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Technical Developments |
1 From the Mallinckrodt Institute of Radiology (C.H., P.K.W., K.T.B.) and Division of Cardiovascular Diseases (G.S.C.), Washington University School of Medicine, Campus Box 8131, 510 S Kingshighway Blvd, St Louis, MO 63110. Received October 2, 2003; revision requested December 10; revision received December 22; accepted January 30, 2004. G.S.C. supported by a research grant from the Vascular Biology Working Group, University of Florida College of Medicine. Address correspondence to K.T.B. (e-mail: baet@mir.wustl.edu).
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ABSTRACT |
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0.87) with, attenuation in the proximal and distal untreated lumen. Estimated values for in-stent luminal diameter were lower with CT than with conventional angiography (P < .001), and the mean error (16.1%) that resulted from estimation based on sharp-kernel CT images was significantly smaller than that (27.3%) from estimation based on medium-smooth–kernel CT images (P < .001). Visualization of the in-stent lumen at CT angiography with a 16–detector row scanner allows assessment of coronary artery stent patency on the basis of measured contrast enhancement. © RSNA, 2004
Index terms: Computed tomography (CT), angiography, 54.12116 • Coronary angiography, 548.1244 • Coronary vessels, CT, 548.12116 • Coronary vessels, stenosis or obstruction, 54.76, 54.812 • Coronary vessels, stents and prostheses
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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The development of noninvasive and less expensive imaging modalities for assessing coronary artery stent patency is therefore of great clinical interest. Magnetic resonance imaging is a versatile cardiac imaging modality, but its ability to depict coronary artery stents is impaired by susceptibility artifacts (6). Electron-beam computed tomography (CT) also has been used to assess stent patency; assessment with this modality, however, depends on indirect time-attenuation analysis in vessel segments distal to the stent, without actual visualization of the in-stent lumen (7,8).
Since its introduction, multi–detector row CT technology for cardiac applications has continuously evolved. With increasing numbers of detector rows (currently as many as 16), CT scanners can provide markedly improved temporal and spatial resolution at coronary imaging. Investigators in a recent in vitro study (9) demonstrated that the in-stent lumen could be partially visualized at CT angiography by using a scanner with four detector rows. The purpose of our study was to prospectively assess coronary artery stent patency and the accuracy of measurements of in-stent luminal diameter and contrast enhancement with multi–detector row CT angiography performed with a 16–detector row scanner.
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Materials and Methods |
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50% and < 100%). The 19 patients had a total of 26 stents placed in the left main (n = 1), left anterior descending (n = 10), circumflex (n = 7), and right coronary (n = 8) arteries. The 26 stents included 17 Express stents (Boston Scientific, Mass), three Bx Sonic Hepacoat stents (Cordis, Miami, Fla), two Bx Velocity stents (Cordis), two Zeta stents (Guidant, St Paul, Minn), and two drug-eluting Cypher stents (Cordis). Labeled diameters of the stents were 2.25, 2.5, 2.75, 3.0, 3.5, 4.5, and 5.0 mm, and lengths ranged from 8 to 28 mm. All stents were made of 316 low-carbon stainless steel. Coronary CT angiography was performed 1–3 weeks after stent placement, and images were compared with those from conventional angiography performed during stent placement.
Multi–Detector Row CT Angiography
All CT examinations were performed by using a scanner with 16 detector rows (Sensation 16; Siemens, Forchheim, Germany). Prior to scanning, to determine whether the use of a ß-blocker could improve the visualization of stents by lowering the heart rate, we intravenously administered 5 mg per 5 mL metoprolol (Lopressor; Novartis Pharmaceuticals, East Hanover, NJ) in two patients who had no contraindications to ß-blockers. Their heart rates decreased from 86 and 83 to 62 and 66 beats per minute, respectively. During scanning, the heart rates of the 19 patients ranged from 41 to 98 beats per minute, with a mean of 64 beats per minute ± 18 (standard deviation). The distribution of heart rates among patients was 40–50 beats per minute in four patients, 51–60 in four patients, 61–70 in five patients, 71–80 in three patients, 81–90 in two patients, and 91–100 in one patient.
