For anyone keeping an eye on advances in cardiac CT, don’t blink. Once dominated by coronary calcium screening and weighed down by controversy, cardiac CT has gotten a second wind, racing past technological obstacles and impressing former skeptics with its clinical promise.

Early research suggests that CT could become the preferred tool for noninvasive angiography, differentiate soft atherosclerotic plaque from its less vulnerable calcified form, and perhaps add a new dimension to myocardial perfusion imaging. Cardiac CT is being cheered on by both radiologists and referring clinicians.

“I’ve never seen a year in cardiac MR as I’ve seen in cardiac CT,” said Dr. Richard D. White, head of cardiovascular imaging at the Cleveland Clinic. “It’s taken off very, very rapidly.”

Electron-beam technology clearly is responsible for showing what CT can do when it images fast enough to stop cardiac motion. But credit for the enthusiasm that propels the field today lies with multidetector spiral CT, along with three-dimensional reconstruction technology, according to Dr. Lawrence M. Boxt, cardiovascular imaging chief at Beth Israel Medical Center in New York City.

“It allowed radiologists who were doing conventional CT to start doing cardiac CT. They didn’t have to buy a machine just for one organ,” he said. “With very fast scanners and the new 3-D reconstruction technology for handling stacks and stacks of data in a convenient manner, people started seeing the coronary arteries-and started thinking about going after them.”

While researchers are most excited about their progress in CT coronary angiography and intrigued by the possibility of perfusion imaging, other applications of cardiac CT have already become part of routine clinical practice. In some cases, CT is taking work away from established forms of cardiac imaging.

MR, for example, is being elbowed aside (see sidebar) as CT takes over the evaluation of large aneurysms of the thoracic aorta, particularly when stent-graft therapy is likely. Imaging to determine whether chest pain results from an abnormal pericardium is another example of an exam MR is ceding to CT, as is the initial evaluation of arrhythmogenic right ventricular dysplasia.

“It’s now commonplace for us to first screen for arrhythmogenic right ventricular dysplasia with CT, rather than MR, because we can quickly detect the likelihood of significant muscle disease of the right heart,” White said. “The first line of diagnostic workup in our institution is becoming CT. Then we go to MR when needed and do a more tailored examination.”

CT may also provide a noninvasive alternative to intravascular ultrasound in evaluating patients for transplant vasculopathy, and it is ideal for guiding certain electrophysiologic procedures, such as catheter ablation of atrial fibrillation. Often the source of the arrhythmia can be found at the opening of the pulmonary vein. CT can help determine which patients are good candidates for ablation by defining the size of that vessel and the pattern of its side branches. Providing guidance during electrophysiologic procedures is also a role CT could claim in the future, White said.

Coronary CTA: No Joke

When talk turns to noninvasive coronary angiography, CT is increasingly the subject matter. Many imagers say they have all but given up on MRA of the coronaries.

“I don’t do coronary MRs anymore. It’s just too time-consuming, and I can’t get a straight answer,” Boxt said.

Instead, he is one of several researchers fueling a flurry of studies into coronary CTA. Boxt expected to begin a study in October comparing CTA with conventional angiography in patients already scheduled for the cath lab. Even before that, he and his colleagues were performing coronary CTA under certain circumstances, such as the evaluation of coronary artery anomalies or low-likelihood stenoses. Boxt said he once considered cardiac CT a joke, but he has been impressed by the results.

“The pictures are just spectacular. When you electrocardiograph-gate, you see everything: the entire course of the right coronary artery, the left main becoming the circumflex, the anterior descending, and side branches,” he said. “We’re seeing incredible detail.”

Dr. Tom Brady, director of the cardiac imaging program at Massachusetts General Hospital, is a little more reserved in his praise of coronary CTA. At press time, he and his colleagues had compared the results of coronary CTA and conventional angiography in about 30 patients with known or suspected coronary artery disease. Early results suggested CTA’s overall sensitivity for coronary artery stenoses was in the range of 70%: better in the proximal portions of the coronary arteries and worse in the distal segments. The right coronary artery can also present a challenge, because it moves out of the plane of acquisition as the heart beats.

“CTA is coming along nicely, but it still needs more work. We need to decrease the temporal resolution of the acquisition and improve a couple of other technical parameters before it’s going to give us a great study every time from the coronary ostia all the way down to the apex. But I’m very bullish on it,” Brady said.

Not everyone is convinced of a clinical role for coronary CTA. Dr. William Stanford, a professor of chest and cardiovascular imaging at the University of Iowa, believes that a patient who has a high score on coronary calcium screening, for example, should probably have a nuclear stress test to look for perfusion defects caused by flow-limiting stenoses.

“That individual probably ought to go to cath, not only to define the anatomy, but also because you can do balloon angioplasty at the same time. I’m having trouble finding where CT angiography—though it’s talked about a lot—has a big clinical use,” he said.

