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Current Evidence in Cardiothoracic Imaging

Computed Tomography–derived Fractional Flow Reserve in Stable Chest Pain

Schwartz, Fides R., MD*; Koweek, Lynne M., MD*; Nørgaard, Bjarne L., MD, PhD

doi: 10.1097/RTI.0000000000000369
Current Evidence in Cardiothoracic Imaging

High-accuracy diagnostic imaging is needed to diagnose and manage coronary artery disease as well as to allow risk stratification for future events. Advancements in multidetector computed tomography and image postprocessing allow for routine computed tomography coronary angiography to provide anatomic luminal assessment similar to invasive coronary angiography, and, similarly, computational fractional flow reserve derived from computed tomography facilitates determination of hemodynamically relevant stenosis comparable to invasive fractional flow reserve. In this review article, we describe the diagnostic performance and the potential impact of fractional flow reserve derived from computed tomography in clinical practice.

*Department of Radiology, Duke University Medical Center, Durham, NC

Department of Cardiology, Aarhus University Hospital Skejby, Aarhus, Denmark

B.L.N. and L.M.K. have received institutional research support from Edwards Lifesciences, Siemens, and HeartFlow.

The authors declare no conflicts of interest.

Correspondence to: Fides R. Schwartz, MD, Department of Radiology, Duke University Medical Center, P.O. Box 3808, Durham, NC 27710 (e-mail: fides.schwartz@duke.edu).

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KEY POINTS

  • In patients with stable chest pain, fractional flow reserve derived from computed tomography (FFRCT) has a high diagnostic performance when compared with invasive fractional flow reserve (FFR) without exposing patients to additional radiation doses or the risk of an invasive procedure.
  • Coronary computed tomography angiography (CCTA) with selective FFRCT testing may improve the management of patients with stable chest pain by identifying functionally relevant coronary disease in one noninvasive diagnostic imaging procedure.
  • Imaging strategies using FFRCT may have a positive economic profile compared with routine care.
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INTRODUCTION

During the last decade, CCTA has undergone tremendous scanner advances and clinical validation with improved image quality and significant reductions in radiation dose. Accordingly, noninvasive imagers are increasingly involved in the diagnosis and management of patients with suspected coronary artery disease (CAD). The diagnostic and prognostic value of CCTA has been documented in multiple studies.1,2 Its negative predictive value for detection of CAD is close to 100%, and CCTA is unique in its ability to detect and quantify atherosclerosis in both nonobstructive diffuse as well as obstructive CAD, providing clinicians with information on the risk that may better guide preventive and therapeutic interventions than routine care.3–5 Accordingly, recent meta-analyses, large-scale registry studies, and randomized trials have shown improved clinical outcomes in stable CAD following initial testing with CCTA when compared with noninvasive functional testing such as stress electrocardiography and radionuclide studies.4–10 Moreover, the US National Cardiovascular Data Registry demonstrated in >400,000 patients that only 45% were correctly identified as having >50% stenosis at invasive coronary angiography (ICA) following traditional functional testing while for CCTA the proportion was 70%.11 Vavalle et al12 demonstrated that, on the other end of the spectrum, 74% of patients with typical angina and a negative stress-test result subsequently had obstructive CAD on ICA.

Accordingly, the updated National Institute for Health and Care Excellence (NICE) stable chest pain guideline (CG95) recommends CCTA in lieu of functional testing as the first-line test strategy in patients without known CAD and atypical or classical angina.13,14 Advanced image analysis algorithms such as FFRCT further expand the imager’s role in the diagnosis and management of CAD by providing information on lesion-specific ischemia.

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HOW GOOD IS CCTA IN THE CLASSIFICATION OF CAD?

Coronary artery CTA has a reported high sensitivity (90%; range: 86% to 93%) and high negative predictive value (98.7%; 95% confidence interval, 97.9%-99.4%), but lower specificity and positive predictive value of 60% and 64%, respectively, for the diagnosis of an obstructive CAD.15 Major limitations are the inaccurate evaluation of vessel lumen in areas of severely calcified disease as well as motion artifacts hampering vessel definition. Thus, in general, CCTA overestimates stenosis severity. Moreover, similar to ICA, CCTA remains a strictly anatomic test and, as such, it is often discordant with measures of lesion-specific ischemia determined by invasive FFR, which is the established gold standard for decision-making in the catheterization laboratory.16,17 Even in seemingly clear-cut cases with low-grade or high-grade stenosis, anatomic imaging alone can be incorrect in characterizing the hemodynamic significance of lesions.18,19 Accordingly, when compared with traditional functional test modalities, CCTA is associated with increasing downstream ICA and revascularization utilization. To overcome these limitations, new diagnostic strategies for assessment of the physiological relevance of stenoses determined by CCTA have been proposed.

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WHAT IS FFRCT?