The contrast medium was administered via an 18-gauge needle in the antecubital vein. A test bolus of 15 mL of ioversol (Optiray 350; Tyco Healthcare Mallinckrodt, St Louis, Mo) was administered first, at a flow rate of 3 mL/sec, followed by sequential scanning of the ascending aorta at a fixed level of the main pulmonary artery every 2 seconds after bolus administration. The time to peak aortic contrast enhancement plus 4–8 seconds was used as the scanning delay. With this delay, spiral scanning was performed with a collimation of 12 sections with individual section thickness of 0.75 mm, table feed of 6.7 mm/sec, rotation time of 420 msec, 120 kVp, 500 mAs, and retrospective electrocardiographic gating. Scanning from 1 cm below the carina to the apex of the heart was completed during a single breath hold. The duration of scanning was 18–24 seconds (mean, 21.5 seconds), depending on the cardiac dimensions. Coronary vessel enhancement was achieved with a contrast medium bolus of 120 mL injected at 3 mL/sec.
Transverse images were reconstructed retrospectively from the raw CT data and electrocardiographic tracings with a temporal resolution of 110–210 msec per image. The reconstruction was gated at 40%–60% of the R-R interval of each cardiac cycle to generate diastolic-phase cardiac images. Effective section thickness and reconstruction increment were 1.0 and 0.5 mm, respectively. Two sets of CT images were reconstructed with convolution during image data postprocessing: One set was reconstructed with a medium-smooth kernel (B30f; 50% value of the modulation transfer, approximately 4.0 cm–1), and the other was reconstructed with a sharp kernel (B46f; 50% value of the modulation transfer, approximately 5.0 cm–1); reconstructions with these two kernels resulted in in-plane resolution of 10 and 12 line pairs per centimeter, respectively (10). Images generated with the sharp kernel are useful in general for visualizing fine details in high-contrast materials such as bone, metals used in stents, and lung, whereas images generated with the smooth kernel generally are used for delineating low-contrast objects such as blood vessels and visceral organs.
Quantitative Analysis of Multi–Detector Row CT Angiograms and Conventional Angiograms
At a digital workstation equipped with software for three-dimensional image display and analysis (Vitrea, version 3.3; Vital Images, Plymouth, Minn), the transverse CT images reconstructed by using the medium-smooth (B30f) and sharp (B46f) kernels and the corresponding curved multiplanar reformatted images were quantitatively analyzed by a radiologist (C.H.) with 5 years of experience in CT coronary angiography. Measurements were performed manually by using electronic calipers available at the workstation. Image noise was measured by calculating the standard deviation of the mean CT attenuation in the ascending aorta. To improve the delineation of the stents, the images were displayed in a zoom mode with the window level at 200 HU (9) and with window width ranging from 700 to 2000 HU. We found that the combination of a 200-HU window level with 1500-HU window width provided a better visualization of stents with respect to both in-stent luminal dimension and stent strut contrast enhancement. Thus, the data obtained in this setting were used for further analysis. In-stent luminal diameter and CT attenuation of the stent strut, in-stent lumen, and coronary artery lumen proximal and distal to the stent were measured by using electronic calipers. At each site, measurements were made on transverse images of three adjacent sections, and the results were averaged. In cases of extensive calcification attached to the stent, or if the axis of the stent was neither perpendicular nor parallel to the imaging plane, the measurements were performed on the curved multiplanar reformatted images that showed a longitudinal view of the stent. The regions of interest used for the CT attenuation measurements had mean areas of 1.4 mm2 for the stent strut and in-stent lumen measurements and 3.5 mm2 for measurements of the coronary artery lumen proximal and distal to the stent.
All patients underwent conventional coronary angiography and stent placement 1–3 weeks before CT angiography. Conventional angiography was performed by using the standard Judkins technique with access via the femoral artery (11). Coronary angiograms were obtained in two orthogonal views by using standard fluoroscopy at a rate of 30 frames per second. The stents were deployed through either 6-F or 8-F guide catheters and at 12-atm balloon inflation pressure. After stent placement, angiography was performed to verify the positioning of the stent; intervention was considered successful in all cases. The post–stent-placement angiograms were interpreted by a cardiologist from the cardiac catheterization laboratory (G.S.C., with 2 years of experience), who used a quantitative coronary angiography system (Integris H5000; Philips, Best, the Netherlands). The minimum luminal diameter (MLD) of the treated coronary artery segment was measured with a single matched view (of a reference segment and the stenotic segment) on diastolic-phase images from conventional angiography. Measurements on conventional angiograms served as the reference standard for in-stent luminal measurements on CT images.
