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Noninvasive quantitative evaluation of coronary artery stent patency using 64-row multidetector computed tomography
Journal of Cardiovascular Computed Tomography (2010) 4, 29–37
Original Research Article
Noninvasive quantitative evaluation of coronary artery stent patency using 64-row multidetector computed tomography Murrad J. Abdelkarim, MDa,*, Naser Ahmadi, MDa, Ambarish Gopal, MDa, Yasmin Hamirani, MDa, Ronald P. Karlsberg, MD, FACP, FACCb, Matthew J. Budoff, MD, FACCa a
Department of Medicine, Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, 1000 West Carson Street, Box 400, Torrance, CA 90509, USA and bCardiovascular Medical Group, Beverly Hills, CA, USA KEYWORDS: 64-row multidetector computed tomography (MDCT); Cardiac computed tomographic angiography (CTA); Computed tomography (CT) density; Coronary stent patency; In-stent restenosis (ISR)
BACKGROUND: Many studies have used multidetector computed tomography (MDCT) angiography to evaluate coronary stents qualitatively but not quantitatively. OBJECTIVES: This study sought to validate a method of quantitatively evaluating stent patency by using 64-row compared with invasive coronary angiography (ICA) and to evaluate the stent size threshold of MDCT in detecting stent patency. METHODS: Stented lesions (n 5 122) in 55 patients (age, 65 6 10 years; 90% men) who underwent both 64-row MDCT and ICA were studied. Density measurements in Hounsfield units (HUs) and stent diameters in millimeters were recorded in the stented segments, with the density of the ascending aorta (AO) taken as a reference. The ratio of the average of stent’s proximal, middle, and distal densities to mean AO density was defined as the AS/AO HU. Threshold values for the detection of stent patency were examined by using receiver operator characteristic (ROC) curve analysis. RESULTS: One hundred six of 122 stents were interpretable. By ICA, 24 stents were found to have instent restenosis (22 interpretable and 2 noninterpretable with MDCT). The ROC curve showed that the optimal cutoff value of AS/AO HU to predict stent patency on MDCT was 0.81 with sensitivity of 90.9%, specificity of 95.2%, and the optimal stent diameter cutoff value was R2.5 mm with a sensitivity of 91.8% and a specificity of 93.8%. CONCLUSION: With 64-row MDCT, coronary stent patency can be evaluated quantitatively with high sensitivity and specificity and with adequate diagnostic accuracy in stents R2.5 mm in diameter. Ó 2010 Society of Cardiovascular Computed Tomography. All rights reserved.
Conflict of interest: Dr. Budoff is on the GE Speaker Bureau. The remaining authors report no conflicts of interest. There was no source of funding for this study. * Corresponding author. E-mail address: [email protected] Submitted May 28, 2009. Accepted for publication October 23, 2009.
Introduction Percutaneous coronary intervention (PCI) with coronary stent implantation has been increasingly used in the treatment of coronary artery disease. In 2005, an estimated 1,265,000 PCI procedures were performed in the United
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Figure 1 Cardiac CT angiography of patent left anterior descending (LAD) coronary stent. (A) Long-axis and (B) cross-sectional views of a left anterior descending coronary stent without any evidence of ISR. All density measurements taken in the marked areas on the longaxis view were between 479 and 490 HU. Also note moderate disease as marked distal to the stented segment well seen on the cardiac CT.
States.1 Despite significantly reducing the occurrence of restenosis2,3 compared with balloon angioplasty alone, the instent restenosis (ISR) rates remain high. ISR can occur in 16%–35% of bare-metal stents4–7 and in 4%–11% of drug-eluting stents.4–7 The ‘‘gold standard’’ for the detection of ISR remains invasive coronary angiography (ICA), although it is limited as an invasive technique. Therefore, there is a need for an alternative, noninvasive technique for the detection of ISR. Many studies have used multidetector computed tomographic (MDCT) angiography to evaluate coronary stents qualitatively but not quantitatively. The qualitative assessment of stent patency with MDCT angiography is difficult, mostly limited by artifact caused by small stent size and design type. Our goal in this study was to develop a method of noninvasive quantitative assessment of coronary artery stent patency with the use of 64-row MDCT angiography by comparison with invasive coronary angiography and to evaluate the stent size threshold of 64-row MDCT angiography in detecting stent patency.
Methods The study protocol and consent form were approved by the Institutional Review Board Committee of the Los Angeles Biomedical Research Institute at Harbor UCLA Medical Center, Torrance, CA.
Study population This was a retrospective analysis of 55 patients who underwent both 64-row MDCT coronary angiography and ICA. Patients were referred by their physicians for MDCT
angiography for evaluation of their stent and native coronary arteries because of the presence of symptoms or an abnormal cardiac stress test. All patients received a subsequent ICA. We assessed the diagnostic performance of MDCT angiography on a per-stent basis, including all patients and all vessels for final efficacy analysis.
