Original Research Article
Influence of slice thickness and reconstruction kernel on the computed tomographic attenuation of coronary atherosclerotic plaque

https://doi.org/10.1016/j.jcct.2010.01.013Get rights and content

Background

The computed tomographic (CT) attenuation of coronary atherosclerotic plaque has been proposed as a marker for tissue characterization and may thus potentially contribute to the assessment of plaque instability.

Objective

We analyzed the influence of reconstruction parameters on CT attenuation measured within noncalcified coronary atherosclerotic lesions.

Methods

Seventy-two patients were studied by contrast-enhanced dual-source CT coronary angiography (330 millisecond rotation time, 2 × 64 × 0.6 mm collimation, 120 kV, 400 mAs, 80 mL contrast agent intravenously at 6 mL/s), and a total of 100 distinct noncalcified coronary atherosclerotic plaques were identified. Image data sets were reconstructed with a soft (B20f), medium soft (B26f), and sharp (B46f) reconstruction kernel. With the medium soft kernel, image data sets were reconstructed with a slice thickness/increment of 0.6/0.3 mm, 0.75/0.4 mm, and 1.0/0.5mm. Within each plaque, CT attenuation was measured.

Results

Mean CT attenuation using the medium soft kernel was 109 ± 58 HU (range, −16 to 168 HU). Using the soft kernel, mean density was 113 ± 57 HU (range, −13 to 169 HU), and using a sharp kernel, mean density was 97 ± 49 HU (range, −23 to 131 HU). Similarly, reconstructed slice thickness had a significant influence on the measured CT attenuation (mean values for medium soft kernel: 102 ± 52 HU versus 109 ± 58 HU versus 113 ± 57 HU for 0.6-mm, 0.75-mm, and 1.0-mm slice thickness). The differences between 0.75-mm and 0.6-mm slice thickness (P = 0.05) and between medium sharp and sharp kernels (P = 0.02) were statistically significant.

Conclusions

Image reconstruction significantly influences CT attenuation of noncalcified coronary atherosclerotic plaque. With decreasing spatial resolution (softer kernel or thicker slices), CT attenuation increases significantly. Using absolute CT attenuation values for plaque characterization may therefore be problematic.

Introduction

Computed tomographic (CT) imaging allows assessment of coronary atherosclerotic plaque in 2 different ways. Coronary calcifications, which are one component of coronary atherosclerotic plaque, can be detected and quantified in noncontrast scans. Their predictive value for future cardiovascular events has been proven in numerous prospective trials.1, 2, 3, 4, 5, 6, 7 In addition, contrast-enhanced coronary CT angiography allows visualization not only of the coronary artery lumen but also of stenotic and nonstenotic coronary atherosclerotic plaque.8 Presuming high-quality images, noncalcified plaque components can be visualized,9, 10, 11, 12, 13, 14, 15 and, to a certain extent, plaque characterization is possible. This includes quantification of plaque volumes,9, 11, 12, 13, 16, 17, 18 assessment of arterial remodeling,19 and the measurement of CT attenuation within noncalcified plaque components.12, 13, 18, 20, 21, 22, 23, 24 Several studies have correlated the CT attenuation of noncalcified plaque components to intravascular ultrasound (IVUS) characteristics of coronary atherosclerotic plaque, including the detection of a “necrotic core,” which has been proposed but not validated.12 In most,12, 20, 21, 22, 23, 24 but not all,13 cases lower plaque density correlates with lipid-rich plaque appearance on IVUS scanning, whereas higher attenuation values correlate with plaques with a fibrotic appearance on IVUS scanning. One study reported lower CT attenuation values for plaques with positive remodeling than for plaques without or with negative remodeling.25 Beyond these imaging correlations, within retrospective studies the CT attenuation values within culprit lesions of patients with acute coronary syndromes are lower than in plaques related to stable coronary stenoses.26, 27, 28, 29, 30 Finally, a prospective analysis has shown that a CT attenuation of <30 Hounsfield units (HUs) was one marker for plaques with an increased likelihood to be the culprit of acute coronary events during follow-up.31

Although existing data suggest that the determination of the CT attenuation within coronary atherosclerotic plaques may be useful for risk stratification, external factors have significant influence on CT densities measured within noncalcified plaque in coronary CT angiography. Phantom studies indicate that intraluminal contrast concentration has a significant effect on the CT attenuation of plaque.32 With increasing intraluminal contrast concentration, the density measured within the simulated plaque also increases, most likely because of partial volume effects and image interpolation. Correspondingly, crude changes in acquired slice collimation (1.0 mm versus 2.5 mm) influence densities measured in atherosclerotic plaque in phantom studies,33 and changes in the convolution kernel alter CT densities measured in atherosclerotic plaques of ex vivo coronary arteries.34 However, the magnitude of such effects in the in vivo situation, and using image reconstruction parameters typically applied for coronary CT angiography, has so far not been evaluated. We thus attempted to systematically analyze the influence of image reconstruction parameters on the CT attenuation of noncalcified atherosclerotic lesions in coronary CT angiography in vivo.

Section snippets

Patients

Coronary CT angiographic data of 72 patients was analyzed. They were selected from consecutive patients who had been referred for coronary CT angiography because of suspected coronary artery disease. Only data sets in which ≥1 coronary atherosclerotic plaque was detectable within a proximal or mid coronary artery segment (segment 1, 2, 5, 6, 7, 11, 12, or 1335) and which were free of visible artifact caused by motion, breathing, poor contrast enhancement, or excessive image noise were included.

Data acquisition and reconstruction

Results

Among the 72 patients, mean age was 61 ± 10 years, and 52 were men. The mean body weight was 81 ± 16 kg, the mean body mass index was 27 ± 5 kg/m2. Mean heart rate during CT acquisition was 58 ± 7 beats/min. Among the 100 plaques identified, 8 were located in the left main coronary artery, 60 in the left anterior descending coronary artery, 5 in the left circumflex coronary artery, and 27 in the right coronary artery.

Mean attenuation in the contrast-enhanced lumen adjacent to the plaques was

Discussion

Among other features of coronary atherosclerotic plaques, such as the extent of calcification, the volume, or the extent of “positive remodeling,” the CT attenuation measured with a plaque may help to define plaques that are at increased risk of future coronary artery disease events.37 Our study shows that image reconstruction parameters, such as the reconstructed slice thickness and the kernel used in the reconstruction algorithm, influence the CT attenuation value measured within noncalcified

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    Conflict of interest: Drs. Achenbach, Lell, Kuettner, Anders, Pflederer, Marwan, Ropers, and Uder have received speaking honoraria from Siemens Healthcare. The other authors report no conflicts of interest.

    The study was supported by Bundesministerium für Bildung und Forschung (BMBF), Bonn, Germany (grant BMBF 01 EV 0708) and by research grants from Siemens Healthcare (S.A., M.L., A.K., and M.U.).

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