Original Article
Transluminal attenuation gradient derived from coronary CT angiography to predict ischemia in SPECT myocardial perfusion imaging: Effect of coronary cross-sectional area

https://doi.org/10.1007/s12350-020-02242-wGet rights and content

Abstract

Background

Coronary computed tomography angiography (CCTA)-based transluminal attenuation gradient (TAG) was suggested to determine the functional significance of a stenosis. However, evidence that TAG acquired by wide-volume scanners can assess the hemodynamic significance of stenosis assessed by single-photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI) is lacking. Moreover, coronary cross-sectional area may influence TAG. Hence, we aimed at assessing the diagnostic value of TAG to predict ischemia in SPECT-MPI and the correlation between TAG and the transluminal cross-sectional area gradient (TCG).

Methods

Patients undergoing CCTA and SPECT-MPI for suspected coronary artery disease were included. TAG and TCG were calculated measuring the mean vessel attenuation and the cross-sectional area along major coronary vessels at 5-mm intervals.

Results

A total of 255 coronary arteries of 87 patients were included. TAG and TCG did not discriminate between coronary arteries with or without ischemia as assessed by SPECT-MPI (p = .44 and p = .25, respectively). The area under the curve to predict ischemia was not increased by adding TAG (0.88, 95% CI 0.83-0.92) or TCG (0.87, 95% CI 0.81-0.90) to CCTA alone (0.85, 95% CI 0.80-0.89). There was a significant correlation between TAG and TCG (r = 0.43; p < .001).

Conclusions

CCTA-derived TAG and TCG do not offer any value in predicting ischemia assessed by SPECT-MPI. TAG is partly affected by differences in the coronary luminal area.

Introduction

Coronary computed tomography angiography (CCTA) is an increasingly available noninvasive method in patients with suspected coronary artery disease (CAD).1 While CCTA is an excellent tool for exclusion of CAD due to its high sensitivity and negative predictive value, it performs moderately in predicting the hemodynamic relevance of a lesion.2 Evidence of hemodynamic relevance of a stenosis is recommended before revascularization procedure to improve outcome in patients with chronic coronary syndromes.3, 4, 5, 6

As a consequence, a functional test is usually performed following CCTA to guide patient management. Cardiac hybrid imaging is a modality that can assess coronary anatomy and functional information at the same time. It combines CCTA and single-photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI), generating added diagnostic and prognostic value.7,8 However, the addition of two methods increases radiation exposure and cost. Hence, in an attempt to assess functional and anatomic information from one single noninvasive imaging modality, some CT based techniques have been proposed, such as CT-derived fractional flow reserve (CT-FFR), CT perfusion imaging, and transluminal attenuation gradient (TAG). While CT-FFR is a method which has been shown to add diagnostic value over CCTA alone,9,10 the value of TAG was discussed more controversially. The concept of TAG is based on the assumption that attenuation along a hemodynamically restricted vessel decreases more than along a regular vessel. Initial trials showed that TAG might have incremental predictive value over CCTA alone for detecting functionally significant coronary artery stenosis,11, 12, 13 but later studies failed to confirm these initial findings.14,15 Although the concept of TAG was initially introduced in a study performed on a wide-volume CT scanner providing single heartbeat coverage,16 the vast majority of the subsequent studies were realized on CT scanners with retrospective electrocardiography (ECG) gating or step-and-shoot technique were the z-axis of the heart is covered over multiple heartbeats.

Moreover, it has been suggested that TAG may be influenced not only by coronary blood flow but also by other parameters such as the coronary vessel length and diameter, and scan timing after contrast media injection.15,17, 18, 19, 20 It has been proposed that the decrease of coronary cross-sectional area or luminal diameter along the course of a coronary vessel, previously expressed as transluminal diameter gradient (TDG), correlates with TAG, which possibly explains the impaired diagnostic accuracy of TAG.14,15,20 However, studies evaluating the diagnostic performance of TAG and TDG compared to SPECT-MPI are scarce.

The aim of the present study was to determine the diagnostic accuracy of TAG and transluminal cross-sectional area gradient (TCG) assessed using a latest generation 256-slice CT scanner with single heartbeat coverage to predict ischemia as defined by SPECT-MPI. Additionally, the relation between TAG and TCG, as well as the differences of these gradients concerning stenosis degree assessed by CCTA was analyzed.

Section snippets

Study Population

The present study is a retrospective study. We included eighty-seven consecutive patients who were referred for hybrid CCTA/SPECT-MPI because of suspected CAD. Exclusion criteria were history of coronary artery bypass graft or coronary artery stenting. The study protocol was approved by the institutional review board (local ethics committee) and written informed consent obtained from all patients.

CCTA Acquisition and Interpretation

Patients underwent contrast-enhanced CCTA with prospective ECG triggering on a stand-alone

Study Population

The baseline characteristics of the study population (n = 87) are summarized in Table 1. Out of 261 major vessels, TAG and TCG were successfully assessed in 255 major vessels (98%). Reasons for exclusion were motion artifacts (n = 4) and myocardial bridging (n = 2). The stress test for SPECT-MPI was either pharmacological with adenosine (n = 72) and dobutamine (n = 9) or by bicyle exercise (n = 6). None of the patients showed signs of ischemia in the stress-ECG.

CCTA and SPECT Findings

CCTA showed obstructive CAD in 64

Discussion

The present study demonstrates that neither the assessment of TAG nor TCG derived from a latest generation 256-slice CT scanner with a single heartbeat coverage does improve the diagnostic value of CCTA based diameter stenosis alone for the presence of ischemia as defined by myocardial SPECT-MPI. Moreover, a moderate but significant correlation of TAG and TCG was found, which indicates that TAG is at least partly dependent on the cross-sectional area. To the best of our knowledge, the present

Conclusion

CCTA-derived TAG and TCG do not offer any value in predicting ischemia as assessed with SPECT-MPI, despite the application of a latest generation 256-slice CT scanner offering single heartbeat coverage. Moreover, a moderate but significant correlation between TAG and TCG was seen, indicating that TAG may be at least partly caused by differences in luminal diameter.

New Knowledge Gained

TAG and TCG derived from a coronary CT scan using single heartbeat coverage do not add diagnostic value to visual CCTA analysis for detecting hemodynamically significant coronary stenosis as defined by SPECT-MPI. TAG and TCG correlated moderately, indicating that TAG is partly affected by differences in the coronary luminal area.

Disclosure

The University Hospital Zurich holds a research contract with GE Healthcare.

Funding

No funding sources.

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    Elia von Felten and Dominik C. Benz share first authorship.

    Philipp A. Kaufmann and Aju P. Pazhenkottil share last authorship.

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