Scan protocol Patients underwent MDCT angiography before conventional ICA. All scans were performed with a 64-row detector Lightspeed VCT scanner (GE Healthcare, Milwaukee, WI). All patients were in normal sinus rhythm at the time of the MDCT scan. Persons presenting with baseline heart rates . 65 beats/min were administered oral and/or intravenous b-blocker therapy. Patients were not excluded from MDCT when the heart rate remained .65 beats/min. After a scout radiograph of the chest (anteroposterior and lateral), a timing bolus (with 10–20 mL contrast) was performed to detect the time to optimal contrast opacification in the axial image at a level immediately superior to the ostium of the left main artery. Nitroglycerine 0.4 mg sublingual was administered immediately before contrast injection. During MDCT angiography acquisition, 60–80 mL iodinated contrast (Visipaque; GE Healthcare, Chalfont St Giles, United Kingdom) was injected with the use of a triple-phase contrast protocol: 40 mL iodixanol, followed by 20–40 mL of a 50:50 mixture of iodixanol and saline, followed by a 50-mL saline flush. Retrospective electrocardiogram (ECG)–gated helical contrast-enhanced MDCT angiography was performed, with scan initiation of 10 mm above the level of the left main artery to 10 mm below the inferior myocardial apex. The scan parameters were 64 ! 0.625 mm
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Figure 2 Cardiac CT angiography of 2 different-sized patent left anterior descending (LAD) coronary stents. (A) Long-axis and (B) cross-sectional views of 3-mm stent; (C) long-axis and (D) cross-sectional views of 4-mm stent. There is no evidence of ISR with all density measurements between 349 and 385 HU.
collimation, tube voltage of 120 mV, effective tube current 350–780 mA. Radiation reduction with ECG tube current modulation was used. After scan completion, multiphasic reconstruction of MDCT angiography scans was performed, with reconstructed images from 70% to 80% by 5% and from 5% to 95% by 10% increment.
MDCT image reconstruction and interpretation Individually scanned raw data sets were uploaded and analyzed on dedicated workstations (Advantage Workstation; GE Healthcare, Milwaukee WI). All stented segments were evaluated by a reviewer who was blinded to the ICA findings. Image reconstruction was retrospectively gated to
an ECG, and the optimal cardiac phase showing the least motion artifact was determined. Stented segments were viewed in their short and long axes by using both the original 2-dimensional axial images and multiple image reconstruction methods. In addition, they were viewed with a wide window width and the minimum image thickness possible. A fixed window width of 1200 Hounsfield units (HU) and window level of 240 HU were used for all interpretations, to minimize interscan differences and to reduce a source of error for measurements. The location of each stent (vessel and segment) was recorded. Image quality of each stented segment was judged as adequate or not for the evaluation of stent patency and ISR. The stent was considered interpretable if the stent lumen was visible and the contrast density of the
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lumen could be evaluated without or with negligible influence of image degrading factors. The reason for noninterpretability was recorded such as partial volume effects, metal artifacts from stent struts (beam hardening), cardiac motion artifacts (banding artifacts), and calcification on the vessel wall. Density measurements in Hounsfield units and stent diameters in millimeters were recorded in the proximal, middle, and distal sections along the long axis of the stented segments in all interpretable stents (Fig. 1 and Fig. 2). Density measurements were determined by using the exact center point in the short axis of all the designated sections. Stent diameters were determined by measuring the length of a straight line segment running from the middle of the stent rim from one end to the other through the center point along the short axis of all the designated sections. The density measurement of the ascending aorta was determined by using a region of interest area (100 mm2) in the center of the shortaxis view and was taken as the reference density measurement. The ratio of the average of the stent’s proximal, middle, and distal densities to mean ascending aorta (AO) density was defined as the average stent to AO (AS/AO) HU ratio. The average of the stent’s proximal, middle, and distal diameters was defined as the average stent size. To determine observer variability, a subgroup of stented segments were reevaluated by the first reviewer and a second who was also blinded to the ICA findings. Thirty stented segments in 20 patients were re-reviewed by the first reader and reviewed by a second reader to determine the intra-reader and inter-reader variability associated with our method of quantitative evaluation of stent patency. With the use of the Pearson’s coefficient of correlation in this subgroup, we found an r 5 0.86 for intraobserver and r 5 0.82 for interobserver evaluations. The mean intrareader variability was 6.9% 6 9.7%, and the mean interreader variability was 9.9% 6 0.4%.
receiver operator characteristic (ROC) curve was calculated for MDCT angiography to identify obstructive coronary artery stenosis at the 50% threshold in the stented segment. A second ROC was used to determine the minimum stent size to predict feasibility of MDCT coronary angiography in diagnosing stent patency. Pearson’s coefficient of correlation (r) was used in the subgroup analysis to evaluate intraobserver and interobserver variability. All statistical analyses were performed with the use of SAS Proprietary Software, Release 9.1 (SAS Institute Inc, Cary, NC).
Results Patient/stent characteristics and angiographic findings A total of 122 stented segments in 55 patients (age, 65 6 10 years; 90% men) were studied, among which 106 stents (86.8%) were interpretable for evaluation of stent patency by 64-row MDCT angiography. Reasons for noninterpretability of the 16 stents were caused by metal artifacts in smaller sized stents (7 stents), motion artifact (4 stents), partial volume effects (3 stents), and calcification on the vessel wall (2 stents). There was no difference in age, sex, or the vessels stented among interpretable and noninterpretable stents. The majority of noninterpretable stents were located in distal arterial locations (56.2%). By ICA, 24 stents were occluded (22 in the interpretable group and 2 in the noninterpretable group). Stent characteristics for the 106 interpretable stents among patients with and without stent patency are shown in Table 1. Patients with patent stents tended to be older
ICA procedure and analysis Selective ICA was performed by standard transfemoral arterial catheterization. A minimum of 8 projections were obtained with a minimum of 5 views for the left coronary artery system and a minimum of 3 views for the right coronary artery system. Figure 3 shows an example of one view obtained of a patent left anterior descending coronary artery stent. All stented segments were qualitatively evaluated by an experienced cardiologist who was blinded to the results of the MDCT. ICA was used as the ‘‘gold standard’’ for determining stent patency, with ISR defined as a diameter reduction of R50% compared with the original stent diameter.
