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Measuring the contrast gradient across an artery in addition to the standard anatomic evaluation via computed tomographic angiography (CTA) may provide an acceptable noninvasive alternative to fractional flow reserve (FFR) for identifying functionally significant coronary stenoses, according to a study published online February 13, 2013, ahead of print in the Journal of the American College of Cardiology.

The novel technique utilizes 320-detector row computed tomography (Aquilion ONE, Toshiba Medical Systems, Tokyo, Japan), which enables single-beat imaging of the entire coronary tree, to determine the transluminal attenuation gradient, or TAG320 for short.

Sujith K. Seneviratne, MBBS, of Monash Medical Centre (Clayton, Australia), and colleagues analyzed 54 stable patients (78 vessels) who underwent CTA with TAG320 and FFR assessment within 2 months.

Thirty vessels (38%) in the overall cohort met the threshold for functionally significant stenosis (FFR ≤ 0.80). In a bootstrap analysis using a retrospectively determined cutoff, TAG320 predicted significant lesions with a sensitivity of 77%, specificity of 74%, positive predictive value of 67%, and negative predictive value of 83%.

The combined assessment approach of TAG320 and CTA had an area under the curve of 0.88. In addition, 2 statistical tests found an incremental value for adding TAG320 to CTA for detection of significant FFR (Wald test P = 0.0001; integrated discrimination improvement index P = 0.002). The predictive value of TAG320 and CTA was independent of the presence of coronary calcification (P < 0.001 for both TAG320 and CTA).

TAG320 was highly reproducible and outperformed another, simpler measurement approach called contrast opacification difference.

Combined Approach May Work

“The current study has demonstrated for the first time the diagnostic accuracy of TAG320 . . . on a 320-detector row CT compared with an invasive functional standard: FFR,” Dr. Seneviratne and colleagues write.

Prior studies have “highlighted the high sensitivity and negative predictive value of CTA” but showed the imaging method comes up short on specificity and positive predictive value, Dr. Seneviratne told TCTMD in an e-mail communication. “Our study finding implies that another component of CTA, which is the study of iodinated contrast across coronary artery stenosis, has the potential to improve the diagnostic accuracy of cardiac CT for the assessment of coronary artery stenosis.”

Using this index “provides acceptable prediction of invasive FFR and may provide a noninvasive modality for detecting functionally significant coronary stenoses,” the authors conclude, adding that larger studies are required to “determine the diagnostic and prognostic value” of the combined approach.

Despite ‘Optimistic’ Results, Questions Abound

In an accompanying editorial, Andrew J. Einstein, MD, PhD, of Columbia University Medical Center (New York, NY), writes that the results are more “optimistic” than what has been found in prior studies, and “further [support] the potential complementary roles of anatomic and physiologic information obtained from [coronary CTA] scans.”

But “it remains to be seen whether TAG still significantly complements the degree of stenosis under [real-world] conditions,” he observes.

Also skeptical of the clinical applicability of TAG, James K. Min, MD, of Cedars-Sinai Medical Center (Los Angeles, CA), told TCTMD in a telephone interview that the strategy is “an interesting method for evaluation of patients who may have significant coronary artery disease. The relatively low sensitivity and specificity of TAG for the diagnosis of vessel-specific ischemia by an invasive FFR reference standard is relatively unsurprising,” he commented, explaining that “invasive FFR measures the hyperemic response to adenosine, while TAG assesses the contrast gradient at rest.”

But for this method to “be more clinically relevant, it should probably be evaluated in the setting of coronary hyperemia. Thus, whether it can be routinely applied in widespread clinical practice remains to be seen, and future large-scale studies will be necessary to improve upon these findings,” Dr. Min suggested. He added that the relative value of TAG compared with other novel noninvasive methods for physiologic assessment of lesions, such as FFR CT, also deserves further evaluation.

Dr. Einstein also proposes developing and validating “more automated image processing software” for a more “practical implementation of TAG. . . . It would not be difficult to integrate such software with curved multiplanar reconstruction that is now standard on workstations used to read [coronary CTA] scans, and thus to routinely determine TAG, at no additional cost, while reading a [coronary CTA] scan,” he writes.

Sources:

1. Wong DTL, Ko BS, Cameron JD, et al. Transluminal attenuation gradient in coronary computed tomography angiography is a novel noninvasive approach to the identification of functionally significant coronary artery stenosis: A comparison with fractional flow reserve. J Am Coll Cardiol. 2013;Epub ahead of print.
2. Einstein AJ. TAG—Is it it? Improving coronary computed tomography angiography with the isotemporal transluminal contrast attenuation gradient. J Am Coll Cardiol. 2013;Epub ahead of print.

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