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Is Coronary Stent Assessment Improved with Spectral Analysis of Dual Energy CT?

Posted Sep 30, 2009 Updated Apr 7, 2023
By Ethan J. Halpern MD , David J. Halpern , Jeffrey H. Yanof PhD , Sigal Amin-Spector PhD , David Fischman MD , Galit Aviram MD , Jacob Sosna MD
9 min read

Rationale and Objectives

The aims of this study were to distinguish stents from iodinated contrast on the basis of spectral characteristics on dual-energy computed tomographic (DECT) imaging and to determine whether DECT imaging might provide a more accurate measurement of true stent lumen.

Materials and Methods

Three stainless steel stents and one cobalt chromium stent were scanned using a multidetector, single-source DECT scanner. Stents 2.5, 3.5, and 4.0 mm in diameter were filled with iodinated contrast, submerged in water, and scanned. Spectral analysis was performed to assess the separation of stents from iodinated contrast. Two independent reviewers measured stent lumen diameter and strut thickness on low-energy (L 0 ), high-energy (L 1 ), and combined-energy (L c ) images. Dual-energy full-width half-maximum edge detection analysis was used to provide an independent assessment of stent luminal diameter and strut thickness.

Results

Two-dimensional graphical plots of computed tomographic attenuation for the L 0 and L 1 images did not demonstrate a sharp separation between the absorption characteristics of stents and iodinated contrast material. Stent lumens were underestimated by approximately 50% on L c images. Observer measurements on L 1 images demonstrated a 24% decrease in strut thickness and a 25% increase in stent luminal diameter compared to L 0 images ( P < .0001). Full-width half-maximum measurements did not demonstrate significant changes in stent luminal diameters or strut thicknesses between L 0 and L 1 images.

Conclusions

Spectral analysis did not clearly distinguish stents from iodinated contrast with the DECT system used in this study. The larger stent lumens visualized by the high-energy components of the x-ray spectrum were not related to improved computed tomographic delineation of stent thickness.

Computed tomographic (CT) evaluation of coronary stent restenosis is limited by blooming artifacts from high x-ray–attenuating materials, such as metal struts and mural calcium. Artifacts from high-density stent material lead to the overestimation of stent strut thickness and the underestimation of stent luminal diameter . In vitro evaluation on a four-slice scanner revealed that luminal narrowing of a stainless steel stent may be as great as 62% to 94.3% . The newer 64–detector row CT scanners provide near isotropic pixels with improved spatial resolution that can reduce blooming artifacts and minimize the importance of stent orientation compared to earlier 16-detector scanners . Nonetheless, imaging of coronary stents on computed tomography (CT) remains a technically challenging task.

Conventional CT scanners display an image on the basis of differences in the x-ray attenuation of different tissues. Dual-energy CT (DECT) imaging improves the differentiation of tissues on the basis of CT data obtained from synchronous CT acquisitions at two different energies . On the basis of the preferential absorption of photons by different materials, high-density structures such as calcium and iodine may be separated. Studies have suggested that the CT characterization of the coronary lumen , calcified coronary plaque , and the coronary stent lumen may be improved with DECT imaging. An additional advantage of DECT imaging may be related to decreases in beam-hardening and blooming artifacts in the high-energy component of DECT imaging. The present study was performed to assess in a phantom model the ability to separate stents from iodinated contrast on the basis of their distinct spectral characteristics and to determine whether DECT imaging might provide a more accurate measurement of the true stent lumen.

Materials and methods

Experimental Overview

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Data Acquisition

Stents

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Phantom Design

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DECT System

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Scanning Technique

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Data Analysis

Spectral Analysis

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Figure 1, Spectral separation. Spectral Viewer displays a color-coded image of the stent (yellow) and contrast-enhanced internal lumen (blue) on the basis of dual-energy classification of the tissues. A graphical scatterplot of the low-energy versus high-energy attenuation values within the computed tomographic (CT) slice is displayed, along with a user-defined line that is used to classify stent material from iodinated contrast. Dual-energy values that fall below the user-defined line are color coded as yellow stent material. (a,b) Dual-energy CT (DECT) classification of a Cypher 3.5-mm stainless steel stent; (c,d) DECT classification of a Vision 3.5-mm cobalt chromium stent. The dual-energy graphical plots do not demonstrate a clear separation between stent and iodine within the stent lumen. Consequently, the positioning of the user-defined classification map is subjective. Slight changes in this user-defined classification map result in obvious difference in stent diameter and strut thickness, as demonstrated in (a) versus (b) and in (c) versus (d) .