Statistical analysis Categorical variables are presented as frequencies and percentages, and continuous variables are presented as mean 6 standard deviation (SD). The area under the
Figure 3 ICA of patent left anterior descending (LAD) coronary stent (patient from CT angiography in Fig. 1), with moderate disease distal to stented segment.
Abdelkarim et al Table 1
Evaluating coronary artery stent patency with MDCT
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Stent Characteristics on MDCT among patients with patent stents and ISR Patent Stents (n 5 84)
Age, y, mean 6 SD Male sex, n (%) Stented vessel Left main, n (%) Left anterior descending, n (%) Left circumflex, n (%) Right coronary artery, n (%) Saphenous vein graft, n (%) Segment Proximal, n (%) Middle, n (%) Distal, n (%) Stent size, mm, mean 6 SD Density AO, HU, mean 6 SD Proximal stent, HU, mean 6 SD Middle stent, HU, mean 6 SD Distal stent, HU, mean 6 SD Average stent, HU, mean 6 SD Proximal stent/AO, HU, mean 6 SD Middle stent/AO, HU, mean 6 SD Distal stent/AO, HU, mean 6 SD Average stent/AO , HU, mean 6 SD
66 6 11 70 (93)
ISR Stents (n 5 22)
P
60 6 12 15 (83)
4 (4.8) 35 (41.7) 9 (10.7) 24 (28.6) 12 (14.3)
0 8 1 7 6
0.02 0.31 0.15
(0) (36.4) (4.5) (31.8) (27.3) 0.06
37 (45.7) 30 (37) 14 (17.3) 2.87 6 0.66
6 (27.3) 7 (31.8) 9 (40.9) 3.26 6 1.07
369.1 6 77.1 397.7 6 145.7 397.2 6 108.8 380.9 6 94.1 391.8 6 95.7 1.10 6 0.34 1.09 6 0.32 1.04 6 0.21 1.07 6 0.24
391.1 6 97.2 132.4 6 167.3 132.1 6 153.6 152.6 6 168.4 121.6 6 120.6 0.33 6 0.39 0.37 6 0.45 0.41 6 0.44 0.37 6 0.37
0.03 0.33 0.043 0.039 0.036 0.0001 0.0001 0.0001 0.0001 0.0001
Stent patency was diagnosed by invasive coronary angiography.
(66 6 11 years verus 60 6 12 years; P 5 0.02). Most stents with ISR were located in the distal segment of the stented vessel (40.9%), as opposed to patent stents that were mostly located in the proximal segment of the stented vessel (45.7%). By ICA, 84 (79.2%) of interpretable stents and 14 (87.5%) of noninterpretable stents were patent; no significant difference was observed among the groups (P 5 0.44).
Density analysis Density measurements for all of the 106 interpretable stents are shown in Table 1. No significant difference was observed in the reference density of the AO among patients
Table 2
with and without patent stents (369.1 6 77.1 HU versus 391.1 6 97.2 HU, respectively; P 5 0.33). The density measurements in all stent sections (proximal, middle, and distal) were significantly lower in the ISR stents than in the patent stents. The AS/AO HU ratio was also significantly lower in ISR stents compared with patent stents (0.37 6 0.37 HU compared with 1.07 6 0.24 HU; P 5 0.0001). The results of ROC curve analysis for the relationship between stent patency and quantitative luminal stent density measurements are shown in Table 2. A mean stent attenuation value greater than approximately 300 HU correlated with stent patency with a sensitivity of 95.2% and a specificity of 84.3% (AUC 6 SE, 0.95 6 0.019; 95% CI,
ROC curves created to assess the ability of density measurements to predict stent patency
Proximal stent Middle stent Distal stent Average stent Proximal stent/AO Middle stent/AO Distal stent/AO Average stent/AO
AUC 6 SE
95% CI*
Cut point
Sensitivity, %
Specificity, %
PPV, %
NPV, %
0.90 6 0.030 0.92 6 0.027 0.88 6 0.030 0.95 6 0.019 0.93 6 0.023 0.88 6 0.033 0.85 6 0.039 0.95 6 0.019
0.83–0.95 0.85–0.96 0.80–0.93 0.91–0.99 0.87–0.97 0.81–0.94 0.76–0.91 0.89–0.98
255 247 219 302 0.85 0.76 0.68 0.81
81.8 81.8 77.3 95.2 81.8 77.3 72.7 90.9
90.5 95.2 96.4 84.3 92.6 95.2 97.5 95.2
90.0 95.3 96.2 86.3 92.1 95.1 97.3 95.7
83.3 84.1 81.3 95.4 83.6 81.2 78.2 91.3
PPV, positive predictive value; NPV, negative predictive value. *P 5 0.0001.
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0.91–0.99; P , 0.0001). The optimal AS/AO HU ratio for the diagnosis of stent patency was 0.81 with sensitivity of 90.9%, specificity of 95.2%, positive predictive value of 95.7%, and negative predictive value of 91.3% (AUC 6 SE, 0.95 6 0.019; 95% CI, 0.89–0.98: P , 0.0001) (Fig. 4).
