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2 decades of imaging
A Cardiology Today Editorial Board member discusses progress in cardiac imaging technologies and capabilities.
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Editor’s Note: Cardiology Today is celebrating its 20th anniversary in 2017. We are reaching out to experts in cardiology for their take on changes in CV medicine since the publication launched in 1997. In this issue, Nathaniel Reichek, MD, FACC, FAHA, focuses on cardiac imaging.
The 20 years since Cardiology Today first appeared have been marked by enormous advances in cardiac imaging. For several imaging technologies, rapid progress is, if anything, still accelerating today. In 1997, 3D echocardiography and cardiac MRI were largely research methods, cardiac CT was limited to calcium scoring with electron-beam CT and nuclear cardiology was heavily dependent on thallium perfusion imaging. Cardiac PET was limited to a few research sites using very short half-life isotopes that required a cyclotron and a radiochemistry team on site. Decision-making in CAD was almost entirely based on invasive coronary angiography.
Prominent technologies
In contrast, today, each of these technologies has assumed new and greater importance in patient evaluation and therapeutic decision-making. Digital harmonic 2D transthoracic and transesophageal imaging provide far greater spatial and temporal resolution than older methods. Real-time 3D transthoracic and transesophageal echocardiography with color flow have become critical methods in performance of procedures such as transvascular valve replacement. In addition, 3D transthoracic and transesophageal echocardiography can provide highly reliable volumetric assessment of cardiac structure and function in any cardiac disorder in patients with adequate acoustic windows.
Whereas measurement variability and geometric assumptions had been important limitations of 2D echo, markedly reduced measurement variability enables digital 3D echo detection of much smaller but real changes in ejection fraction, left ventricular volume and mass than older echo techniques. Echo strain imaging, first with tissue Doppler and more recently with speckle-tracking methods, emerged over a decade after cardiac MRI (CMR) methods, but is so widely available now that it has begun to be an important clinical tool as well.
In nuclear cardiology, thallium has been largely abandoned. Radiation doses for rest-stress imaging have been reduced and stress-only perfusion evaluation for normal studies has been validated. Importantly, the emergence of rubidium PET as an alternative to conventional single-photon emission CT imaging provides reduced radiation, reliable perfusion quantitation and relative freedom from major distortion by tissue attenuation. This greatly improves reliability of nuclear cardiology in patients with morbid obesity, a key issue given the current obesity epidemic. In addition, use of PET makes possible incorporation of fluorodeoxyglucose studies for myocardial viability, detection of inflammatory sarcoid and, potentially, other inflammatory processes. Further, rubidium PET does not require a nearby cyclotron because it is generator-derived, so the cost of PET studies has been dramatically reduced.
Cardiac MRI
Fittingly enough, the Society for Cardiovascular Magnetic Resonance was also founded in 1997, by a small number of investigators from the United States and Europe. Although the capabilities of CMR had been explored and applied for research purposes at a limited number of sites since the early 1980s, there had been little clinical impact. Since that time, CMR has come to the fore as an immensely powerful clinical technology. Key steps in that direction included the development of what are now definitive so-called “gold standard” methods for imaging MI, necrosis and viability, as well as replacement fibrosis in nonischemic disorders, based on the development of inversion-recovery late gadolinium enhancement imaging by Orlando P. Simonetti, PhD, and colleagues.
In addition, the development of high contrast bright-blood cine imaging using steady-state free precession sequences has made CMR the leading method for evaluation of abnormalities of cardiac chamber size and function. Applications of multiple methods for CMR strain, including tagging, feature tracking, displacement encoding with stimulated echoes/strain encoding and tissue velocity mapping, have become key research methods, whereas tagging and feature tracking are widely available enough to play a significant clinical role.