Traditionally, adjudication of lesion-specific ischemia is performed invasively by FFR, which assesses the ratio of flow across a stenosis during maximum vasodilatation. FFR is associated with the clinical outcome in a continuous manner and has been shown in multiple randomized trials to guide revascularization in a cost-effective manner.20,21 While FFR is a robust tool for the adjudication of the hemodynamic significance of a stenosis, it is limited by its invasiveness and costs; hence, in real-world practice, it is used for coronary revascularization decision-making in only a minority of patients.22 Recent advances in image postprocessing with the integration of computational fluid dynamics and quantitative anatomic and physiological modeling now enable simulation of patient-specific hemodynamic parameters including blood velocity, pressure, pressure gradients, and FFRCT from standard acquired CCTA data sets without the need for additional procedural planning, medication, or radiation.23 FFRCT testing requires offsite computer processing (HeartFlow, Redwood City, CA) requiring 2 to 6 hours.23–26 However, significantly faster FFRCT processing times resulting from software improvements are expected in the near future. Recently, there has been renewed interest in past generations of reduced order computational fluid modeling versions that are less computationally intense and, when coupled with less comprehensive anatomic modeling than for FFRCT, enable on-site analysis with reduced analysis times (<1 h). While noninvasive on-site CT-derived FFR has shown interesting results in small, single-center, retrospective studies,27–29 further investigations in prospective multicenter trials are needed in order to determine the actual diagnostic performance of these techniques.30

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HOW ACCURATE IS FFRCT IN THE ASSESSMENT OF CAD?

The diagnostic performance of FFRCT in patients with proven or suspected stable CAD has been tested in 3 prospective multicenter trials.24–26 A total of 609 patients and 1050 vessels have been investigated. An overview of the study design, populations, and estimates of FFRCT diagnostic performance in these 3 trials is presented in Table 1. Overall, FFRCT revealed both high per-patient and per-vessel discrimination for the presence of ischemia with blinded comparison with invasively measured FFR. Moreover, the diagnostic performance of FFRCT was superior when compared with anatomic interpretation alone. In the most recent trial (NXT), which used an updated FFRCT analysis software, a marked increase of 34% to 79% on a per-patient and 60% to 86% on a per-vessel basis was shown in diagnostic specificity when moving from stenosis (>50%) to physiological assessment by FFRCT (≤0.80).26 Thus, on a per-patient level, FFRCT correctly reclassified 68% of CCTA false positives to true negatives. Of note, in the NXT trial, >90% of the coronary lesions were in the intermediate range (30% to 70%), in which the discrepancy between anatomical and physiological assessment is most profound. Relevant patient examples are shown in Figure 1.

TABLE 1

TABLE 1

FIGURE 1

FIGURE 1

In the NXT trial, there was a good direct per-vessel FFRCT to FFR correlation (Pearson correlation coefficient 0.82).26 Moreover, it has been demonstrated that FFRCT has high diagnostic performance in the presence of coronary calcification. In an NXT trial substudy, there was no difference in diagnostic accuracy, sensitivity, or specificity of FFRCT across Agatston score quartiles, including the highest quartile of patients with Agatston scores ranging between 416 and 3599.31 No prospective head-to-head comparisons between the diagnostic performance of FFRCT and traditional noninvasive functional modalities against FFR have been published. In a recent meta-analysis by Danad and colleagues, FFRCT demonstrated higher sensitivity than stress echocardiography (90% vs. 77%), ICA (90% vs. 69%), and SPECT (90% vs. 70%), while showing equal sensitivity to cardiac MRI (both 90%). The specificity of FFRCT (71%) was similar to stress echocardiography (75%), ICA (74%), and SPECT (78%) but significantly lower than MRI (94%).15

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HOW DOES FFRCT IMPACT PATIENT MANAGEMENT DECISIONS?

The impact of noninvasive coronary imaging on patient management and outcome has implications for best practice use and appropriate resource investments.

Recently, we have seen FFRCT advance from diagnostic testing to the realm of clinical utility (Table 2). The prospective, multicenter PLATFORM trial examined the clinical impact of FFRCT to guide patient management compared with usual care.32–34 The primary endpoint was the rate of finding no obstructive stenosis (≥50%) among those with planned ICA. A total of 584 patients with new-onset chest pain, no prior history of CAD, and an intermediate pretest likelihood of obstructive CAD were enrolled. In patients scheduled for ICA, 73% of usual care patients had no obstructive CAD compared with only 12% of patients guided by FFRCT, an 83% reduction (P<0.0001). Although clinicians in the PLATFORM study were not protocol driven to utilize FFRCT test results, in 61% of patients with planned ICA, the angiogram was cancelled after receiving FFRCT results. In a recent prospective study by Jensen et al34 of 774 patients with stable chest pain, a diagnostic strategy of first-line CCTA with selective FFRCT testing in intermediate range CAD, ICA was deferred in 91% versus 75% in patients with low-intermediate and high pretest probability of CAD, respectively. In an observational analysis from real-world practice of 3500 patients with intermediate stenosis on CCTA by Nørgaard et al35, it was shown that replacing an adjunctive test strategy of myocardial perfusion imaging with FFRCT led to lower downstream ICA utilization, an improved diagnostic yield of ICA, and improved physiological guidance of revascularization. Moreover, the potential clinical utility of FFRCT has been evaluated in 2 retrospective studies. The RIPCORD-FFRCT study demonstrated that 30% of the subgroup of patients planned for coronary revascularization based on the CCTA result after FFRCT could be managed with medical treatment.36 In addition, the target vessels for coronary interventions were adjusted from the CCTA-based management plan in 18% of patients after FFRCT data were made available. In a substudy of the PROMISE cohort by Lu et al37 it was demonstrated that reserving ICA for patients with FFRCT≤0.80 would reduce the proportion of ICA procedures with the finding of no obstructive disease by 44%, and increase the proportion of ICA, leading to revascularization by 24%. Importantly, all data on the clinical utility of FFRCT have been derived from patients with stable chest pain. At this point, no data have been published to support the use of FFRCT in patients with acute coronary syndromes.38