Stent size threshold analysis The optimal stent diameter needed to evaluate stent patency on MDCT angiography was R2.5 mm (Fig. 5). At this threshold, stent patency was detected with a sensitivity of 93.8%, specificity of 91.8%, positive predictive value of 92.1%, and negative predictive value of 93.8% (AUC 6 SE, 0.97 6 0.01; 95% CI, 0.93–0.99; P , 0.0001).
Discussion The current literature available on the use of MDCT for the qualitative evaluation of coronary artery stent patency shows a range of sensitivity from 86% to 100% and specificity from 91% to 98%.8–14 Despite this apparent accuracy, concerns exist over the ability to translate these findings to daily practice, given the need for qualitative assessment and a requirement for advanced reconstruction and postprocessing techniques. As such, an objective and quantitative method of evaluating stent patency with 64-row MDCT would potentially enable greater confidence in evaluation of stent patency. In this study, we have shown that a mean stent density R300 HU and a difference in CT density from the reference vessel (AO) to the coronary stent lumen of %19% correlated with a patent stent with a high sensitivity
Figure 4 ROC curve of average stent/AO HU ratio. PPV, positive predictive value; NPV, negative predictive value.
Figure 5 patency.
ROC curve of stent size and interpretable stent
and specificity. With the use of this method, the evaluation of coronary stents R 2.5 mm in diameter appears feasible. Advantages of this method include its quantitative nature and the ability to apply it without special reconstruction techniques. Furthermore, display parameters such as window width and display levels do not affect Hounsfield unit measures, so this is independent of reader preference and workstation settings. This study marks the onset of an era in which MDCT angiography has emerged as a viable and effective option in the noninvasive evaluation of patients with coronary stents after PCI (Fig. 6). Prior studies have applied densitometric analysis for the analysis of coronary stents. In a study evaluating a complicated method that compared density measurements in the proximal, mid, and distal regions inside each stent, 60% ISR was detected with a sensitivity of 54.5% and specificity of 94.7%.8 An alternative method, proposed by Kitagawa et al,15 evaluated ISR with the use of quantitative density analysis in 16-row MDCT angiography. A density measurement of R5 region of interest (area %1 mm2) positioned in a low-density area of the stent, compared with reference vessels (sites proximal and distal to stent), suggested ISR in the present of high variability of density values (75 6 10 HU to 270 6 7 HU). These initial studies led to a subsequent study by Kitagawa et al,16 in which regions of interest were assessed in a reference segment 5–10 mm proximal to the stented segment and compared with regions within the stent. The study found that the accuracy for detection of ISR was high with the use of either a mean CT density cutoff value of 280 HU (sensitivity of 83% and specificity of 83%) or a DCT density cutoff value of 75 HU (sensitivity of 100% and specificity of 96%). As in the study by Kitagawa et al,16 we found that a mean in-stent attenuation value of 300 HU was highly
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Figure 6 Cardiac CT Angiography of a proximal left anterior descending (LAD) stent with ISR. (A) Long-axis and (B) cross-sectional images showing a dark center in the proximal portion of the stent measuring 216/239 HU, far below the aortic comparison of 414 HU in this study. (C) This evidence of ISR correlated well with the invasive coronary angiogram.
accurate for the exclusion of ISR. In addition, the evaluation of relative attenuation within the stent compared with an aortic reference segment avoided the confounding effect of interpatient variability of cardiac output and volume of distribution of intracoronary contrast. We chose to use aortic attenuation as a reference segment because this region is less influenced by various artifacts (partial volume effects, beam hardening from nearby stent struts, banding, and vessel calcification) that may result in either an overestimation or underestimation of D CT density. With the use of this method, we have shown that quantitative assessment of ISR can enable the evaluation of stent diameters of %2.5 mm. Despite the superiority of 64-row MDCT compared with older generation scanners, limitations in coronary stent
evaluation include inferior spatial resolution compared with ICA, relatively low temporal resolution, and increased false-positive rates for detection of ISR in smaller stents. Stent design (diameter, length, strut thickness, and material) is the main limiting factor for the application of MDCT in the detection of ISR. Smaller diameter stents remain a challenge. In one recent study evaluating longterm outcomes of drug-eluting compared with bare-metal stent diameter data in 13,890 stents, 30% of implanted stents were ,3.0 mm in diameter and 4.8% were ,2.5 mm in diameter.17 Trends in stent design and size will have an effect on MDCT stent assessability. In a study by Schuijf et al,18 90% of stents . 3.0 mm were interpretable, compared with only 72% of stents , 3.0 mm. In addition, 89% of stents with strut thickness , 140 mm were interpretable,
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2.
3.
4.
5.
Figure 7 A high-definition cardiac CT angiogram showing patency of multiple stents with good distal filling. This new technology applying thinner slice thickness with specialized reconstruction techniques may allow better visualization of stents with fewer artifacts.
compared with only 41% of thicker strutted stents. The future may see refinements in MDCT scanner (Fig. 7) or stent technology that further enhances the accuracy of noninvasive stent patency evaluations. Our study has several limitations. This was a retrospective study; therefore, complete information was not available on stent design and the time between stent implantation and MDCT angiography acquisition. The lack of established criteria for quantitatively evaluating stent patency and the overall small size of our patient population make it difficult to draw generalized conclusions for the application of our method to all patients with coronary stents receiving MDCT angiography. Finally, inherent problems or limitations in the MDCT angiography technique exist such as radiation exposure, use of iodinated contrast, and the absence of functional or perfusion data.
Conclusion The use of 64-row MDCT angiography coronary artery stent patency can be evaluated quantitatively with high sensitivity and specificity and with improved diagnostic predictability in stents R 2.5 mm.
