Noninvasive Imaging for Suspected CAD: Which Test When?

Nandan S. Anavekar, MB BCh; Amber N. Boler, MD


March 28, 2018

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Amber N. Boler, MD: Greetings. I'm Dr Amber Boler, cardiology fellow at Mayo Clinic. Today we will be discussing noninvasive imaging for suspected coronary artery disease (CAD). I'm joined by my colleague Dr Nandan Anavekar, who specializes in this area. Welcome, Dr Anavekar.

Nandan S. Anavekar, MB BCh: Hi, Amber. Thank you so much for having me here and for the chance to chat about imaging in CAD.

Noninvasive Imaging Options for Assessment of CAD

Boler: What are the noninvasive imaging studies available to assess CAD?

Anavekar: Several noninvasive tests are available for the assessment of CAD, and all are aimed at helping the risk stratification process. The mainstays that we are all familiar with are echocardiography, which uses ultrasound technology to produce images, and nuclear cardiac imaging, which uses radiation to produce images. Echocardiography provides information regarding function and hemodynamic parameters, whereas the strength of nuclear cardiac imaging is in its ability to demonstrate function, perfusion, and metabolic phenotypic features of myocardial performance.

Not so new are cardiac MRI and cardiac CT. However, these are not typically mainstay imaging modalities used in practice. MRI uses the magnetic properties of protons to produce images, and it provides useful information regarding cardiac function and potentially hemodynamic abnormalities. Most important, it is able to help distinguish ischemic from nonischemic forms of ventricular dysfunction. Cardiac CT uses X-rays to produce images, and its major strength is in its ability to noninvasively delineate the coronary anatomy.

To summarize, the mainstays of noninvasive imaging modalities available to assess CAD include echocardiography and nuclear cardiac imaging, and cardiac MRI and cardiac CT at centers with advanced cardiac imaging technologies.

Which Modality When?

Boler: When would you choose a noninvasive imaging test to evaluate for CAD?

Anavekar: This is a great question, and it pertains to the underlying disease. CAD is an important diagnosis to be aware of, because it is related to both major morbidity and mortality. However, both primary and secondary preventive strategies as well as contemporary treatment of acute CAD have improved outcomes in ischemic heart disease overall, and this is something that the wider cardiovascular community can really be proud of.

When to choose a noninvasive test for suspected CAD, in reality, depends on the type of practice that one operates in. For example, my inpatient practice is in the niche of cardiac intensive care, and therefore essentially all of my patients with ischemic heart disease present with an acute ischemic syndrome. In this situation, the goals of imaging are to assess left ventricular function, structural complications of ischemic heart disease, and alternative diagnostic possibilities for the presentation.

In the outpatient setting, the goals are intuitively different. The two major considerations are the evaluation of the patient presenting with symptoms and the evaluation of the asymptomatic patient who is at risk for coronary events. In these settings, imaging usually is undertaken to facilitate risk stratification and the decision to pursue invasive angiographic assessment. In the majority of instances, this may be in the form of functional cardiac imaging using a stress test, or anatomical imaging using noninvasive coronary computed tomography angiography (CTA).

The two major considerations are the evaluation of the patient presenting with symptoms and the evaluation of the asymptomatic patient who is at risk for coronary events.

Boler: How do you choose a noninvasive study for these patients?

Anavekar: Two important factors determine one's choice of noninvasive test. The first is the availability of the technology, and the second is the availability of the expertise. If both are present, the choice depends upon acuity of illness and the goal of testing.

Let's first consider the acute coronary syndrome (ACS) in the inpatient setting. In most instances, one does not need imaging to make the diagnosis. However, it may become necessary when there may be diagnostic ambiguity—we do not see this infrequently. The major diagnoses in the acute setting beyond myocardial infarction include stress cardiomyopathy, myocarditis and/or pericarditis, pulmonary embolism, and the acute aortic syndromes. If there is any doubt about the diagnosis, then imaging may become very important in the evaluative process.

The two important modalities available in the inpatient setting essentially 24/7 in most places are echocardiography and CT imaging. Echocardiography, in particular, is very useful because it can be brought to the bedside, which is important in the care of our unstable patients. Once the diagnostic goal has been established, most imaging in ACS takes place after ischemia has been eliminated. It is focused on assessment of ventricular function and monitoring of complications, which can include both acute structural complications and also chronic changes in size and shape of the pumping chambers (which holds prognostic relevance).