Quantitative myocardial perfusion with CMR is now as definitive as PET perfusion imaging, but with higher spatial resolution. It has been shown in a recent study to provide quantitative stress perfusion data competitive with invasive fractional flow reserve for optimization of outcomes in making decisions on whether to invasively treat coronary stenoses. The combination of T1 and T2 mapping, early and late post-gadolinium contrast enhancement, and strain imaging provides a powerful armamentarium for detection of myocardial tissue abnormalities due to ischemia, infarction, fibrosis, inflammation and infiltrative myocardial diseases. In particular, it has had high impact in improving detection of acute myocarditis, myocardial sarcoidosis and cardiac amyloidosis.
CMR faces a number of challenges today. There is a pressing need to reduce the cost and increase the speed of CMR studies, which is being addressed with development of volumetric imaging using sparse data sets. These approaches have the potential to reduce imaging time to 20 minutes or less. Another issue, especially in the United States, is expanding availability of CMR. Although some issues are economic, availability is also limited by widespread turf issues between radiologists and cardiologists. Unless solved, these will continue to reduce appropriate utilization of CMR and weaken patient care.
Cardiac CT
By 1997, only coronary calcium scoring (CAC) had emerged as a promising application of cardiac CT. Since that time, a number of large-scale long-term studies, including the St. Francis Heart Study, have documented the powerful independent prognostic utility of CAC in assessing future risk for adverse atherosclerotic events.
However, the clinical impact of cardiac CTA is even greater than that of CAC. Cardiac CTA surfaced in the mid-1990s with multiple electron-beam CT studies at the University of Erlangen, Germany. Stephan Achenbach, MD, first author of most of the work, went on to be a key leader of the field. In the early 2000s, the limitations of electron-beam CT technology were laid to rest with emergence of 64-slice helical CT, which led to major advances in image quality. More recently, we have entered an era of volume CT imaging and multifrequency image reconstruction on scanners capable of high speed, low radiation dose imaging and as many as 640 CT slices from a single acquisition.
Contemporary cardiac CTA morphologic imaging results generally compare very favorably to invasive angiographic results, save in extremely calcified lesions, and permit quantitation and tissue characterization of noncalcified as well as calcified plaque. Current imaging and post-processing are capable of identifying vulnerable but nonstenotic plaque as well. Even more powerful have been the emergence of both CT stress perfusion imaging and, especially, determination of FFR using coronary CT, based on mathematical modeling of flow. Thus, cardiac CTA is becoming positioned as the most powerful noninvasive diagnostic and gatekeeping method for assessment of coronary atherosclerosis. It excels at determination of which patients, other than those with evident acute MI, are candidates for PCI or CABG. Since cardiac CTA with CT-derived FFR reduces both false-negatives and false-positives encountered using SPECT or echocardiographic stress testing, the beneficial impact on patient care and health care economics promises to be very substantial. The availability of cardiac CTA in the United States is better by far than that of CMR, but there is still a need for enhanced collaboration between cardiologists and radiologists at many sites.
Extraordinary growth
Overall, the era since the start of Cardiology Today has been one of extraordinarily rapid growth in the power and utility of noninvasive cardiac imaging in ways that have made major contributions to the most important advances in diagnosis and treatment of CVD. But the field is afflicted with issues of both overutilization and underutilization, as well as quality control and missteps, in efforts by insurers and regulators at assuring appropriate use. Technical and clinical research advances in the utility of imaging technologies have the capacity to continue at a rapid pace. One hopes that development of optimal availability, utilization, quality and economics can do so as well.
– Nathaniel Reichek, MD, FACC, FAHA
Cardiology Today Editorial Board Member
St. Francis Hospital – The Heart Center, Roslyn, New York
- References:
- Nagel E, et al. Featured Clinical Research I. Presented at: American College of Cardiology Scientific Session; March 17-19, 2017; Washington, D.C.
- Polonsky TS, et al. JAMA. 2010;doi:10.1001/jama.2010.461.
- Simonetti OP, et al. Radiology. 2001;doi:10.1148/radiology.218.1.r01ja50215.
Disclosure: Reichek reports no relevant financial disclosures.