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Original Research Article
Noninvasive quantitative evaluation of coronary artery stent patency using 64-row multidetector computed tomography Murrad J. Abdelkarim, MDa,*, Naser Ahmadi, MDa, Ambarish Gopal, MDa, Yasmin Hamirani, MDa, Ronald P. Karlsberg, MD, FACP, FACCb, Matthew J. Budoff, MD, FACCa a
Department of Medicine, Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, 1000 West Carson Street, Box 400, Torrance, CA 90509, USA and bCardiovascular Medical Group, Beverly Hills, CA, USA KEYWORDS: 64-row multidetector computed tomography (MDCT); Cardiac computed tomographic angiography (CTA); Computed tomography (CT) density; Coronary stent patency; In-stent restenosis (ISR)
BACKGROUND: Many studies have used multidetector computed tomography (MDCT) angiography to evaluate coronary stents qualitatively but not quantitatively. OBJECTIVES: This study sought to validate a method of quantitatively evaluating stent patency by using 64-row compared with invasive coronary angiography (ICA) and to evaluate the stent size threshold of MDCT in detecting stent patency. METHODS: Stented lesions (n 5 122) in 55 patients (age, 65 6 10 years; 90% men) who underwent both 64-row MDCT and ICA were studied. Density measurements in Hounsfield units (HUs) and stent diameters in millimeters were recorded in the stented segments, with the density of the ascending aorta (AO) taken as a reference. The ratio of the average of stent’s proximal, middle, and distal densities to mean AO density was defined as the AS/AO HU. Threshold values for the detection of stent patency were examined by using receiver operator characteristic (ROC) curve analysis. RESULTS: One hundred six of 122 stents were interpretable. By ICA, 24 stents were found to have instent restenosis (22 interpretable and 2 noninterpretable with MDCT). The ROC curve showed that the optimal cutoff value of AS/AO HU to predict stent patency on MDCT was 0.81 with sensitivity of 90.9%, specificity of 95.2%, and the optimal stent diameter cutoff value was R2.5 mm with a sensitivity of 91.8% and a specificity of 93.8%. CONCLUSION: With 64-row MDCT, coronary stent patency can be evaluated quantitatively with high sensitivity and specificity and with adequate diagnostic accuracy in stents R2.5 mm in diameter. Ó 2010 Society of Cardiovascular Computed Tomography. All rights reserved.
Conflict of interest: Dr. Budoff is on the GE Speaker Bureau. The remaining authors report no conflicts of interest. There was no source of funding for this study. * Corresponding author. E-mail address: [email protected] Submitted May 28, 2009. Accepted for publication October 23, 2009.
Introduction Percutaneous coronary intervention (PCI) with coronary stent implantation has been increasingly used in the treatment of coronary artery disease. In 2005, an estimated 1,265,000 PCI procedures were performed in the United
1934-5925/$ -see front matter Ó 2010 Society of Cardiovascular Computed Tomography. All rights reserved. doi:10.1016/j.jcct.2009.10.014
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Figure 1 Cardiac CT angiography of patent left anterior descending (LAD) coronary stent. (A) Long-axis and (B) cross-sectional views of a left anterior descending coronary stent without any evidence of ISR. All density measurements taken in the marked areas on the longaxis view were between 479 and 490 HU. Also note moderate disease as marked distal to the stented segment well seen on the cardiac CT.
States.1 Despite significantly reducing the occurrence of restenosis2,3 compared with balloon angioplasty alone, the instent restenosis (ISR) rates remain high. ISR can occur in 16%–35% of bare-metal stents4–7 and in 4%–11% of drug-eluting stents.4–7 The ‘‘gold standard’’ for the detection of ISR remains invasive coronary angiography (ICA), although it is limited as an invasive technique. Therefore, there is a need for an alternative, noninvasive technique for the detection of ISR. Many studies have used multidetector computed tomographic (MDCT) angiography to evaluate coronary stents qualitatively but not quantitatively. The qualitative assessment of stent patency with MDCT angiography is difficult, mostly limited by artifact caused by small stent size and design type. Our goal in this study was to develop a method of noninvasive quantitative assessment of coronary artery stent patency with the use of 64-row MDCT angiography by comparison with invasive coronary angiography and to evaluate the stent size threshold of 64-row MDCT angiography in detecting stent patency.
Methods The study protocol and consent form were approved by the Institutional Review Board Committee of the Los Angeles Biomedical Research Institute at Harbor UCLA Medical Center, Torrance, CA.
Study population This was a retrospective analysis of 55 patients who underwent both 64-row MDCT coronary angiography and ICA. Patients were referred by their physicians for MDCT
angiography for evaluation of their stent and native coronary arteries because of the presence of symptoms or an abnormal cardiac stress test. All patients received a subsequent ICA. We assessed the diagnostic performance of MDCT angiography on a per-stent basis, including all patients and all vessels for final efficacy analysis.