White holds the opposite view. Even if it takes several years for CTA to fully overcome its limitations, its potential value remains high, he said. Just being able to tell clinicians that proximal arterial segments are clear may be enough to eliminate unnecessary conventional angiography in many cases, saving the patient from an invasive procedure and reducing healthcare costs.

“We don’t necessarily have to shoot for the stars to have an impact,” White said.

Soft Plaque

The proper role of coronary calcium screening in determining the risk of heart disease has been controversial and remains hotly debated. The subject is the center of a technological tug-of-war between electron-beam and multidetector technology. Just as studies increasingly supported its value as a cardiovascular risk factor, the attention of clinicians and researchers shifted to the identification of soft plaque.

Many researchers are observing what they believe to be soft plaque on CTA images. Since soft plaque does not show up on conventional angiography and is more likely to be unstable than calcified plaque, this finding has sparked intense interest. So far, CTA can’t reliably determine which soft plaques are stable and which are likely to rupture and cause a heart attack, but research is moving in that direction.

“Detecting segmental enlargement of the coronary artery and the presence of soft plaque is a pretty ominous sign, and we’re able to pick up on that even now,” White said. “I think we can get a hint of a less-than-desirable situation-one that hasn’t presented with symptoms yet-and maybe use this to monitor therapies directed at plaque progression.”

Perfusion Imaging

Mention myocardial perfusion imaging and CT in the same sentence, and the typical response is a blend of interest and skepticism. Dr. Ting-Yim Lee plans to turn skeptics into believers.

“CT perfusion imaging is here already. We can calculate blood flow maps in an ischemic model, and where you expect the ischemia to be, it’s there,” said Lee, a Ph.D. researcher at the John P. Robarts Research Institute and the Lawson Health Research Institute, both in London, Ontario. “The challenge facing us is to prove to the world that it really works.”

Lee has developed a method to quantify myocardial blood flow and distribution volume using contrast-enhanced multidetector CT. The results, displayed in pseudocolor maps, show perfusion defects and reveal the presence of infarcted tissue. But they also take advantage of CT’s spatial resolution to suggest whether the infarction is transmural or extends only partway through the myocardium. That’s something PET, perfusion imaging’s gold standard, can’t do.

“I’m very excited about this,” Lee said. “We’re using an ordinary CT scanner, we are injecting contrast using standard techniques that CT techs use day in and day out, and the time of scanning is less than 30 seconds. And out of that you get all this information.”

So far, Lee and his colleagues have studied dogs with experimentally induced ischemia, but they anticipate beginning studies in human heart patients next year.

The perfusion imaging protocol teams a four-slice multidetector CT scanner, ECG gating, and retrospective reconstruction of projection data selected from the end diastolic phase of the heart cycle, when the heart is nearly motionless. Perfusion studies are done following an intravenous injection of contrast. CT tracks the rate at which contrast passes through the aorta into the myocardial capillary network and then through various regions of the myocardium. From these two pieces of data, separate software that Lee has developed and licensed to GE Medical Systems-known as CT Perfusion 2-calculates blood flow, blood volume, mean transit time, and leakage of contrast from the capillaries into the myocardial interstitial space, and then creates a pseudocolor perfusion map.

For determining myocardial distribution volume, CT scanning is done first without contrast, then again after a continuous 30 to 60-minute infusion. Baseline images are subtracted from contrast-enhanced, steady-state images. An above-normal distribution volume would indicate the breakdown of myocardial cell membranes and leakage of contrast into the intracellular space, a sign of myocardial infarction. CT’s spatial resolution is high enough to show whether the increased distribution volume-and, hence, the infarct-extends through the myocardial wall.

Lee’s next step will be to validate his blood flow measurements against those determined with radiolabeled microspheres that have a diameter of about 15 micron-just large enough to pass through the coronary arteries and lodge in the myocardial capillaries. Assuming these animal studies go well, Lee plans to validate his technique in humans using PET as the quantitative gold standard.

White and his colleagues have had some success with CT perfusion imaging, detecting a few cases of myocardial infarction from perfusion defects observed while conducting contrast-enhanced CT of suspected aortic dissection. Still, he is convinced that CT perfusion imaging must overcome several obstacles before it can be accepted clinically. Lee’s technique requires slowing the heart rate to 60 to 80 bpm by administering medications like beta blockers, something that White prefers to avoid in sick patients. In addition, improvements in contrast agents that would enable them to pass less quickly through the coronary circulation would be helpful, he said.

“You can’t overlook some of the limitations of CT, including its speed. It’s not so fast that you can necessarily appreciate a first-pass effect, which is what is needed, given the agents at hand,” White said.

The imaging industry is advancing quickly to give researchers increasingly sophisticated tools, developing scanners capable of acquiring eight to 16 simultaneous slices of imaging data. In what could be an even bigger technological leap, volume CT systems are under development.

“Within the next five years we’re going to see the next generation of CT going the extra length,” White said. “I think we’re really seeing a new CT.”