busy since June.
This summer saw the commercial debut of 16-slice scanners from three major vendors, and a fourth company was aiming for an October release. Although scanner specifications vary, all pair submillimeter slice widths with temporal resolutions that hover around the 100-msec mark and drop well below that in some modes.
Dramatic technical improvements in the newest generation of CT scanners means imaging will be faster and better in all parts of the body. Cardiac imaging, which stretches CT to its limits, has the most to gain, however.
Thin slices create excellent spatial resolution in all three imaging planes. The resulting tiny isotropic voxels capture fine detail and create dazzling 3D renderings of coronary arteries that wind in and out of the imaging plane.
Because the same volume of tissue can be scanned more quickly with a 16-slice scanner, breath-hold times can be cut dramatically, often by half or more. Whereas a four-slice scanner might take 40 seconds to image the entire heart, for example, roughly 20 seconds will do the job with both an eight- and 16-slice scanner. For some patients with respiratory compromise, this advantage could mean the difference between success and failure.
And temporal resolutions of close to 100 msec-as low as 65 msec in GE's cardiac burst modes-mean that patients with a wider range of heart rates can now be imaged without blurring.
"We used to think it was a miracle to look at the heart with CT. Now we can routinely acquire microvoxel data in a single breath-hold and look at it in 3D without ever sacrificing resolution," said Jonathan Murray, global manager of premium CT for GE Medical Systems.
One of the first things users notice about a 16-slice scanner is crisper imaging of the coronary arteries. Previous generations of multidetector scanners were, in some cases, nearly fast enough to freeze the motion of the heart. None, however, combined high speed with spatial resolution fine enough to adequately image arteries 1 to 2 mm in diameter.
Now, with slice thicknesses of 0.5 mm on the Toshiba Aquilion 16, 0.625 mm on GE's LightSpeed 16, and 0.75 mm on the Philips Mx8000 IDT and Siemens Sensation 16, today's top-end multidetector scanners not only produce cleaner, more detailed pictures of even tortuous vessels, they offer the possibility of imaging the elusive distal segments of the coronary arteries.
"Everything is resolved better spatially and temporally. Motion-related blurring has decreased, and we're able to see finer anatomy than we've seen before," said Dr. Richard White, head of cardiovascular imaging at the Cleveland Clinic.
At the same time, GE's electron- beam scanner continues to improve on its established strengths in noninvasive angiography. In addition to the 10 line-pair/cm resolution introduced on an earlier version, the C300 offers multiphase electron-beam angiography, a technique in which the scanner acquires three sequential diastolic images per heartbeat. The diastolic phases are displayed so as to simulate the visual effects of a cine loop and provide additional diagnostic confidence during angiographic interpretation.
OUTSIDE LOOKING IN
Even as CT extends it reach along the length of the coronaries, it strives to more precisely define conditions in the arterial lumen. Sharper resolution may mean that CT can distinguish gradations of stenosis more reliably, rather than merely detecting the presence or absence of high-grade lesions.
"The new 16-slice scanner provides unprecedented image quality and a more precise analysis of the proximal lesions as well as the distal ones," said Dr. Jean-Louis Sablayrolles of the Centre de Cardiologie du Nord in Paris. "Moreover, by acquiring the entire heart volume, we are able to examine coronary lesions as well as the impact of these lesions on the myocardium; for example, ischemia, necrosis, or postinfarct aneurysm."
The ability to image the arterial lumen is particularly important in patients who have undergone coronary intervention and stent implantation. About one fourth to one third of such patients develop restenosis and chest pain within the first few months, typically prompting an urgent trip back to the cath lab.
CT now has an important clinical opportunity in the triage of stent patients experiencing a new bout of chest pain. With resolution measured in just a few pixels, CT can often visualize through the stent to assess whether the artery remains patent or has become obstructed. Previous generations of scanners simply weren't up to the job.
"You want to be able to see inside the stent and determine whether there is good blood flow through it. If you have a stent that's a millimeter and a half in diameter, you've got to have significantly better resolution than that to get good visualization of the details," said Bryan Westerman, Ph.D., clinical sciences manager for Toshiba America Medical Systems.
Noninvasive angiography in other parts of the body-or of the entire body-will also improve with 16-slice scanners. Although CT angiography of peripheral vessels has become routine since the introduction of four-slice scanners, advanced technology enables users to better balance the trade-offs between speed and spatial resolution. That means imagers can choose head-to-toe imaging or acquisition of extremely detailed images in a smaller territory or, with variations in slice width and speed, some balance between the two.
With such versatility CT is primed for an enhanced role in preoperative screening. The evaluation of vascular health before major surgery is one possible application of whole-body angiography. Patients with atherosclerosis in one part of the body are a likely target group, as the disease is systemic, often silent, and potentially deadly under physical stress.