Scan protocol Patients underwent MDCT angiography before conventional ICA. All scans were performed with a 64-row detector Lightspeed VCT scanner (GE Healthcare, Milwaukee, WI). All patients were in normal sinus rhythm at the time of the MDCT scan. Persons presenting with baseline heart rates . 65 beats/min were administered oral and/or intravenous b-blocker therapy. Patients were not excluded from MDCT when the heart rate remained .65 beats/min. After a scout radiograph of the chest (anteroposterior and lateral), a timing bolus (with 10–20 mL contrast) was performed to detect the time to optimal contrast opacification in the axial image at a level immediately superior to the ostium of the left main artery. Nitroglycerine 0.4 mg sublingual was administered immediately before contrast injection. During MDCT angiography acquisition, 60–80 mL iodinated contrast (Visipaque; GE Healthcare, Chalfont St Giles, United Kingdom) was injected with the use of a triple-phase contrast protocol: 40 mL iodixanol, followed by 20–40 mL of a 50:50 mixture of iodixanol and saline, followed by a 50-mL saline flush. Retrospective electrocardiogram (ECG)–gated helical contrast-enhanced MDCT angiography was performed, with scan initiation of 10 mm above the level of the left main artery to 10 mm below the inferior myocardial apex. The scan parameters were 64 ! 0.625 mm
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Figure 2 Cardiac CT angiography of 2 different-sized patent left anterior descending (LAD) coronary stents. (A) Long-axis and (B) cross-sectional views of 3-mm stent; (C) long-axis and (D) cross-sectional views of 4-mm stent. There is no evidence of ISR with all density measurements between 349 and 385 HU.
collimation, tube voltage of 120 mV, effective tube current 350–780 mA. Radiation reduction with ECG tube current modulation was used. After scan completion, multiphasic reconstruction of MDCT angiography scans was performed, with reconstructed images from 70% to 80% by 5% and from 5% to 95% by 10% increment.
MDCT image reconstruction and interpretation Individually scanned raw data sets were uploaded and analyzed on dedicated workstations (Advantage Workstation; GE Healthcare, Milwaukee WI). All stented segments were evaluated by a reviewer who was blinded to the ICA findings. Image reconstruction was retrospectively gated to
an ECG, and the optimal cardiac phase showing the least motion artifact was determined. Stented segments were viewed in their short and long axes by using both the original 2-dimensional axial images and multiple image reconstruction methods. In addition, they were viewed with a wide window width and the minimum image thickness possible. A fixed window width of 1200 Hounsfield units (HU) and window level of 240 HU were used for all interpretations, to minimize interscan differences and to reduce a source of error for measurements. The location of each stent (vessel and segment) was recorded. Image quality of each stented segment was judged as adequate or not for the evaluation of stent patency and ISR. The stent was considered interpretable if the stent lumen was visible and the contrast density of the
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lumen could be evaluated without or with negligible influence of image degrading factors. The reason for noninterpretability was recorded such as partial volume effects, metal artifacts from stent struts (beam hardening), cardiac motion artifacts (banding artifacts), and calcification on the vessel wall. Density measurements in Hounsfield units and stent diameters in millimeters were recorded in the proximal, middle, and distal sections along the long axis of the stented segments in all interpretable stents (Fig. 1 and Fig. 2). Density measurements were determined by using the exact center point in the short axis of all the designated sections. Stent diameters were determined by measuring the length of a straight line segment running from the middle of the stent rim from one end to the other through the center point along the short axis of all the designated sections. The density measurement of the ascending aorta was determined by using a region of interest area (100 mm2) in the center of the shortaxis view and was taken as the reference density measurement. The ratio of the average of the stent’s proximal, middle, and distal densities to mean ascending aorta (AO) density was defined as the average stent to AO (AS/AO) HU ratio. The average of the stent’s proximal, middle, and distal diameters was defined as the average stent size. To determine observer variability, a subgroup of stented segments were reevaluated by the first reviewer and a second who was also blinded to the ICA findings. Thirty stented segments in 20 patients were re-reviewed by the first reader and reviewed by a second reader to determine the intra-reader and inter-reader variability associated with our method of quantitative evaluation of stent patency. With the use of the Pearson’s coefficient of correlation in this subgroup, we found an r 5 0.86 for intraobserver and r 5 0.82 for interobserver evaluations. The mean intrareader variability was 6.9% 6 9.7%, and the mean interreader variability was 9.9% 6 0.4%.
receiver operator characteristic (ROC) curve was calculated for MDCT angiography to identify obstructive coronary artery stenosis at the 50% threshold in the stented segment. A second ROC was used to determine the minimum stent size to predict feasibility of MDCT coronary angiography in diagnosing stent patency. Pearson’s coefficient of correlation (r) was used in the subgroup analysis to evaluate intraobserver and interobserver variability. All statistical analyses were performed with the use of SAS Proprietary Software, Release 9.1 (SAS Institute Inc, Cary, NC).
Results Patient/stent characteristics and angiographic findings A total of 122 stented segments in 55 patients (age, 65 6 10 years; 90% men) were studied, among which 106 stents (86.8%) were interpretable for evaluation of stent patency by 64-row MDCT angiography. Reasons for noninterpretability of the 16 stents were caused by metal artifacts in smaller sized stents (7 stents), motion artifact (4 stents), partial volume effects (3 stents), and calcification on the vessel wall (2 stents). There was no difference in age, sex, or the vessels stented among interpretable and noninterpretable stents. The majority of noninterpretable stents were located in distal arterial locations (56.2%). By ICA, 24 stents were occluded (22 in the interpretable group and 2 in the noninterpretable group). Stent characteristics for the 106 interpretable stents among patients with and without stent patency are shown in Table 1. Patients with patent stents tended to be older
ICA procedure and analysis Selective ICA was performed by standard transfemoral arterial catheterization. A minimum of 8 projections were obtained with a minimum of 5 views for the left coronary artery system and a minimum of 3 views for the right coronary artery system. Figure 3 shows an example of one view obtained of a patent left anterior descending coronary artery stent. All stented segments were qualitatively evaluated by an experienced cardiologist who was blinded to the results of the MDCT. ICA was used as the ‘‘gold standard’’ for determining stent patency, with ISR defined as a diameter reduction of R50% compared with the original stent diameter.