Statistical Analysis
Image noise was compared between CT images reconstructed with B30f and B46f kernels. The in-stent luminal diameter measured on the CT images, or CTD, was compared with the MLD on the conventional angiograms. The error in the diameter measurements, expressed as a percentage derived from the equation [(CTD – MLD)/MLD] · 100, was compared between the B30f-kernel and B46f-kernel CT images. CT attenuation was compared between different measured sites and between the B30f-kernel and B46f-kernel CT images. The correlation between MLD and in-stent CT attenuation, or the error in in-stent luminal diameter measurements, was assessed. The CT attenuation and the error in the diameter measurements, as well as the actual MLD of the stents, were compared between individual coronary arteries. Continuous data were presented as means ± standard deviations. Comparison was performed with the two-sided paired Student t test and analysis of variance (ANOVA). Repeated-measures ANOVA was used for comparison of multiple measures per coronary artery. In assessing differences between coronary arteries, statistical significance was adjusted for multiple stents per patient by using the correction factor C defined by Gonen et al (12). The Pearson product moment correlation coefficient was calculated to assess the association between CT attenuation values measured in the stent lumen and in the untreated coronary artery segments proximal and distal to the stent. P < .05 was considered to indicate a statistically significant difference. All statistical analyses were performed by using statistical software (JMP, version 5.0; SAS Institute, Cary, NC).
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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The mean image noise measured on the B30f-kernel images (26.9 HU ± 3.2) was significantly lower than that on the B46f-kernel images (52.0 HU ± 4.9; two-sided paired t test, P < .001). Despite the increased background noise on B46f-kernel images, however, the stents were more sharply delineated (Fig 2).
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.008). The CT attenuation measured on the B46f-kernel images was higher than that on the B30f-kernel images at all sites except the in-stent lumen, where CT attenuation was lower and closer to that of the untreated lumen proximal and distal to the stent. On both B30f-kernel and B46f-kernel images, CT attenuation significantly differed between the sites (repeated-measures ANOVA, P < .001; two-sided paired t test, P < .001 for all pairs). The CT attenuation measured in the stent lumen was higher than that of the contrast-enhanced coronary artery lumen proximal and distal to the stent but lower than that of the stent strut. However, the in-stent CT attenuation measured on both B30f-kernel and B46f-kernel images highly correlated with the contrast-enhanced attenuation of the coronary artery lumen proximal and distal to the stent (Table 2). There was no association between the in-stent CT attenuation and the MLD of the stents (r = –0.01, P = .96). The in-stent CT attenuation, however, differed significantly according to stent location in the coronary arteries (ANOVA, P = .016); for example, mean in-stent attenuation values measured on the B46f-kernel images were 378.4 HU ± 22.2 for the right coronary artery, 339.0 HU ± 45.0 for the left anterior descending artery, and 303.8 HU ± 33.6 for the circumflex artery.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and Methods ResultsDiscussionREFERENCES |
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The diameter of coronary artery stents corresponds to that of major coronary arteries and their branches. CT of the in-stent coronary artery lumen, however, is more challenging than that of the native coronary artery lumen because of partial-volume–averaging artifacts caused by the metallic stent struts. High in-plane and through-plane spatial and contrast resolution, as well as small partial-volume errors, are essential for overcoming the technical challenges to this application. Advances in CT technology, such as submillimeter spatial resolution (0.5 x 0.5 x 0.6 mm) provided by scanners with 16 detector rows, could play a valuable role. The technical superiority of 16–detector row CT scanners over the previous generation of scanners in the diagnosis of coronary artery stenosis has been demonstrated in recent investigations (13,14).
The in-stent lumen depicted on 16–detector row CT images allows us to directly assess stent patency and to measure contrast enhancement in the in-stent coronary artery lumen. Because of partial-volume–averaging artifacts from the highly attenuated stent struts, the CT attenuation values measured in the stent lumen were significantly higher than those measured in the coronary artery segments both proximal and distal to the stent. The in-stent contrast-enhanced attenuation measured on sharp-kernel images was closer to that measured in the proximal or distal lumen than was the in-stent attenuation measured on the smooth-kernel images. This is largely a result of a reduction in partial-volume averaging with the sharp kernel. In addition, we found that the in-stent contrast enhancement differed significantly according to the coronary arteries in which the stents were placed. This effect was also reported by previous investigators (15,16).
