Author + information
- Nathaniel Reichek, MD⁎ ()
- ↵⁎Reprint requests and correspondence:
Dr. Nathaniel Reichek, Cardiac Imaging Program and Research Department, St. Francis Hospital, 100 Port Washington Boulevard, Roslyn, New York 11568
- coronary artery disease
- high-dose dobutamine stress testing
- myocardial strain rate reserve
- strain-encoded magnetic resonance imaging
The hunt for optimal approaches to assessment of myocardial performance has been longstanding. However, assessment of segmental myocardial function lagged for decades and was largely qualitative and subjective, on the basis of endocardial excursion or wall thickening, not quantitative myocardial deformation. As the era of revascularization began, the need for better assessment of segmental myocardial function stood out. In experimental models, it was simple: implanted sonomicrometry crystal pairs and arrays could depict segmental deformation. In humans, it was more challenging. Ultimately, in the late 1980s, cardiac magnetic resonance (CMR) with saturation band tissue tagging emerged, first at Johns Hopkins, then at the University of Pennsylvania, enabling evaluation of segmental deformation of the myocardium noninvasively in humans (1,2). Research applications emerged rapidly, as did more advanced 2-dimensional and 3-dimensional analytic approaches (3–6). However, limited availability of CMR, the need for both conventional cine and tagged imaging in each patient, limited temporal resolution, and lengthy post-processing made real clinical applications impractical. In the late 1990s, tissue Doppler and then speckle tracking led to rediscovery of strain by echocardiologists, making clinical applications feasible. But despite the wide availability of echocardiography, progress toward wide clinical adoption has been relatively slow. The first obvious target, assessment of asynchrony and biventricular pacing, has proved more challenging and the results more controversial than anticipated. The limited reproducibility, angle dependence, and limited transthoracic sampling that complicate tissue Doppler have been part of the problem. Speckle tracking avoids some of these limitations, but creates additional challenges. Given the limitations of both CMR and echocardiographic strain and strain rate imaging, a skeptic might conclude that, like too many other innovations in cardiac imaging, these are not destined for prime time. However, mastery of technical challenges takes time, and the drive to achieve practical, high-impact clinical tools based on strain and strain rate imaging is far from over.
In this issue of the Journal, Korosoglou et al. (7) from Heidelberg, Germany, using methods developed at Johns Hopkins, report striking data demonstrating that CMR dobutamine stress with strain and strain rate imaging provides improved detection of coronary stenoses, with an increase from 84% to 96% in sensitivity without loss of specificity and improved long-term risk stratification of patients, with an increase in chi-square from 39.3 for combined clinical variables and wall motion abnormalities to 50.7 for segmental strain maps and 52.5 for segmental strain rate reserve (both p < 0.001). This is actually the third paper in a series that previously demonstrated both detection of ischemia at lower dobutamine doses and improved diagnostic performance with CMR strain and strain rate imaging. They conclude appropriately that “Strain-encoded MRI aids the accurate identification of patients at high risk for future cardiac events and revascularization procedures, beyond the assessment of conventional atherogenic risk factors and resting or inducible [wall motion abnormality] on cine images.” As important, the imaging technique used, strain-encoded magnetic resonance imaging (SENC), permits rapid, objective, and operator-independent image post-processing methods, and key prognostic information is immediately available as color maps.
Thus, the approach is suitable for widespread clinical use where expert CMR is available. Regrettably, while the impact of this approach will be high in many European centers, slow diffusion of CMR into routine practice in the United States, where it is entrapped in turf battles and economic pressures, is likely to limit access to these very promising methods to a handful of academic centers. Nonetheless, this report dramatically advances CMR strain imaging with stress and clearly demonstrates substantial potential clinical value. The results appear to be more robust than those currently available for echocardiographic strain imaging with stress, whether using tissue Doppler or speckle tracking techniques. While further prospective testing in larger and more diverse multicenter studies is highly desirable, the basic response to this study should certainly be “point proven.”
It has been a long time coming. The paucity of real clinical applications for CMR tissue tagging >20 years after its emergence has been due in part to technical challenges, but also to hesitancy by industry to devote substantial resources to the challenges of CMR given the initial modest size of the market for CMR imaging and post-processing software. The contribution of the turf wars between radiology and cardiology in the United States and elsewhere over CMR to the slow development of the field cannot be overestimated. In addition, since the technical development needed to advance the field has often been rooted in the proprietary domains of major technology corporations, National Institutes of Health funding mechanisms may not have contributed as much to advancement of the field as would have been desirable. Nonetheless, even the earliest studies of segmental myocardial deformation during progressive dobutamine infusion in normal humans offered intriguing insights (8), leading almost linearly to the current report (7).
Those early data pointed to the fact that circumferential shortening became maximal at very low doses of dobutamine, even before heart rate acceleration, suggesting that absolute strain would not be the ideal parameter for assessment of contractile state. In contrast, strain rate showed a linear relationship to dobutamine dose and, presumably, inotropic state over a wider range of dobutamine dose and heart rate. Such observations resonated with the wide recognition in the 1990s that imaging stress testing, despite >2 decades of development, was still not performing optimally, particularly in women. These factors set the stage for further efforts to make CMR strain analysis fast enough for use during stress testing. It took the emergence, from the academic world rather than industry, of the next major advance in CMR strain imaging and post-processing: harmonic phase MRI (HARP): HARP shortened data reduction and analysis time by a factor of 10, compared favorably for quantitation of strain with conventional tag analysis, and also demonstrated sensitivity to provocable ischemia in animal models (9). Evidence from animal models of provocable ischemia and myocardial viability then reconfirmed the longstanding expectation that strain analysis was more sensitive than conventional wall motion analysis in detection of impaired myocardial perfusion reserve. The emergence of new variants on tissue tagging such as SENC has also created new opportunities to advance the field.
Where do we go from here? Technical development continues. A promising alternative to tagging, CMR DENSE (displacement encoding with stimulated echos) imaging, may permit more progress. Echo speckle tracking has been adapted to conventional CMR cine images as “feature tracking” and gives results similar to those of HARP while eliminating the need for additional imaging. Ongoing market-driven advances in the echocardiography software will spill over to CMR, offering faster analysis and better visual displays. In addition, competition between echocardiography and CMR will continue. More definitive large-scale multicenter trials using both technologies are needed to evaluate their relative utility and cost effectiveness. Lastly, economic pressures in healthcare threaten to prevent wide clinical application of advanced echocardiography and, especially, CMR strain methods unless large, compelling multicenter trials showing clinical and cost effectiveness can persuade the health policy and Center for Medicare Services constituencies of real value. Perfection of strain imaging will not come easily. But then, to borrow a famous phrase, cardiac imaging ain't for sissies.
Dr. Reichek has reported that he has no relationships relevant to the contents of this paper to disclose.
↵⁎ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
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