Statistical analysis Categorical variables are presented as frequencies and percentages, and continuous variables are presented as mean 6 standard deviation (SD). The area under the
Figure 3 ICA of patent left anterior descending (LAD) coronary stent (patient from CT angiography in Fig. 1), with moderate disease distal to stented segment.
Abdelkarim et al Table 1
Evaluating coronary artery stent patency with MDCT
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Stent Characteristics on MDCT among patients with patent stents and ISR Patent Stents (n 5 84)
Age, y, mean 6 SD Male sex, n (%) Stented vessel Left main, n (%) Left anterior descending, n (%) Left circumflex, n (%) Right coronary artery, n (%) Saphenous vein graft, n (%) Segment Proximal, n (%) Middle, n (%) Distal, n (%) Stent size, mm, mean 6 SD Density AO, HU, mean 6 SD Proximal stent, HU, mean 6 SD Middle stent, HU, mean 6 SD Distal stent, HU, mean 6 SD Average stent, HU, mean 6 SD Proximal stent/AO, HU, mean 6 SD Middle stent/AO, HU, mean 6 SD Distal stent/AO, HU, mean 6 SD Average stent/AO , HU, mean 6 SD
66 6 11 70 (93)
ISR Stents (n 5 22)
P
60 6 12 15 (83)
4 (4.8) 35 (41.7) 9 (10.7) 24 (28.6) 12 (14.3)
0 8 1 7 6
0.02 0.31 0.15
(0) (36.4) (4.5) (31.8) (27.3) 0.06
37 (45.7) 30 (37) 14 (17.3) 2.87 6 0.66
6 (27.3) 7 (31.8) 9 (40.9) 3.26 6 1.07
369.1 6 77.1 397.7 6 145.7 397.2 6 108.8 380.9 6 94.1 391.8 6 95.7 1.10 6 0.34 1.09 6 0.32 1.04 6 0.21 1.07 6 0.24
391.1 6 97.2 132.4 6 167.3 132.1 6 153.6 152.6 6 168.4 121.6 6 120.6 0.33 6 0.39 0.37 6 0.45 0.41 6 0.44 0.37 6 0.37
0.03 0.33 0.043 0.039 0.036 0.0001 0.0001 0.0001 0.0001 0.0001
Stent patency was diagnosed by invasive coronary angiography.
(66 6 11 years verus 60 6 12 years; P 5 0.02). Most stents with ISR were located in the distal segment of the stented vessel (40.9%), as opposed to patent stents that were mostly located in the proximal segment of the stented vessel (45.7%). By ICA, 84 (79.2%) of interpretable stents and 14 (87.5%) of noninterpretable stents were patent; no significant difference was observed among the groups (P 5 0.44).
Density analysis Density measurements for all of the 106 interpretable stents are shown in Table 1. No significant difference was observed in the reference density of the AO among patients
Table 2
with and without patent stents (369.1 6 77.1 HU versus 391.1 6 97.2 HU, respectively; P 5 0.33). The density measurements in all stent sections (proximal, middle, and distal) were significantly lower in the ISR stents than in the patent stents. The AS/AO HU ratio was also significantly lower in ISR stents compared with patent stents (0.37 6 0.37 HU compared with 1.07 6 0.24 HU; P 5 0.0001). The results of ROC curve analysis for the relationship between stent patency and quantitative luminal stent density measurements are shown in Table 2. A mean stent attenuation value greater than approximately 300 HU correlated with stent patency with a sensitivity of 95.2% and a specificity of 84.3% (AUC 6 SE, 0.95 6 0.019; 95% CI,
ROC curves created to assess the ability of density measurements to predict stent patency
Proximal stent Middle stent Distal stent Average stent Proximal stent/AO Middle stent/AO Distal stent/AO Average stent/AO
AUC 6 SE
95% CI*
Cut point
Sensitivity, %
Specificity, %
PPV, %
NPV, %
0.90 6 0.030 0.92 6 0.027 0.88 6 0.030 0.95 6 0.019 0.93 6 0.023 0.88 6 0.033 0.85 6 0.039 0.95 6 0.019
0.83–0.95 0.85–0.96 0.80–0.93 0.91–0.99 0.87–0.97 0.81–0.94 0.76–0.91 0.89–0.98
255 247 219 302 0.85 0.76 0.68 0.81
81.8 81.8 77.3 95.2 81.8 77.3 72.7 90.9
90.5 95.2 96.4 84.3 92.6 95.2 97.5 95.2
90.0 95.3 96.2 86.3 92.1 95.1 97.3 95.7
83.3 84.1 81.3 95.4 83.6 81.2 78.2 91.3
PPV, positive predictive value; NPV, negative predictive value. *P 5 0.0001.
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0.91–0.99; P , 0.0001). The optimal AS/AO HU ratio for the diagnosis of stent patency was 0.81 with sensitivity of 90.9%, specificity of 95.2%, positive predictive value of 95.7%, and negative predictive value of 91.3% (AUC 6 SE, 0.95 6 0.019; 95% CI, 0.89–0.98: P , 0.0001) (Fig. 4).
Stent size threshold analysis The optimal stent diameter needed to evaluate stent patency on MDCT angiography was R2.5 mm (Fig. 5). At this threshold, stent patency was detected with a sensitivity of 93.8%, specificity of 91.8%, positive predictive value of 92.1%, and negative predictive value of 93.8% (AUC 6 SE, 0.97 6 0.01; 95% CI, 0.93–0.99; P , 0.0001).
