Additional value of transluminal attenuation gradient in CT angiography to predict hemodynamic significance of coronary artery stenosis

Abstract

Objectives: The current study evaluates the incremental value of transluminal attenuation gradient (TAG), TAG with corrected contrast opacification (CCO), and TAG with exclusion of calcified coronary segments (ExC) over coronary computed tomography angiogram (CTA) alone using fractional flow reserve (FFR) as the gold standard.

Background: TAG is defined as the contrast opacification gradient along the length of a coronary artery on a coronary CTA. Preliminary data suggest that TAG provides additional functional information. Interpretation of TAG is hampered by multiple heartbeat acquisition algorithms and coronary calcifications. Two correction models have been proposed based on either dephasing of contrast delivery by relating coronary density to corresponding descending aortic opacification (TAG-CCO) or excluding calcified coronary segments (TAG-ExC).

Methods: Eighty-five patients with intermediate probability of coronary artery disease were prospectively included. All patients underwent step-and-shoot 256-slice coronary CTA. TAG, TAG-CCO, and TAG-ExC analyses were performed followed by invasive coronary angiography in conjunction with FFR measurements of all major coronary branches.

Results: Thirty-four patients (40%) were diagnosed with hemodynamically-significant coronary artery disease (i.e., FFR ≤0.80). On a per-vessel basis (n = 253), 59 lesions (23%) were graded as hemodynamically significant, and the diagnostic accuracy of coronary CTA (diameter stenosis ≥50%) was 95%, 75%, 98%, and 54% for sensitivity, specificity, negative predictive value, and positive predictive value, respectively. TAG and TAG-ExC did not discriminate between vessels with or without hemodynamically significant lesions (-13.5 ± 17.1 HU [Hounsfield units] × 10 mm(-1) vs. -11.6 ± 13.3 HU × 10 mm(-1), p = 0.36; and 13.1 ± 15.9 HU × 10 mm(-1) vs. -11.4 ± 11.7 HU × 10 mm(-1), p = 0.77, respectively). TAG-CCO was lower in vessels with a hemodynamically-significant lesion (-0.050 ± 0.051 10 mm(-1) vs. -0.036 ± 0.034 10 mm(-1), p = 0.03) and TAG-ExC resulted in a slight improvement of the net reclassification index (0.021, p < 0.05).

Conclusions: TAG did not provide incremental diagnostic value over 256-slice coronary CTA alone in assessing the hemodynamic consequences of a coronary stenosis. Correction for temporal nonuniformity of contrast delivery or exclusion of calcified coronary segments slightly enhanced the results.

Keywords: coronary artery disease; coronary computed tomography angiography; fractional flow reserve.

Figure 1. Example of TAG Analysis of a LAD With a Hemodynamically-Significant Stenosis

Coronary computed tomography angiography (CTA) (A and C) and cross-sectional images with the region of interest indicated by the green circle (D), invasive coronary angiography with FFR (B), and results of 3 transluminal attenuation gradient (TAG) analyses (E). Note the decrease of contrast density along the coronary artery, which is compatible with the presence of an obstructive lesion in the left anterior descending coronary artery (LAD) with a noncalcified hemodynamically-significant stenosis (fractional flow reserve [FFR] = 0.43). CCO = corrected contrast opacification; ExC = excluding calcified coronary segments; HU = Hounsfield units; HUa = aortic Hounsfield units; HUc = coronary Hounsfield units.

Figure 2. Example of TAG Analysis of a LAD Without Stenosis

Coronary CTA (A and C) and cross-sectional images with the region of interest indicated by the green circle (D), invasive coronary angiography with FFR (B), and TAG analyses (E) of a normal left anterior descending coronary artery without stenosis (FFR = 0.93). Note that TAG values are in the same order of magnitude as the patient with an ischemic lesion as depicted in Figure 1. Abbreviations as in Figure 1.

Figure 3. Scatter Plot Showing the Relations Among TAG, TAG-CCO, and TAG-ExC

Correlations of TAG, TAG-CCO, and TAG-ExC were highly significant (p < 0.001 for all correlations). Abbreviations as in Figure 1.

Figure 4. Diagnostic Performance of Coronary CTA Alone and With TAG, TAG-ExC, and TAG-CCO

Receiver-operating characteristic (ROC) curve analysis with corresponding area under the curve (AUC) and 95% confidence interval (CI) displaying the diagnostic performance of coronary CTA, coronary CTA and TAG, coronary CTA and TAG-ExC, and coronary CTA and TAG-CCO for the detection of hemodynamically-significant coronary artery disease as indicated by FFR. Abbreviations as in Figure 1.

Figure 5. Example of TAG Analysis of a Highly-Calcified RCA

Coronary CTA (A and C) and cross-sectional images with the region of interest indicated by the green circle (D), invasive coronary angiography with FFR (B), and TAG analyses (E) of a highly calcified right coronary artery (RCA) with a hemodynamically significant stenosis (FFR = 0.27). Note that TAG analyses were comparable and omission of calcified segments did not alter the results. In this patient, TAG did not show a decline in contrast density along the artery despite a severely compromised hyperemic distal pressure. Abbreviations as Figure 1.