Dual-energy CT of the heart—Principles and protocols

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Abstract

The introduction of coronary CT angiography (cCTA) has reinvigorated the debate whether management of patients with suspected coronary artery disease (CAD) should be primarily based on physiological versus anatomical testing. Anatomical testing (i.e., cCTA or invasive catheterization) enables direct visualization and grading of coronary artery stenoses but has shortcomings for gauging the hemodynamic significance of lesions for myocardial perfusion. Conversely, rest/stress myocardial perfusion imaging (MPI) has been extensively validated for assessing the clinical significance of CAD by demonstrating fixed or reversible perfusion defects but has only limited anatomical information. There is early evidence that contrast medium enhanced dual-energy cCTA (DECT) has potential for the comprehensive analysis of coronary artery morphology as well as changes in myocardial perfusion. DECT exploits the fact that tissues in the human body and iodine-based contrast media have unique absorption characteristics when penetrated with different X-ray energy levels, which enables mapping the iodine (and thus blood) distribution within the myocardium. The purpose of this communication is to describe the practical application of this emerging technology for the comprehensive diagnosis of coronary artery disease in the context of the currently used tomographic imaging modalities (cCTA, nuclear MPI, MR MPI).

Introduction

Coronary artery disease (CAD) remains one of the most important causes of morbidity and mortality in western societies [1], [2]. Since the introduction of 201Thallium (201Tl) myocardial perfusion imaging as an adjunct to ECG ergometric studies in the mid-1970s [3], [4], functional cardiac imaging plays an increasing role in the diagnosis and management of CAD. Existing imaging modalities for the diagnosis of CAD can be classified according to the mechanism underlying the detection of coronary artery stenoses: Functional imaging modalities rely on the visualization of myocardial hypoperfusion or related regional wall motion abnormalities and comprise stress echocardiography, nuclear myocardial perfusion tests (201Thallium-, 99m-Tc-Sestamibi-SPECT and 18FDG-PET imaging) and the more recently introduced MRI myocardial perfusion imaging. Coronary CT angiography (cCTA) and invasive coronary angiography, on the other hand, allow direct visualization of the coronary artery lumen and are thus categorized as anatomical imaging modalities (Table 1). The latter are generally considered to be more sensitive for the detection of CAD, however, it proved surprisingly difficult to predict hemodynamic significance of stenoses based on anatomical data. Recent research has demonstrated the potential of dual-energy CT (DECT) for the characterization of tissue composition and organ perfusion [5]. Early evidence supports the utility of DECT for comprehensive imaging of coronary artery disease: With a single contrast-enhanced exam DECT coronary angiography permits the assessment of coronary artery anatomy, the detection of coronary artery stenoses and the evaluation of myocardial perfusion, thus demonstrating the hemodynamic significance of non-invasively detected coronary artery disease [6].

Section snippets

Functional imaging modalities

Functional imaging modalities for the detection of CAD utilize the fact that stenoses become hemodynamically more relevant as overall coronary blood flow increases: The rationale of these imaging modalities is to demonstrate the hemodynamic effect of existing coronary artery stenoses on myocardial perfusion and function, which can be accentuated using ergometric exertion or with the administration of pharmacological agents simulating vasodilation (adenosine or dipyridamole) or exercise

Combined functional and anatomical imaging of the heart

Ongoing scientific efforts concentrate on defining the best imaging workup for CAD; currently, no established modality combines reliable functional and anatomical information in a straightforward manner.

Stress echocardiography requires an adequate acoustic window, findings are largely subjective and observer-dependent, and this technique is exceedingly limited for the assessment of the coronary arteries in the adult population.

Since nuclear perfusion imaging detects myocardial hypoperfusion,

Dual energy CT for the diagnosis of CAD: protocols

It has been recognized early on that the penetration of human tissues with X-rays of different energy spectra permits the extraction of information beyond traditional attenuation values and thus facilitates a more detailed characterization of tissues. Initial experiments applying this technique to computed tomography have been performed more than two decades ago [44], [45], [46] and have shown that by dual energy scanning the average atomic number of each voxel can be estimated [5], [47]. The

Dual-energy CT post-processing, display, and analysis

The reconstructed data sets are further analyzed using a dedicated analysis platform (Syngo-Multi-Modality Workplace, Siemens). Coronary artery morphology and stenoses can be assessed based on the merged images of the high-voltage and low-voltage data sets. These can be interpreted using all commercially available software solutions for the post-processing of coronary CTA data (e.g., Circulation, Siemens), for instance based on a combination of transverse sections and curved multi-planar

Dose considerations

In our recently reported initial experience [6], dual-energy CT applied a mean total effective radiation dose equivalent of 15.2 ± 2.7 mSv across the population (n = 35) for the comprehensive workup of CAD, including cCTA workup and myocardial perfusion imaging from a single scan. This is in ranges well comparable with the average effective radiation dose equivalent applied during single-energy 64 slice CT [40,59,60] and significantly less than the combination of morphological and functional tests

Dual energy CT for the diagnosis of CAD: performance

Our initial experiences [6] suggest that DECT perfusion analysis correlates very well with fixed perfusion defects (i.e., perfusion defects at rest) at 99m-Tc-Sestamibi-SPECT. Compared with SPECT we found 91% sensitivity and 91% specificity with 91% accuracy for detecting myocardial perfusion defects on a segmental basis [6]. Somewhat more surprising is our observation that without the administration of any pharmacological stressors DECT can detect most of the reversible perfusion defects that

Conclusion

As with the introduction of every new imaging modality, large evidence-based trials are needed to determine its accuracy in a real-life context. It will be interesting to evaluate the impact of this method for comprehensive CAD work-up on the diagnostic and therapeutic efficacy in clinical routine and – finally – on health outcomes and costs. Our initial experience supports the notion that DECT has the potential to reliably assess the luminal integrity of the coronary artery system, myocardial

Conflict of interest

U.J.S. is a medical consultant to Bayer-Schering, Bracco, General Electric, Medrad, Siemens, and TeraRecon and receives research support from Bayer-Schering, Bracco, General Electric, Medrad, and Siemens. S.V. and B.S. are employees of Siemens Medical Solutions. P.C. is a medical consultant to Bracco and receives research support from Siemens. P.L.Z. receives research support from Boehringer-Ingelheim, Bracco, Bristol Myers Squib, and Siemens.

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