Discussion The current literature available on the use of MDCT for the qualitative evaluation of coronary artery stent patency shows a range of sensitivity from 86% to 100% and specificity from 91% to 98%.8–14 Despite this apparent accuracy, concerns exist over the ability to translate these findings to daily practice, given the need for qualitative assessment and a requirement for advanced reconstruction and postprocessing techniques. As such, an objective and quantitative method of evaluating stent patency with 64-row MDCT would potentially enable greater confidence in evaluation of stent patency. In this study, we have shown that a mean stent density R300 HU and a difference in CT density from the reference vessel (AO) to the coronary stent lumen of %19% correlated with a patent stent with a high sensitivity
Figure 4 ROC curve of average stent/AO HU ratio. PPV, positive predictive value; NPV, negative predictive value.
Figure 5 patency.
ROC curve of stent size and interpretable stent
and specificity. With the use of this method, the evaluation of coronary stents R 2.5 mm in diameter appears feasible. Advantages of this method include its quantitative nature and the ability to apply it without special reconstruction techniques. Furthermore, display parameters such as window width and display levels do not affect Hounsfield unit measures, so this is independent of reader preference and workstation settings. This study marks the onset of an era in which MDCT angiography has emerged as a viable and effective option in the noninvasive evaluation of patients with coronary stents after PCI (Fig. 6). Prior studies have applied densitometric analysis for the analysis of coronary stents. In a study evaluating a complicated method that compared density measurements in the proximal, mid, and distal regions inside each stent, 60% ISR was detected with a sensitivity of 54.5% and specificity of 94.7%.8 An alternative method, proposed by Kitagawa et al,15 evaluated ISR with the use of quantitative density analysis in 16-row MDCT angiography. A density measurement of R5 region of interest (area %1 mm2) positioned in a low-density area of the stent, compared with reference vessels (sites proximal and distal to stent), suggested ISR in the present of high variability of density values (75 6 10 HU to 270 6 7 HU). These initial studies led to a subsequent study by Kitagawa et al,16 in which regions of interest were assessed in a reference segment 5–10 mm proximal to the stented segment and compared with regions within the stent. The study found that the accuracy for detection of ISR was high with the use of either a mean CT density cutoff value of 280 HU (sensitivity of 83% and specificity of 83%) or a DCT density cutoff value of 75 HU (sensitivity of 100% and specificity of 96%). As in the study by Kitagawa et al,16 we found that a mean in-stent attenuation value of 300 HU was highly
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Figure 6 Cardiac CT Angiography of a proximal left anterior descending (LAD) stent with ISR. (A) Long-axis and (B) cross-sectional images showing a dark center in the proximal portion of the stent measuring 216/239 HU, far below the aortic comparison of 414 HU in this study. (C) This evidence of ISR correlated well with the invasive coronary angiogram.
accurate for the exclusion of ISR. In addition, the evaluation of relative attenuation within the stent compared with an aortic reference segment avoided the confounding effect of interpatient variability of cardiac output and volume of distribution of intracoronary contrast. We chose to use aortic attenuation as a reference segment because this region is less influenced by various artifacts (partial volume effects, beam hardening from nearby stent struts, banding, and vessel calcification) that may result in either an overestimation or underestimation of D CT density. With the use of this method, we have shown that quantitative assessment of ISR can enable the evaluation of stent diameters of %2.5 mm. Despite the superiority of 64-row MDCT compared with older generation scanners, limitations in coronary stent
evaluation include inferior spatial resolution compared with ICA, relatively low temporal resolution, and increased false-positive rates for detection of ISR in smaller stents. Stent design (diameter, length, strut thickness, and material) is the main limiting factor for the application of MDCT in the detection of ISR. Smaller diameter stents remain a challenge. In one recent study evaluating longterm outcomes of drug-eluting compared with bare-metal stent diameter data in 13,890 stents, 30% of implanted stents were ,3.0 mm in diameter and 4.8% were ,2.5 mm in diameter.17 Trends in stent design and size will have an effect on MDCT stent assessability. In a study by Schuijf et al,18 90% of stents . 3.0 mm were interpretable, compared with only 72% of stents , 3.0 mm. In addition, 89% of stents with strut thickness , 140 mm were interpretable,
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2.
3.
4.
5.
Figure 7 A high-definition cardiac CT angiogram showing patency of multiple stents with good distal filling. This new technology applying thinner slice thickness with specialized reconstruction techniques may allow better visualization of stents with fewer artifacts.
compared with only 41% of thicker strutted stents. The future may see refinements in MDCT scanner (Fig. 7) or stent technology that further enhances the accuracy of noninvasive stent patency evaluations. Our study has several limitations. This was a retrospective study; therefore, complete information was not available on stent design and the time between stent implantation and MDCT angiography acquisition. The lack of established criteria for quantitatively evaluating stent patency and the overall small size of our patient population make it difficult to draw generalized conclusions for the application of our method to all patients with coronary stents receiving MDCT angiography. Finally, inherent problems or limitations in the MDCT angiography technique exist such as radiation exposure, use of iodinated contrast, and the absence of functional or perfusion data.
Conclusion The use of 64-row MDCT angiography coronary artery stent patency can be evaluated quantitatively with high sensitivity and specificity and with improved diagnostic predictability in stents R 2.5 mm.
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