Chicago, IL - Imaging companies have in recent years competed furiously for supremacy in the booming business of cardiac computed tomography (CT) with the aim of producing the best, diagnostic-quality CT angiograms. Now, the big four—Siemens, GE, Philips, and Toshiba—seem to be evolving along slightly different paths, although the goal remains the same. Abstracts presented at last month's American Heart Association (AHA) 2006 Scientific Sessions, as well as the Radiological Society of North America 2006 meeting, both in Chicago, indicate that companies are gradually chipping away at technological or procedural factors that have kept growth of cardiac CT in check.

A patient with contraindication for beta blockers scanned for atypical chest pain. Scan time <10 s, heart rate 85 to 93 bpm. (Source: Siemens)

"I think all four vendors are going in different directions here," Dr Matthew Budoff (Harbor-University of California, Los Angeles Medical Center, Torrance, CA ) explained to heartwire. "Toshiba is going after the whole-heart imaging in one scan, which would be nice because arrhythmias would be less problematic. Siemens is trying to address the temporal-resolution limitations with a dual-source CT. GE has the new snapshot pulse technology, which lowers the radiation to 1 to 4 mSv . . . . And I know Philips is working on new detectors and dual-energy imaging, trying to expose the same artery to different energy forces so that you can expose calcium differently from contrast and differently from plaque to distinguish various structures in the coronaries better."

Dr Sandra Halliburton (Source: Cleveland Clinic)

The Siemens "dual-source" technology employs two X-ray sources with two sets of detectors, mounted 90º from one another (other commercially available systems use just one X-ray source and detector configuration). Whereas on standard single-source systems the acquisition time is half the gantry rotation, having two sources for data acquisition cuts this acquisition time in half, to one quarter of the gantry rotation. On the Siemens dual-source system—the Somaton Definition—this means 83 ms instead of 165 ms.

"The big advantage of this for cardiac imaging is that the dual-source capability enables you to acquire the images in less time, and that's a big deal because you've got an organ that's moving," Dr Sandra Halliburton (Cleveland Clinic, OH) told heartwire. "And the longer it takes to capture an image, the more susceptible you are to motion artifacts, so the shorter you can make that interval of time, the better the chance we have of catching the heart when it's during its slower diastolic phase. Even in 83 ms the heart is still going to be moving, it's not like you can freeze-frame the heart, but it's going to be moving less. That's where the excitement is for cardiac imaging."

Dual-source capability enables you to acquire the images in less time, and that's a big deal because you've got an organ that's moving.

Three abstracts presented at the AHA 2006 meeting provide an early glimpse of what this improved temporal resolution might mean for cardiac CT. In one study, Dr Annick C Weustink (Erasmus Medical Center, Rotterdam, the Netherlands) and colleagues compared the diagnostic accuracy of dual-source CT (DSCT) angiography against the gold standard of traditional coronary angiography in 100 patients with atypical chest pain, none of whom took beta blockers to slow the heart before their CT scan. Weustink reported that the sensitivity and specificity of DSCT for the entire cohort of 100 patients were both 95% on a per-patient basis. Per segment scanned, these values remained high, even in patients with higher heart rates, and none of the coronary segments were nonevaluable.

Similar results were reported by Dr Alexander W Leber (University of Munich, Germany) who found that DSCT could correctly diagnose or rule out coronary stenoses >50% and could also identify patients requiring coronary revascularization. A third study, presented by Dr Ulrike Ropers (University of Erlangen-Nurenberg, Germany), comparing patients who took beta blockers and underwent 64-slice multidetector (MD) CT with patients who were not premedicated with beta blockers and underwent DSCT, found that the sensitivity, specificity, negative predictive value, and positive predictive value of both tests were no different and the number of evaluable segments was significantly increased in the DSCT group as compared with the MDCT group (98% vs 91%, p<0.001), despite the fact that heart rates were significantly higher in the DSCT group.

In an interview with heartwire, senior author on the study presented by Ropers, Dr Stephan Achenbach (University of Erlangen-Nurenberg), emphasized that the ability to obtain diagnostic-quality images in people with higher heart rates is a "major advance." Not only does premedicating patients with beta blockers pose logistical and staffing hurdles, but the agents are contraindicated in patients with asthma or heart failure, for example. Some patients may be unwilling to take beta blockers, and some imagers are reluctant to prescribe them, particularly radiologists, if they are less familiar with the use of these agents.

With dual-source technology, Achenbach says, "you can guarantee better image quality in patients you couldn't scan before, and also, increasingly, people are using this in ACS patients who come to emergency department with chest pain. In those patients you might not want to take the time to wait for beta blockers to take effect, so that's an advantage of dual-source CT. . . . This will be a tremendous advantage."

Halliburton emphasized to heartwire that at least for the time being, dual-source CT doesn't spell the end of beta blockers. "I don't think we're ready to say that you can abandon beta blockers with this technology. I would say that this could mean that for patients in whom beta blockers are contraindicated, this gives them a chance, whereas before, we wouldn't even want to image them; now we're willing to try to image them. But it doesn't necessarily mean that in a patient who can receive a beta blocker that we won't want to give it, because slowing the heart can still improve images with a dual source."

Dr Matthew Budoff

But Budoff, for one, says the beta-blocker issue isn't the biggest stumbling block for imaging specialists and patients. "Far and away, the number-one concern is not the need for beta blockade or amount of contrast, it's the high radiation dose that we're exposing patients to," he said. "That's the push-back I get routinely."

