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.

Diagnostic performance of transluminal attenuation gradient and fractional flow reserve by coronary computed tomographic angiography (FFRCT) compared to invasive FFR: a sub-group analysis from the DISCOVER-FLOW and DeFACTO studies

“…Therefore, abnormality by TAG is likely to be associated with severe anatomic stenosis [24] as well as limited coronary flow [23], and TAG may fail to detect stenosis that limits flow as assessed by invasive FFR during pharmacologic stress but does not reduce resting coronary flow. This underlying mechanism may also explain why the combination of TAG and CCTA stenosis did not provide a greater diagnostic performance over CCTA stenosis alone in the current study and the aforementioned study [20].…”

“…TAG is a novel method developed to assess the hemodynamic significance of CAD and is calculated using a linear regression model of luminal contrast attenuations in a coronary vessel obtained by manual or semi-automated techniques [9,10,[19][20][21][22]. To date, several studies have examined the relation of TAG to invasive FFR and have shown a variety in the diagnostic accuracy of TAG in predicting lesion-specific ischemia by invasive FFR \0.80.…”

“…The discordance of the study by Wong et al and the results of our study as well as Yoon and Choi's study may be explained by variations in contrast along vessels using scanners that require sequential heart cycles for image acquisition, which does not occur with the 320 slice CT scanner that covers the whole heart within one heartbeat. For solving the issue in assessing TAG by limited CT slices, Stuijfzand et al [20] prospectively examined the comparative utility of TAG and TAG with corrected coronary opacification (CCO) (TAG-CCO), which potentially corrects the temporal contrast variations in a coronary artery between scans, among 85 patients undergoing 256 slice CCTA [23]. However, the diagnostic performance of TAG-CCO did not differ from that of CCTA stenosis alone (AUC: 0.88 vs. 0.85).…”

“…However, the diagnostic performance of TAG-CCO did not differ from that of CCTA stenosis alone (AUC: 0.88 vs. 0.85). Another potential issue may be that an optimal cutoff of TAG has not yet been established and the range of thresholds of TAG to predict lesion-specific ischemia has varied widely, ranging from -0.654 HU/mm (64 slice) to -15.1 HU/10 mm (320 slice) [9,10,20]. Although optimal thresholds for TAG were derived from ROC curve analysis among their limited cohort in these previous studies, the cut-off of TAG including various CT slices derived from ROC curve analysis was -1.1 HU/mm in our study, which is in line between these ranges.…”

Fractional Flow Reserve and Coronary Computed Tomographic Angiography

“…69 A subsequent comparison with invasive FFR revealed no incremental value for TAG (69% sensitivity, 44% specificity, 27% PPV, and 83% NPV) over CTA alone (95% sensitivity, 75% specificity, 54% PPV, and 98% NPV) in 253 vessels in 85 patients; the addition of CCO and exclusion of calcified segments only slightly improved the results. 70 Finally, in a 3 part study of phantoms, dogs and 152 vessels in 62 patients undergoing ICA, no significant TAG differences were noted between stenotic and normal vessels. Proximal to distal decreases in attenuation were a function of vessel diameter rather than stenosis.…”