Andreas H Mahnken Department of Diagnostic Radiology, Rheinisch Westfälische Technische Hochschule-Aachen University, Aachen; , Joachim E Wildberger Department of Diagnostic Radiology, Rheinisch Westfälische Technische Hochschule-Aachen University, Aachen; , Peter R Seidensticker Bayer Schering Pharma AG, Berlin

Originally printed in:
» European Cardiovascular Disease 2007 - Issue 1 - June 2007

Coronary artery disease (CAD) represents a relevant economic burden to modern medicine. In 2003 more than 1.41 million diagnostic coronary angiograms and an additional ¹.24 million percutaneous coronary angioplasties were performed in the US alone.₁ While coronary angiography is considered the gold standard for diagnosing CAD, several non-invasive techniques have been evaluated for assessing CAD. Over the last decade, multi-slice spiral computed tomography (MSCT) has emerged as a serious alternative for noninvasive cardiac imaging. The rapid technical development from 4-slice technique to dual-source computed tomography (DSCT) fuelled the quest to establish computed tomography (CT) as a routine method for coronary imaging. While 4-slice CT already allowed acceptable visualisation of the proximal coronary segments,² 16-slice CT permitted reliable exclusion of relevant coronary stenosis (>50%) with a negative predictive value (NPV) of 99%.³ The introduction of 64-slice CT with further improved spatial and temporal resolution was the breakthrough for routine implementation of coronary CTangiography. ⁴ Currently, DSCT is state-of-the-art in cardiac CT imaging. This new generation of CT scanners with two X-ray sources provides a temporal resolution as low as 42msec at submillimetre spatial resolution.5 Thus, DSCT outperforms the spatial and temporal resolution of electron beam CT. Within the first year after its clinical introduction, contrast-enhanced DSCT became a key technology for non-invasive imaging of CAD.

Technical Basics of DSCT

DSCT utilises two tube-detector systems that are mounted at orthogonal orientations in the gantry. The 180° data in parallel geometry are split into two sinograms, which are simultaneously acquired in the same phase of the cardiac cycle at the same anatomical level with a 90° angle between the detectors. The projections for reconstruction of one image slab can thereby be obtained in half the time needed by a single source CT. This results in a constant temporal resolution of 83msec when using data from a single RR-interval only. Avoiding the problems of multisegment image reconstruction algorithms, the constant high temporal resolution permits an efficient pitch adaptation, which allows a heart rate-dependent dose reduction. Detector design and spatial resolution are identical to the geometry of a 64-slice CT scanner.⁵

Coronary DSCT-angiography

Non-invasive coronary angiography is the most common indication for cardiac DSCT. Since 1998 multiple studies have demonstrated the value of contrast-enhanced MSCT-angiography for the assessment of coronary artery stenosis. Recent studies on 64-slice CT reported sensitivities and specificities in the range of 90–98% for the detection of coronary artery stenoses.^4–7 In all of these studies the positive predictive value (PPV) for the detection of relevant coronary artery stenoses was lower than the NPV, which is typically about 98%. Nevertheless, single-source CT is hampered by the limited, heart ratedependent temporal resolution. Consequently, rigorous patient selection is recommended and even with 64-slice CT, approximately 10% of coronary segments have to be excluded from analysis.

With the introduction of DSCT, the robustness of coronary CTangiography improved to a degree that allows even patients with heart rates ?80bpm to be examined at diagnostic image quality (see Figure 1). Several phantom and patient studies confirmed these improvements.^8–10 A first study comparing DSCT coronary angiography with invasive coronary angiograms for the detection of coronary artery stenosisWith the introduction of DSCT, the robustness of coronary CTangiography improved to a degree that allows even patients with heart rates ?80bpm to be examined at diagnostic image quality (see Figure 1). Several phantom and patient studies confirmed these improvements.^8–10 A first study comparing DSCT coronary angiography with invasive coronary angiograms for the detection of coronary artery stenosis in a high pre-test likelihood population confirmed these initial observations. With a sensitivity of 96.4% and a specificity of 97.5%, a NPV of 99.4% for the exclusion of coronary artery stenosis was achieved. Only 1.4% (6/420) of the coronary artery segments had to be excluded from analysis.¹¹ These results are remarkable as the first-time patients with irregular or elevated heart rates were not excluded from the study and no beta-blockers were administered. in a high pre-test likelihood population confirmed these initial observations. With a sensitivity of 96.4% and a specificity of 97.5%, a NPV of 99.4% for the exclusion of coronary artery stenosis was achieved. Only 1.4% (6/420) of the coronary artery segments had to be excluded from analysis.¹¹ These results are remarkable as the first-time patients with irregular or elevated heart rates were not excluded from the study and no beta-blockers were administered.

So far there are no data on the DSCT assessment of coronary artery bypass grafts (CABGs) or coronary artery stents. However, for both indications only minor improvements are to be expected. While singlesource CT already provides excellent results for the assessment of CABG, coronary stent imaging may benefit from the improved temporal resolution. Spatial resolution, which is the key to assessment of the stent lumen, equals that of 64-slice CT with 0.4x0.4x0.4mm3. Thus, the potential for improvement is limited.