"In the setting of undetected atherosclerosis, there can be substantial risks to surgical patients, from the standpoint of both the anesthesia and the hemodynamic instability that ensues during the procedure," said Dr. Geoffrey Rubin, cardiovascular imaging chief at Stanford University.
Surgical planning is another key clinical opportunity. With high-end scanners depicting anatomy in increasingly fine detail-not just main vessels and branches, but secondary and tertiary branches-and offering the advantage of 3D display, invasive angiography faces an increasingly serious challenge.
"Conventional angiography has always had good resolution, but vessels twist and wind all over the place, and you can't necessarily tell where that vessel is in space," said William Kulp, manager of product marketing for Philips Medical Systems. "With CT, you can look at a volume-rendered 3D view and really see the relationships of the vessels. That's very important for vascular surgeons."
Improved spatial resolution offers new opportunities in plaque imaging as well. Calcified plaque has been relatively easy to detect with less sophisticated multidetector scanners because it appears in stark contrast to the surrounding vasculature. Soft plaque, which is believed to pose a greater danger of rupture and myocardial infarction, presents bigger challenges. Its composition and appearance make it difficult to distinguish from the vessel wall. Fine spatial resolution is necessary to detect the soft plaque, define it within a region of interest, and determine its CT number without volume-averaging with nearby tissue.
"To be able to make those very fine measurements, you have to have that basic spatial resolution in your data sets. Otherwise, you've got a moderately pretty picture, but it's not as useful as it might be," Westerman said.
By indicating how calcified or soft the plaque is, the CT number could become a way to risk-stratify patients. A reliable track record in plaque imaging could also open the door to using CT in pharmaceutical research. It could be used to gauge the efficacy of lipid-lowering and plaque stabilization medications by demonstrating their ability to track changes in plaque size and composition over time.
CT can detect coronary artery calcification with ease and is making rapid headway in coronary angiography. Add functional and myocardial perfusion imaging, and CT might just become the noninvasive one-stop shop visionaries have been talking about for years.
Cardiac software packages already churn out functional analyses, including wall motion, left ventricular ejection fraction, wall thickness, and end-systolic and end-diastolic volumes. Myocardial perfusion imaging has been a tougher nut to crack, though researchers are making rapid headway, and many feel it is within the grasp of multidetector CT.
Detector size is a major limitation of multislice scanners. For perfusion imaging, it's important to capture all or most of the organ at one time. Toshiba's detector provides 32 mm of z-axis coverage, Philips' and Siemens' detectors capture 24 mm at a time, and GE's captures 20 mm: None comes close to the 12 cm that would be necessary to image the entire heart.
Electron-beam scanners' design offers a clear edge in myocardial perfusion imaging. In this application, the electron beam sequentially sweeps along each of four tungsten rings positioned beneath the patient. In doing so, it covers 8 cm of the heart without moving the patient through the scanner. This coverage is enough to image the left ventricle.
With EBT scanners, perfusion imaging comes as a bonus of angiography and requires no additional contrast. Instead, perfusion studies use the small amount of contrast injected to determine circulation timing, just before commencing the angiographic study.
The ability to do perfusion imaging gives EBT the right to claim a one-stop CT shop, according to Jeffrey Sorenson, EBT marketing manager for GE. A comprehensive cardiac exam has four components, and only EBT can hit the mark on all four.
"You do coronary calcium screening to find high plaque burden, angiography to find stenosis, perfusion to find what the impact of that stenosis is, then wall motion to see how well the heart is functioning," he said. "If you can't do perfusion, you don't have a comprehensive cardiac exam, and nobody else can do perfusion."
With that range of capabilities comes the potential to play a key role as gatekeeper to the cardiac catheterization laboratory, potentially eliminating some unnecessary invasive diagnostic procedures and improving the productivity of both the CT suite and the cath lab.
SAFETY AND COMFORT
Advanced CT clearly enhances patient comfort and safety by reducing breath-hold times and, in some instances, obviating invasive angiography. There are other opportunities as well, however. Scanning the same anatomy with 16 slices reduces radiation dose by about one third compared with four-slice scanning. Faster scanning means that less contrast is needed and that critically ill patients can be imaged quickly and then returned to the intensive care unit. Finally, in cardiac studies, the speed of today's scanners may enable imaging over a wider range of heart rates, which would reduce the need to administer beta blockers.
Today, cardiovascular CT is characterized by innovation, energy, and promise. It is also marked by great challenge. Whether recent technological advances are enough to propel CT into the mainstream of cardiovascular imaging will soon become apparent, as early users take up the challenge of turning opportunity into reality.
"Cardiac imaging is really what stretches the limits of CT," said Kulp of Philips. "It has huge promise, but there's still an awful lot of market proof and acceptance that needs to be built up."