Optimal contrast enhancement in the coronary artery lumen is crucial for evaluation of stent patency. Acquisition timing has a major impact on the quality of vascular contrast enhancement. In single–detector row CT angiography, the time to peak enhancement, measured after a test bolus injection, is considered the scanning delay to be used after the full bolus injection. This approach, if used in multi–detector row CT angiography, could result in insufficient contrast enhancement because of the much shorter acquisition time. An additional delay of several seconds (eg, 4–8 seconds in our study) is necessary to achieve optimal contrast enhancement (17).
Partial-volume averaging may affect not only the measurement of the in-stent attenuation but also that of the in-stent luminal diameter. As a result, the in-stent luminal diameters measured on the CT images were smaller than those measured on the conventional angiograms. As shown in previous studies (18), the absolute lumen dimension is an important predictor of angiographic restenosis after coronary artery stent placement. The thin-section collimation of 16–detector row CT and the high-resolution image postprocessing algorithm help to decrease the effects of partial-volume averaging. The results of our study demonstrate that the sharp kernel reduced the error in diameter measurement from 27.3% to 16.1%, and both error levels are substantially lower than the error range (62%–100%) found in a four–detector row CT study (9). The stent boundary was depicted more sharply and thus was more easily identified on the B46f-kernel images than on the B30f-kernel images. Furthermore, a larger window width to suppress the high attenuation of the stent strut seems to have contributed to better delineation and more accurate measurement of the in-stent lumen. Nevertheless, a consistent underestimation of the in-stent diameter may limit the usefulness of CT for the detection of in-stent restenosis.
Our study had several technical limitations. First, the temporal resolution of multi–detector row CT, even with 16 detector rows, is insufficient for coronary artery imaging in patients with high heart rates, as observed in a previous study (19). At 16–detector row coronary CT angiography, a heart rate–dependent data acquisition window of 110–210 msec and a gantry rotation time of 420 msec are used, resulting in a higher temporal resolution than at four–detector row CT. However, in our study, cardiac motion artifacts caused substantial image quality degradation in three patients with heart rates of more than 70 beats per minute. The four stents in these patients were blurred on CT images and therefore could not be evaluated. Lowering the heart rate by means of a ß-blocker is usually an effective way to obtain artifact-free coronary CT angiograms, as has been shown in previous studies (13,14). Second, extensive and highly attenuated coronary calcifications attached at a stent site affected the visualization of the stent by exacerbating partial-volume–averaging effects and obscuring the in-stent lumen. Third, because the stent assessment and the measurements in this study were made by only one investigator, the interobserver variability of the technique is unknown. Last, the present study did not assess the capability of 16–detector row CT for depicting stent restenosis. This technically challenging but clinically crucial application requires further clinical follow-up studies and advances in CT technology. Previous investigators (20) demonstrated that coronary artery plaques can be detected and characterized by means of the difference in CT attenuation between them and the enhanced blood in the arterial lumen. It is possible that a lumen restenosed by intimal hyperplasia could be differentiated from a healthy lumen by means of contrast-enhancement measurement. However, accurate evaluation of in-stent restenosis with the current generation of multi–detector row CT scanners appears quite challenging. A proposed clinical study design may include a comparison of concurrent CT and conventional angiograms from examinations performed 4–6 months after stent deployment. On a separate note, drug-eluting stents, which are now commonly used, are reported to be associated with low rates of restenosis (21).
In conclusion, the coronary artery stent lumen can be visualized by using 16–detector row CT angiography, which allows assessment of stent patency by measuring contrast enhancement in the stent lumen. The in-stent luminal diameter measured on CT images was smaller than that on conventional angiograms. The accuracy of the CT measurements can be improved with use of the high-resolution (B46f) kernel at image reconstruction.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Authors stated no financial relationship to disclose.
Author contributions: Guarantors of integrity of entire study, C.H., K.T.B.; study concepts, C.H., K.T.B.; study design, all authors; literature research, C.H., G.S.C.; clinical studies, all authors; data acquisition and analysis/ interpretation, C.H., G.S.C.; statistical analysis, C.H.; manuscript preparation, definition of intellectual content, editing, and final version approval, C.H., K.T.B.; manuscript revision/review, all authors
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