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Quantitative Measurements of Stent Lumen and Strut Thickness

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Figure 2, Multiphase display of a 3.5-mm stent that was imaged in an oblique orientation to the scan plane. The display is magnified to the maximum possible degree and rotated to create a long-axis thin slice through the center of the stent lumen. The top row of images demonstrates combined-energy (left) , low-energy (middle) , and high-energy (right) computed tomographic (CT) images acquired in standard-resolution mode. The bottom row of images demonstrates combined-energy (left) , low-energy (middle) , and high-energy (right) CT images acquired in high-resolution mode.

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Figure 3, Multiphase display of a 3.5-mm stent illustrating the technique used for quantitative measurement. The display is magnified to the maximum possible degree and rotated to create a long-axis thin slice through the center of the stent lumen. The top row of images demonstrates combined-energy (left) , low-energy (middle) , and high-energy (right) computed tomographic (CT) images acquired in standard-resolution mode. The bottom row of images demonstrates combined-energy (left) , low-energy (middle) , and high-energy (right) CT images acquired in high-resolution mode. The stent lumen is measured in (a) , and the strut thickness is measured in (b) .

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FWHM Measurement

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Figure 4, Full-width half-maximum measurement of a 3.5-mm cobalt chromium stent. To measure the stent lumen (a) , the reviewer defined a box perpendicular to the stent lumen. To measure the strut thickness (b) , the reviewer defined a box perpendicular to the stent strut.

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Statistical Analysis

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Results

Spectral Analysis

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Quantitative Measurements

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Combined Conventional Imaging

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Table 1

Stent Lumen by Stent Size and Measurement Method

Stent Lumen (mm) Stent Size (mm) L c L 0 L 1 FWHM L c FWHM L 1 FWHM L 1 2.5 1.2–1.1 0.7–0.9 1.2 1.3 1.3 1.3 3.5 1.7–1.8 1.6 1.8 1.9 1.6 1.9 4 1.7 1.4 1.7 1.8 1.9 1.9

FWHM, full-width half-maximum; L c , combined-energy image; L 0 , low-energy image; L 1 , high-energy image.

Table 2

Strut Thickness by Stent Size and Measurement Method

Strut Thickness (mm) Stent Size (mm) L c L 0 L 1 FWHM L c FWHM L 1 FWHM L 1 2.5 0.9–1.0 1.1–1.3 0.8–0.9 1.0 1.0 0.9 3.5 1–1.2 1.2–1.3 0.9–1.1 1.0 1.3 1.1 4 1.0–1.1 1.1–1.4 0.9–1.0 1.2 1.2 1.1

FWHM, full-width half-maximum; L c , combined-energy image; L 0 , low-energy image; L 1 , high-energy image.

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Low-energy Versus High-energy Imaging

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Resolution Mode

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Imaging Plane

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Table 3

Stent Lumen by Stent Size and Stent Orientation Relative to Scan Plane

Stent Lumen (mm) Stent Size (mm) Imaging Plane L c L 0 L 1 2.5 Oblique 1.6 1.4 1.6 2.5 Parallel 1.1 0.6 0.9 2.5 Perpendicular 0.8 0.5 1.1 3.5 Oblique 1.8 1.7 1.8 3.5 Parallel 1.8 1.6 1.8 3.5 Perpendicular 1.7 1.5 1.8 4 Oblique 1.7 1.5 1.6 4 Parallel 1.7 1.3 1.7 4 Perpendicular 1.7 1.4 1.8

L c , combined-energy image; L 0 , low-energy image; L 1 , high-energy image.

Table 4

Strut Thickness by Stent Size and Stent Orientation Relative to Scan Plane

Strut Thickness (mm) Stent Size (mm) Imaging Plane L c L 0 L 1 2.5 Oblique 0.9 1.0 0.7 2.5 Parallel 0.9 1.2 0.9 2.5 Perpendicular 1.2 1.4 1.0 3.5 Oblique 1.0 1.2 1.0 3.5 Parallel 1.0 1.1 0.9 3.5 Perpendicular 1.3 1.5 1.2 4 Oblique 1.0 1.2 1.0 4 Parallel 1.0 1.2 0.9 4 Perpendicular 1.1 1.3 1.0

L c , combined-energy image; L 0 , low-energy image; L 1 , high-energy image.

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FWHM Measurement

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Interobserver Correlation

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Table 5

Pearson’s Correlation Coefficients Between the Two Independent Reviewers

Variable L c L 0 L 1 Strut thickness 0.67 0.88 0.50 Stent luminal diameter 0.92 0.95 0.90 Outer stent diameter 0.90 0.93 0.95

L c , combined-energy image; L 0 , low-energy image; L 1 , high-energy image.

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Discussion

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Academic Radiology, Volume 16, Issue 10
Journals General Radiology
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