Medication Management

In Cardiac Imaging, Things Are Looking Up for CTA

Computed tomography angiography is challenging functional tests, and some versions now include fractional flow reserve.

Thomas Reinke
Contributing Editor

The first cardiac imaging tool, the electrocardiogram, was invented in 1902. It took another 50 years before the next modality, the echocardiogram, came along in 1953. Today, though, cardiac imaging, like all forms of technology, is advancing at breakneck speed. The current advances are not so much brand-new modalities as previous ones that now produce almost unimaginably detailed, vivid images. They put on a show that rivals a major Fourth of July fireworks display. It doesn’t stop with the pictures: 3D imaging, virtual reality technology and gear, and artificial intelligence are all being incorporated to manipulate the images.

A lot of the advances are occurring for equipment used in cath labs and operating rooms. One example is Philips Healthcare’s use of Microsoft’s HoloLens headsets to create mixed reality 3D images that physicians can use while they are performing interventional heart and vascular procedures. Three-dimensional images of the heart’s anatomy derived from computed tomography (CT) scans or angiography can be projected as virtual holograms above the patient. The physician can rotate these 3D images on any axis to see the heart or coronary arteries from different angles. Seeing these images in three dimensions rather than just two is supposed to aid physicians while they are performing interventional procedures.

Cardiology is the sharp end of the imagery trend, but this new technology is working its way into other medical and surgical specialties, including oncology, orthopedics, and neurosurgery.

CTA coming on strong

But the adoption of new technology is rarely smooth; learning curves create jagged edges. The gee-whiz factor of technology can easily get ahead of true utility and effectiveness. And, of course, there’s the expense. Hospitals and other providers need to be ready to plow millions in new machines, especially when it comes to imaging.

So many of the actual developments in cardiac imaging these days are a little more on the incremental side of the change spectrum. All of the noninvasive imaging modalities are being tweaked: CT, ultrasound, cardiac magnetic resonance imaging (CMR), single-photon emission computed tomography (SPECT), and positron emission tomography (PET).

CT imaging, for example, is undergoing technological changes that is making it increasingly competitive with the mainstay of the catheterization lab. Computed tomography angiography (CTA) provides a 3D view of coronary artery plaques and stenosis in coronary arteries. Several months ago, Canon received FDA approval for an ultrahigh-resolution CT system that it says doubles the resolution of the current crop of cardiac CT images.

Philips says its new machine puts CT angiography on a par with top-quality traditional angiography that involves snaking a catheter to the area to be imaged, usually up through the femoral artery.

Traditional angiography has had a leg up on CTA because a cardiologist can also use it to measure fractional flow reserve. Fractional flow reserve is a calculation of blood flow through a narrowed artery that can help guide treatment decisions. But CTA is catching up. A company called HeartFlow has developed software that measures fractional flow reserve using a CTA scan. The tool, approved by the FDA in 2014, uses algorithms from fluid dynamics. It takes data from a CTA to produce measurements of ischemia to show a 3D model of the arteries supplying blood to the heart. This add-on shows anatomical characteristics of the blood flow through lesions to aid the cardiologist or surgeon in a possible future intervention. The enhancement adds approximately $1,500 to the price of a CTA.

Fierce competition

But the CTA versus traditional angiography comparison is only part of the story. CTA is also in a cage match with all the other noninvasive cardiac imaging tests—CMR, SPECT, PET, and so on. So far, CTA is winning the contest as the best test for stable chest pain patients who are at low to intermediate risk for coronary artery disease. CTA is used to measure coronary artery calcification. Last month, the United States Preventive Services Task Force said there wasn’t enough evidence to endorse adding coronary artery calcification to risk assessment of asympto­matic patients, but the American Heart Association said that “coronary artery calcification (CAC) by chest computed tomography is an excellent CVD risk marker.” And the new, ultra-high resolution CTA machines may give CTA an edge over its imaging rivals.

But the American health care system isn’t so quick to change, and evidence supporting a change can lag behind. Two years ago, an Agency for Healthcare Research and Quality comparative effectiveness review concluded that “limited evidence from RCTs found no clear differences between CCTA [the same thing as CTA] and other strategies in clinical outcomes across risk groups.” Interestingly, the review did not attempt to control for post-imaging patient care, which is likely to have affected clinical outcomes. The review dismissed the importance of these variables with the statement: “The absence of information on post-test risk stratification and subsequent decision making precluded evaluation of the impact of testing on patient management or outcomes.” Moreover, the review downplayed the traditional diagnostic test performance measures (i.e., sensitivity, specificity, positive/negative predictive value, and positive/negative likelihood ratio).

The American College of Cardiology has a more constructive and pragmatic approach. In May, it posted competing pro and con articles for the use of CTA as the first line test for coronary artery disease. The articles are a quick read and provide insight into many of the competing arguments. Current guidelines say that functional tests, like stress echo rather than “anatomic testing” such as CTA, is first line. The ACC has stayed away from recommendations of specific modalities for specific situations in part because of the lack of good head-to-head studies.

The pro article advocates for CTA to serve as the first test in many patients with stable chest pain. The article cites the chest pain guidelines from Great Britain’s official health care cost-effectiveness agency, the National Institute for Health and Care Excellence, which made CTA the first test for all patients without coronary artery disease who present with typical or atypical angina or with nonanginal chest pain plus an abnormal resting electrocardiogram.

The con article argues against CTA as an initial test, citing lack of evidence about its superiority over functional tests, the need for clinicians to work harder at initial risk assessment, and the possibility of increased referral to expensive traditional angiography.

Marketing CTA

But there may be no way to stop technology. In 2011, MedPAC began highlighting the rapid growth in advanced imaging and it recommended action to curtail it. In 2014 Congress passed legislation that required physicians to consult a clinical decision support system prior to ordering advanced imaging across all specialties, but implementation has been repeatedly postponed. It’s now scheduled for 2020.