TABLE 2

TABLE 2

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HOW DOES FFRCT IMPACT CLINICAL OUTCOMES AND COSTS?

In the PLATFORM trial, clinical outcomes for patients in whom ICA was deferred on the basis of FFRCT were favorable with a 1-year risk of all-cause death, myocardial infarction, or unplanned hospitalization with urgent revascularization of 1%.33 This finding is in accordance with a recent single-center real-world study of 110 consecutive patients with stable chest pain, moderate CAD, and deferral of ICA in those with FFRCT≥0.80, among whom no adverse cardiac events occurred over a median of 12 months’ follow-up.39 In the Lu PROMISE FFRCT substudy, “blinded” FFRCT≤0.80 was predictive of subsequent revascularization and major adverse cardiac events with a significantly higher hazard ratio than CCTA stenosis assessment alone (hazard ratio, 4.3 vs. 2.9; P=0.033).37 Historical cost simulation analyses indicate that FFRCT guidance for selection of ICA and decision-making on coronary revascularization may reduce costs in stable CAD.40,41 In the PLATFORM trial, among patients with planned ICA, the mean costs were 32% lower in the FFRCT group than in the usual care group.32 The ongoing prospective multicenter multinational longitudinal “Assessing Diagnostic value of Non-Invasive FFRCT in Coronary Care” (ADVANCE) registry will further delineate the clinical utility, prognostic aspects, and cost-effectiveness of FFRCT-guidance in 5000 patients with or without known CAD.42

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WHAT ARE THE CHALLENGES TO THE IMAGER?

Incorporation of cardiac CTA and FFRCT into routine clinical use requires these examinations to provide accurate means of classification and high confidence for interpretation. Significant CT imaging artifacts such as motion, low contrast, or blooming may impair the diagnostic performance of CCTA and thus of FFRCT. However, these issues can be minimized by adhering to CCTA image acquisition guidelines, particularly by administration of heart-rate–lowering medication and sublingual nitrates before image acquisition.43,44 In recent studies from real-world practice, it was demonstrated that a conclusive FFRCT result was obtainable in >95% of consecutive stable chest pain patients with intermediate range lesions.35,39

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WHAT IS THE FUTURE?

The impact of FFRCT for next-step patient management is evolving. Standardized criteria on the clinical utility and interpretation of FFRCT have not been published. However, at this point, FFRCT testing seems to be a valuable gatekeeper to the catheterization laboratory for physiological assessment of intermediate-range lesions determined by CCTA in patients with stable chest pain. Currently, preservation of 80% of flow across a lesion is considered not flow limiting for both FFRCT and FFR. Luminal narrowing associated with a drop of flow between 75% and 80% is considered indeterminate for lesion-specific ischemia.39 Further understanding of the downstream management of patients following FFRCT testing as well as the impact of other CT-derived measures associated with flow obstruction such as coronary plaque volume and characteristics19 and ratio of blood volume to myocardial mass45 are yet to be determined. Moreover, further studies are needed to assess the relative cost-efficiency in the clinical practice of CCTA-FFRCT versus conventional noninvasive functional testing.

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CONCLUSIONS

FFRCT is a novel noninvasive method using computational fluid dynamics for the calculation of FFR from patient-specific modeling derived from CCTA data sets. During the last 5 years, FFRCT has undergone remarkable advancements in technology and clinical validation, challenging conventional ischemia testing as the adjunctive test strategy in patients with moderate CAD determined by CCTA. These trends in combination with continued technical developments and emerging data supporting the clinical utility and safety of FFRCT will move imagers to the forefront of early anatomic and physiological assessment of CAD for an ever-growing patient population.

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Keywords:

coronary computed tomography angiography; fractional flow reserve; computational fluid dynamics; stable chest pain; coronary artery disease; noninvasive cardiac imaging; noninvasive diagnostic testing

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