In the outpatient setting, the two important technologies that are probably the current mainstay include echocardiography and nuclear cardiac imaging, because there is such extensive global experience and availability of these technologies.

Most of the outpatient assessment revolves around functional imaging. Cardiac MRI is evolving and is a growing presence in the field of functional cardiac imaging. It provides absolutely beautiful images in any plane, and the imaging resolution is so good that it allows assessment of subtransmural perfusion defects. CT is also developing a role in the assessment of individuals where alternative diagnoses are being considered and where the manifestation of ischemia may be due to another etiology (eg, myocardial bridging or the detection of congenital coronary arterial abnormalities).

Another controversial area is viability imaging. In my practice, I have utilized echocardiography, PET imaging, and cardiac MRI, although my preferential bias is toward PET imaging because I feel that in my experience at Mayo, we tend toward PET imaging for viability assessment. Having said that, it is extremely important to be cognizant of the relative paucity of data supporting routine viability testing and the lack of data regarding the role of the different imaging modalities in viability testing.

If we were to summarize, my choice of imaging in the inpatient setting in the majority of cases utilizes echocardiography and CT imaging. In the outpatient setting, my practice preference has been echocardiography and nuclear cardiac imaging. I utilize cardiac CT if I am considering alternative diagnoses for chest pain or for screening, and I consider cardiac MRI, echocardiography, or nuclear cardiac imaging if I am considering an assessment of myocardial viability in a patient with established CAD with significant left ventricular dysfunction.

Limitations of Noninvasive Imaging Modalities

Boler: This is fascinating. What are some of the pertinent limitations of each of the studies you mentioned?

Anavekar: Let's take this by imaging modality and first consider echocardiography. Remember that echocardiography depends upon ultrasound to develop images. Therefore, if there are limited acoustic windows, then we will have limited image quality, and this will certainly impact the interpretive sensitivity of the test. I would consider this to be the major limitation of echocardiography.

With nuclear cardiac imaging, the major limitation is radiation exposure, which becomes of particular importance especially in our younger patients with CAD.

For cardiac MRI and CT the major limitation remains access to technology. Beyond that, for MRI, there are both patient- and technology-related limitations. For example, in a patient with claustrophobia, MRI probably would not be a good choice. In a patient with renal disease, one has to be cognizant of the fact that this represents a contraindication to the administration of gadolinium-based contrast agents. Although over the past 5 years or so we have been able to image individuals with MRI who have cardiac devices, I still see this as a relative contraindication to cardiac MRI, especially if alternative imaging strategies are available.

For cardiac CT, there are several important factors to consider. First, CT uses radiation to produce images. Most cardiac CT scans in contemporary practice use dose-reducing protocols to minimize X-ray exposure. But having said that, this remains an important consideration. Cardiac CT imaging requires electrocardiographic gating; therefore, the presence of arrhythmias may affect the quality of images produced. Finally, iodine-based contrast agents are used, and this is contraindicated in individuals who have iodine allergies and is relatively contraindicated in those with significant renal disease.

Future Developments

Boler: This is a rapidly evolving field. Can you discuss some ongoing areas of research in this area?

Anavekar: This is a challenging, but important, question to answer. Simply, there is a large volume of research in the field of ischemic heart disease. In the context of cardiac imaging, areas of interest relate to the performance of one modality against another in diagnostic and prognostic capacities. These goals must also be pitted against the phase of the disease that one is interested in: acute versus chronic, and symptomatic versus asymptomatic. CT perfusion imaging is rapidly becoming an important adjunct to coronary CT angiography for both anatomical and functional assessment of CAD using a single modality.

My second area of interest would be viability imaging, because it pertains to the evaluation of a group of patients who are at the highest risk for poor outcomes but who paradoxically have the greatest to gain from potential revascularization strategies. Viability imaging has the potential to connect the different modalities of cardiac imaging and provides us the opportunity to understand the different phenotypic manifestations of coronary heart disease.

There are so many other research areas in CAD that it would take a symposium to just touch upon. So beyond my personal practice interests, I would say stay tuned for more to come.

Boler: Thank you, Dr Anavekar, for this excellent discussion and these very important insights into noninvasive imaging for CAD.

Anavekar: Thank you, Amber, for the chance to be here today.

Boler: Thank you all for joining us on on Medscape Cardiology.


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