Heart image of seven-year-old male with right coronary artery to pulmonary artery fistula; dose: 0.75 mSv on GE Healthcare LightSpeed VCT XT (Source: Dr James Earls, Fairfax, VA)

Budoff says the "most impressive" improvement he's witnessed with cardiac CT this year has been GE's snapshot technology. While all of the commercially available machines have dose-modulation functions and/or the capacity for dose-saving algorithms to be applied so that radiation exposure is reduced during the scan, GE's 64-slice LightSpeed VCT XT scanner uses software that converts the scanner mode from conventional helical acquisition to something it's calling "step-and-shoot" mode with prospective gating, in the which the table (as well as the patient) is "stepped" incrementally, so that the heart can be imaged in as few as five heart beats.

"The snapshot pulse actually turns the radiation off completely except during the imaging you want to get. With dose modulation, the X-ray beam is always on, it's just on lower or higher, but with the snapshot pulse you're obtaining a snapshot picture, then turning the X-ray beam off until the next heartbeat. It represents a 70% to 80% dose reduction as opposed to the maybe 40% dose reduction that we typically hope to get with dose modulation. I've seen images at meetings where the dose was under 1 mSv to obtain the CT scan, which is back to the calcium scan era or back to dental X-rays," Budoff said.

Elsewhere, the buzz is building over whole-heart imaging. Here, Toshiba is leading the way and last month announced that it would be installing the first 256-slice scanner in the US, a beta system based on the company's Aquilion platform, at Johns Hopkins University, in February 2007. The machine can cover 128 mm of anatomy with 0.5-mm slices, producing high-resolution images within a single gantry rotation.

"By covering the heart more quickly, there's less chance that the patient is going to move or have an arrhythmia or a premature ventricular contraction that will ruin your scan," Dr Todd C Villines (Walter Reed Army Medical Center, Washington, DC) "This reduces the breath hold, and instead of taking five seconds to scan the whole heart it can do it in one heartbeat."

Dr David Bush

Dr David Bush (Johns Hopkins University School of Medicine, Baltimore, MD) is also enthusiastic about 256-slice scanners. Having the entire heart covered in a single sweep of the gantry has several advantages, he told heartwire. For one, "the table doesn't have to move—it will be a nonhelical scan. Helical scanning, which allows good spatial resolution, requires overlapping acquisition to capture the heart in all of its phases of motion; this greatly increases the radiation exposure. Since a 256-slice scanner captures the entire heart in one half-rotation, the radiation dose will be substantially reduced."

Indeed, if coupled with prospective gating, radiation exposure could be as low as 1 to 3 mSv, Bush said. What's more, in addition to less artifact due to arrhythmias and the advantage of a shorter breath hold, capturing the entire heart in a single heartbeat using a single pass of the gantry also means lower volumes of contrast, Bush said.

"We hope to capitalize on the much lower radiation exposure with the 256-slice scanner to repeat a scan a few minutes after the initial scan. This may allow us to detect differences in perfusion resulting from coronary stenoses so that the physiologic significance of stenoses can also be determined. Having anatomy and perfusion in a single study would clearly be a major breakthrough in cardiac imaging."

Philips, meanwhile, is leading development of simultaneous multienergy spectral CT, in which the detectors themselves consist of scintillators that can simultaneously detect either lower- or higher-energy X-rays. A company representative told heartwire that more than 600 patients have now been scanned using simultaneous multienergy CT, and early studies suggest that dual-energy CT permits improved tissue characterization and the ability to distinguish between coronary lumen and atherosclerotic plaque and, ultimately, plaque components.

That's the promise with cardiac imaging, but I say promise, because currently the scanners in the US and Europe are not equipped to perform dual-energy scanning for cardiac imaging.

Dual-source technology, like that of Siemens, may also be able to employ dual energy to image cardiac and coronary structures down the road.

"Tissue has different attenuation properties, depending on the X-ray-tube voltage that's used," Halliburton explained. "So, for example, the standard X-ray-tube voltage for cardiac imaging is 120 kV, but you could potentially use two kV settings, something like 80 kV on one tube and 140 kV on a second tube, and use those simultaneously to get different attenuation properties from your tissue. It would give you two images, acquired simultaneously, but the tissue would look different because you've changed those acquisition properties, and that might help you in being able to separate different tissue types."

For cardiac imaging, the main focus for dual-energy CT would be plaque, and, it is hoped, being able to differentiate between plaque components. "That's the promise with cardiac imaging, but I say promise, because currently the scanners in the US and Europe are not equipped to perform dual-energy scanning for cardiac imaging."

"Every vendor has advantages and disadvantages, and they're each going after this in a different way," Budoff said. "They're all doing something a little different to give themselves a competitive edge but also to expand the number of patients who can be scanned with CT."

Proprietary issues aside, some of the technologies could be combined, Budoff noted, with the exception of dual-source CT, in which the sheer physical limitations of having two detector systems rotating simultaneously would make it difficult to up the speed of the rotation or number of detector rows.

All of the companies are also constantly revamping software, making it faster and more powerful and able to interpret scans differently, depending on the tissue of interest. Villines points out that clinicians, too, are also pushing the field forward, exploring the possibility of reducing radiation dose through different methods of breast shielding or by experimenting with reduced tube output in smaller patients to see how much tube output can be lowered and still yield an interpretable scan.

For now, Villines believes the developments in cardiac CT are exciting, but many will take years to come to fruition. "People ask me a lot, should I buy a 64-slice scanner or is it going to be out of date? And what I would say is that with the current technology as it is, with a rotation time under 420 ms, with 64 sub-mm slices, the quality is extremely good, and you're going to be able to use a 64-slice scanner for a long time."